As Vlad pointed out, this is a matter of opinion. To date, no one has instrumented a VVT head to measure the temperature gradiant across it while running a rear-therm setup with the stock head gasket. So we can only speculate thusly:

1. The rear-therm reroute is designed to solve the problem of a majority of coolant flowing straight up from the water pump and right out the front of the head again, without circulating through the rear of the engine. It does this by re-establishing the coolant path which the B6/BP engine was originally designed with in the FWD applications for which it was developed.
   
   
2. The change to the HG design in the '01-'05 Miata engines effectively eliminates this problem by forcing the coolant to take the long way through the engine, back into the #2/3 area in order to flow up and out of the front of the head. Thus, performing a rear-therm reroute solves a problem which does not exist in these engines.
   
   
3. At best, there will be no significant improvement in cooling performance by doing a rear-therm reroute on an engine with a stock '01-'05 head gasket. At worst, the resultant stagnation of coolant all around the #1 chamber and towards the front of the #2 chamber will cause the front of the head to run significantly hotter than the rear, effectively creating a mirror-image of the problem encountered on the '90-'00 engines in stock form.

We have empirical evidence from a few people who have done rear-therm reroutes on VVT engines with stock head gaskets which shows that this doesn't result in immediate, catastrophic failure. I posit, however, that this modification does not significantly improve the cooling performance of these engines, and may in fact seriously degrade it, causing the front of the head to run significantly hotter than the measured coolant temperature, and decreasing the margin of safety for overheating.

I've run lots of race hours on unopened BP6D motors with reroutes. It's fine.

:likecat::likecat::likecat::likecat::likecat::like cat:

I think the effect of this is to add pressure behind the radiator and impede cooling.

From a theory standpoint, I've never liked that FM steering rack-mounted oil cooler design. Local friend of mine who installed one for track duty had poor results.

Then with your experience, would you say it helps over the stock setup?

Poor in what way? Not enough cooling?

The two issues with the FM mount are protecting it from rocks (under tray is pretty good at this) and there's only enough room for a fairly small cooler.

--Ian

Small cooler.

-8 lines.

Either in an extemely low-airflow area or, if you pipe air to it, you compromise delta-P across the radiator.

Street bling for the m.net crowd IMHO. YMMV.

Seems to work OK on my car on the track, although I only used the bracket (I'm using -10 lines and cooler). I don't have definitive before/after oil temp numbers, though.

--Ian

Dcamp2, let me know how the re-route works with the BP6D head gasket.

Tracked down some factory fans. Going to see what they do first. Then will buy an OEM undertray and see what that does. If those don't fix it, then I'm going for the re-route. Will probably use the one from 949 so that I can continue to use the EGR tube.

-Remove A/C condenser

This is what I did also. I had a lot of issues prior to this. When you remove the AC condenser that allows you to push the Rotrex oil cooler back, which allows you to push the FMIC back which allows more air to get to the radiator

Wasn't really looking to pull out the AC but it's certainly sounding like that may be one of the options I have to pursue.

Airflow through the radiator requires pressure differentials. At the moment there are several things working against you to minimize your pressure differential:

a) the replacement undertray does nothing but act as a fancy skidplate. It has no sides at all. The wheel wells are high pressure areas, so that pressure is also behind your radiator. The OEM undertray encloses the entirety of the bottom of the engine bay, and has a couple of nifty diffuser exits at the very back to extract air.

b) fan shrouds are a mistake for hard-driven miatas. At speed, they restrict airflow.

c) you need a real fan. "A", one, uno, real fan. This single spal cooled my car at idle just fine with no shroud (stack was IC/rotrex cooler/AC condenser/radiator; radiator was FM crossflow): Spal 3010.2029 Spal 3010.2029.

There is a huge difference between a random 12" radiator fan and a real fan designed to move air through a restrictive stack of heat exchangers under load.

d) your FM oil cooler is causing two problems. 1- the 8an lines are causing you restriction and pressure drop unsuitable for a car driven hard. 2- the placement and the "air scoop" are again increasing pressure BEHIND your heat exchangers, which is the opposite of what you want.

Mobius' solution to your cooling needs:

Buy the spal 3010.2029. Remove the FM oil cooler, sell it on m.net for profit. Use the stock undertray. At this point your radiator should be ok; if not TSE radiator and you're done. Or supermiata crossflow, or FM crossflow.

It's all about pressure differential, and at the moment you have very little.

Caveat - my car had AC but I rarely had to use it for prolonged temperatures in hot, traffic-jam type conditions. If you experience those conditions, you might want to add the spal 3010.1522 Spal 30101522 at Jegs

Mobius - thanks for the info. I've already removed the shrouding and installed the OEM fan. This alone made a huge improvement. I've gone ahead an ordered the M-tuned reroute also and will be picking up the OEM under tray from the dealer next week. With regards to the Rotrex oil cooler - you mention removing it and selling it, what would it be replaced with and where would that be mounted?
Appreciate the insight - thanks

He said FM oil cooler

Oh, he must of have misread then, because I don't have an FM oil cooler, just the Rotrex oil cooler. Thanks

Oh yeah sorry Zaphod has the FM cooler (Zaphod get rid of it :))

The other thing I didn't mention is ducting. Seal your exchanger stack together. Give the air only one path - through the exchangers. You can get the bottom and partway up the sides pretty easily with vinyl tub & shower liner from Home Depot Racing, and some aluminum HVAC tape:

https://www.miataturbo.net/attachmen...ine=1444601821

Some foam window insulation on the sides of the radiator. A cover to seal over the gaps at the top between the radiator and the bumper. Things like that will go a long way and are quite inexpensive.

Along those lines, the replacement air guide I built is working well.

https://www.miataturbo.net/build-thr...8/#post1244564

@gsyk59 my car runs cool on track.

20 minutes of driving as hard as I can (personal best 5 seconds under SM record) temp gauge doesn't budge. Temp was about 75 at 5000ft elevation.

Will do over the winter - but I'd like to keep the oil cooler - so where to put it instead? Behind the water cooler? In front of the passenger side wheel with an air duct coming from the radiator opening?