Reroutes, and gaskets, and data...
 

The recent thread about the new Qmax coolant reroute package has re-awoken an old thought scratching around in the back of my head. It's about the differences in the stock head gasket between the 01-05 engines and all the other NA/NB engines.

There was a big thread about this back in 2010 or so on M.net. In was mostly Jason_C and myself documenting the locations of the coolant passages on the blocks, heads and gaskets of various BP-series engines. The short version is that most of us are familiar with the shortcomings of the cooling-system design in the NA and NB engines, stemming from the fact that when Mazda took what was originally a transverse FWD engine and turned it 90° to stuff it into the Miata, they wound up relocating them thermostat and radiator feed from the "back" (flywheel side) of the engine to the "front" (pulley side). This design deprives the rear of the engine of coolant flow when the thermostat is open, thus causing the back of the head to run much hotter than the front, and decreases the efficiency of the system as a whole by allowing significant amounts of coolant exiting the pump to travel straight up and then right out the front of the head again, bypassing most of the block and head, and not picking up nearly as much heat in the process as it otherwise could.

What few people (myself included) had noticed until Jason brought it up was that in 2001, Mazda apparently acknowledged this shortcoming, and re-designed the head gasket to block off quite a lot of coolant passages, with the effect that the coolant was now forced to take the long way around the engine, thus permitting it to actually do its job.

I wound up creating the following illustration which depicts the active coolant passages between the block and the head, taking the gasket into account, for a '90-'00 engine vs. an '01-'05 engine. In these images, we are using our X-ray vision to look straight down through the top of the head:

https://cimg7.ibsrv.net/gimg/www.mia...1990a9dc18.png



For anyone who has spent a significant amount of time pondering the design of the cooling system, and the various aftermarket solutions to it, the ramifications of this should be pretty obvious. Assuming a totally stock configuration, the '01-'05 gasket design means that the coolant can no longer just flow right up the front of the engine and then out the thermostat. It's forced to travel back through the block into the #2-3 area, then flow upwards, and then flow out both the front (thermostat side) and rear (heater core side) of the head.

Seems like a pretty clever solution which, from Mazda's point of view, incurred zero additional cost and required changing only one part; the head gasket. (It should be noted that this head gasket design is totally unique to the '01-'05 Miata, whereas the '90-'00 head gaskets are the same parts shipped on all of the various FWD implementations of the B engine.)



For those who have NOT spent a significant amount of time pondering the design of the cooling system, the ramifications, and indeed the very core of the problem, may be non-obvious or even counter-intuitive. I'm not going to offer a primer here, merely an apology: if you don't already know what we're talking about, then sorry, but you're in the wrong thread.




And this got me to thinking...


1: Does the stock design in the '01-'05 engine actually suffer from the same basic inadequacy as its predecessor? Was Mazda's solution vis-a-vis the new head gasket design every bit as effective as a rear-thermostat reroute in the earlier engines? Or do these engines still have the potential to benefit from the relocation of the thermostat back to where it belongs?

2: On the obverse, can doing a rear-therm reroute on an '01-'05 engine (with the stock head gasket) actually do more harm than good?



My reasoning is as follows: With regard to the image above, on the side labeled '03, visualize the flow of coolant as it exits the pump and is forced into the front of the block, around the #1 cylinder. We can see that the coolant is being encouraged to travel down the block until it reaches the #2/3 cylinders, then turn upwards and flow into the head. Now, were the thermostat still at the front of the head, the coolant would take two diverging paths. Some of it would flow towards the rear and exit through the heater-core feed, picking up heat from the #3 and 4 combustion chambers. And some of it would turn towards the front and exit the thermostat housing, picking up heat from the #1 and 2 chambers.

But imagine that we have relocated the thermostat to the rear, and completely blocked off the front coolant port. Now, all of that coolant flowing upwards around the #2 and 3 cylinders is going to turn towards the back, picking up heat from the # 2-4 chambers on its way out the exit. But look at the #1 chamber; its basically a dead zone, particularly at the front and intake sides.


To my overly analytical and yet data-deprived mind, this would seem to actually create a problem exactly opposite that which a rear-therm reroute solves in the earlier engines. We're now depriving the #1 combustion chamber of coolant, and forcing it to run hotter than the rest.


But of course I have no data to either prove or disprove this theory.


I've read a number of anecdotal accounts, in which people have said "I did a rear-them reroute on my '01-05 engine, with the stock gasket, and it didn't blow up." Ok, I'm totally on-board with this. We know that rear-therm rerouting on an '01-'05 engine doesn't destroy it.

