Stronger at 16psi than 23psi...WTF?

[VadersV6]

Ever since I got my new MAF, the car is running better than it ever has. I was never able to stay in the throttle for long because it would seem to fall on its face. I always thought it was bad valve springs or maybe a clogged exhaust. Now I can go WOT and stay there for the first time. But Ive noticed that the car pulls harder at 16-18psi than it does at 23. Ill tune it so the o2s are good with either boost level, and Ive got alky spraying the same at both boost levels. So I dont know what the deal is. I can get low 800's, high 700's just fine with 23psi and no knock. If I tune the same with 16-18psi, the car pulls like a freight train. The other way, it tears the tires loose than just feels flat even though my o2s show no sign of going lean and no knock. Think maybe the springs are bad and maybe the extra boost is cracking the valves off their seats? It looks like they have LT1 springs but the car did sit for 4 years before I bought it, so it probably had a few springs being compressed for 4 years.

 

[turbo nasty]

If I tune the same with 16-18psi, the car pulls like a freight train. The other way, it tears the tires loose than just feels flat even though my o2s show no sign of going lean and no knock. QUOTE]


Get it to hook and go from there its hard to nail down the timing when the motor gets unloaded from tire spin.

 

[22lbChevelle]

Does it fall flat as it reaches high boost or as it reaches higher rpm (boost already high).

The extra force on the valve at high boost: (8lb/in^2)(pi)(0.75^2) = 14.1 lbs... I guess maybe it could be doing it to your springs, since they are only 100lb springs.

Maybe you're running out of spark..

Edit: the force on the valve is for a 1.5" valve ( I think thats what we have). Also doesn't account for the valve stem.

 

[2 QK 4 U]

Just my .02 but....

If all your numbers look good, then maybe your engine is making the extra power that the rest of your driveline cannot handle.

My GN has a similar problem. It pulls hard up to 10-15psi and then falls flat and don't pull.

Eventually the engine would slam up on the rev limiter and the transmission would not shift. Dropped the trans pan and found black fluid with converter material littered everywhere. the converter was not holding the extra power level I was making at the engine, therefore was not transfering all the power to the trans. I am putting in a brand new converter soon and it should be night and day difference.

Just something to think about because it was the last thing on my mind considering it's a Precision Industries converter. But anything man-made can go bad at anytime.

HTH Good luck,
Jason

 

[22lbChevelle]

Good point ^. I just figured he would have noticed that.

 

[Welby]

Same problem with my Limited. I ran a 13.6 with 17lbs, then added alky and with 24lb's ran a 13.4@105. Pretty discouraging. I'm sure it's because I don't know what I'm doing. I sent out for a matching alky chip, so once I get that back I'm going to start from scratch and see what I can do. The car feels amazingly flat for 24lbs....

 

[BlackMetal]

I sent out for a matching alky chip, so once I get that back I'm going to start from scratch and see what I can do.

Spending money on mods is pretty pointless if you're not going to match the chip right away, so that's why you weren't impressed with the performance. Spraying alky with your old chip did nothing except over-fuel the car, not to mention an alky chip will turn up the timing and help make extra power.

Install the matching chip before you get discouraged.

 

[Welby]

Spending money on mods is pretty pointless if you're not going to match the chip right away, so that's why you weren't impressed with the performance. Spraying alky with your old chip did nothing except over-fuel the car, not to mention an alky chip will turn up the timing and help make extra power.

Install the matching chip before you get discouraged.

I know, I know, lol... When I sent the chip to Eric I noticed it was for 91 octane, 14-17lbs of boost and (I think) 21 and 18° timing. So no wonder it's not right, eh? I thought I could get away with alky on a standard chip. It's my own fault for being impatient. I can't wait to get the chip back and start fresh with the right stuff

 

[BlackBandit]

Spending money on mods is pretty pointless if you're not going to match the chip right away, so that's why you weren't impressed with the performance. Spraying alky with your old chip did nothing except over-fuel the car, not to mention an alky chip will turn up the timing and help make extra power.

Install the matching chip before you get discouraged.

While I agree with this statement overall, it's not entirely true, especially when it come to alky. If you have a pump chip designed for 15 pounds of boost and add alky you should see a significant gain when you turn up the boost. This is because the ecm isn't compensating for the increase in airflow when you turn up the boost so it should go lean. The extra fuel needed will come from the alky. Is it ideal? No. But it should still work fairly well and the car should definitely NOT be over fueled. This is assuming stock ecm and the only time I can think of that this wouldn't be true is if you have an extender and I'm not even sure about that. HTH. james

 

[cjmatt]

The ecm will still add the right amount of fuel per the reading from the maf, the alky goes over the top of that and will richen it further

 

[BlackBandit]

The ecm will still add the right amount of fuel per the reading from the maf, the alky goes over the top of that and will richen it further

Depends on the chip. If your not running an extender what happens when the maf hit's 255 gps? It guesses based on preset values. If those values are written assuming 17 psi and you're running 23 psi you go lean.

 

[ngc414]

Have you checked for boost leaks? I had a similar problem with a twin turbo. The car would pull then hit a wall around 4k rpm. I thought the same thing valve springs. When I did a boost test it would hold boost to a point then when it hit its breaking point (in my case 12psi) I could hear it start to hiss then the guage would rapidy drop. I replace 2 hose clams and the car ran perfect.

 

[GN-231]

I don't see any comments about fuel pump upgrades to your car. I assume you are using a hot wired stocker or any upgraded pump?

I have the same issues and noticed that my fuel pressure drops because it can support the volume required at the higher RPM's with extra boost. I need to upgrade the pump soon. I also get good O2 numbers on the scan master, but I think the lack of fuel pressure could still be the cause. At the 16 to 17 psi level, my fuel pressure will not drop off...only at the high boost/high RPM.

 

[Terry_H]

High boost can also interfere with the spark if the plug gap is too large. Easy to close the gap and test.