Replying a little late to this comment, but I haz data and this is one of my favorite little simple experiments I've done. I've shared this elsewhere but since removed it for various reasons. Anyway.

The first iteration of the 949 reroute had a larger cheater hole. I think it was 3mm OD. This produced enough cheater flow through the rad to cause some significant issues with slow warm up as well as dropping below thermostat temp on long descents like we have here in WNC.
If you do some quick envelope math, the cross sectional area of a 3mm hole vs that of the ~1/2" bypass hose, that cheater hole represents about 6% of the open flow area during thermostat-closed condition.

Back in early 2024 I tested this on a cool-but-not-cold morning on my commute to work using the same route and same gentle driving both times, as one would expect from a commuter.
- Blue line is the original large cheater hole with the AC side of my radiator blocked by cardboard.
- orange line is the cheater hole sealed with RTV and no cardboard.

Conclusion - sealing the cheater hole with RTV...:

• improved warm-up time to temp
• improved the ability of the thermostat to retain minimum temperature control on long descents and cold days
• had no impact to race track performance or peak temperatures on hot days
• adds no issues when combined with the use of cardboard to cover half the rad during winter months

IMO, the yo-yo effect exists but in my mild temperatures is not concerning, as the rate of change in temperature (shock) is within reason similar to what you'd expect by turning on the radiator fan. If you live somewhere with real winter and sub-freezing temps, you may care about that cold shock more than I do. But that's what a garage and a winter beater is for.


949 has updated their cheater hole to be smaller like OP has, 1.5mm OD IIRC, and apparently it's better. I have the new part on my shelf but have not felt the need to tear into things and install it because my sealed RTV plug has been holding up fine for 2 years now.
The smaller 1.5mm cheater hole represents about 1.5% of the open flow area during a closed-thermostat condition. Big improvement there - kudos to 949 for listening to customers, understanding the functions of their product, and making the correct improvement. It sounds silly to give props for something this minor, but R Theory's recently released coolant reroute shows that aftermarket race shops *still* fail to understand the fundamental functions required in a cooling system. Arguably FM also.

I understand the purpose of the cheater hole but I wish the design had incorporated an included roll pin or something so users can choose to 95% block it if they don't plan to run a racecar setup without the bypass loop.

 

I'm glad you brought that back up, as I'd had some more thoughts on the reroute.

As both of us noted, the bypass notch got smaller at some point, despite previous posts from Emilio saying they had no intention to change the bypass notch. That said, even if you got rid of the hole/notch there is still significant bypass around the entire thermostat. Every stock thermostat housing and gasket I have used creates a solid seal around the entire thermostat diameter. The circlip used in the Qmax does not. I sealed the entire diameter of the thermostat entirely with silicone, not just the notch.

Running at steady state around town in mild 60ish degree weather, the readings off of the OBDII feed will oscillate between 185 and 195* over the course of 20-30 seconds. Previously, my car would hold steady at 195 in most any condition, from 45 degree days to 95 degree 45 minute track sessions at VIR.

I have not confirmed this, but I am choosing to blame the thermostat itself rather than my install, or the QMax design. Stant Superstats used to be the gold standard, and they have little V notches that create a more gradual change in coolant opening and flow as the thermostat opens. Stant no longer seems to make their own thermostats and just repackage Motorad now, and I'm not sure if those notches are still there on this thermostat. Every time it opens, a big slug of coolant coming off of a 36mm rad overcools the engine until the thermostat shuts again, repeating the cycle. It may be more use of the thermostat but I prefer this to overcooling on warm up.

I wish I had monitored coolant temps during the last track day, as it seemed to be running hot (205 indicated in the pits) compared to previous experience, but I only opened up the OBD reader after the misfires started. I'm not going to call that a normal operating condition if some of the combustion gasses could have been escaping into the intake manifold with my tulipped intake valves and increasing the overall operating temp.

 

If it were me, I'd buy a full set of intake valves, and pop them in myself.
A valve spring compressor is pretty cheap, and you can use sharpie as a witness ink to make sure the valve is contacting the seat in the right spot.
Likely the contact will be fine on the valve/seat as far as location goes. and a SLIGHT lapping can ensure the seat/valve are sealing well.

If the contact area is only on the outer edge of the valve, or inner edge of the seat... then it's to the machine shop again to get seats replaced.

Then you can do the bucket/shim dance on what fits where, and order new for ones that happen not to fall in where you need the lash.

note: I'm more cheap than lazy, and i'm pretty damn lazy.