But does it actually do any good? And, more specifically, does it, in fact, cause the situation which I've postulated above, to wit the temperature gradient across the head actually becomes more uneven than it was before?


I'd love to see this data. Has anyone actually instrumented an '01-'05 head with multiple temperature sensors, and done an A:B comparison before and after a rear-them reroute, to actually determine whether we're making the situation better or worse?




And we finally come to the post which triggered this whole line of intellectual gluttony. It's copied from a different thread, so paging @yossi126:

Ok, but was this with or without a rear-them reroute?

If without, then when you are saying makes total sense, and supports my presumption.

If with, then this totally throws my theory on its head, and also contradicts much of what has been said on the matter by knowledgeable persons, to wit (paraphrased): "The '94-'00 gasket with a reroute is best, but the '01-'05 gasket with a reroute is better than nothing."

Joe, I too was wondering about this. I actually, once remember explaining to someone asking about this, exactly what you just posted. This is why you don't reroute with a 01+ headgasket.

It is my belief as well, that this would stagnate coolant flow in the exact same places you listed, effectively duplicating the problem we had in the 94-00 cars in the cylinder 4 area, just on the opposite side of the engine.

One glaring exception, the reroutes do not relocate the CLT sensor to cyl 1. If on the 94-00 vehicles with no reroute, stagnant coolant flow to cyl 4 began to be a problem, it would reflect in the CLT reading. Not so in this case. I feel the line of thought that has appeared (Read: A reroute is ALWAYS better no matter what headgasket) may not be well supported. I myself have not seen validation of CLT temps at the front of cylinder 1 post reroute on a 01-05 gasket.

How effective is the 01-05 gasket? I don't know. I have not done any track days on my built engine, so I can't tell you that perspective. But my N=1 experience, hooning my car around in 105 degree heat, then idling for 30 minutes on searing hot pavement in essentially grid lock with one cooling fan down did not result in a CLT reading over 205* My eyes are always on my shadow dash in those situations.

That being said, I still believe the 94-00 gasket with a reroute to be the optimal cooling solution for the BP engines.

WHAT ASSHAT JUST QUOTED ALL THAT. I DONT EVEN HAVE THE TIME TO SCROLL UP AND FIND OUT. if I still had mod powers...there's a warning for being a retard!

u mad brah :giggle:

"Ok, but was this with or without a rear-them reroute?"

Yes, both cars are running rear-therm reroute.

Am I understanding correctly that Yossi's sample of 1 event, 2 cars, tends to indicate that the best choice is NB2 gasket with re-route? If, of course, one looks at Temp at back of head only.

One other approach is to use NB2 gasket, without re-route, on all NA and NB's. EDIT: Inthink it has already been shown that, in all cases, a reroute is better than none.

Those two cars had different radiators, fans and no mention of how good the ducting is....... That isn't a data point.

The easy test here is to add a temp sensor to the front of the engine and compare temps pre and post reroute.

This is what I suspect. And yet I can't prove it.




I actually believe that the situation may be worse in the case of the '01-'05 engine.

In the early cars, there wasn't a lot of coolant flow around the #4 combustion chamber when the thermostat was open, but at least there was some. The rear outlet to the heater core, while narrow and restrictive, was at least always open. By comparison, when you rear-them an '01-'05 engine, there is literally no laminar flow whatsoever around the front and intake side of the #1 chamber, merely whatever convective and turbulent transfer might happen to occur by chance.

Is this causing '01-'05 engines to be destroyed en masse? Clearly not. We have enough anecdotal evidence to reach this conclusion.

I merely ask whether the rear-therm reroute has any benefit at all on the '01-'05 design, and posit that it may actually cause some small amount of harm.




And that's why I'm leery of all of the "data" which to data supports this configuration. It would not be especially hard to settle the question; just tap a hole in the blockoff spacer which is normally installed at the front of the head when a rear-therm reroute is done (assuming the water neck is being removed entirely) and stick a temp sensor into it. Log this sensor alongside the rear sensor, and post a graph showing how much the two spots deviate from one another, how much the temp at one location lags / leads the other as overall temp changes (eg: due to movement of the thermostat), and so on. Repeat for an engine with an '01-'05 gasket (and an otherwise identical configuration), and that'd pretty much settle the issue.

Sadly, I lack both the physical health and the number of Miatas required to perform this test at the moment.





Huh, turns out you're right. :giggle:




I believe you are, given that:

1. There's obviously a big difference in radiators between the two cars,
2. "Overheating" is not defined in this context,
3. Yeah, we're probably talking just about the temp behind #4, which totally ignores the problem which I suspect may be occurring, to wit; the temp at the front of #1, and
4. I'm not entirely sure how much I trust one anecdotal datapoint supplied by yossi126.

