During Speed Channel's broadcast of the Grand Am race from NJMP on Sun. (8/31), Dorsey Schroeder mentioned that the RX-8 rev limit was reduced by the rules to 8600RPM as of that race. He then said that the RX-8 cars had been experiencing excessively high egt. One of the cars had even broken a header due to the high egt.



His next statement puzzles me. I understood him to say that the high egt was due to the reduced rev limit which allowed the hot gases to remain in the header too long. He then said that if they could have used the old rev limit the gas would have passed through the header so quickly the egt would have been lower.



My conclusion, as it concerns me, is that if I start shortshifting by 1000rpm to keep water temp safe on a really hot day I can expect to see the egt go up.



Is Dorsey's statement correct?



I thought those revs would not have any effect. The high egt would be due to wrong jetting or timing. I would appreciate it if someone could confirm Dorsey's statement and explain it a little more.

 

A great race car driver, and a good comentator, but couldn't get a wheel off a car with help.



The best power mixture for any race car is just a hair rich of peak EGT. That is very hot. So in piston engines you hear comments like I leaned it out too far, and melted a piston. Or how about this: it was running great just before it blew up.



Yep, it probably was. Because best power is just rich of peak EGT. And a long pull at peak EGT in a piston engine, melts the piston. Best power means complete combustion in the chamber, so timing is advanced far enough to get the burn done before the exhaust port opens. That means the longest time with the highest heat trapped in the chamber. So, best power is off the table for piston and rotaries.



In the rotary, tuning and timing has more to do with EGT than in the piston engine. There is no valve head in the flow path to absorb heat and slow the flow, so an EGT probe 3" out from the port gives you a real good look at the combustion process. So, if the mixture is close to ideal and the timing is right on for the RPM, the rotary can do over 1,900 degrees. Not a good idea. The apex seals will overheat in the center, bow up and start leaking badly. So we actually don't go there. We tune well rich of peak EGT. About 300 degrees if you can do it.



Power is off a few percent but it will run all year (or two) with no attention. So, shoot for 1600 degrees or less for road racing. You can go up to 1,800 with ceramic seals.



Now if you have had your peak RPM reduced, and before that the extra weight added, so the Porsche or Pontiac can win once in a while, then you tune closer to peak EGT (best power). The only downside for the engine is oil and water cooling demand is up, and if you run ceramic apex seals nothing can go wrong. Right? However if you forget to blow more air on to the headers, even stainless can get soft and squishy when overheated, so it did.



So reducing the rev limit did nothing much to the EGT. Leaning closer to best power did in the headers, that was all that happened. My guess is that the stainless headers come out and Inconel headers go in.



Short shifting usually lowers oil temps more than coolant temps, but both come down.



There is no such thing as too much cooling in a road racer. About 160 oil and 180 water (coolant) is ideal.



This is three 13 row Setrabs for oil cooling.



Lynn E. Hanover

 

Thanks for clarifying and elaborating, Lynn. Nobody had asked Dorsey, on the air, a question. I have no idea why he felt compelled to offer that piece of information.



I use one of those coolers in each sidepod.