I have a suspicion that I may be experiencing valve float or at least some loss of valve control around 5700 to 6000 rpm. I don't take the engine over 6000 but that last 300 rpm seems difficult to achieve. The rate at which the engine is accelerating slows noticeably. It also sounds and feels slightly different.
I've read that valve float sounds like a misfire. I wouldn't describe it like that. More like a hesitation, maybe that's what misfire feels like at 5800 rpm. Anyone experienced this that can describe what the symptoms were?
I'm currently using comp 987 dual springs with a 370 lb/in rating. This is a little more than the cam called for at 330 lb/in. I am also using Howards cams roller lifters on a medium intensity 270 duration roller cam with 219@.050. The lifters are rated for 6500 rpm so not worried there. That rating I suppose is going to hinge on valve train control. Figured the 987 springs would give me enough pressure to control the heavy roller lifters, maybe not.
Could be the cam is all done at 5700 rpm?

I upgraded the ignition module to eliminate that possibility and it seems a little better but still hesitant.
Maybe beehive springs if this is valve train issue?
Any advice from those that have been there would be appreciated. Thanks.

 

From what you describe, it could be either valve float, lifter pump-up, or running out of fuel. Either will feel as if the engine was losing power almost to the point of a miss. I am not sure how to specifically diagnose the issue further, but I learned not to trust the specs of hydraulic lifters or aftermarket springs unless I can test them myself.

You could be running out of gas at high RPM's if your fuel system isn't set up for it.

Just thinking out loud...

 

Temporarily hook up a fuel pressure gauge and watch it as you hit those higher RPM's. You also dont mention what carburetor but check the float level and the condition of any internal fuel filter.

 

My first guess would be that your cam is such a low duration that it is way passed it's peak at 5700 rpm. The amount of cylinder filling is negliable.

Other than this 370# number, what is your closed pressure and installed spring height?

The 370 lb/in rating is only a small part of the story. If you set up that spring for 100# closed and lift the valve .500 inches you only have 285# at max lift which can cause valve float with heavy H-rollers. but lets say you shim the spring with the same installed height and place 140 pounds closed your are at 325 pounds at .500 lift. Which might not float.

 

you are platoing... what heads? what exhaust system? what intake? you may be hitting your peek hp do to restrictions in the cam, intake, or exhaust. i have had this feeling, for me it was my springs, and when i went to the beehives, it was night and day... i was peeking at about 5500K, after the new springs i could run all the way to 7K RPM and still pull strong... very strong.... but then again, i snapped my crank. :-) where does your cam supposed to peek at?

 

i agree with George about your cam...

 

Fuel pressure or spark but valve float shouldn't be an issue with those springs.
It's possible you just need a few more degrees of timing or close the plug gap. if it hesitates at 5700 rpm in first gear. Fuel usually will show itself when winding out second gear.

 

My camaro with a 396 would float the valves at 5500 it sounded like a really bad miss backfire sound and fall on its face. It would never rev past 5500 the way it was set up. At the time my fix to get it to rev higher was a solid lifter cam.

 

not enough details on the engine to diagnose.

 

W/gkull that cams done at 5800

When they do float it goes "flat" as you described. If you can "hear it" its real bad back off immediately

Just to be safe take a look at your valve tips see if they are a little beat up make sure no retainers look like they are "sunk" or wanting to pull through.

Had a motor similar to what yours does it would pull hard to about 5k then seemingly take forever to go any higher, valve float (larger cam)

 

Funny you should mention that. I did have a fuel pump failure recently and I know how that feels. The engine would shut off the restart each time it starved for fuel and then got it back. Running 6.5 lbs of fuel pressure with pressure regulator.

Float level and fuel pressure up to the task. I have an inline gauge on the fuel line and reads 6.5 psi. Don't believe it's running out of fuel.

I also suspect this may be happening. And maybe lifter pump down? I haven't had it on a dyno so I don't know for sure where it peaks.

Seat pressure is 130# at 1.8" installed height. Open at .550" spring pressure is 330#. My cam/rocker combo opens valve to .549". Max open is .600" so not much shim room left.

You may be right. The heads are Dart SHP 180cc heads. I cc'd the intake runner though and it is more like 197cc instead of 180 so I guess you could really call them 200cc heads. Flow is advertised at 248 cfm on the intake and 170cfm on the exhaust. I pocket ported the exhaust so it is a little more. No flow numbers though.
Exhaust is Hooker headers with side pipes using STS baffles. Probably no restriction there.
Biggest restriction is likely the edelbrock 2101 performer intake. It's rated to 5500 rpm. I did message it a little too but it may still be holding me back.

This would be good. Easier to deal with weak spark issue than valve train control.
My other suspicion is that the stock coil is not able to fully saturate at those rpm levels and is getting weak. CR is 9.9:1 so spark does need to be strong enough to get the job done. Guess I could close up the gap a bit and see what happens.

 

Ok, it's not doing that. If that's the symptom of valve float then I'm in good shape there. Thanks

 

What did it feel like when it peaked at 5500? I mean would it go higher but only with a lot of effort or was it like a wall?
What kind of intake are you running?

 

It wouldn't go any higher . the engine sounded like it was running on 6 cylinders really rough and you could feel the car stopped pulling

 

edelbrock 2101 performer – Is miss named. It is lack of performance. They were made for pickups….. I tried one 20 years ago
and my motor hit a wall. So I thought that it needed a bigger carb. The bigger carb did not help. So when a manifold says 5500 rpm max it really does mean it.


So your bottle necks are your manifold and the low duration cam. Maybe the heads are also a limiting factor.


Look at the bright side…………. Your motor can’t spin its self fast enough to blow up!

 

it was an Edelbrock RPM performer.

at about 5K it just did not gain power.. it tabled flat and a little futher up the rpm tree it just ran bad

 

I suspected this was going to be an issue. Thought maybe I could get a little more out of it and I guess I am at 5700 rpm. Unfortunately it is the one that fits under the stock hood.

Self preserving intake Eh? I just wanted that extra 300 rpm for the shift to third. It's a long pull on a th350 with a 3.08 out back.

sounds like I have encountered the limits of the intake system. It can't breathe any better with whats on it. 100 shot nitrous?

 

I had a stock hood with a weiand single plane, K&N dropped base open element air cleaner. It was something like #7531 or what ever the shortest low rpm model is. I think was a 7200 rpm or something.

 

I use the Weiand 7530 sp. This is the one gkull mentioned above. Carb height: Front 3.63", Rear 4.56". My Howards HR .560" lift, 227* @ .05 pulls well past 6000 with a 650 Street HP. It all fits under the stock '76 hood with a 1/2" to spare.

 

I think I'll save that intake for a higher performance build. It looks like it could be a real dog on this engine until 3000 rpm. Guess a guy can't have his cake and eat it too all the time.
Think I could get a Q-jet on that intake with an adapter and still have room?
Pretty much when I look for a dual plane spread bore intake, the performer stands out, at least in the idle to whatever rpm range of intakes.
There are two summit brand intakes that claim idle to 6000rpm. Kind of wonder if that is true.
Or the Edelbrock 7104 which is rated from 1500rpm to 6500rpm. Maybe that is what I should get if I want to rev to 6000. I don't want to give anything up at the bottom either though.
Anyone run one of these on a similar build and have a performance report?