Of course, maybe there's some external factor that I'm completely blind to which, in the case of the VVT engine, makes none of this matter. Maybe there's a lot of thermal conduction happening through the MLS gasket, maybe the #1 cylinder in these engines just naturally wants to run cool. Maybe Kanye West actually does posses some modicum of talent. I can't prove any of these things, but I also can't disprove them.


Replacing the head gasket is a lot of work. Bolting on a reroute kit is relatively quick and easy.

I wouldn't posit that the NB2 gasket with front-therm is superior to the earlier gasket with rear-therm. I'd accept that the two configurations might be close in both overall system performance and linearity of temperature across the head. But, again, I have no data to support any of this, nor does anyone else appear to have any data to refute it. And that's just frustrating as hell...

^^ Oh, I suspect that Andrew and Emilio will be on shortly (California time) to set this all into perspective.

Just so my reference for "overheating" is clear, the point which we both set a rev limiter on is 225f or 107c, at which point we backed off. I was never going to go into the science bit of things, as there are too many variables between us. And to the guy who wondered, no ducting or hood vents on neither of the cars.

I wish someone would. Emilio & I have discussed this concept offline some time ago, but so far as I'm aware, no one has yet gathered any hard data on the matter.





So, as per DNMakinson's remarks, we're looking at the temp at the back of the head only.

This is interesting.

I'm not ready to accept that a rear-therm '01-'05 config is superior to a rear-therm '94-'00 config in any way. In fact, I flatly reject this hypothesis.

I can buy that the 949 radiator is probably the best unit out there, and that numerous other tedious differences likely existed between the two vehicles.

And I'd have really, really loved to have had some data as to the temps up front at the #1 end of both of these engines while the #4 end was heading towards the top of your scale.


But alas...

Don't you have an 01+ Miata? Install qmax reroute (when the lung reinflates) and tap a few holes for thermocouples in the 4 corners of the head.

You seem to underestimate both my cheapness and my apathy.

For what it's worth, which may not be much, I've done some combustion chamber cooling design work, and at least intuitively, I agree with Joe. The NB2 head gasket with a coolant reroute seems likely to result in dramatically reduced flow around cylinder 1. According to the nifty x-ray diagram, only one corner of the cylinder would see any coolant, which would result in a very poorly and unevenly cooled combustion chamber. I would think such a non-uniform cylinder temp could potentially cause issues for the ring pack too. In fact, this seems to be a flaw with the NB2 gasket block-off trick in general, since 3 out of 4 cylinders only see coolant on one side. That being said, my (somewhat limited) experience in power cylinder design comes from industrial diesel and natural gas engines that are required to last 80k hours, so "little" things like uneven cooling are a big deal. Nonetheless, for that reason, I would say the NA/NB1 gasket with the coolant reroute would be the most effective.

I would also say the yossi data point is worthless because of the different radiator setups, unless the delta T across each radiators could be known with some accuracy and taken into account. Even then, though, the cyl 4 temp sensor will only tell part of the story, and it's the cyl 1 temperature that's concerning.

*edit*
When I say "see coolant" I should be saying "see coolant flow." I understand there will still be coolant there, but if it's not flowing, it's not doing much cooling.

If cyl#1 in the 01-05 config is seeing less flow and the pressure differential in both setups is similar, would you expect to see more flow reverted past cyl#4? Could that then result in lower temp measurements at the sensor?

I'm wondering if the front cylinders are seeing a sort of "heat engine" effect. The pump itself may not be producing much flow, but the temperature differential helps. There could also be some pressure differential induced flow as well. Water "exiting" the system leaves a lower pressure area and brings some of the high pressure (hotter) water into the flow with it. Its been a long time since I've studying thermodynamics, so I could just be pulling random bits out of my head.

you bought a keyboard and you use it. that would be the extent?

I had a 2001 with no reroute and it overheated. Mostly because the water turned into liquid iron and started turning freeze plugs into weep plugs.

Prior to that clt temps were pretty solidly in the 220-230F range when it was running hot. It never asplode.

Lower than what?

If you mean "lower than the temp around cyl #1," then yes, that's exactly what we're talking about.

Liquid Iron? I have no porpoises.

But I have been keeping a close eye on CLT in my stock '04 since noticing the leak (which I still haven't fixed) a few weeks ago. As reported by OBD2, seems to hover around 210-215° on the street at 80-90° ambient with the aircon running, which is pretty much ideal. (This is mostly freeway driving, not stop-n-go.)

