johntorg

I have run my 951 widebody at the track for the first time and after starting at around 10, I gradually leaned out the AFR to 12.0 on the straight at Roebling Road. I would like to know what other racers are using for an AFR target when running on 91-93 Octane.

johntorg

I guess either no one knows or no one wants to share.

Larry Herman

Not an expert on A/F ratios, but 12 under full load sound reasonable to me. It gets too expensive if you try to lean it out any more, and I don't think that there is too much more power available anyway. You might want to PM Geoffrey Ring, as he IS an expert on this.

d15b7

hi!

i'm not a 944 Turbo expert, but i have lots of experience with turbos on track in general.... my Mitsu EVO (sold it last fall; boo hoo!!!) liked to run at 11.3 to 11.5 WOT; at 20 psi boost, there were zero knock events at that A/F ratio. that was running 93 octane fuel. on my honda civic racecar, with 2 liter RSX-S motor and Greddy turbo (12-14 psi) i run A/F of 11.5. that car runs on 110 octane fuel, but it is also 11.8:1 compression, so it really needs the high octane fuel. zero detonation events. typically, on all the turbos i run on track, i aim for 11.5 or less on the A/F. the fuel helps cool things, and keeps detonation way down. you want little or no det events; it cracks the ringlands, and kills pistons, and blows headgaskets, and hammers rod bearings. bad all the way around!!!

good luck with your racecar!
todd

PS always check on the track, with a wideband sensor. i've seen the A/F really vary from what it read on the dyno, to what it is actually doing at the track, at speed....

Van

Truthfully, I haven't got the slightest idea... My check engine light has never come on... and I haven't blow the engine up yet - but I suppose I'm unlikely in a stock setup.

eclou

be in the 11's and watch your EGT if possible

johntorg

I can get it back to 11.5 or so pretty easily. Is anyone running leaner than this? I have an EGT gauge, but need to have a bung welded into the header. I saw a knock counter thread in Rennlist and will probably put one together. The car has a wideband with a new sensor and I am logging it through the Mafterburner. I have found that tailpipe sniffers like those used by dyno operators sometimes read leaner than those in the exhaust tubing.

hp18racer

I don't run anything under 100 octane at the track. Somewhere b/t 11 and 12 and I'm happy.

eclou

+1. I always run a mix of 104 with 93 just to give a little headroom for detonation. Last time to the track I forgot my own rule and burned up an exhaust valve in 3 laps.

johntorg

I just ran 83 laps at Roebling without any problems. The last 30 or so were at 11.8 to 12.2. of course it was pretty cool out. I have to detune the engine a bit to fit in NASA GTS4, so I will continue to work on a 93 Octane tune. I'll go back to 11.5 and install a knock counter to see how many I have. My understanding is that high EGT is the result of retarding the ignition timing. If timing is a problem, I won't have any contol over that unless I order a custom chip, or use Russell's Maxtune setup (which I have). I actually don't think this will be an issue with the Vitesse chipboard currently in the car.

dand86951

I have mine set to be at 11.8 around 4000rpm decreasing to 11.2 at redline. My egts at 1bar boost are generally less than 1600F. I read some information on the newer ZO6 engines that said GM set the AFRs down to the mid 10s after the throttle was wide open for ? secs. My 951 seems happy to run at these numbers without any overheating of oil or water. Also, my intake temps stay below 60C.

hp18racer

With higher octane at a given boost level, you can advance your timing for lower EGT (exhaust valves last longer) and still be knock safe. On the track I'm looking for under 1700F EGT. Tonyg and Chris White have put up some really good posts on EGT, worth searching for them on the 944 turbo forum.

Oddjob

I have run as low at mid 10's to low 11's at around 15 psi boost. EGTs (in the x-over pipe) will creep up to a little over 1600F by the end of a session/race. Noticed that the EGTs are not affected by adjusting fuel within the 10-11.5 AFR range, and not much by dropping boost down to 12 psi either. Very similar number regardless of what chips I am playing with (APE, 951MAX, Factory Cup). Oil temps in the pan will also creep up to 260F even with dual oil coolers, which is higher than I would like.

I have used the simple curtis event counter setup, and I am not sure it shows or indicates actual knock signals. It might just be picking up noise. May be interesting to keep track of possible knock counts along with other data logging to see if there is correlation with other variables, but I would not rely on it as a single source of data.

Note: my in-car wideband AFR (zeitronix) does not match the AFRs that have been charted/measured at the tailpipe during dyno runs. My wideband typically shows a quite linear/flat AFR from midrange to redline, but the dyno sniffer showed a real rich midrange and lean on the top end. Not sure which to trust at this point.

HP18 - I will look for those EGT threads, I dont recall seeing them in the past, thanks.

Geoffrey

That is exactly the opposite of what you would want...4000rpm is at or near peak torque of a 944 engine and where peak VE occurs and the highest cylinder pressure. You'd want the engine richest at peak torque and can be leaned out as the torque goes down due to less heat being produced.

The question for the OP is how much boost are you running and what is the configuration of the engine ie TRUE MEASURED compression ratio. Most importantly, have you had the engine tuned on a dyno (chassis or engine) where the minimum best timing has been found for every operating cell?

I have several 944 Turbos in various configurations running 1.0 - 1.4 bar of boost and at 1.0bar they are running in the .83 lambda range and the 1.4 bar in the .80 lambda range. Don't forget that if you are running street fuel which has ethanol, you need to adjust your AFR from pure gasoline because stoich is different and you'll be lean if you are using AFR for gasoline.

A single EGT probe in one cylinder does not provide very useful information, I wouldn't waste time on installing one.

johntorg

Thanks to all, I appreciate the input. This car is a club racer and at its current weight needs to be about 348 WHP to run in NASA GTS4 (if I've calculated correctly).

Geoffrey: Thanks, I am running a Vitesse Chipboard and so my timing is wherever John sets it. Boost is now 15lbs. I didn't build the engine and so don't know the compression ratio. Re: the "true measured" compression ratio, I don't know what that is unless you're talking about measuring the CCs of the combustion chamber. Re: The single cylinder EGT probe. I already have one from another car so I'll install it anyway. Without a standalone you can't do much single cylinder timing adjustment anyway. I know its been covered, but what's your view of installing the probe in the cross-over pipe?