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Questions about rev limiter strategies

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I recently set up my BMW M20 B27 on ECU master EMU classic with coil near plug full sequential injection and spark. GTX 3076R, 45 mm external wastegate appropriately sized intercooler, about 18 PSI of boost....The car runs really well I think the tune is mostly very good.... the thing with the EMU classic is that it has of course spark and fuel cut limiting and also a by cylinder option which is more gentle for SR engine than the like with potential valve train problems. I had my son at his first drift event in my drift car and he spent a fair time on the rev limiter and probably too low of a gear and what I noticed was that the engine flash heated on the rev limit. I do pull about 4° of timing out of it near the rev limit to soften the rev limit but I'm wondering if this is sending my EGT through the roof? It got hot enough to push out maybe 2 l of coolant we would catch and it top it up. It never seems to get terribly hot by the gauge and I think the combustion chambers and coolant passages above them or flash heating... Mind you this is traveling at a speed of 10 or 15 mi an hour on the rev limit in first gear so the cooling system is not working at its best to say the least.....

I have all three of these limiters set at 6,500 RPM I want to be gentle with the engine even on the limiter so I think the by cylinder limiter is the best one for me to use so I may increase spark and fuel limiter to say 7,000? Any input on this would be greatly appreciated thank you for your patience with sort of a long question :-)


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You need a fan to help the cooling. Normally, airflow when traveling more than about 40mph is sufficient for most radiators. But if your radiator isn't aimed in the direction of travel (drifting), or even traveling slowly the fans need to do the job of creating a pressure differential across the radiator. For your application, I would think that a full shroud for the radiator fan would be an advantage (not always so for high-speed track and road cars).

Thank you very much for taking the time to answer my question. I clearly understand this :-) I was running a modified E46 fan clutch and fan assembly which I machined on my mill to fit between the radiator and the engine. When I went to sequential fueling and spark the cam sensor took up some of the room at the front of the engine and I'm running an electric fan.

Literally at 20 miles an hour the engine temperature rapidly comes down. I have a very large radiator in front of the engine with the tubular support that I fabricated that made room for a clutch fan and a much larger radiator...

To be clear here I am a 54-year-old man who has been in the automotive trade for 40 years and tuning for about 20 I say this not to boast on my knowledge obviously I'm asking for help here :-) But to let you know that I understand the basics. My question more directly was about the rev limit strategy and if pulling timing out before the revlimit increases engine temperatures substantially....

Thank you again so much for your time :-)


Pulling timing would be expected to slightly reduce the coolant temp's, as the heat is produced later in the cycle meaning less exposure to the ccylinder walls and head, and more expelled out the exhaust.

I would be looking at some other things.

a/ cavitating or faulty water pump - while this is usually a problem when the engine is operating significantly above the OEM rpm range, you may find some improvement running a larger water pump pulley, if possible with that engine.

b/ collapsing bottom hose - these are subject to a lower pressure as the water pump 'sucks' the coolant and they can partially close up, restricting the coolant flow.

c/ while either, or both, of the previous conditions can cause overheating, and may, or may not, have been a factor, you're well past that point!

Something has to be getting into the coolant system to displace the coolant - this IS going to be either air being drawn into it from a join or the waterpump and/OR combustion gases getting into the cooling system, displacing the coolant. I expect the latter is going to be your problem - ESPECIALLY since you're reporting it isn't showing up as overheating/boiling, as such, so something else must be displacing the coolant. Further, once it happens, the probable lack of coolant is likely to allow much higher local temperatures to be reached and worsen the problem.

At this point, I'd start with a sniffer test, this will check for combustion gases in the coolant (carbon monoxide, IIRC) and if it's positive I'd work from there, removing the head and looking for gasket failure(s) and, as it's been abused like that for a while, head warpage and cracking - you may even find the block requires resurfacing - certainly carefully check it for cracks, too.

In future, take any blowing of coolant seriously - a very small amount, which stops after the initial blow, isn't unusual, but anything more is usually a sign of a more significant problem.

I thank you for that :-) again I appreciate you guys taking the time to answer my question. It is possible that I'm pushing coolant out with combustion gases. I'm waiting on ARP head studs ..... a couple of months ago I actually put three head gaskets on it in one month finally resolving that I was exceeding the stock head bolts ability to clamp the gasket....I have oringed the deck and backed the boost down until the head studs get here ..... the reason I had sort of ruled out combustion gases was that the car was being driven in such a low gear that it's not really building any boost and figured it was related to the rev limit. Also that I had done some full boost pulls in a tall gear with no problem multiple times before this incident with no issue.....I understand now that the later timeing puts the heat out the exhaust and not into the cylinder head like I had thought before and I appreciate you pointing that out. It may very well be a leaking head gasket or pump cavitation I will look into that :-) again thank you

Oh, completely forgot something - it could be partially due to a faulty radiator cap.

Sometimes they don't hold their rated pressure and that may allow the pressure generated by the high rpm water pump to lift it off it's seal. They usually also have a secondary vacuum valve to allow coolant, or air depending on design, back into the cooling system to prevent hose collapse, which may not be sealing properly. Also, and this has caught me out personally, the distance between the flange and the sealing surface does vary, with some being deep and others shallow, and this will affect the sealing force and hence pressure seal. A 'short' cap in a 'tall' receiver may not even seal the pressure, but it should be reasonably obvious, as, IIRC, there's around a 1/4" difference and you would feel little, if any, resistance when fitting it.

If you're lucky, this may be the basic problem, and the water pump may still have been circulating enough coolant around the top end to keep it within a safe heat range. A sniffer test would still be worthwhile, as I'm a bit paranoid about coolant losses.

Of course, these are just what came to mind - might be something else entirely, but hopefully it brings something to mind for you to check out.

Yeah it's not the cap :-) The expansion tank and cap on these are different than what you're thinking of. Not the typical radiator cap. But yes I get concerned about the actual temperature around the combustion chamber and cylinder head one of the head gaskets I put on was after a 3-day drift event where the temperature and resultant detonation was bad enough that it pitted the cylinder head and broke a ring land on number 6 cylinder :-(

I wanted to thank everyone for their time... I have been lifting my head ... there was coolant emerging from the side of the engine at the head gasket ... this is with orings and stock bolts because ARP are not available... I used some Mcmaster bolts for the time being and this is holding for now

thank you again for your time in the matter I appreciate it

take care