I find myself being annoyed by the relatively greater complexity of removing and installing the cam cover on the VVT engine as compared to the earlier models...

It is unbolting the hard oil line at the back of the head, and then unplugging the VVT pigtail. Then no harder then any other VC on a miata. Well maybe having to remove the stock COPS.

I agree.

No, lower relative to the same car running a 94-2000 head gasket (both inc reroute).

The real bitch is not dropping those copper washers. They have magical properties that make them attracted to places you cant see or reach.

​​​​​​​Wait, yes. The cyl 4 temp would be lower, what we are worried about is what is happening on cyl 1.

Since I doubt anyone has done the exact testing that would be needed to give an answer to your questions, perhaps gathering data on failures and the cooling setups in place at time of the failure would be worthwhile. For example, if everyone without a reroute was nuking #4 stock pistons, while those with a reroute the distribution was random, that would suggest the reroute on those cars fixed a hotspot issue and the reroute had positive real world affects.

Well, I think one of us needs to man up and do it and acquire the data. Anything less, then we may as well resort to techsalvager style coolant manifolds for guaranteed even coolant distribution.

The problem is, many such as myself are having zero issues, have little motivation to work on their cars, and only tend to do such when something breaks, or the value of the upgrade supercedes our desire to enjoy our car and we believe we would see some tangible benefit from it. Be it improved torque, more reliability, or the unparalleled throttle response we get from datalogging with our sharpie.
When you confound this with possibly adding another unintended issue to fix, such as head removal for a 94-00 gasket. Heads explode.

Tl;dr, we are wondering why we should when we are seeing no issue, and another issue could potentally exist that anyone that has made it through 2 years of engineering school, or just has a general working idea of how water works could spot.

As a newbie VVT owner, this is the kind of stuff I need to learn.

The FSM, for instance, says that I need to remove the oil pipe from the top of the cam cover, then remove the oil control valve (VVT solenoid), and THEN remove the cam cover as I would on a non-VVT engine. And of course there are a lot of gaskets and O-rings in the mix which are all supposed to be replaced...

Before I went into the hospital, I was about to buy FM's "complete timing belt kit," and I noticed that none of this stuff was included. I emailed Keith Tanner asking about this, and the response was basically (I'm paraphrasing) "Wow, you're right. Never noticed all that stuff wasn't included. I'll update the kit."

So either very few NB2 owners have actually bothered to replace a timing belt, or I'm just totally clueless. Either way doesn't fill me with confidence, and doesn't mesh with Scott's advice that one simply unbolt the hardline at the back of the head and proceed as normal.

This is what I get for being so far behind the curve, and having gone several years without owning a car of any kind, much less a Miata. Lot of catching up to do on the basics...

But I'm getting off track...

Ok, let me make sure that we're both on the same page here as far as assumptions. If I read you correctly, you're saying the following:


Referring to my image in post #1, you assume two engines, one with a pre-'01 gasket and one with an '01-'05 gasket, both with a rear-therm reroute. And I understand you to be positing that the CLT sensor at the back of the head should read lower on the '01-'05 engine than on the pre-'01 engine? Is this correct?


I don't see why this would be the case. Or why it would matter.

If we assume the total coolant flow (litres / minute) to be a constant, and assume that all other variables (load, radiator design, overall health of engine, etc) are identical, then I'd expect the temperature registered by the CLT sensor to be pretty much the same.

But this misses the point I was trying to make in Post #1. Specifically, that it really doesn't matter what specific value the CLT sensor is reading. What I'm concerned with is how much the temperature varies between the #1 and #4 combustion chambers.

In the early engines, with the stock (front-therm) design, we know that the #4 chamber runs a lot hotter than the #1 chamber. This is well-documented, and makes perfect sense when you visualize the flow of coolant through the engine. We also know that the overall performance of the cooling system is compromised by the fact that we're allowing quite a lot of "cold" coolant to exit the pump, flow straight up through the front of the engine, and then go right out the thermostat into the radiator again, having not passed through the majority of the engine.

Based solely on speculation and analytical thought, I posit that the redesign of the head gasket in the '01-'05 engines essentially solves this problem. I base this both on having poured a lot of neurons into thinking about the design, and also on the assumption that Mazda actually bothered to spend some engineering time analyzing the problem before arriving at this solution which, again, is completely unique to the Miata was was not implemented on any of the various FWD implementations of the same engine.


And thus, I return to the two basic questions:

1. Does the cooling-insufficiency problem which plagued the '90-'00 engines even apply to the '01-'05 engines, and
   
2. Does applying the same "fix" to the '01-'05 engines actually make the situation worse, and create a new problem vis-a-vis a lack of laminar coolant flow around the #1 and #2 combustion chambers?