 

Regarding fixing the engine, I've had some time to research and think. I haven't made a decision but I'm getting closer.

Used heads: Two different people have offered me used BP6D heads. Neither is a pristine example, and they're not local. Their pricing is fair but I want to get on track reliably, not just check boxes.

Rebuild this head: I'm going to revisit the machine shop today and explain what happened. Neither of us knew that stock valves have a very narrow margin where they can be refaced and machined, and I am blaming that for the issues I'm having. On one hand, he's not a Miata focused shop and this is pretty esoteric knowledge. On the other hand, I am paying for expertise and I've had a failure related to his work. I don't want money back, but if he would put me at the front of the line and knock out the work in two weeks or so, I can get back on track pretty quickly. Manley valves are actually less expensive than stock ($440/set), and I would get valve guides replaced as well ($120/set) just out of an abundance of caution. I called Manley and they told me their valves have a reasonably large margin for machining. I'd probably prefer to adjust with shims if possible to sidestep that entirely, which could be a pain to source though. I spoke with another shop that multiple people pointed me towards and he didn't make any promises besides 1-2 months before my head would be worked on.

Spec Miata Shops: Treasure Coast Miata charges $1600 for a head that gets the exact same work done I paid for - no replaced valves. That leads me to questions about if this could happen again. East Street Racing wants $3,000 for a fully serviced head built to the limit of the Spec Miata rules. I want all parts replaced but I don't want to pay for expensive, tedious optimizing to rule sets I'm not bound by while ignoring cheaper, lower hanging fruit that's banned by SM rules. But I would get back on track real quick.

I'm going to try to get this head off ASAP. It has to happen regardless of my next steps, and if the valve seats look good, I may end up trying to do as much of the work myself as you point out @Ironhydroxide. It would fit the overall theme of this thread... I feel the need to do everything myself.

 

The heads off and I'm not thrilled with what I've found.

First bit of concern is this oil at the throttle body. There's clear signs it's coming through the pre-throttle valve cover breather, as there's streaks in the intake pipe.



The reason I'm actually doing this - tulipped valves from cylinder 4.



Some cylinders had this yellow/bronze sheen to them. It did not wipe off.


I knew this when it went together, but for some reason the cross hatching has distinct steps in the cross hatching reflections most obvious in the back of cylinder 4 below. This is not the result of different lights in my garage reflecting funny, its down to the surface finish.

The yellow bronze coloration is still visible but less obvious.



Bronze coloration in my intake ports are the result of oil.



Every intake port had oil all around the sealing surface.



It seems to me that I've got sub optimal ring seal, giving me blowby and pushing oil into the intake. While I did open up the valve cover baffles to clean them out, I don't imagine this is just the result of a bad seal there.

Prior to the bad idle, it was making decent power, and I never noticed oil levels dropping significantly. So even if there is some blowby, it's probably best to just throw a used cylinder head on it and go forward. I don't want to spend another $1k on rebuilding a cylinder head only to find my ring seal is garbage and the whole thing has to come out again.

 

This sucks. And blows.

Honestly (and I haven't had my coffee yet, so my thinking isn't all there yet), I think the easy button to maximize track time this year would be to yank the rest of the motor and drop in a used engine.

Drop in Sixshooter's engine, even though it's not VVT. (As long as you can deal with the ECU side of things) It's just for a season (or two) and honestly, VVT isn't worth that much for a "fun" track car.

Your bottom end needs work. It's not going to get any better with another year of track time on it and it could get much, much worse and take out your head (again).

 

I hate how right you are.

Someone has offered me an engine out of their old track car for $1100, along with a spare head. His maintenance records sound good, my only concern is 175 PSI compression on cylinder 1 compared to 195 on the other 3 cylinders. He's going to a K swap. And so I find myself contemplating a third trek to the DC area related to buying a Miata engine in less than a year.

I just cannot seem to get a win with this car lately.

 

Ugh, dude that's brutal. I think TBB is right, though. If that $1100 engine is sound, that's a steal. Especially with a spare head. It's a VVT as well just like yours?

Was gonna say it'd be nice to have leakdown results before you pulled the head, but then it occurred to me that you'd already have low results from the intake valve conundrum, and that'd mask some, if not all, of the issues with your ring seal.

Do you have any sort of a control test with how much PCV sludge your motor would originally emit over a similar period of time? I obviously only have the above photos to go off of, but I'm wondering if the amount of oil/sludge you're seeing is "within spec", especially since the motor was fresh and you just finished breaking in the rings.

