Off idle stumble...................

[XfireZ51]

Originally Posted by tpepmeie

Jeff,
Boy do I know what you are talking about. I was totally perplexed by this for several years on my modified '91. Several "little" things can help, but not totally eliminate this. For what it's worth, my pristine stock 7000-mile 93 does the same thing. I am (almost) convinced that this is something all LT5's experience. My personal view is that it is a function of the lengthy inlet tract behind the throttle blades, and the plenum volume. There is surely an inertial "lag" of the airflow from when the throttle is cracked to when the engine responds, but I can't get my head around the engineering explanation.

Some things that help, but don't totally eliminate the phenomenon:

1) Denso IT-22 plugs, or equivalent, gapped at no more than 0.040" AC Rapidfires seem to contribute to a sag, in my experience.

2) properly adjusted TPS sensor, and throttle blades. I like less than 15 IAC counts at idle.

3) extensively re-worked Delta TPS, and Delta MAP Acceleration enrichment and some other values in the calibration.

This sag clearly shows up as a lean spike on a wideband O2 trace. The task, therefore is to get the fueling to respond quicker to the initial tip-in. But not too much fuel as to cause a rich bog after the tip-in. It will try your patience to get it just right.

Short answer... you're not alone. It is just something to live with unless you want to spend hours working with the calibration to improve it.

Todd

Todd,

Do the Delta TPS/MAP AE have any "lag filters" or coefficients? How about the VAFPR? EBL has them which allows you to adjust how quickly these params will react to the changes. Shortening the lag improved throttle response significantly. I would think that the fuel would arrive sooner than the airflow given that the injector is closer to the valve than the TB. By that logic I wouldn't expect to see a lean spike. But as you described the stumble is caused by AFR leaning out.

[tpepmeie]

Originally Posted by XfireZ51

Todd,

Do the Delta TPS/MAP AE have any "lag filters" or coefficients? How about the VAFPR? EBL has them which allows you to adjust how quickly these params will react to the changes. Shortening the lag improved throttle response significantly. I would think that the fuel would arrive sooner than the airflow given that the injector is closer to the valve than the TB. By that logic I wouldn't expect to see a lean spike. But as you described the stumble is caused by AFR leaning out.

Yes, there are filter coefficients for these inputs, and they can be adjusted to affect the timing of the enrichment pulse.

[XfireZ51]

Originally Posted by tpepmeie

Yes, there are filter coefficients for these inputs, and they can be adjusted to affect the timing of the enrichment pulse.

Todd,

So they work by affecting how quickly the params react to new inputs, correct? Are those the ones you changed and to what result? Or did you just add more AE?

[tpepmeie]

Originally Posted by XfireZ51

Todd,

So they work by affecting how quickly the params react to new inputs, correct? Are those the ones you changed and to what result? Or did you just add more AE?

Sort of. Both AE types are driven by delta-x inputs. I.e., delta-tps and delta-MAP. The delta values are partly determined by the filter coefficients. So by changing the way the delta-x is calculated, you can increase or decrease the amount of enrichment applied for a given (actual) change in input.

There are many variables involved in fine-tuning the throttle response and tip-in. Off hand, I would say I worked over maybe 12 different variables to achieve the response I wanted. It took a lot of experimenting.

Todd

[Z51JEFF]

I dont know if there related but the car has stalled several times.Which injectors are the best and whats a set go for?

[lbszr]

Originally Posted by Z51JEFF

I dont know if there related but the car has stalled several times.Which injectors are the best and whats a set go for?

I doubt if fuel injectors would cause it to stall. Rough or miss or hard to start maybe. Have you done a leak down test after turning the engine off? I was still in denial that my oe inj. would last forever and it still ran good but noticed a little roughness. Found one inj resistance out of limit cold and 6 more out when heated up in the oven to 170. Anyway ended up with Accel and happy. It's smoother than it's ever been. From Atlanticspeed for about 530, heard afterward's with netregistry membership can get for below 500. 21# p/n 150121

[Z51JEFF]

I haven't done a leak down test but just plan on replacing the injectors.It looks like a pretty straight forward job.It would give me a chance to clean up some things while Im at it.Whats the deference between the RC injectors and the ACCELS besides price?

[tomtom72]

I may be mistaken.....but I think the RC's and the Accels are made from the same s/s injectors made by Lucas....I know the RC's start out as Lucas s/s injectors. I have the RC's in my 90 and think they have served me well. If the Accels were offered when I had to do my injectors, I would have bought them as they look to be the best deal for the $. My RC's were $1068/16 in 9/05!


Tom

Oh, I don't really know enough about injectors & EFI to say any more than I wrote. All I know is I have no off idle stumble...I went out to play with my toy this morning....for sure no off idle stumble that I can detect and I tried several times just to be sure!

[XfireZ51]

Originally Posted by Pete

Yeap, that's our ZR-1's some guys snap there foot on the throttle expacting a pro stock engine response not in our Z's ported or stock.

Pete

Pete,

What's interesting about the ZR calibration is that AE is based on MAP. There doesn't appear to be a table for Delta TPS%. There are some TPS% Constants in oorder for MAP AE to take place but nothing like the AE PW based on Delta TPS%. When I tuned my 84 using EBL, I relied more on TPS % change than I did on MAP % because it reacted faster and then I would decay out the AE. But that's on a wetflow TBI system. In talking to some other tuner friends it seems that GM may have used the VE tables to add AE. And the Decay for the AE PW in my calibration is immediate. Once the
AE PW is delivered, ITS OFF. Also, the AE PW is the same in the 92 AYWT bin regardless of Delta MAP. I'm sure part of the reason for this is the second injector. It makes up for lack of AE fairly quickly.
Is the stumble you have all experienced more likely at lower or higher rpms?

[tpepmeie]

Originally Posted by XfireZ51

Pete,

What's interesting about the ZR calibration is that AE is based on MAP. There doesn't appear to be a table for Delta TPS%.

Not true. There is a delta TPS AE factor. I do agree with your prior experience, the TPS-based enrichment does provide a better response off-idle.

For what it's worth, going from memory, the TunerCat tables/constants are not exactly labeled right for some of the AE items. What you see is indeed the Delta MAP AE table. If I recall the earlier calibrations have a rather flat enrichment regardless of D-MAP, while the later (BMCB) is more sloped. I think some of the other TC items are labeled wrong. I don't have it installed, so can't check it anyway.

A final "for what its worth", the very early development calibrations relied exclusively on D-TPS enrichment. The D-MAP feature didn't get put in until later, from what i can tell.

Todd.