And I really wish that I had some real-world data to look at here, as opposed to speculation. But nobody on the SEMA-side of our community seems to be asking the same question, and I'm far too cheap and lazy to sacrifice a head to generate this knowledge. All I can say with any degree of certainty is that this '04 I'm driving now, with a totally stock cooling system, doesn't seem to be plagued by the same overheating problems (even with a functional aircon system) that I experienced in my previous NAs (without aircon.) This anecdotal evidence does not specially address the delta-T question, but it does suggest that the issue of overall insufficiency in the cooling system was in fact ameliorated by in the '01 gasket redesign, and this presumption is reinforced by theoretical analysis of the differences in coolant flow through the engines between the two designs.

You probably want to re-anneal those copper washers before reinstalling them, BTW.

My thoughts on the VVT head gasket issue are:

- If you're running NA, use a good rad (I like the TSE one), ducting, etc, but don't reroute. That's probably good enough.
- If you're running FI, pull the head to swap the gasket for a 99 one, and that makes it really easy to install the reroute without having to wedge your arm between the back of the head and the firewall.

--Ian

Well... I have no tangible benefit to offer this thread, but it is timely for me. Perhaps I can supply some data in the near future.

I have an N/A NB2 that I track regularly. The only cooling system mod it has is a Koyo radiator. In up to 95F ambient heat, I have never seen the coolant temp, as measured at cylinder 4, exceed 206F on the track, and I data log every session. That has caused me to conclude that I do not need a re-route, as long as I stay N/A. However, with an eye toward F/I in the future, I just installed the M-Tuned re-route kit. Now, I wonder if I have done more harm than good with that expenditure. (With the transmission still out, I could quickly and easily reverse that situation.) It seems the thing to do is install a temp sensor in the water neck block-off plate and wire it up to the MS for logging.

You likely won't get usable data from putting a sensor on an area with no flow like the block off plate.

And I never studied thermodynamics but it would seem to me that the re-route with the early gasket would be better since you would not be forcing hot water back to the #1 cyl which is already being limited in flow and especially on the intake side. Wasn't there a pic in a thread about JDM (yo!) pistons where it showed the piston missing a chunk at the intake side on cyl1? Will see if memory is correct when I find it. Not empirical data obviously but maybe a sign.

And Joe, running 210-215 in traffic sounds a bit high. Can't remember off the top of my head but I believe I run ~200 in city and at the t-stat opening on the highway. This is an 01 engine in a 95 car with the AC on so both fans pulling air as yours would be. I'd say your rad is shot.

Edit: looks like cyl 1. Like I said, not empirical data, etc. and I have no idea of re-route or not, or the gasket used, or.......
https://cimg7.ibsrv.net/gimg/www.mia...c3ec81d696.jpg

I don't remember much about fluid dynamics, but I do remember how unpredictable flow is in many circumstances. The only way to know how much flow is at the front of the head with a re-route installed is to measure it. Who knows what that sensor might see? If the water is stagnant in that location, all the better. Compared to my previous situation, it might tell me something whether a re-route hurts or helps an NB2. Front of the head vs. back of the head data is useful to me in its own right.

I'm happy to see that I've been able to instill fear, uncertainty and doubt in someone. :giggle:

But I would love to see instrumentation of the front of the head vs the back of the head under load, preferably after the two (presumably different) sensors had been simultaneously calibrated in a pot of boiling water so that we can differentiate measurement error from actual data.

ME majors, tell me if I'm in the weeds here, but I *think* that this test should give a reasonable indication of the delta-T across the metal of the head casting as well, which is what we *really* care about.

Well I did state "likely" and "usable" but I did not ask what block off plate you were using.

If the block off plate is on the t-stat housing, I would think it would not be usable data. The stagnant water temp change would come by way of convection so I can't imagine it being useful but since you did study this stuff I will defer to you.

Now if the block off plate deletes the t-stat housing, then that would be usable data since it "should" register temp from flowing water since the sensor would be in the coolant port.

Either way, I would do the early head gasket if doing a re-route and could guess that it would be better than the 01-05 gasket with or without a re-route.

It might tend to bias the results upwards in that location.

But if the coolant temp is higher in that region, we can assume that the head temp is also higher, because second law of thermodynamics.



On the one hand, I know for a fact that the radiator is shot and the system isn't holding pressure. I posted a pic of the leaking coolant a few weeks ago. Got a 949 crossflow radiator sitting in the garage waiting for me to be physically healthy enough to install it.

