Valve float due to exhaust back pressure?

[Merc Man]

I had a BBC engine dynoed recently and did experience what seemed to be valve float when the curve went almost straight down at 6200 rpm.
Up to 6000 the engine is running fine doing 1.35 hp/cu in.

Engine has a .640" lift HR cam around 245°@.050", nothing extreme. Springs are set at 145 lb on seat with a 430 lb/in spring rate.
Cam manufacturer recommends 130 lb on seat and 350 lb/in spring rate so I was hoping to have some positive margin here.

The engine was run as it was supposed to go into the car, with mufflers, and hp was a little on the low side. We removed the mufflers and gained over 40 hp so the mufflers will end up in the dumpster.

Question is, could the exhaust restriction be a part of the valve float?

[mt-engines]

I had a BBC engine dynoed recently and did experience what seemed to be valve float when the curve went almost straight down at 6200 rpm.
Up to 6000 the engine is running fine doing 1.35 hp/cu in.

Engine has a .640" lift HR cam around 245°@.050", nothing extreme. Springs are set at 145 lb on seat with a 430 lb/in spring rate.
Cam manufacturer recommends 130 lb on seat and 350 lb/in spring rate so I was hoping to have some positive margin here.

The engine was run as it was supposed to go into the car, with mufflers, and hp was a little on the low side. We removed the mufflers and gained over 40 hp so the mufflers will end up in the dumpster.

Question is, could the exhaust restriction be a part of the valve float?

how did you rule it to be valve float? a single cylinder missfire could be 80 hp causing your power curve to go straight down also.

[1980RS]

I had a BBC engine dynoed recently and did experience what seemed to be valve float when the curve went almost straight down at 6200 rpm.
Up to 6000 the engine is running fine doing 1.35 hp/cu in.

Engine has a .640" lift HR cam around 245°@.050", nothing extreme. Springs are set at 145 lb on seat with a 430 lb/in spring rate.
Cam manufacturer recommends 130 lb on seat and 350 lb/in spring rate so I was hoping to have some positive margin here.

The engine was run as it was supposed to go into the car, with mufflers, and hp was a little on the low side. We removed the mufflers and gained over 40 hp so the mufflers will end up in the dumpster.

Question is, could the exhaust restriction be a part of the valve float?

Valve float is more likely due to Roller rockers and heavy valvetrain. I just tested out some BBC heads I did with 11/32 valves and lighter retainers and the BBC I have will spin to 7K if I wanted it to, there is no power there but I can still spin it that high. I did a test last year when my other BBC heads went into valve float. I switched to a stock rocker and the problem stopped.

[Merc Man]

I had a BBC engine dynoed recently and did experience what seemed to be valve float when the curve went almost straight down at 6200 rpm.
Up to 6000 the engine is running fine doing 1.35 hp/cu in.

Engine has a .640" lift HR cam around 245°@.050", nothing extreme. Springs are set at 145 lb on seat with a 430 lb/in spring rate.
Cam manufacturer recommends 130 lb on seat and 350 lb/in spring rate so I was hoping to have some positive margin here.

The engine was run as it was supposed to go into the car, with mufflers, and hp was a little on the low side. We removed the mufflers and gained over 40 hp so the mufflers will end up in the dumpster.

Question is, could the exhaust restriction be a part of the valve float?

how did you rule it to be valve float? a single cylinder missfire could be 80 hp causing your power curve to go straight down also.

Well, not that it excludes misfire but the engine has run repeatedly upp to 6100 and does so every time without any signs of misfire so by logic both I and the other guys in the room instantly said valve float.

[Merc Man]

I had a BBC engine dynoed recently and did experience what seemed to be valve float when the curve went almost straight down at 6200 rpm.
Up to 6000 the engine is running fine doing 1.35 hp/cu in.

Engine has a .640" lift HR cam around 245°@.050", nothing extreme. Springs are set at 145 lb on seat with a 430 lb/in spring rate.
Cam manufacturer recommends 130 lb on seat and 350 lb/in spring rate so I was hoping to have some positive margin here.

The engine was run as it was supposed to go into the car, with mufflers, and hp was a little on the low side. We removed the mufflers and gained over 40 hp so the mufflers will end up in the dumpster.

Question is, could the exhaust restriction be a part of the valve float?

Valve float is more likely due to Roller rockers and heavy valvetrain. I just tested out some BBC heads I did with 11/32 valves and lighter retainers and the BBC I have will spin to 7K if I wanted it to, there is no power there but I can still spin it that high. I did a test last year when my other BBC heads went into valve float. I switched to a stock rocker and the problem stopped.

Heavy valve train is what we're always fighting on BBC, this one has AFR heads with 11/32" valves.
Still I tried to add some safety margin on springs compared to the cam card. At the same time I want to be fairly easy on the lifters since this is supposed to be a daily runner.
If I can get it to 6200-6300 safely there are maybe 10 more hp there because the torque drops quite slow after peak.