Here's more conjecture on my part based only off of personal experience, but these are the intake ports on my original 2.5 Duratec after ~7k miles and 15+ track events. I cleaned the valves and ports before dropping the engine in, and ran a whole can of seafoam through a vacuum port a week after getting it running, so the valves were pretty spotless at the beginning of its journey. I ran the engine with no PCV system and both breather ports VTA as well, so no PCV gases were entering the intake tract. Intake valves and ports still looked like this when I pulled the engine out:



Maybe my comparison is null, because it's possible the cam overlap on this engine was allowing some combustion gases to reenter the intake ports at idle. However, in a car with a standard PCV system, I'd expect at least a bit of oil/carbon in the intake tract to be present after breaking in new rings and basically a full day on track?

 

not sure If I'm seeing what you're seeing.
The steps I can see are from where the rings stop at the top of the stroke, you can see the top and 2nd ring stop points pretty easily.

Or is that not what you're referencing?

How long on fresh rings?
A new hone/rings will definitely have greater blowby until the rings seat correctly.
How did you do the ring breakin? if broken in too hard you can overheat the rings, lose temper and never get the ring to seat after.

 

I'm referring to the reflections shown here. It is difficult to capture in a single picture, but this is not the result of multiple lights reflecting off of different spots in the bore. Notice how abrubtly the reflections stop and change. It looks like the surface finish left by the hone has fundamentally different textures left in different spots on the bore. There is not a uniform surface finish across the entire bore like I've seen in pretty much every other engine I've opened up.



Another angle.



Again, it's not just a trick of the particular angle of this shot. As you move your head/camera and change perspective, there remains an obvious change in the reflections at these lines. In other words, I don't think the surface finish of the bore hone is identical throughout the cylinder wall.


The engine break in was a bit aggressive. I drove a 4-5 miles for the engine to get up to temperature (190 coolant temp), then followed FM break in guidelines, albeit with more like 10-12 runs instead of 5. I used Lucas 30W break in oil. Here's their most relevant guidance.

"Because you want to take your warmed-up car out on the road, find a nice straight stretch and do a couple of heavy throttle* runs in third or fourth gear from about 2500-5500 rpm. Each time you hit 5500-6000 rpm, snap your foot off the gas and let the car coast down to 2500 rpm while in gear to pull high vacuum in the cylinders. Repeat this step about five times, and you should have a nicely mated set of rings and cylinders."

The oil was changed immediately after the ring seating procedure (14 miles), I filled with break in oil again, did another 45 miles of easy 35-55 mph b roads, changed with cheap 10w40 conventional, and after 1500 miles another oil change to Rotella T6 5w40 for the track day.

How to Break-in a New Engine

 

AH, now i'm seeing it.

If you had a bore gauge I'd say measure spots there and see if the bore varies....
Kinda looks like a manual hone job, with inconsistent feeds, and a tri-stone hone instead of ball hone.
The crosshatching looks pretty shallow down further in the bore on that last pic.... almost as if they dwelled the stones there instead of constant movement.


And that breakin should be fine, so long as you let it engine brake back down between heavy pulls ((and the pulls weren't up a very steep grade)
where I see people overheat rings is taking way too much to heart the "break it in hard" and do steep grade, multi gear pull (like stopped to 100mph)
Or brake boost pull (using brake to hold speed while engine is at WOT)

 

Yeah that was my assumption as well on the stone hone.

For the ring seating, there was no significant extended grade and they were 3rd gear pulls, limiting the overall length of time they were under load.

I'm strongly considering it. Just messaged the guy to see if he's free this weekend, as I have Good Friday off.

I wish I had done something to get a bit more info, but with such obvious problems in the spark plugs and borescope pics, I figured it was academic. Now I wish I had tried to do a leakdown test on 2/3 so I could at least have some idea of how those rings are doing.

Well, it's not the first time I've done this sort of extended track day and I never had any oil in the intake prior to the TB. Even with two cylinders having absolutely awful rust damage. It did consume oil, I assume mostly due to said damage and other leaks. I did wonder about how the valves could be contributing, but given the oil is coming out of the breather tube and is evenly distributed across all four intake ports, I feel like its a systemic issue not isolated to just the 1/4 cylinders with no valve lash.

 

 

Last night I went and looked at the sad state of the car for the first time in a while. It pissed me off. The oil is all over every part of the intake manifold and combustion chamber. What a load of *****.