On the other hand, I'm reporting what's being told to me by a freeware Android app communicating with a $15 Chinese OBD-II adapter reading a 13 year old CLT sensor.

And on the gripping hand, 210-215° is pretty much dead in the middle (well, maybe the upper third) of the normal operating range for a modern-ish engine. It's kind of the whole reason that these cooling systems are pressurized, as opposed to the one in the Ford Model T.

Uh, no. American and European cars, yes. They run at 210-230. Japanese cars, especially older ones, usually run right at thermostat temperature or a little above. So 190-200.

I honestly think the whole reroute question will never be answered. You can never control the ducting, radiator, engine, fans, weather, coolant mix, or driving conditions enough to get accurate results. We found a potential problem, fixed it, and now it's up to the owner of the car to monitor and maintain healthy coolant temperatures.

I've seen rerouted cars over heat due to minor leaks, poor ducting, etc, along with a number of non-rerouted engine.

We have an enduro car that has a SM style air dam, great ducting, and a rerouted 01 engine, with 94-00 head gasket. Obviously no AC, Koyo's older 52mm radiator, and we run distilled water+water wetter. We were concerned recently with ~215 degree coolant temperatures in 90* heat, that's right on the brink of blowing a head gasket, but it survived for 8 hours of racing.

The shop I work at also has a town bicycle type Miata, 94 with a VVT swap. I believe it has the 01-05 head gasket, with no rereroute and a stock radiator that we run coolant in. Manages to survive a small amount of daily driving, and recently survived our track day, where it was driven by a few people all day long in 80+ degree heat.

The point is I don't seem to see any connection, however I do recommend a reroute to anyone who wants to seriously track their Miata.

Hopefully that's a typo, and you mean more than 215. Perhaps 255?

Humor me for a moment. As I said upthread, I track my car roughly twice a month in ambient temps up to 95F, and I have no issues with coolant temps. I have the Koyo 36mm radiator with some sealing. Everything else is stock, and I have run straight FL-22 up until now. Highway cruising temps are right at 194F, which leads me to believe the thermostat and sensor are both accurate. Peak coolant temps on track are 205.8F at the back of the head in up to >=90F weather. These data lead me to believe the re-route is unnecessary in a N/A NB2 with a good radiator.

The question in my mind right now is whether the re-route does more harm than good in a car with a 00-05 head gasket. I'm sure it's fine, but is it really any better at all, or even a little worse?

I see my name has been invoked in vain.
My solution with my 01 head was to take the 99 gasket and add 4 holes to the gasket in the area near the exhaust valves, with a 1/4" punch. Green circles below, added to Joe's diagram. (With a coolant reroute, of course). My reasoning is turbos run the exhaust valves hot.
Not much data, but after installing the 01 head (with the added gasket holes) the coolant is noticeably cooler than the 99 head under the same street conditions. Something like typically 4-5°C cooler.
The big thread of this discussion is here
https://forum.miata.net/vb/showthread.php?t=350182

I had written these rules:
-----------
The following is true for any block 94-05. (I don't know about the MSM)

Here is the general rule:
The only time an 01 gasket should be used is on an 01 head *with no coolant reroute.*

All other combinations, use a 99 gasket.

Do NOT use an 01 gasket with a 99 head. Doesn't matter if you have a reroute or not.

If you have a reroute and an 01 head, use a 99 gasket. And consider the modification I described for the gasket.

Here's the problem of the 01 gasket on a 99 head:
The (missing) holes in the 01 head gasket, DO NOT (all) line up with the (missing) holes in the 99 head. You will end up with fewer functional coolant holes than either.
---


https://cimg6.ibsrv.net/gimg/www.mia...68330a6021.png

In the back of my mind, I was remembering that there was discussion about the holes in the heads changing, as well as the gaskets.

Thanks for re-aquatinting us with the original thread.

There is a reason i put that in a sticky. Shittons of good info.

If I were running this test, I would put thermocouples into the metal of the head itself, not into the coolant. Coolant temp isn't actually what you care about; hotter coolant exiting the head could just mean it's pulled more heat out of the head, which means the head would be cooler, which is what you want. It could also mean that the system overall is much hotter, though, which is not what you want. Cooler coolant temp could mean that a bunch of coolant is bypassing most of the engine and artificially lowering the temperature the sensor is reporting...you get the point. Thermocouples in the head would eliminate this uncertainty.

