Yesterday night, I input the bone stock timing map and PE values from the LT4 Corvette into my current calibration. Whoa! What a difference!

(BTW, I'm still running that 30lb injector on #2 cylinder- 24's everywhere else- and I have another 30 lb coming for the #1 cylinder as well in order to richen the front two cylinders the way GM does on the sequential LT1/LT4's.)

The idle quality is now at least 70% better than what it was when in drive, and 90% better in park. It'll be interesting to see what happens when I put the additional 30lb injector on the #1 cylinder.

And the torque.... YIKES! I thought this thing was fast before! Just from my butt-dyno it's clear this change picked up a very significant amount of torque- particularly below 2500 rpm.

Any rate, I'm practically completely convinced now that I have a major fuel distribution problem with this Miniram. My combination of cam, heads, and exhaust must interact very poorly with the Miniram at idle speeds.

The only curious thing though, when I compare the VE tables, the LT4 has what seems to be 30-40% higher values than what I'm currently running, yet I have very close to 128 BLMs almost everywhere. Otherwise I would have put the LT4 VE in as well. If I put the LT4 VE in, my BLMs would probably drop down below 110.



The 30lb injectors of course aren't a solution to the problem, but merely a mask for it. But it's certainly encourages me to proceed ahead with the LT1 PCM conversion. Now if Mrs. ULTM8Z and I could only get our new baby to settle down and sleep, I could actually get some work done on the Camaro!

 

I have 2 questions, did you try the Lt4 SA before going to the 30lb injector to isolate the issue and secondly how is it a 30% change in VE doesn't peg your BLM's to it's limits? I can't get away with more than 10% either way before the BLM's will peg to my set limits of 108-148. Very odd.
In anycase, glad to hear you're making headway and the Lt1 pcm should work well in your application.

 

Well, I'm only guessing about the effects of the 30% change in VE. Like I said, I didn't implement that change yet- not sure I will either. You're probably right that it would peg the BLMs too rich.


I did the 30 lb injector thing about a month ago. It ran significantly better, so I left it in (with respect to idle quality, not really any more power though). When the 2nd 30 lb injector gets here, I'll put the 24 back on #2 and run a test with the LT4 spark map before putting 30's on #1 and #2. I'm wagering it won't run nearly as good as it is now- even with the LT4 SA map.

Looking at GM's idle fuel trim on LT1/LT4's, on an LT4 Vette, there's as much as a 13% richer trim on the front two cylinders as opposed to the rears. For an LT1 (95SS) there's as much as a 20% richer trim! This tells me that these short runner manifolds have the front two cylinders hogging a heck of lot of air from the remaining six. For me, putting 30lb injectors on the front two cylinder is kind of a poor-man's way of approximately duplicating GM's fuel trim, at least for testing purposes.

 

Is the 15-20% richer fuel trim in the factory code just at idle/low speeds, or across the boards? You have something like 25% richer trim globally on the problem cylinder(s) with your 30 lb injector placement. This sounds highly unusual for a street car EFI setup. What does your cranking compression and leak down look like across all 8 cylinders?

 

I know.... I'm running richer than it needs to be during off-idle operation. Remember, this is only a test for idle conditions.


It's been a while since I ran the compression and leakdown tests, but I do remember that the #2 cylinder was actually a little better than some of the others. There is no mechanical problem with the engine. I'm already convinced of that.

The 95SS LT1/LT4 is a sequentially injected engine and look at how rich they make the #2 cylinder with respect to the rear cylinders. Mine's batch fire, which makes it all the worse at idle speeds. Though I want to get hold of a 92-93 batch fire LT1 to see what GM does at idle. The batch-fire LT1's are about as close to my engine as GM got.

95 SS Camaro Idle and Off Idle fuel trims. NOte that GM also slightly trimmed the fuel trim off idle as well.:

 

Well, my other 30lb injector arrived. So I ran the test you suggested , and sure enough it ran worse going back to 24 lb on #2 with the LT4 SA. I then put the old timing map back in and ran it again- this time to compare timing maps with uniform 24 lbs on all cylinders. The LT4 SA does idle "a little better". It runs 25-27 deg at idle conditions as opposed to the 23 I was running. I put that in quotes because it's not at all a slam dunk conclusion- just appeared to run a little better. There was no scanner data to back up that "butt-dyno" feeling. However, I can say this difinitively: The car runs better with a 30 lb injector on the #2 cylinder.


I then put the LT4 SA map and 30lb injector back on #2. The same improvement was observed as before.

Putting the second 30lb injector on the #1 cylinder yielded an even further improvement. The improvement is marginal compared to what I saw previously, but it did help.

 

Here's something you might want to try if you noticed increased SA clearing up the idle... add more.
I helped a guy with an Lt4 hot cam TPI vortec 350 and at idle we have 28, 30, 32, 34 in the 400, 600, 800, 1000 RPM cells. It's a good solid idle. When we first started tuning the idle we tried 18 degrees and it started to misfire from fouling the plugs. Increase the SA and the plug stays hot enough to stay clean. With that being said, I'd go ahead and try 30 degrees of timing at idle and see how it does. Be careful not to add a lot of timing in the low RPM high load area's. Stay under 20 degrees below 1000rpm and 80-100kpa.

 

Sounds like you covering the symtons of a poor ignition system with trying to cascade the reaction sooner with more fuel and timing, to make up for the reaction not being able to get off to a good start otherwise.

 

Maybe so.
His setup wanted that much timing, 36 was too much and 20 felt slouchy with tip in so we crept up to 36 and backed off 4 degrees. Gets 18mpg average mostly city with a 2800stall full weight.
We tried leaning out the startup fuel to see if that was the original issue but it got worse so we added the timing. The plugs look perfect now as apposed to the dark brown almost black that would ensue from not running enough timing at idle. We didn't change the fuel, just the timing and the plugs cleaned up nicely and the MAF was reporting it's lowest gps with the ~30 degrees. If it were my car I would have played with it more but he's happy and it is his car. What would you have tried?