Why am I running out of breath at 4-5000rpm

[C0V3R]

I've been running in my new 5.0 for a while now. I've noticed that it seems to run out of breath up top, seems to slowly stop pulling as compared to the 2-4000rpm range.

The build is as follows:

Brand new 5.0 roller shortblock
TFS Stage 1 Cam
TFS TW Heads
Weiand 7515 single plane
Edelbrock 1406 carb (600cfm)
Full MSD ignition & diz
Tri-y headers (and crappy press bent exhaust)

I am running around 15deg initial timing, and am up a couple of jets on the mains from stock.

Do you think the carb is too small, exhaust may be a problem or maybe something else?

 

[OrLackThereof]

That single-plane should really want to rev...Im not sure what kinda single plane it is, but generally dual-planes are better except for race apps.... May just be a mismatch of parts that's keeping you from your potential.... I mean when I think single-plane, usually I think of my buddies running sick forged N2O 306's, with single-plane Vic Jr's and Holley 650 hp DP's

 

[C0V3R]

Yeah thats what I thought too... bar the carb and maybe exhaust the core package should be good for 5-6000 rpm I would think.

 

[OrLackThereof]

Yeah thats what I thought too... bar the carb and maybe exhaust the core package should be good for 5-6000 rpm I would think.

Im in agreement there. Could always play around with the tuning a little and see what you can do.

You sure the cam timing is right? Maybe it's retarded a few degrees

 

[C0V3R]

Pretty sure its spot on straight up. I had the timing markers aligned exactly vertically.

 

[dragnazz5.0]

what made you decide to go up with the jets in the carb? it could be just too rich up top. whats the carb setup at? what jets and metering rods.

 

[conv91lx]

Before turbo kit, I used to bounce off the rev limiter easily with same heads and trick flow stage 1 cam. Fuel injected though. Carb maybe a little too small but I doubt it, not much of an expert with carbs. Unless exhaust is like 2" pipe or something, the crappy bends should'nt be that much of a restriction.

 

[C0V3R]

I went up in jets as the previous motor it was on ran lean and had a lot less air flow capacity.

 

[dragnazz5.0]

ok....well it makes sense since you have trickflow heads...i must have missed that part before. how many stages rich are you anyways? i doubt the carb is too small since my buddy was running the same carb on a 383 that had some monster heads on it. what jets/rods/springs are you running anyways?

 

[C0V3R]

0.95 secondaries (stock) - Cant remove these easily, one is stuck in the carb body.

Cant remember exactly on the primaries, I think about 3-4 stages rich on cruise and power (about 15% IIRC). Will check and post back. Running stock step up springs (orange I think). I don't have any noticeable surging or flat spots.

I have the fuel pressure spot on 5psi from an electric pump

 

[GTOreturns]

what kinda gears and tranny?

 

[C0V3R]

2.79's and c4

 

[GTOreturns]

2.79's and c4

Mite wanna go up a tad.323's or even 355's!

 

[C0V3R]

GTO - I dont think gear should have anything to do with the car going flat at 4-5000rpm. Sure it would make it quicker but its not solving the root cause of the problem.

 

[dragnazz5.0]

no changing the gear wont do anything...now does this problem happen when you are just full throttle or any time the car hits 4k....like does it do it if you just give it some gas and let the motor wind up that high or does it only happen at full power

 

[C0V3R]

Thats a good question, but it rarely gets that high unless you're on the throttle as the tranny shifts. I'd say it does it under both circumstances though at g guess

 

[dragnazz5.0]

have you tried putting the shifter in low 1 and trying it out. it should hold that gear and not shift. also...what is your total timing....maybe its too much total and thats causing it.

 

[GTOreturns]

GTO - I dont think gear should have anything to do with the car going flat at 4-5000rpm. Sure it would make it quicker but its not solving the root cause of the problem.

This is true. Whats your fuel pressure at?
*Scratch that last question.
I would suggest bring it to a dyno and have a dyno tune

 

[C0V3R]

I've got the smallest amount of total timing I can get in the MSD diz given the 15 deg base. IIRC this is 18 degrees making for 33 total. I'll have to double check later to make sure.

Also I have the stiffest springs so the mech advance will come in relatively slowly.

 

[C0V3R]

Got the AEM wideband installed today and went for a run. I'm idling at 11.6:1 and cruising around 12:1

On most WOT runs it starts off around 12:1 and moves up to around 13.6:1 at around 4500-5000 although I did see a 14.x once.

Would being a little lean up top cause it to stop revving?