I think you could get away with just one at either end, since it seems likely that cylinders 1 and 4 will be the extremes of the head temp. However, I'm not sure if it's strictly correct to say that it's the delta T across the head that we care about; what we're interested in is ensuring an equal and adequate amount of coolant is getting to all 4 cylinders, and it so happens that cylinders 1 and 4 seem to be the ones getting shorted in the various coolant strategies. So I would say it's the absolute value of the temps of cylinders 1 and 4 that we care about, not necessarily the difference between them.

I agree 100%. If I had the time, money, and inclination to really answer this question, that's exactly what I'd do. One thermocouple drilled into the casting at the front and the back, in both a front-therm and rear-therm configuration.

But I don't.



Presupposing that we're talking about delta-T across the metal, then these two concepts are functionally interchangeable. If anything, I'd actually be less interested in seeing the coolant flow than in knowing the actual temperature differentials, as in the end, we care more about the temperature of the head casting than the flowrate of the water surrounding it.


On an unrelated note, I can't help but think about how much this whole thread reminds me of the sort of thing that I used to mock and despise 25 years ago when I was totally into air-cooled engines... :giggle:

I agree; I guess I was ambiguous in this comment. What I should have said was that we care about the actual temps of the metal of each cylinder, and figuring out how to make those temps as low as possible, which is accomplished by getting an adequate amount of coolant to each cylinder.

I've been thinking about this a lot. And right now, I'm wishing I'd stayed awake in thermodynamics.

I don't think that keeping the temp in each spot along the head as low as possible is the most important goal. I think keeping the temperature across all points on the head as consistent as possible is the most important goal. It reduces the mechanical stress due to expansion (I think), and perhaps more importantly, the hottest chamber is the one that's gonna kill the engine first. Better to have three cylinders at X° and one at X+30°, or all four at X+10°?


The more I'm thinking about Steve Dallas' proposal, the more I think it has merit. Sure, the temperature of the coolant outside the combustion chamber doesn't give us an accurate reading of the temp in the metal. We can skew that relationship by adjusting the flowrate of the coolant so that it's uneven across the head.

Which is exactly what using an '01+ gasket with a rear-therm reroute does.

BUT, it is relatively safe to assume that the rate at which heat is being generated in the head is fairly consistent across all four cylinders. And 2nd Therm tells us that the rate of heat transfer between two objects is proportionate to the delta-T across them. So if we do measure the temperature of the coolant at two locations, and find that it's significantly higher at point A than point B, we can reasonably deduce that less heat is being transferred from the metal into the coolant at point A. And that means that, by definition, the metal at point A is being cooled less effectively and must, therefore, be running hotter.


I really want to see that data...

It's definitely better to have all 4 cylinders as close in temperature as possible, which is why I originally said "equal and adequate" amount of coolant. Even if the hottest cylinder is not knocking, it still leads to inconsistent combustion characteristics across cylinders. I'm actually working on a project now in which we're trying to minimize the coefficient of variation across 12 cylinders...even assuming we can design the intake to get the same amount of air into each cylinder, which is very difficult, casting and assembly variations are enough to cause unacceptable amounts of variation. Anyway, getting consistent combustion in each cylinder is very important.

Your application of the 2nd law of thermodynamics is correct, however, I'm not sure I understand your conclusion. If the coolant is flowing from point A to point B, the coolant temperature at point A is always going to be lower than the temperature at point B, because it's going to be picking up heat as it flows along the head. If you mean that, because of this, point B is always going to less efficiently cooled than point A, then that is correct. However, this doesn't actually tell us anything interesting, since we can easily deduce that just from looking at the direction of flow.

I may be misunderstanding your point though.

Really glad this thread was started -- it is very timely for me. That is all, I have nothing else to add.

Put another way:

Let us presuppose that:
We don't need to know the actual temperatures of the metal of the head in order to deduce that since each part of the head is being heated equally, and the front of the head is less able to reject heat into the surrounding coolant, that the metal at the front of the head is going to run a lot hotter.



I think we're saying the same thing here, and I'm not trying to convince you of anything. Just kinda typing it for my own benefit to make sure that it makes sense, and also to clarify my line of reasoning for anyone else who might not have followed the same train of thought.

Yup, we're on the same page :-) So, the application of this, then, is to take coolant temp measurements in both front and back of the head in the following configurations:

(A) Front Therm / Pre '01 gasket
(B) Rear therm / Pre '01 gasket
(C) Front Therm / '01+ gasket
(D) Rear Therm / '01+ gasket

Based on our discussion, it seems likely that the results will show the following:

(A) #4 much hotter than #1
(B) 1 and 4 pretty close together, and overall cooler
(C) not as good as B, but close
(D) #1 much hotter than #4

Now...someone with no children and an ungodly amount of time and number of Miata's on their hands, get to work proving (or disproving) this hypothesis.

