TimZ28

Here's the situation...

I was asked to tune a C5 (owned by a performance driving school in FL). The car is a 383 shortblock, AFR heads, Comp 256/260 cam, FAST intake, headers.

On the Dynojet (loaded) the car is only doing about 350 ft-lbs and 380 RWHP dropping quickly after 6K. On some runs the car starts to miss at the higher RPMs but this does not happen on every run. The engine builder acknowledged this happened on the engine dyno as well but when they boosted the fuel pressure to 70+ it stopped and picked up power. None of the dyno runs indicated a lean condition anywhere. The base tune was in the low 12s across the board. The engine builder also claimed that this combo made 600 HP on their engine dyno.

I overlaid my stock GTO graph over this one and it was idential up to about 5K where the C5 was up about 30 HP till redline. The graph is also poor... at 4500 the peak torques... then falls... then comes back up around 5400.

Any ideas on what could be going wrong here?
Tim

2000 Tran Zam

iTrader: (2)

what kind of valve springs are you using? have the dyno graph? maybe youve got valve float with such an aggresive cam in that size motor?

383LQ4SS

iTrader: (33)

is this speed density?

My old 385 solid roller had some issues similair to this when we got started tuning. In my car it was the FAST TB spinning on its shaft and causing it to go over the voltage threshold for the TPS. I had to mod it to get it to stay. You may check the TPS voltage at WOT. It was way out on mine...but it wouldnt throw a code because it was only WOT for short periods. Or the code would come and then go away after a few starts. It would pull hard on the dyno as you went WOT and as soon as the TPS voltage went out of range the car would drop 30 hp...it would then continue to climb....but slower.

Other things obviously are valvetrain related. Springs, rocker arm and rocker geometry depending on what top end combo are being used.
Can you give an exact list of top end components including springs, rockers, pushrods, lifters, etc?
The thing is though even with some valve float I would expect more Tq at the peak TQ rpm range.

Possible cam degree issue?

All cylinders/injectors running even? Maybe a laser thermometer on each primary to see if there isnt an injector issue? That may make sense with them boosting fuel pressure.

Any codes?

gtodoug

iTrader: (1)

Valve control is key, I'd be wondering what the valvetrain setup on the car is. That is alot of cam, I should know I have it and tuned it and did the tuning as well. I street tuned it in open loop with a wideband.

256/260 .660/.643 114LSA

Here is a copy of the graph:

TimZ28

GTODoug,

Thanks. This car is nowhere close to the graph you posted. I wish I had grabbed the Dynojet file when I left the shop. Unfortunately I do not know how the valvetrain is setup at the moment.

Tim

beardWS6

iTrader: (1)

GTODoug. How is the car running? I found some info on our ET heads and the restrictions of the intake. Mike at G-Force has been talking with me about a LS2 intake with our heads. Vette Doctors put a 408 together with ET heads and LS2 intake and made freaking awsome power. The LS2 flowed better than my ported LSX 90/90. I`m planning to call them to make sure what was all done and try it. Low cost test sorta! Mike did say they had a 383 with et heads make 480rwhp with LS2 intake!!

Phil99vette

iTrader: (11)

Valvetrain stability is key. Good springs 3/8" pushrods are necessary for good control.

gtodoug

iTrader: (1)

Hey Beard, everything good here. I made it out to the track a couple months ago, but bad air (+3000 DA) and driving (2.2 60') netted me 13.0's at low 120's with 4015lb raceweight.

I'm currently looking into making the setup work a little better. I should have some info within a couple weeks.

dougie

pewter 01 SS

iTrader: (5)

hey i thought i was the only one with this problem with the tps in wot.. the guy who tuned it was saying that he had to reset the tps many times ever few hours.. he said that he never had a problem like this before and didnt really know how to fix it.. any tips you can give inorder to fix this? i bought a new tps and same thing.. i know have nick williams 90 mm tb with fast intake and havent figured it out..

383LQ4SS

iTrader: (33)

just have him check the actual voltage output of the TPS at WOT. It must be within the range specified. If its not youll have to fix it so it is within range and stays within range. Im not sure what adjustments you have on the NW TB but it shouldnt be too difficult. The problem with the FAST is the hardware used to hold the various pieces in place werent strong enough and everytime a mashed the gas pedal the shaft would spin.
So whatever you have to do to get the right voltage range and keep it there.

And I am wondering about Tims customers issue being vavle float. It doesnt make any TQ in the lower rpms. 350 ft/lbs for a 383 with AFR heads is very bad. (is this a big stall auto?) You sholdnt have float at 4500 rpm unless the valvetran geometry/spring setup is just terrible. Which of course is certainly possible. But to be that low of tq at that rpm it would be BAD!!!