cal30_sniper

iTrader: (16)

I'm hoping some of the more experienced tuners here can help me out with this problem.

I have been chasing my tail on a throttle tip-in or low-rpm acceleration backfire in my speed density code. I'm about 95% certain this means I still have the AE setup too lean, but I've tried fattening that up significantly and the problem still exists.

The biggest problem areas:
1. Throttle tip in from idle in neutral. Several small back fires in the intake and a large flat spot in torque following through about 2000rpm. If I gently add throttle to get the engine above 2400rpm, and then tip-in, I don't have a backfire. This problem gets worse with the vehicle in gear under load.
2. Adding throttle from an overdrive cruise in 6th gear, again, less than 2000-2400rpm. Above that RPM, no noticeable problem.

My setup is:
350 with Vortec Heads
LT4 Production Cam
Holley Stealth Ram Intake
24lb injectors
T56
3.45:1 gears

I've changed the following AE tables Tables:

1. AE Delta TPS Mult vs. Delta TPS (increased the values, with a heavier emphasis on the lower %TPS values)


2. AE Delta MAP Factor Mult. Vs. TPS (changed the 0% TPS multiplier to match the 12.5%):


I am using a hybrid Spark Advance table that combines the L31 Truck Spark Advance and 9C1 Caprice (LT1) spark advance tables, using whichever value is higher at each RPM/load point. I then added about 20% spark below 2000rpm in order to compensate for the slightly larger cam and lower compression ratio (9.3:1) of this engine.

My Current Spark Advance Table:


The VE tables started with factory LT1 data, and have been tuned through about 10 iterations of ~20,000 reference point datalogging. All of my BLMs are between 126 and 132 at this point.

My current VE Table:


The remaining tables and values are the same as a factory 92 305 5-speed car (aside from injector constant, which I modified to get the closes BLM values before I started shaping the VE table, and a few other odds and ends which are not related to fueling or air delivery.

If someone could find it in the goodness of their heart to make some recommendations on how to go about tuning this issue out, I'd really appreciate it.

-cal30sniper

ULTM8Z

Before you start chasing your tail on tuning, I'd HIGHLY recommend verifying your setup. Especially the ignition. I had the exact same issue and it turned out to be faulty spark plug wire. Tuning around a malfunction is a huge PITA...

cal30_sniper

iTrader: (16)

New Plugs, AC Delco Coil, and 3 year old 8mm Summit Racing spark plug wires. Original GM Ignition Module. Distributor cap and rotor are recent MSD units with very little visible wear.

It's popping through the pipes on deceleration, which could also point to a problem in the ignition system, but I wouldn't really know where to start looking that hasn't already been replaced. I guess I could pull the ignition setup off my '88 GTA, which I know is in good shape, and see if that fixes the issue. Still, the motor spins to 6000rpm with no problem, has zero misfires at idle or any other steady RPM that I can find. The only real trouble area is in the throttle tip-in, which to me points a big glaring arrow towards my tune. This was a V6/T5 car. I built this engine and swapped in the T56, and have been trying to get the setup tuned using the speed density 730 ECM with a v8 memcal.

ULTM8Z

You could e-mail me and I'll send you my Miniram bin. It has ZERO tip in issues and runs really well. Your set up is a little different, but it should be fairly close... Might be a good place to start.

ultm8z@yahoo.com