Recenly had the car dynoed and it made 400rwhp @ 6200rpm. After 6200 the curve falls off dramatically. I loose probably 50rwhp w/in 300rpm (run made to 6500rpm). I know the default answer is usually valve float. I've tested 4 of my valve springs and they are exactly in spec. I know I should have all the springs tested, but since it's virtually impossible w/ the heads on the engine... I was wondering could perhaps one cylinder be the culprit with such a sudden loss of power? Engine is still trying to make power before it hits a brick wall.
I watched the wide band throughout all the runs and A/F was w/in the green. Car was being scanned and watched throughout as well w/ nothing abnormal happening.
I'm using LPE LT4 ported heads/intake w/ the GM847 cam and Crane 99893-16 springs. I have Comp R lifters in the engine now, but even when dynoed with stock lifters before it fell off at the same rpm at the same rate.
I'm now wondering if the lobes on the cam are hammered...
Any insight?

 

VE problem (Volumeric efficiency) exhaust/ intake restriction maybe.

 

Could be one cylinder but by the sound of it to me it says there is some valvetrain issue, either deflection or a harmonics problem.

Bret

 

MachinistOne,
According to LPE the heads flow 276cfm on the intake @ .600. I'll have to look up the exhaust side.

What's meant by deflection? Meaning rockers making contact w/ springs? If so, I've checked all the drivers side bank and there doesn't seem to be any contact made. As far as harmonics.. since different springs have different harmonic tendencies could a different spring with the same seat/open pressure help out?

Thanks

This would be hilarious if my air filter or even intake elbow were collapsing at those engine speeds...

 

What exhaust components do you have? Still have stock dual cats?

That dual spring has the right rate for a hydraulic roller, but what was your installed height?

Does the curve fall off as you said, or just go flat and lay down? For how long did it carry peak power?

 

Sorry about not including exhaust...
I have Hooker LT's going through an off-road pipe that is 3" but not mandrel bent. Wondering if that could be an issue...
The install height is 1.81 with 150lbs of seat pressure and 375lbs @ .600 lift.
There is no plateu of any kind on the curve. It reaches 400hp and immediately starts falling off. The fall off is as smooth as the ramp.
I'll see if I can scan the sheet in a few hours.

 

Here's the dyno:

http://www.geocities.com/ssteal96/latestdyno.jpg

Copy/paste if it doesn't hotlink.

Sorry for the quality. I don't have one that's in rpm either.

 

what size is the engine we're talking about here, and have you verified wot when the pedal is to the floor. have you hooked up a vacuum gauge during a run to verify that there are no major restrictions? and have you checked out your fuel system durring the runs to make sure you maintain pressure all the way through. just some ideas to check besides valvetrain stuff.

on another note that graph does look like it could be a valvetrain issue. the springs are probably haveing a big of a harmonics problem in those rpms. by deflection what they mean is your pushrods rockers etc are flexing from the loads put on them. i've seen a couple of pretty strange spintron tests where cams lost .100 peak lift from deflection and lifter issues, a valvetrain is a very strange creature and doesnt always behave the way you would think.

 

350cid
Yes.
That is a hell of an idea. I'll try that this weekend.
Very much appreciated! A fuel guage was hooked up and pressure looked good (45psi). Wouldn't you be able to tell if fuel pressure fell off via wide band as well?
I have CC HiTech rockers with CC chromoly pursh rods. I would think they could handle the load? I would now not be surprised if it was a harmonics problem...

 

When a motor "hits the wall" like that it's almost always one of 3 things:

1. Outta fuel (or the fuel maps are all screwed up at those RPM levels)

2. Outta spark (or it's retarding for some reason- false knock, timing tables pulling it back at high RPMs, flaky ignition amplifier box if applicable, etc.). Don't overlook this. A pooped-out ignition can act very similar to valve float and there isn't always popping out the exhaust to give it away.

3. Outta valvetrain (flat out not enough valve spring or something weird like Ace said- deflection or a weird harmonic of some kind).

Roller cam setups (especially hydraulic rollers) are tricky. And when something's not matched right, 6000 RPMs is about where the trouble usually begins. The more of a "Frankenstein" you've got for a valvetrain the greater the chances something won't want to work with the rest of the pieces. Once you rule out fuel and spark the next most obvious place to look is at valvesprings that are actually recommended for the cam. Having equivalent pressures from a different set of springs is NOT a get out of jail free card. Obviously, valve spring isntalled height must be verified on EVERY valve if you're going to be thorough.

It is also often helpful to put on standard 1.5 rocker arms, at least for diagnostic purposes, if that's what the cam was designed around. Gotta take the unknown variables out of the equation.

 

Damon,

Thanks for the info. I would say that my setup is pretty Frankenstein'd.
The coil is the original w/ approx 110K miles and I'm not running an ignition box. The injectors are 30#'ers and I got them used.
I'll make some data logs and post them up as soon as I can.

 

Quickly - I will look at it more in a sec, but that is not a VE problem. Intake/exhaust issue would have a more gradual decline.

 

that was the next thing i was gonna ask is if you were using stock injectors. if you have a scanner it would be nice to know what type of duty cycle you're running. as for the fuel presure you'd have to put a gauge on it and then rev it out under a load, thats pretty much the only i can think of to check that other than looking at the a/f vs rpm. as for the rockers and pr's thats the parts i usually spec if guys have the coin, and they can take quite a beating and are plenty stiff. assuming its not a spring issue which is looking slimmer all the time, there are some guys here who can offer you some very good help if you run a data logger while you dyno the car.

 

It's not valve float, those pressures are on the very high side for a hyd roller. Your torque starts to drop quickly there, and the HP is having some trouble for a while before it makes it's nosedive. I would seriously check into an ignition breakdown or spark-scatter issue. If the AFR is holding 13:1 through the rpms your fuel pressure is most likely ok unless you are chasing a fuel pressure problem with pulsewidth and loosing atomization.

Looks electrical or programming.

Doesnt the dyno shop have an ignition scope setup where they can look at the waveform of each firing event on each cylinder?

 

He says it is stock ignition only shouldn't he need a box at these power levels?