Just built a 408 with Ls3 heads, intake and 631/617 cam. On the dyno it makes power up until 5200 rpm then the curve drops hp and goes flat for the remainder of the pull.

We ruled out the new converter, cam is dot to dot, afr is good, not showing a fuel supply issue and no air restrictions.

Any suggestions on possible issues will be appreciated.

Thanks for your time.

 

Does the power flat line or fall off a cliff.

What PCM? If it flat lines It could be in the tune, TM setting. (depending on the PCM or if it's DBW)

If it falls off a cliff I'd look at the valve springs. Which rocker arms are you using?

 

I believe the springs are Pac 660 but unsure. The heads were bought assembled and will message the company monday about the spring brand.

Pcm is a Gm 411, Dbw and stock Ls3 rockers with trunion upgrade.

 

Do you have a log by chance?

I'm wondering if it's a TM issue and it's closing the throttle.

 

Apologies it was late when i made the post and responded. It is a 98 camaro and dbc. The compete wiring is out of a 01. This is how it looks on the pull but I don't have a complete log. Timing was at 28.

 

I had the same issue with my first H/C combo on my C5 and it was the cam head combination. It would fall on its nose after about 5500 rpms. So I had to go with a completely different combo and it worked out. I hope that's not the issue but I've seen many of build like this.

is that a reverse split cam?

 

Can you share cam details please? Is the cam a reverse split?

 

This is the cam. At this point I'm leaning to the cam and or springs.

 

Springs or pushrods would be my first guess

 

How does it feel on the street?
weird that the wheel speed surges up as the power drops off. Is that a auto or manual?

 

What pushrods?

 

What heads are you using and chamber size what is the lift and bind on your springs what size pushrods. What compression are you running.

 

A HPT data log from that run would be extremely helpful.
We have no idea what AFR and timing are doing at the point it goes south.

Ron

 

What lifters and what was the preload for the lifters?

 

I ran a ls3 headed 421 a few years back with the LS3 intake and it also fell off a cliff in the mid 5000 rpm range. Swapped the intake for a Fast 102 with the high hp runners (mid length) and it fixed the problem. Picked up my hp, moved my powerband up, and knocked 3 tenths off my 1/4 mile time. Don't get me wrong I think the factory intake is a wonderful piece for stock displacement, just not for a stroker.

 

I doubt it's the cam. I doubt it's spark and fuel. I doubt it's the intake as well, that wouldn't make sense to me....flatlines? Not valvetrain instability...maybe your pushrods are way too short?

 

I would have dialled the cam in, not relied on dot to dot.

 

I seen this once on here like six years ago -- a crappy intake hose was being used and the motor was sucking the tubing in and choking the motor off.

 

I've had a similar issue in my C4, the original rubber intake boot from the LT1 will suck shut occasionally, it does feel like hitting a brick wall. Only happened when the underhood temps warmed up enough to soften the rubber, and only about ~5000 rpm. Increase airflow needs (running a 403" LS2 now) + 30 years of age is took much for mine

@ the OP, check your MAP readings from your run, you should be about at ambient pressure (not sure your units in your setup, but 95+ kPA for me), it is drops drastically at that point, something blocking the airflow.

Because that curve doesn't look natural. Its a little small on the intake for that cam, but not crazy. Shouldn't fall off so soon, and it wouldn't kink the curve like that. I run a 239/247 in my 403.

 

So far we've checked these things
Fuel- never leans or goes rich throughout the pull.
Airflow- manifold pressure is good and the hose isn't sucking to.
Cam- number matches the card so it's correct.
Timing chain- dot to dot.
Converter- locked or unlocked issue is still there.
Replaced spings with new BTR 660 and no change.
​​​Plugs are AC Delco iridium gapped on .040

The car doesn't miss or trip any code throughout the pull. It just isn't making power past 5200rpm.