Why You NEED a Reroute (and why it should NOT be a BEGI racer reroute)
 

This is an outgrowth of Concealer404's thread on overcooling with a BEGI "Racer" reroute. As part of that thread, I got on Visio and drew up some cooling diagrams. They present the whole reroute issue so clearly that I thought they were worth posting for general interest. As they say, a picture is worth a thousand words.

First, let's start with the OEM coolant routing:

https://www.miataturbo.net/attachmen...1&d=1393525833

Mazda's dilemna was fitting a FWD transverse engine to a RWD chassis. To preserve maintainability, they wanted the thermostat in the front. They had room to fit a large radiator, so why not?

Note that with the OEM configuration, the most even flow of water through the engine is with the thermostat closed. As the thermostat opens, the path of least resistance to get out of the engine/head moves towards the front cylinders which tends to starve the rear cylinders of cooling. Not ideal.

In addition, it means that the radiator never sees full flow no matter how open the thermostat is because some coolant always takes the path through the heater core. That means radiator efficiency is also compromised -- which is one reason our radiator is more than twice as large as a Civic's.

So, the Mazda engineers paid a heavy price for that front thermostat. But they weren't dumb. They sized the components so that the engine retained adequate cooling water (at OEM power levels) with the thermostat fully open in hot conditions.

Alas, who among us is happy at OEM power levels? #4 gets awfully hot when you bump it up. Hence . . . the traditional reroute:

https://www.miataturbo.net/attachmen...1&d=1393514555

Since we're men and don't fear blood on our knuckles, the traditional and best way to reroute is to restore the flow of coolant to the FWD transverse configuration. This is really the optimum configuration. Every bit of water goes through the entire engine from front to back cooling every cylinder. Doesn't get much simpler or effective than this. On my cars, I do this with a BEGI spacer, KIA waterneck, GM truck hose and TSE waterneck block-off. BAM!! (Relatively cheap too).

The above seems so obvious. So, what might be this BEGI "Racer" reroute I'm reading about? Went to BEGI's website to check it out. Aaaannnddd . . . here it is in all it's glory:

https://www.miataturbo.net/attachmen...1&d=1393525833

In this setup, all water always goes through the radiator with some of it going through the heater core as well. So, expect LOOOONNNNGGG warmup times (under some conditions you may never warm up). Plus, the deficiency with water balance through the engine remains. WAIT!!! Actually . . . it's worse!!! How can that be?

Look carefully. The highest pressure point in the system is the water pump outlet. Likewise the lowest pressure point in the system is the water pump inlet. The water pump belt provides the mechanical power to make this happen. Now, with the OEM system the flow through the rearmost cylinders is:
1. pump outlet
2. rear cylinders
3. heater core
4. pump inlet

But, with this system, the flow is:
1. pump outlet
2. rear cylinders
3. heater core
4. radiator
5. pump inlet

The benefits of this system is that it increases water flow through the radiator and doesn't mix water from the heater core (which may not have been cooled) into the water pump inlet. Only the radiator feeds the water pump inlet. But it doesn't fix the cooling balance problem and may make it worse. And, of course, it may never warm up. Not good.

Some recommend "fixing" this reroute by moving the T into the lower hose. Here's what that looks like:

https://www.miataturbo.net/attachmen...1&d=1393527676

Bears an uncanny resemblance to the OEM system, doesn't it? LOL. At least it's better than a T in the top hose.

TL;DR . . . if bumping power, install a traditional reroute. Avoid the BEGI racer reroute.

Propped for truth. This is sticky-worthy

I hear the hyper reroute is the best.
https://www.miataturbo.net/attachmen...ine=1376345263

I think your diagrams for the begi "reroutes" are incorrect. but im too lazy to actually bother.

It's a little more complicated than this, but yeah, pretty much right.

It does fail to distinguish between thermostat open routing and thermostat closed routing, and where it routes through the block.

For instance, in the OEM routing, it doesn't accurately show how the water doesn't go through the block when warm, instead mostly going right back out the front neck.

https://www.miataturbo.net/attachmen...ine=1393518873

https://www.miataturbo.net/attachmen...ine=1393518873

Basically I like the colored pictures.

NVM.

NVM

And what changed with the '99 and the different openings in the head gasket?

From 2001 onward, the OEM head gasket restricted flow to the front of the cylinder head to force more water to the rear. This amounts to "water balancing." Definitely beneficial at OEM power levels. May not be enough at elevated power levels.

949 recommends changing to an earlier headgasket if a traditional reroute is installed.

Sorry to potentially mislead. According to TSE, it was 2001, rather than 1999. I think I will do a little research.

Good, a confusing thread like all the others. My job here is done.

But they aren't correct how you drew them.

I think these more accurate represent them:

https://www.miataturbo.net/attachmen...1&d=1393522718


The Rear Thermostat reroute they sell, is the exact same thing as the typical reroute diagram you posted with the additon of the water bypass, which is silly.

