redgto4u

iTrader: (8)

Hey all,

Just got my GTO on the dyno and was kinda shocked at the power curve. The numbers themselves don't concern me, but I've never seen a dyno graph look like this. The power seems to be falling off BAD after 6200 rpm. All my relevant mods are in my sig. I'm thinking it might be valve float from either weak PRC dual springs, these comp 1.75 roller rockers, or a simply the AI 243 heads not able to keep up, maybe a combo of all three?? The springs are about 4 years old 25k miles, the lift on the TX Giant cam is around .640 with the 1.75 roller rockers. The springs are rated to .650

What do you guys think? Its and auto with a big stall so I see why the graph shoots up in the beginning, but the top end looks jacked up! Car runs fine and the scans look good too. Just seems to fall on its face up top.

Always2Slow

iTrader: (8)

Valve springs! Might I suggest a good valve spring on the car like a set of pac1207x. I wouldn't run the car anymore otherwise your going to find a nice surprise inside.

ranmas2004

iTrader: (1)

I've seen plenty of Ai 243 head dynos and NONE of them ever looked like this. I would suspect something wrong with the valve springs as well. The Ai 243s flow just fine. NICE power!!! Almost 480whp!!!

Are those the 226 or 232cc heads? I got some 232cc Ai 243 heads milled to give 11.5:1 CR, dual springs titanium retainers. Car still being built so we shall see how she goes. Hoping to hit 500whp.

JHR Performance

I highly doubt there is an issue with your cylinder heads. The springs or heavy comp rockers may be the culprit however.

Lockup converter? If so was it locked on the dyno? Almost looks like it was and the converter could not stay locked during the pull.

Zach

redgto4u

iTrader: (8)

The converter was commanded unlocked. I have PAC dual springs in it. I've been using them for like 6 years...maybe 25k miles. Maybe they are just tired.

Will a Comp or PRC dual spring fit onto my PAC bases and Ti retainers? I don't want to buy a kit, just springs. Only the springs are worn. Seems like the PRC .675 springs might be the way to go for me.

Also will a 5/16 pushrod with a thicker .105 wall help things aswell?

Always2Slow

iTrader: (8)

Hate to break it to you, but valve springs wear out a lot faster than you would imagine.

redgto4u

iTrader: (8)

No biggie. Thats an easy fix. I just want the car to run how it should!

redgto4u

iTrader: (8)

No biggie..easy fix. I just want the car to run like it should!

moeZ28

iTrader: (7)

I'd swap back to stock rockers and try again first...eliminate the 1.75's as the problem before swapping springs.

redgto4u

iTrader: (8)

Thanks for the tips. I'm going to try new springs. BTR Platinums soon to arrive. If that doesn't completely fix the issue then back to the stock rockers with trunnion upgrade. I need it to be running right before I put the shot on it.

Sales@Tick

iTrader: (3)

Lifters and springs. Those BTR Platinum spring do not have enough seat or open pressure for roller rockers IMO.

BTR makes a nice Max Pressure spring that would work in your application.

I set up roller rocker set-ups totally differently than stock rocker set-ups for this reason exactly.

Factory lifters also normally cannot keep up with roller rockers either. Some roller rockers seem to do better than others in this regard due to differences in the MOI(moment of inertia) between the rocker arms.

I always use aftermarket lifters, PAC 1207X or 1208X springs and at a minimum 11/32" push rods with roller rockers. I prefer dual taper 3/8-5/16.

Roller rockers get a bad rap, but with the right set-up they can work very well.

redgto4u

iTrader: (8)

Thanks Martin. I can see why now rollers get a bad rap. If not set up for everything to work together they can cause issues.

the extra lift is not worth it for me at this point. So I'll use a stock rocker with trunnion upgrade on my new springs.