I changed from a hot cam to a Comp cams extreme. 224/230 .503/.510
I have an Ed Wright chip for the hot cam. What values do you think will need tweeked?

 

I am putting the top end of this engine together today, and if all goes well I will start it tonight or tomorrow. The reason I ask, is that the Extreme cams have a much more aggresive lobe design. I am curious what the engine will want. More timing? More fuel? If I get it running I will make some Diacom runs and see. I'm not sure how I will know if it needs more timing.

 

it definately needs something. It hunts at idle and just about won't idle in gear.

 

Check your data logs - surging at idle sometimes means it's too lean; might want a little more spark as well.

As for how much to change tables & where, the only way to know for sure is to measure and record what it does, try a little different calibration and measure and record what it does, try a little different calibration and measure and record what it does, etc. IIRC, Traxion's part-throttle tuning guide (on the 3rd Gen board) is pretty comprehensive on how to get started. Lining out a new cam well takes some effort.

HTH

 

Thanks DR. J, I haven't had much time to work on it, but I am getting there.

 

My solution to this problem is to fall back on what I would do if it was a non computerize setup. Computer operation allows you to do some stuff that you can't do with a non computer engine.

1 - Get the idle setup
Warm up the engine completely and find a good idle with the IAC disconnected. Then adjust the spark advance when the throttle is closed to get as much vacuum as possible. If you have a pre 92 car, you can do this by moving the distributor and noting the timing where this occurs. Then reduce the idle to a point that will pass emissions or make you happy. Adjust the idle vs temp curve to match the value you manually set. For cold start, adjust the fast idle as needed. Be sure you have a smooth transition in the idle settings from cold to fully warmed up.

Then tweek the part throttle and full throttle spark curves. Larger cams will allow you to put in more spark advance at lower rpms for part and full throttle. The LSA of the cam also affect this. Tighter LSA will allow for more spark advance at very low rpms. The full advance at WOT and part throttle will stay the same so spark advance above 2500 - 3500 rpm will not change.

Speed density is another story at part and full throttle. Fuel flow with Speed density is based on the volumetric efficiency (VE) of the engine. A larger cam will reduce the VE at lower rpms and raise it higher rpms. The best thought I have for this is to put the engine on a dyno and take readings of the fuel air ratio with the calibration for the old cam at approx (use the table indexes for the values) 1000 rpm increments and three load values (map values). Its easy to get the rpm you want and it maybe possible to get the desired map value by controlling the load on the dyno. You should end up with about 15 to 18 values for the fuel to air ratio.

Take those numbers and figure out how much of a difference there is between the measured fuel air ratio and desired fuel air ratio. Raise or reduce the value in the ecm table by the factor from the fuel ratio. Smooth the transistion between VE values in the tables and the ones that you compute. Be sure to run open loop during this time and use a wideband O2. I would shoot for 14.5:1 at part throttle and 12.5:1 for WOT. This should get the tables close to the correct values. The ecm will adjust the part throttle ones for stochiometric operation and passable emissions. You can tweek the fuel to air ratio at WOT values for full throttle operation.

Here is a sample calculation:

M - measured value
D - desired value
V - value in VE table

new table value = V * (M/D).

Too rich example:

M = 12
D = 14.5
V = .52

New Value = .52 * (12/14.5) = .43

The richer fuel air means the engine is taking in less air than the table value has.

Too lean example

M = 16
D = 14.5
V = .52 (12/14.5) = .57

The lean value means the engine is taking in more fuel than the table value.

Be sure to do a reality check. A larger cam should resutl in an engine that is richer at lower rpm and leaner at higher rpms.

 

Wow, thanks for the in depth reply. The car is running pretty good right now. I have done a few things to it and it seems to be getting better. It does need more fuel at WOT. At what pulsewidth are the injectors getting to their max duty cycle? I have 24lb FMS injectors, and I run 10.8ms at WOT. I don't think they are tapped out, I am just curious. I have them set at 48lbs of fuel pressure also.

 

Assuming I did the math correctly. If the on time was taken at at 6000 rpm (50 ms between each spark in the same cylinder), the injector is on 54% of the time. If the time was taken at 5000, the injector is on 45% of the time. No your not close to running out. This assumes that the on time the computer reports is the total time power is applied to the injector.