I recently had my early 460 redone, stock lower end internally balanced D1VE block, Keith Black 138 Hypereutectic pistons,comp 280h flat tappet cam, aluminum roller rockers, 3/8 hardened pushrods & guideplates, Weiand Stealth manifold. 9.9 compression ratio. C9VE heads, 2.25 intakes, 1.72 exhaust valves, (No exhaust porting). Procomp Electronic distributor, Pertronix flamethower coil. custom built long tube headers.
new lifters, straight 30 weight oil, new standard flow oil pump.

dyno run shows

(w 2in carb spacer)
489.6 ft/lb torque, 436 hp at the flywheel.

but. This cam profile says good to 6000 rpm, but we experience valve float at 5100-5200 rpm. ( blow back thru the carb). measured on the engine at the dyno, the seat pressure is 100-110lbs (hot), where the specs say 130. open pressures are supposed to be around 350lbs. Cam calls for comp 926-16 springs, single spring coil with damper.

so, back to the machine shop we go.. but they measure the spring pressures at spec.. (cold) the machine shop will be testing the calibration of their spring tester today..

is there a problem with hot vs cold measurements?
what else should I have them look at?

based on my reading of the term float, this is the inability of the spring to close the valve in synch with the cam profile. So this is really open (over the nose) pressure that we are concerned with. (seat pressure will be involved with valve bounce, right?)

this is not a race engine, and the vehicle (truck) its in (pictured in my avatar) will not be raced.. but I want it right..

Sam

 

I see no mention of replacing the valve springs. When installing an aftermarket cam, ALWAYS use the cam kit which will include springs or if there is no kit, get and use the springs recommended by the cam manufacturor. Old valve springs get tired and won't even have the pressurse need to handle stock cams.

 

Sorry, the springs were also replaced, and have been tested at the machine shop for both seat and open pressure to spec. (130 & 350). Comp says there should be no problem at these pressures, and shimming an additional .030 might be possible to increase the pressures to 140 & 365 respectively.

Someone has also mentioned that ignition might be a related problem.. this is a new Procomp electronic distributor, new wires, new coil.

Sam

 

Just because it is new, doesn't mean there can't be a problem with it.

 

true

sam

 

If you actually have valve float, heavier springs are the only cure. It could still be the ignition though. Any arcing inside the cap? What is total advance? Both mechanical and vacuum if you are running a vacuum can.

 

running mechanical advance only, 36 degrees total at 3000rpm. 12 degrees initial.

we don't get any bang/pop.. but do get fog back up the carb, typical of float. we did have a bad coil above 4800rpm which produced crackling noises but replaced that and the noise went away.

here are the graphs and tabular data from the dyno run.
this is the last run we made compared to the first time with the newer carb.

there were 3 runs in between but we had the same fall over at 5100/5200.
and we ran out of time.

nothing ever got above 5300, altho we had set the dyno limit to 5800.

there was no arching in the cap. (also new). If the distro had failed, I would have expected a more erratic result.

also attached is the Comp camquest simulator curve for this cam.

Sam

 

The fog above the carb is just an indication that your intake length is wrong for the rpm you are running, not valve float. It's caused by reversion pulses getting back to the carb at the wrong time. Same type of thing that happens in the exhaust headers when the length is wrong.

 

HI, sounds like you have built a nice motor, your valve spring pressure sound's good and you hope your new ignition system is good ??? BUT with this combination of parts why have you NOT opened up the exhaust ports, with your cam, headers and intake the exhaust ports of most ! early heads are a restriction. this may be your problem ? have fun

 

ok, but the intake should not be a problem.. 5100rpm is WAY away from the top end of the manifold.
how do I resolve the reversion problem?
or is this the addition of the 2in spacer causing the length problem.. (gain low end torque, lose higher end)?

Sam

<table class="part" width="90%"><tbody><tr><td colspan="2" style="font-weight: bold; text-align: left;">• 8012
• High rise/dual plane
• Idle to 6800 RPM</td> </tr> <tr> <td colspan="2" style="text-align: left;">Stealth Series: Weiand's top-of-the-line dual plane manifold combines the low-end throttle response of a 180 degree design with the top-end power you'd expect of a single plane. Extensive dynamometer testing confirms that "Stealth" is the ultimate dual plane on the market, with a power range from idle through 6800 RPM. Best suited for engines that are equipped with aggressive camshaft profiles, headers, high performance ignition systems, etc. Features square-bore carburetor pad for aftermarket 4-barrel carburetors.</td></tr></tbody></table>

 

I decided NOT to have the exhaust ported cause I wasn't going for top of the top end. Everything I've read said to get over 550hp I need to port. Ok, I'm not trying to go there.. just to 500 maybe. And the rotating mass should be able to go to 6000 rpm even if not over. Even a 5800 redline is fine for me.

I was going for 2.19 intake valves, but 2.25's showed up and the shop decided to install them for the same price.. ok, cool. I know I can't leverage all of that without exhaust porting, but again, I'm not trying to get get ALL.

Sam

 

HI, remember this is only my opinion i hope it helps. this may not be the cause of your problem. BUT #1 very good intake throw the 2" spacer, #2 big valves, #3 reasonable cam, #4 large headers, these all promote good air flow but stock exhaust ports are like a 1" tap connected to a 3" hose a mismatched component that will not let the others work properly

 

I like opinions, so thank you.

I had hoped to not get to this edge.. we'll see where we go from here..

Sam