Richen mixture to reduce cylinder head temp?
 

I am running a supercharged 3.6, and have a question.

Does richen up the mixture (part throttle and cruise) reduces cylinder head temperature? If so, how much?

Right now, it's 14.7 as controlled by the O2 sensor in closed loop. Wondering if I richen it up to about 13.5 would that reduce cht and probably prevent detonations too.

WOT is at/below 12 AFR.

thanks,
anthony

Sounds like your mixture settings are perfect. Why mess with perfection? I think going from too lean to a perfect stoichiometric (in which the fuel is burned completely) can reduce CHT but adding more fuel that will remain unburned during combustion will just increase exhaust, cat, and muffler temps as it burns outside of the cylinder.

Mark

Car is running very good. But on long hill climbs at partial throttle, cht can climb up. I want to see if richening up the mixture would keep cht in check since high CHT can lead to detonation and other bad stuff.

How is hot is hot? As I recall 250-350 is normal, 400 is a watch, and 500 is a shutdown.

BTW, don't you expect that the CHT would go up during a hill climb, just as it would go down as you head back into the valley?

Mark

Transmountain road here in El Paso is a steady climb over several miles. My cht gets to 400F at the top of the hill on a hot day. That is where I shut off.

Down hill is 330F and cruise temp depends on ambient 350-370F. Of course, this is for the hottest cylinder which is #2.

Want to reduce cht by about 20F through richening the mixture at partial throttle.

Oil temp and air intake temp is kept very cool with the coolers, intercoolers, and fans that I have installed.

How's your timing?

Timing is advanced/retarded by Motronic.

Those temps are high, but not unexpected if the ambient temp is hot. I've seen my CHT in the same range when ambients are 90-100. And that was with me driving rather gently on the highway.

Another data point, on track (where I was into boost WAY more than you'll ever be on the street) most of the CHT data I logged was clustered around 340, with a peak of 377. It was a sunny day with ambient temps in the low to mid 60s. On a 90+ degree day, I have no doubt that I'd be seeing well over 400 CHTs, and that's why I'm not tracking the car in the summer (will do spring and fall events only).

I do plan to monkey with ignition timing just a little, but my ultimate plan to reduce CHT is to add water meth injection.

I would expect WOT to produce a lower CHT (at least on my motronic car) since WOT AFR is 12, ie. very rich mixture which cools the cylinder.

Part throttle and cruise where AFR is 14.7 is where heat can build up.

I am looking at this as a possible alternative to AW injection for part throttle. Just hate having to constantly fill the bottle.

I would think a change to the straight blade fan combo the 965 TURBO engine used would do the trick for you. The factory must have had similar issues with sustained cruise.

Stoich is the hottest burn and a bit leaner is a bit cooler but with added emissions.

IIRC the 964t blade was for less sound?

No the curved blade is for less sound but is less efficient. The straight blade fan was used on the TURBO to increase cooling - just what is needed to reduce CHT at sustained part throttle operation.

The 1994-94 Turbo straight 11-blade fan does not just drop into the 964/993 fan housing!
You can purchase the ~$600 fan housing, too, and then find out it doesn't fit the 964 or 993 plastic fan shroud. The fan housing that does is the 993TT -- and new you're over $1,000 new, plus the fan strap, plus...

Ruf's RCT's didn't run hot, and neither do our 964/993 Turbo conversions. Better to determine why it's getting too hot in the first place under these long, light loads. Confirm fuel and timing. What is your oil temp -- do you have enough oil cooling?

I've asked this question in other posts regarding CHT. How do you know, how many have CHT to read?

This is a NA 993 running a supercharger. Timing is somewhat pulled back by the piggy back computer under load. Mixture at part throttle is in closed loop and controlled by 02 sensor, although there is a 7th injector. I'll try to check timing in these situation, but Durametric software is not good/fast enough for me to see & log timing.

Mixture I see in closed loop is around 14.7 which drops upon throttle tip ins. Oil temperature is actually under 200F when I activate the cooler fans. IAT is only 15F above ambient. These numbers are very good.

The main reason I brought this up is that I was seeing cht climbing up to 400 on the hottest cylinder during a long hill climb. Normal highway cht is about 370F at 95F ambient temperature.

Anyone know what CHT the ruf or factory turbo run at part throttle? WOT? The only data I find is from the VW and air cooled airplane engines which recommend keeping cht <400.

Same for me. I flew EGT and CHT.

Oil temp and CHT do not directly relate. Neither can fix each other if one is extreme.

Is your car street & track? Do you know what your CHT would be driving on the highway on a hot day?

CHT seemed to be a good thing to measure, but there is not much data available for a turbo/sc'ed street car.