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High load, bad stumble

Road Tuning

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Alrighty here we go!

2001 LS 4.8

112/112 110lsa cam.

I'm having an issue while tuning my ve tables. It is very reproducible. I've noticed that under high load 70 kpa+ my engine absolutely will not accelerate. I begin seeing as much as 8° of knock retard being pulled from these areas. This is really frustrating, because as you all know, when taking off from a stop you reach these load areas fairly quickly. If I can keep the engine below 60kpa it runs like a dream. But anything 70 kpa or higher and it starts stuttering, loses all acceleration/power and I'm getting what sounds like a popping noise, cant tell if it's coming from the intake or exhaust, maybe both. Any ideas where to start? I've heard this may be to the ignition being too advanced which would make sense because I'm seeing so much timing being pulled for knock? So is this a take 2° out of the timing in the high and low octane table in these areas until the problem ceases, get the VE all tuned up then go back and add later if I can?

As stated, anywhere below 60 kpa, and this thing runs like a champ, but whether in a high gear, low gear, whatever throttle position, any rpm, if I go above 70kpa it turns to garbage. For example, if I take off it does fine, but if I dont have the rpms up high enough when it shifts and the kpa goes high, it just doesnt want to go. I should also mention that my fuel pressure tested out alright, it is reading 50psi at the rail when key on engine off and 55+ when running. I just looked another post that where Arghx7 (Raymond) had mentioned that if you change the cam, it could affect the low load areas of the table, could the same be true for the high load areas?

Thanks for the help as always guys.

What are your wideband readings doing?

Are you confident with cam timing?

Don't have a blocked catylist or collapsed flex in the exhaust?

My wideband readings are saying I'm lean in that area. I'll have to look at one of my recent logs I made of it and see what its says. But all my values are fairly close to one another in adjacent cells in the high load area. I'm getting a lot of knock retard and it's just acting like it's got none there. But everywhere else, it runs cherry. When cruising or idling, it stays pretty close or kind of snakes the line for my commanded.

As for the cam timing, I feel pretty confident with it, the manufacturer said I could do it dot to dot. I suppose if I absolutely had to I could get it to tdc on one again and pull the water pump and front cover during my next oil change to reassure myself.

I'm running no cats right now, and all my exhaust is new. No kinks or collapsed sections.

Ignition dwell hasn't changed?

Both rich and lean missfire will read lean on the wideband so you need to go off what you target is and how close it reads before it plays up to have a good idea.

Did you throw more ve at high load at it before trying to run with the cam?

I'd be tempted to pull 6 degrees timing and add 10-15% more fuel near WOT than on the factory cam as a starting point if you didn't make preemptive changes.

If you cut valve relief pockets in the pistions did you break sharp edges to avoid hotspots?

Idk what you mean by ignition dwell, I've seen that table but never touched it. I have thrown a little more fuel at it. No real improvement. I have not modified my internals in any way.

I got a good question for you, how do you remove 6 degrees of timing from the whole high load area with only the multiply sign and addition signs? Or would I have to manually put it in all the cells? Sme question about the VE. I know if I'm lean I multiply by 1.04 so if I'm rich in a certain are what do I multiply by? The negative number?

Depends on the tuning platform you interpolate between selected rows/colunms or usually at least highlight a block of cells and either bump up/down with a shortcut key or multiply the whole group. I haven't used any of the packages for editing factory GM stuff but I'm sure there will be a website or help file with quick tips somewhere.

55+ SPI pressure when "running" - but what is it when you have it under load, especially when it seems to stumble? Is it referenced to boost?

Thanks for getting back to me guys! Found some info for taking timing out so I may give that a shot and see what she does. And as for my fuel pressure when it's doing this, no idea Haha. I used one of those little kits with a guage that comes right off the rail, was not able to test it while running it in this high load area. It's a factory vacuum referenced system, so it should stay the same throughout correct?

If it is manifold pressure referenced, naturally aspirated, highest gauge pressure should be key on engine off, it should then drop when running by the vacuum the engine is generating, scaling back up to close to the engine off pressure at wide open throttle, boting that it is normal to have some "regulator droop" with reducing bypass flow (when the engine is consuming more fuel) of possibly a few psi depending on the reg (and intake restriction not alling the manifold to come right up to atmosphere), but more than that usually indicates a pump/filter problem or inadequacy.

The fuel pressure testing equipment I have has a long enough hose to reach the front doors for monitoring while driving - try begging, borrowing or buying a good quality guage, line and fittings as without KNOWING what the pressures are doing you are just pissing into the wind. Yout problem is screaming fueling problems and so that is the first thing I would be investigating - I assume you at least have replace the fuel filter(s) recently?

After some decades of faffing around with cars I've learned NEVER ASSUME anything - what you just assumed was OK is often the thing that will come back and bite you in the bottom.

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