The GT350 has many temp gauges, all in your choice of digital or scaled analog. One of which is cylinder head temp, like virtually all piston engined aircraft. What it does not have, is a scaled coolant temp gauge. Just a needle arc on an unscaled display, separate from the center dash where all the real temp gauges are.

My takeaway is that Ford is more concerned with actual CHT than CLT and thinks I should be too. They know more than I do about engines I think. So the Supermiata S1 (EFR6258) race car we're putting together will get not one, but two CHT sensors. You guessed it, #1 and #4.
NB2 head with 94-00 gasket though so a worthless data point.

Perhaps, amongst our many overlapping and overdue projects, we can stick two CHT's on a stock NB2 with a QMax and do some collectin'.

Your mention of aircraft jogged my memory, no drilling is necessary. You get the temps of the source of most pre-ignition by simply getting the spark plug temp.

Type K - CHT Cylinder Head Temperature Sensor - Ring Terminal Type

Interesting. An aluminum ring would have minimal total effect on the thermal conductivity of the plug to the head. Compression ratio theoretically affected but I think it would be nominal. Smart thing would be to put sensor rings under all plugs, even if you were only logging 1 and 4.

It's how most piston aircraft are retrofitted with individual cyl. Temp sensors if they didn't come from the factory that way. Have a few friends with planes and one that builds experimental aircraft with auto engines. These thermocouples are what I noticed on all of them. They can catch a cylinder going cold before they totally loose fire. They literally treat an engine like each cylinder is it's own sub system, so if you loose one the rest can get you home. I'm talking individual ignition and injection controllers on every cylinder.

I searched and was unable to find a CHT ring that would fit into the miata head without either hogging a channel into the aluminum or cutting a channel in the boot. The common rings outer diameter even have to be ground down just to fit down the bores.

Possbly an interesting data point: as a result of my CHT plug ring sensor attempt fail, I mounted the sensors adjacent to each intake port on a 1.6 w/reroute. I did not log the data, but used an aircraft grade gauge from Westach. Never observed more than a few degrees difference between any of the sensors. They all measured within 5-10 degrees of the CLT reported by the MS.

Edit: the sensors I used were from the above linked vendor.

I can grab an IR gun for my next track day. I recall doing that a few years ago after doing my begi racer reroute and I did see a difference in temps. Will not be accurate data but can give an idea of the delta perhaps?

IR guns are not really reliable in measuring absolute values. However, if measuring under the same conditions, they can give a decent qualitative measure (what is hotter)

I was just about to post something like this, as my mind went back to my early days learning to fly in N6066R, a 172 Skyhawk.

But not for that reason.

Back when I was heavily into air-cooled VWs, sensors such as these were very popular for modified engines. VDO Cylinder Head Temperature (CHT) 14mm Sending Unit and Harness, V323701 - Aircooled.Net VW Parts


I don't have any surplus BKR5s lying around... Is it possible to remove the crush washer without destroying it? This is as close as we're likely to come to getting some real, hard data.

Not sure if this will fit on a Miata spark plug "well". However, there are a few similar thermocouples in various dimensions, which could be bolted at various locations outside the engine block as an alternative; in fact we use them to measure battery cells temperature in our electric racecar, and they work well.

I had a question about how all this pertains to the MSM, since that's my (semi)-dedicated track car. I looked through the thread until I found this post, the executive summary of which reads:
Further into the thread, Joe Perez notes:
I was considering adding the Qmax re-route but, given the lack of solid information (plus reasonable conjecture/supposition about possible harm), I'm going to leave well-enough alone.

Even with a stock cooling system, the car appears to run fairly cool on the track (jibing with its reputation for having the bestest OEM cooling in the NA/NB world). Of course, the idiot gauge doesn't give me any real temps (and I haven't datalogged the Hydra yet) and I do feel the timing being pulled towards the end of a 20 minute session. I may do the rad in the future, possibly Spal fans/shroud and, definitely, some ducting).

Skip the fans and just do hood vents.
Stock coolant gauge doesn't give away anything until the motor is in the 240's.

Yes - it is: "Cold" "Warm" "Blown Head Gasket"

For another data point here is a picture of the gasket that came with my MSM top end kit, and a '99 gasket that I installed because I went with a DIY reroute on my MSM. There is a picture of a MSM head in my build thread if you want to check out the coolant passages (I dont want to clog up this thread with too many pictures)https://cimg9.ibsrv.net/gimg/www.mia...8d8557f8fd.jpg