89vette

I know that the correct answer to my question is "give the motor what it wants" but here is the deal. On October 31st, I have all day access to a brand new Dynojet complete with a wide band O2 sensor at a local technical school. My use is free and all I have to do is let the classes watch as I record data, change and burn new chips. Cool deal huh? My efforts will be focused on WOT timing and fuel /air ratios. What I would like to know is what is a good baseline to shoot for? 12.5:1? 12.8:1? I have heard that motors like a richer setting at peak torque and a leaner setting at peak horsepower. How lean usually is lean? 13.0:1 at peak HP? The shop that built my motor has an engine dyno. He claims that the 396 LT4 motors he builds generally like about 12.5:1 at peak tq and about 12.8:1 at peak HP. What about after peak HP? Do you richen it back up? Just looking for some rules of thumb here. My intent is to baseline the car to see how close I have it now and then shoot for a specific fuel air ratio. Then if all goes well, try it a little leaner and then richer to se if it picks up or gains anything. I do not suspect much power will be in my timing. I run about 35 degrees total advance currently with no knocking as picked up by the knock sensor.

Thanks,
Jason

Grumpy

89 vette,
AL heads?.
You're probably going to be at too much timing at 35d, and again it's what the engine wants

Best torque = Best BSFC

So you be going richer at rpm increase, GENERALLY. Exhuast system will come into play on that. Can get into alot of factors, ie baro presure will drive the self EGR'ing of the engine up as it rises, so gotta be careful in what your **assuming**. Starting in the low 12 would be a min.. Lots of things in chassis dynoing, and then remember you'll have too much timing, and probably be a tad lean when it actually comes time to drive down the road. You can't thermally load an engine on a chassis dyno like in a car, and the airflow, underhood, can't even come close.

First rule in tuning is never accept dyno results are always being best. Again, it's just one tool for you to use.

Doctor J

Jason-

I'm not sure if this is right, but I did some tuning on the street early this summer - using Diacom to measure RPM vs Time (limited to 2nd gear on the streets around here). The methodology was this:

First I set all the VE tables as well as I could with no PE spark or fuel.

Then I added a nominal amount of PE fuel (like 8 to 10%) across the board in order to get to some improved timing settings for WOT.

Lastly I examined PE Fuel vs RPM band as best I could, and wound up with a curve that looked like this:

http://temp.corvetteforum.net/c4/doc...ix/pe_fuel.jpg


I varied the fuel up and down a few % in each RPM band, and this was about the optimum curve I could measure (so far).

To make a long story short, I was looking at PE fuel % adder vs RPM rather than at some specific AFRatio -

These results are empirical, and haven't yet had the benefit of the WB O2 sensor or Dynojet time, and of course are specific to my particular engine set-up, but it it might give you a test method to think about.

HTH
drj

89vette

Why too much timing at 35 degrees? LT1's run 38 degrees. My car runs sluggish at 28-30 degrees total. Seems to like 35 degrees. I thought aluminum heads can take a little more timing than iron ones.

Grumpy

The LT1 is again a different story, and are you talking iron or AL LT1?. On the one I just finished tuning, 95 Caprice, we're running 32d timing, and 14.8s with a 4020# car (stock but exhuast).

Often slow feeling is faster, at the strip (to a degree).

It's not a mater of how much timing you can run, it's a matter of best HP. You can run too rich with too much timing and seriously mess up your motor, might ask Traxion about his tuning episode.

If it's just about timing, I really must be off since in winter tune now, I'm only running 15d <g>.. Applications make a difference.

The idea is to run the min amount of timing consistant with Max HP. Not the max amount that the engine will tolerate. Huge difference in parts bills that way.

89vette

Are you saying you are running 15D total timing at WOT? The LT1 I was talking about was an aluminum head version. Just looking at the stock bins there was as much as 38 degrees total timing. I understand your point. Are you suggesting that I reduce my timing? Car has 10.8:1 compression and aluminum AFR heads. I thought I read somewhere that the LS1 motors only have 28 degrees total? Is this correct? I'm still in the dark. My dyno time will be somewhat limited so I want to know what to shoot for. I guess I'll baseline the car like I said to make sure F/A ratios are around 12.5:1 and then back off a few degrees of timing to see what happens. How does that sound?

Jason