[PRH]

When there is adequate spring load, It’s usually related to lifter function........ either pump up during some degree of valve lofting, or lifter collapse from insufficient oil supply or degraded oil “integrity”(entrapped air bubbles in the oil, etc).

Heavy valvetrains and high rocker ratios are generally more susceptible to having these issues....... which is a big reason why short travel lifters exist.

I’ve seen it many times where the motor won’t get past the mid-5000 range with an aggressive lobe design.

[F-BIRD'88]

Higher relative exhaust port pressure at near the point of exhaust valve seating would act as a air cushion and assist the valve spring in seating the valve, at that point.

Retest using solid roller lifters on your hyd cammed motor.
They are net lighter weight and allow more spring seat and open pressure. That cam profile combined with your valves, rockers etc may just need more raw spring force to run at high rpm.. At some point the hyd roller lifters cannot stand that much spring force.

Smoother cam lobe dynamic design. will help

Short duration with big open area and peak lift with heavey valvetrain needs more valvespring force

Your springs look pretty tame for the job there.

[F-BIRD'88]

You should have started with a STREET mechanical roller lifter set up in the first place, if you want street strip high rpm real performance with reliability on a big block chev.

Do not confuse with a race only roller cam setup.
Thats limited run 1/4 mile stuff.

A mechanical STREET roller cam+ kit is designed to perform on the street and out perform BBC hyd roller setups where rpm (and more power) is desired.

[F-BIRD'88]

If the first dyno test of your session is good but further additional test runs seem to get progressivly worse (rpm limit and power degrades look for excess oil aireation.
Air bubbles are getting in the oil causing hyd lifter plunger collapse at high rpm.
Your engine bearings take a beating also under these high load, rpm conditions with oil aireation.
Air in the oil is bad.

[MadBill]

...Question is, could the exhaust restriction be a part of the valve float?

Short answer: No. First, cylinder pressure on the exhaust stroke will go up in snych with B.P., leaving the seating force differential unchanged. Second, if it was possible, enough backpressure to 'float' the exhaust valves would have a crippling impact on power beginning much lower in the rev range. Third, if you tested without mufflers you already know whether zero backpressure will allow it to pull higher than 6200.

Was airflow monitored on the dyno?

Any chance the spark box has a 6200 RPM chip in it?

To test for pump-up, set the lifters at zero hot lash*.To test for aeration (which would likely come on more gradually than you report) adjust the lifters to just shy of bottomed out for a quick test. *Do the zero lash first as pump-up from bottomed could result in piston-valve contact.

[My427stang]

What were you running for oil? And how close to coil bind?

Which lobes are you running?

[ptuomov]

Short answer: No. First, cylinder pressure on the exhaust stroke will go up in snych with B.P., leaving the seating force differential unchanged. Second, if it was possible, enough backpressure to 'float' the exhaust valves would have a crippling impact on power beginning much lower in the rev range. Third, if you tested without mufflers you already know whether zero backpressure will allow it to pull higher than 6200.

We are seeing exhaust valve seated load requirements going up with turbo engines as the exhaust back pressure grows. The exhaust valve bounces off the seat when the pressure waves arrive at unfavorable time. It starts at some rpm and in some cylinders first and then power goes south. Which cylinders and at which rpm depends on exhaust manifold configuration. Problem appears to come earlier with higher exhaust manifold pressure, whether that is caused by high boost or restrictive turbo-back exhaust. I’ve connected a bunch of dots to come to this conclusion, but increasing the seated loads on the exhaust side keeps the high rpm torque curve smoother and pointing in the right direction.

[mag2555]

What where the Exh temps seen.

How do the plugs look if you shut the motor down after a 10 second run 6K?

Enough reversion will increase Exh valve temps and that heat gets dumped in to the valve , the seat and the valve spring and that heat causes PSI drop from the springs especially if they are not cooled by enough oil flow over them!

[My427stang]

What were you running for oil? And how close to coil bind?

Which lobes are you running?

Sorry for the short post, was driving...tired to use voice and was difficult

On an FE valvetrain, which parts are sometimes even heavier than a BBC, if major intensity is much shorter than mid-50s they get harder to control. That cam has some decent lift, my guess is that it's some sort of aggressive ramp, likely causing the issues.

Additionally, 145 on seat should be good for bounce, but depends on the lobe and the parts. Finally, Morels and other-branded Morels, oil is a factor. Thick stuff makes them act funky.

If you know your .006 and .050, can give a little more advice, but 6500+, never mind 6200 should be easy with HRs and the right lobe/supporting parts and setup.

[PRH]

and the right lobe

Imo, that’s where it starts.
If the lobe is too fast, it can be a big challenge to figure out a way to have the motor turn higher rpms with “regular” HR lifters and “normal” spring loads.