Iwent to the track last night, and noticed a big isssue. The car is lazy at high rpms, and in 4th gear it starts to break up around 5000 rpm. The cam i have (comp xe274 230/236, .487/.490, 110lsa) should pull hard to 6000. In the old motor the thing lived above 6000 rpm, now it doesn't wanna pull after 5000.

I'm thinking it's the valve springs. they are 1.47" singles that came with my trickflow heads, there about a year old. reason i suspect them is because i used run 1.6 rockers, and they tapped constantly. I could never get the tap out of them. I went back to 1.5 rockers and it went away.

Could it be the valvesprings that are responsible for it's sluggishness at high rpm?

If so, what should i get as a replacement. I was looking at the comp 986 duals, but i don't know if thats too much spring or not. Comp recommends a 981 spring but with the 1.52 rockers i have, they might be past there limit (.490).

Thoughts? oh yeah, what else besides springs am i gonna need if i do the swap? locks, retainers...all that crap....

Eric

 

Locks, retainers, may want to think about changing seals while you are at it. Have you checked your ignition coil condition? I would say change your springs, they are probably causing "float" above 5k on the tach. Hope that helps.


P.S. What size head studs are you using?

 

I had the exact same problem with my stock LT-1 in my race car, pulled great up to about 5000 and fell on it's face. I replaced the ignition coil and all is well now.

 

You are on track thinking it's your valve springs. For your flat tappet hydraulic cam, the 986 spring will do the trick. Yes you will need retainers, locks are cheap insurance. Go with 10 deg locks.

www.geocities.com\dzperf

 

Is there ever any time that it revs good to 6000 RPM's?
say for instance if you're just cruising half way through second and then go to WOT?
Or does it break up after 5000 RPM always?
If it does it always then I'd suspect the valvesprings.
If it goes to 6000RPM sometimes then it's not the springs. It could be the carb running out of fuel as stated in another post.

 

it doesn't always break up persay, but it is ALWAYS lazy above 5000 rpm. It used to pull like a raped ape all the way to 6500...now it sucks up top.

I highly doubt ignition is the blame, coil, distributer, wires and digital 6 are all brand spankin new.

Eric

 

i think a REV kit would be a good idea right about know

 

Don't forget to make sure your getting enough fuel at that RPM...

 

i got plenty of fuel...or so says the AF guage.

i went to the track again tonight, and it's not breaking up like it was the other night, but it is still kinda lazy in the upper rpms.

Eric

 

could it be too rich maybe?

 

AF guage says the mix is good. I was having a problem with it leaning out when i got to 3rd gear, but raising the float level on the secondary bowl cured that problem.

Eric

 

How about when there is low load on the engine
like when you're in the burn-out box. Does it rev okay then or the same as if under load?

 

seems to rev fine in the burn out box, but i have a 2 step set for 4500rpm, so it would be hard to tell...

Eric