cpt_koolbeenz

<<<EGT's go up when you retard timing, goes down when you advance timing. >>>

Isn't that the other way around?

NZ951

I would agree with you Chris...

dand86951

The relation of EGT and timing depends on several things, but lets say at a fixed afr and boost level and rpm, if you then started retarding timing your egts would start to rise. If you increased timing egts would start to lower, but then you run into knock at some point. Egts go up due to the fact that as you retard timing the fuel doesn't completely burn in the chamber and then the exhaust valve opens letting the burn complete in the exhaust pipe and around the exhauset valve.

Mike S

I agree with what everyone else has said with the ~12.0 to one final ratio....however if you look at some of the recent posts there have been some suggestions and even a dyno sheet showing improved spool up times with a leaner zone around 2-3.5k rpm's....this lean zone starts at about 15:1 and richens up to 12:1 by about 3500rpm. I beleive that this is supposed to reduce lag a bit.

Dwayne Williams

This is by no means a definitive answer, but I've been tuning my car over the past couple of weeks and I richened up the "boost build" zones around 2-3K to 13ish and it did slow down boost build up (I have 15 PSI at just a tick over 3K). I was running 14.7 to 15 in those same zones before and now I've moved boost build back 200 rpms or so - could be more. It's hard to tell when you're driving... So I'd say based on that, yes a richer mix when you start to build boost will slow things down a little as the exhaust temps are probably lower. But I wouldn't go too lean in those zones either.

rage2

No, it's not. Otherwise, I'd be able to retard timing like crazy, run more boost, and not break anything .

rage2

Yes, Danno mentioned that this was how his chips were tweaked too. I run a 14.5 ratio up to about 0.9bar absolute pressure (not boost), by 1.1bar absolute I'm at 13.0 a/f, by 1.3bar absolute I'm at my 12.2 a/f. This helped spoolup by about 300rpm on my car, as compared to a 12.2 a/f ratio starting from 0.9bar absolute.

I haven't played with timing yet in the area, not sure if I wanna be retarding (less power, but more heat) or advancing (more power, less heat) to help spoolup. Maybe Danno can reveal that side of the secret?

cpt_koolbeenz

The way I figured it was advanced timing causes detonation, so does high EGT (the nature of detonation), so I assumed they were linked. But dand's explanation says the EGT will get higher because of combustion occurring after the exhaust valve opens - so that is not an indication as to what the combustion temps are. So high EGT in that case may/may not indicate pre-detonation. In that instance, my "logic" fails because the EGT reading is not an accurate portrayal of combustion chamber temps.

dand86951

High EGTs are indicative of high combustion temperatures, it is just that with retarded timing the combustion process continues in the exhaust port thereby putting extra heat load on the exhaust valve seat and valve. Keep in mind there are several variables that affect EGT and one of them is timing. You could use way retarded timing at partial throttle and low boost and your EGT would still be low. But if you advanced the timing at the exact same boost and rpm and throttle opening you would get lower egt up to a point.

I can say that if you have high egts measured at a typical 1 to 2 inches from the exhaust port then you can be sure you have high combustion temps. Whether they are high enough to cause knock depends again on other factors. Also like rage2 said you don't really want to exceed approximately 1600F for egt. Those kind of temps start degrading all the metals from the head to the turbo. Why do our engines have sodium filled exhaust valves? It is to carry away the exhaust heat faster to keep from destroying the exhaust valve.

cpt_koolbeenz

Thanks dand-
Do you (or anyone else) know why retarded timing makes EGT's rise? And visa versa? Just curious...

What kind of AFR's are you guys running at idle and crusing conditions?

Bri Bro

Might want to look to airplnes for more info on EGT. One link has a chart on EGT vs AFR vs head temp.

( From GEM Series Technical support)
Why does EGT drop during detonation. Shouldn't it rise?
Detonation is an abnormally rapid form of combustion that follows ignition induced combustion. It occurs under conditions of high compression and temperature and overly lean mixture. By the time the exhaust valve opens, the rapid combustion of detonation is significantly more advanced than normal, resulting in lower EGT and higher CHT indications.



http://www.lycoming.textron.com/main...mperature.html

http://www.lycoming.textron.com/supp...ngEngines.html


Max HP vs AFR, were the rectangle touches the curve.

dand86951

At idle, mine stays around the 15 to one and then at cruise it is probably around 14:1.