Today, I looked at Car-Part dot com.





I went and checked it out. This car was somebody's baby until they crashed it in June 2025. It's a 5 speed LS, it has BC Racing coilovers, a very clean interior that didn't even need a vacuum, but definitely needed new airbags, and a clean view under the oil cap. Nice clean new NGK plugs.







The only bad thing I could find was this small little mark in #4. Cylinder #2 looked great. I rotated the engine over a few times and it was smooth.



It's a 24 year old engine and I doubt I can find anything cleaner short of buying a $5-7k brand new Spec Miata engine. $1,200 gets me the bare engine with no accessories and a 30 day warranty.

I put down a $100 down payment and they said it would not be ready for pickup before Thursday. That's fine with me, I need to get the old engine out and get ready. My current plan is basically the same as when previously needed to replace my engine 1 year ago - Swap the good parts over, reseal, slap it in. I'm not sure about the reroute - it was a pain to install the engine in with it, and I have plenty of radiator.

I want my #$*%@ing car back.

 

It made me sad to see my car sitting for so long, so I feel your pain.

Now if only I could rock down there and pick up the rest, seats, top, maybe buy the wreck less engine, keep what I want/need, sell off the rest ....

 

$1,200 for a 130k mile motor is a pretty bangin' deal at this point. Fingers crossed the issues are all behind you once you get it installed!

What's the plan for your current engine? Probably don't even wanna think about it at the moment, huh?

 

I asked if they would sell the whole thing, but unfortunately not. It was in better shape than mine, besides the accident damage.

"Other than that, Mrs. Lincoln, how was the play?"

Eventually I'd like to rehone, new rings, and rebuild the head myself with Manley valves. But with a good engine powering the car, I can take my time to do the work when I choose without the pressure of missing another season. My main concern is if the valve seats are ready for new valves without remachining or replacement. It's an awful expensive core at the moment.

Then it can just be a spare, or maybe even swap into my NA chassis. But I can't move on that project until this thing stops hogging the main bay of my garage.

 

I picked up the engine on Friday. I got the old block removed that evening, and started prepping the new engine. Weekend plans were made for me, so I didn't get as far along as I'd like towards installing it.

In the interest of having as good of a chance with the (very very hopefully unneeded) 30 day warranty, I decided I'm not doing the timing belt and water pump until I verify it runs. That means no oil pump swap. I decided to do an oil pan swap to get the temp sensor and baffles moved over to the new engine.



And now I'm wishing I hadn't even done that. As a reminder, I'm running two seperate baffles together - a horizontal Spec Miata style baffle from SageSpeed, and a vertical wall rubber flap style from DCBE Tech.

The rubber baffle flaps have shrank and hardened significantly in the time they've been installed, and they no longer completely cover the holes.



Based on some feedback from other forums, I'm guessing some nitrile flaps snuck into the DCBE supply chain. I've already reached out to Harry at DCBE. He was very apologetic and said this wasn't an issue in his initial testing with parts from that supplier. He's sending me some replacements that should be genuine Viton, as well as reaching out to other customers that got parts from the same batch. I think that's definitely worthy of kudos and can't ask for anything more.

 

"Everyone's got a plan 'till they get punched in the mouth." -Mike Tyson

Damn, that's super annoying. Hopefully that's the only hangup you run into this time around. You've had enough hangups at this point lol. Did he give an ETA on when you can expect the replacement?

 

I have a shipping code from Mountune USA but no estimated date of arrival. I imagine it'll be faster than shipping from Greece though, so I appreciate that. They are described as Hydrogenated Nitrile, not Viton, which is surprising to me. At this point I may just remove the rubber flaps and install the metal portion of the baffles to slow down the oil somewhat, or save them for a later project like my old NA6 engine for the red car. If I had Viton ones on hand I would have installed the pan yesterday, but I tend to stop for the night when I reach a question like this in the evening to sleep on it.

At the end of the day the baffles are a nice to have, not a necessity, and the Spec Miata style flat baffle is still an improvement over stock. If I'd have known, I'd have just swapped the rear main seal and clutch and gotten around to installing the engine into the car. I don't expect issues with this engine that'll make a warranty claim necessary, but I've learned to expect less than I used to and I want the least amount of fighting possible if I need to use said warranty. There's a lot of much more annoying "If I had only known ___ I would have ___" in this engine saga, so I'm not going to stress about it any more.