TPI GUY,

Hey I was digging through some posts and think you're the guy to answer my question
On the 355 I'm building Its going to have a cam with duration in the neighborhood of 220, lift over 500, 112 lsa. What I'm wondering is what prom "tuning" allows the more radical cam, just an idle speed change? or is there more? And with prom tuning being availiable to me is the specs on my cam as limitless as carb or will the duration always be hindered by the computer? (not taking the LTR problems into the equasioin)

thanks.

 

My First Call Out


1.What I'm wondering is what prom "tuning" allows the more radical cam, just an idle speed change?

Tuning involves much more than just an idle speed change, although that is definitly part of tuning. The GM ECM gives you the ability to preprogram spark and fuel tables based on engine load, throttle position, and a few other variables. If you get into PROM burning, you have the ability to change the amount of fuel delivery by varying injector pulse width. This is akin to changing jet sizes and/or idle mixture on a carb. The advantage of the ECM is precision and near infinite adjustability.

You also have the ability to program a more precise spark curve. Conventional methods employed with a carb involved the use of a set of "centrifugal" springs and weights to advance the timing within the distributor, and sometimes a vaccuum advance canister to advance timing in part throttle conditions. This meant a lot of guesswork lining up springs, weights, and fine tuning not necessarily total advance, but the optimum rate at which the advance was brought in. With the computer, there are no springs, there are no weights, just initial timing, you program optimum spark curve, and the computer takes care of the rest.

Many times, engine load is measured in manifold vaccuum. When a hairy camshaft is installed, vaccuum at idle is significantly less. The computer proceeds to flip out "...less vaccuum means I am under load, but the throttle position sensor says that I should Idle, but with this new camshaft, I can't idle at 500 rpm anymore ..."

Needless to say, the parameters of operation must be redefined.

The computer does not hinder the camshaft selection. Both the ECM (speed density) carb run off a vaccuum signal.

A good example of a large cam running off of a stock computer with a modified PROM is Traxion.
Traxion is running a cam with about 230 degrees of duration at .050". That's a pretty sizeable cam for 350 standards, but the car runs.

If I haven't answered any of your questions, let me know and I'll attempt to clarify.

 

I think you did just fine

If it clears anything up though I'll be programming the '165 a few months before swapping to a '730


Thanks !