Yeah, those diagrams are better. Shows how water goes into the block and out from the head. Main writeup has been corrected to show this important detail. Thanks.

Yes, it was 2001. 99/00 have the same head gasket as 94-97.

You don't want to run a traditional reroute with an 01+ head gasket.

--Ian

Hey guys, what if you did a reroute with the thermostat on the back of the head, but also left the thermostat/housing in the stock location!? Then plumb them together into the radiator.

It would be like double the cooling.

A true revolutionary innovation! Dare I call it The Holy Grail of reroutes!?

I'm still confused on the consensus on 01+ and a reroute...haha. I was running the reroute awhile back, but took it off when HG design was discovered and it was understood that it was a "bad" idea. That was also when livin in the hot n' humid Southeast. Now that I'm in the much cooler and dryer PNW, I feel the stock setup with the standard large radiator should be good.

BEGi actually suggested that for a while. I think using two temperature stats.

Haven't you had your motor open twice now? Why are you still running an '01 head gasket?

Engine #1 was a swap with a stock engine, not rebuild. This time I'm hopefully buying an assembeled "budget" built engine....dunno which HG was used...will ask though.

Fixed the main writeup based upon Brainy's comments.

MT.net standard reroute parts list:

Spacer ($93): BEGI Rear Thermostat Spacer 1990-2005

Waterneck ($23.50): Miata Thermostat Housing

OEM Thermostat Gaskets x 2 ($3.62 each): GASKET,THERMOSTAT (B621-15-173) - $3.62 - B62115173

Front Waterneck Block-Off ($20): Trackspeed Engineering

Radiator Hose ($15 -- partsgeek.com link provided but get wherever): 05 2005 Cadillac Escalade Radiator Hose - Cooling System - AC Delco, Dayco, Gates, MacKay, Lower, Upper - PartsGeek (Dayco Radiator Hose -- Upper)

Plus some odds and ends. If your thermostat is old, replace it while you're doing this. Look at the condition of your heater hoses and bottom radiator hoses too. Lots of pictures of the reroute I did in my Silver car build thread. I also removed and plugged all those little, annoying coolant lines that run all over the engine and tossed the 1.6L Air Valve (made an aluminum block-off for that). Bottom line, uber-reliable.

I have an MSM engine with the stock headgasket and did some measurements by fitting an additional water temp sender in the front thermostat housing using the Maruha spacer.

On a local track here with lots of long straights, I never saw more than 2 degrees C delta between the front and the back of the head:

https://www.miataturbo.net/attachmen...ine=1401270187

Obviously ignore this data if you have an older engine.

Cheers

Well, the thread's been bumped anyway, so...

To clarify, the BEGi "Rear Thermostat" reroute does not always look like the right-most image. In their original design (which is the best one, incidentally,) the heater core return goes to the mixing manifold just like it does in the OEM MX5 configuration, the OEM 323 configuration, and the traditional DIY rear-therm reroute.

This business that they recently added of the "Bypass Option" basically adds the worst feature of the "racer" reroute to the street version, causing heater-core water to enter the radiator 100% of the time, even when the thermostat is closed.

So this is what a standard BEGI rear-therm config looks like:

https://www.miataturbo.net/attachmen...1&d=1401289633

This shows that Mazda's engineers did a good balancing job with the NB headgasket.

But the point of a reroute on an NB is to increase radiator efficiency by increasing radiator flow. A rear head reroute will change the balance of water going to the radiator vs. the heater core with the thermostat open. More flow to the radiator increases its heat transfer capability. The downside, in my experience, is that your heater doesn't work as well in the wintertime because you don't get as much flow to the heater core.

Thanks for posting the data. Data = goodness!

Yeah thanks for sharing Tim, that's actually very interesting info indeed

And back in the Pleistocene era I suggested improving the standard reroute by adding an oil t-stat to the outlet of the heater:

https://www.miataturbo.net/attachmen...1&d=1401290804

So when the the coolant is hot the flow through the heater will stop, and all coolant will flow through the radiator for maximum cooling. If you then turn on the heater, the coolant in the heater outlet will cool down, and the t-stat will open, so you will have heat in the cabin, and when overheating the heater can function as an auxiliary radiator.

Shaikh has tested this and found no problems with heat in the cabin. When the engine is hot the temperature of the heater line after the oil t-stat is indeed cool.

Sorry, nobody in SoCal can claim to have positively tested heater effectiveness. LOL.

This is exactly how I have mine for a number of years now. thermostat on the heater core line doesnt let coolent flow out of the heater core until the coolant coming out of it is below I think it was 160F. coolant only flows through the heater core if it is transvering out sufficient heat otherwize the bulk of the coolent flow goes to the more efficient radiatior.

Bob, is this on your mega power track car?

If you have the time I'd be very keen to see some pics of how you have it installed and a link to the thermostat you are using!

Cheers