Think in terms of "effective" CR. It doesn't change the real CR, but it produces results comparable to a much lower CR.

If a 10.5:1 engine takes in only 85% of the air MASS (not volume - you're right, volume doesn't change, but density changes the mass, and mass is what builds cylinder pressure and makes power) then compresses the air at 10.5:1, the effective mass of air in the combustion chamber is only 85% of the mass at sea level. In effect, the same mass of air you would see at sea level, with a 9.0:1 CR.

At that elevation, you can only get 91-octane fuel because there is no need for higher octane fuel. The "effective" CR is so low, that you don't need it. That's why you shouldn't be screwing around with the A/F ratio or the timing, until you track down the source of knock retard. Try some higher octane "racing" (unleaded) fuel and see if the knock retard drops.

 

Thanks injuneer, effective CR is what I meant. I'm going to play with it more this weekend if I get a chance. Only 14 datalogs left.

 

You don't need tuning. You need to diagnose and fix whatever is broken.

And, as others said, you need to step away from the tunercats until you have done a lot more reading.

Why do you have a 5200 rpm redline?

 

Well, I have a PCMforless tune that I believe is too agressive. It is trying to use 40degrees advance from 2k to redline. Therefore I'm pulling timing and adding fuel. Not sure what would be "broken".

I shift at 5.5k on the factory tach to be on the safe side, no need to squeeze this thing to redline on the street.

 

So I finally figured out how this fueling works. I went and did the calculations for what AF ratio the PCM is targeting based on the percentage tables based on WOT AFR = 14.7 / (1 + %Change vs Cool/100 + %Change vs RPM/100). I thought it used the "open loop AFR ratio vs rpm vs coolant", but apperantly after it first goes into closed loop, it ignores that table.


RPM Target AFR
1600 12.2
2000 12.4
2400 12.4
2800 12.4
3200 12.4
3600 12.4
4000 12.8
4400 12.8
4800 12.8
5200 13.2
5600 13.3

I think its obvious that 4k plus is set too lean.

 

12.8 is not too lean. You most likely have other issues here. Your not going to get an "aggressive" tune from a mail order tuner. If anything they will always stay on the safe side and give up power. If you have "hefty knocking" then you need to look into what's causing it. You can either choose to ignore very good advice that has been given to you here by more than one person and keep pulling timing and adding fuel until your engine falls flat on it's face or you can determine what is causing the knock and fix it. You could be getting false knock from your exhaust, rockers, etc. Do you have an LT4 knock module installed in your pcm? you can also desensitize the knock sensor if it is in fact false knock.

 

Could be false knock maybe. I don't hear the engine pinging, but its showing counts and 7 degrees retard, so I assume its there. Going to log again tomorrow.

 

Ok, let's do this. If your datalog is showing knock counts and pulling 7 degrees of timing to get rid of it try taking out 7 degrees of base timing and log it again. If the knock goes away and the pcm doesn't pull timing then your done. If the knock is still there then you can stop looking at the timing/mixture and go find what is actualy being picked up as "knock".

 

I already suggested it may be false knock, and one way to check. And, if the knock retard system is doing its work, you won't hear it "pinging".... that's the purpose of the knock retard - to prevent knock. You won't hear it - the knock sensor hears it way before you could, and the PCM eliminates it.

Knock counts have nothing to do with actual knock. It appears to be an internal calculation used to detect operating conditions that MAY produce knock. Its very normal for the knock count field to increment up, without an knock actually ocurring.

 

Hey, I'm the author of those tips someone posted here.

Don't discount that it's the amount of timing set in the PCMforless tune you have. I have seen several of their tunes from people coming to me from them who weren't satisified, and basically they are left stock except for jacking up the timing at WOT (same goes for several other "tuners"). I've seen some set at 37* in the main timing tables at 95-100KPa, which unless you're running 94+ octane at sea level, will almost certainly cause a good bit of knock.

Since you're at such a high altitude, at 91 octane, you can probably get by with 37-38* at 85KPa. Where's it set right now?

I've also seen old knock sensors cause false knock. They can get pitted/corroded over a lot of miles and replacing it with a new one can often fix a lot of false knock (seen it with several people already).