does anybody have an idea of what the pinch point cross section area should be.David Vizards "go fast news" article Porting school#8 says 86.5 to 89.5% of the valve diameter. Thats about 2.0 CSA. anybody got any experience with this. Wallace racing computer gives this CSA a peak torque of around 4600 rpm. i am not looking to go speed demon here. The 305 is goin in a 40 chevy truck. Just wanna smoke some tires now and then.

 

The "pinch point" is the bowl right behind the valve.

85 - 90% is the rule of thumb. You're not going to notice a difference anywhere in that range. Only way you could tell, is if you took about 30 IDENTICAL engines and cars, and put half of em at one end of the range and the other half at the other; collected dyno and/or track data; and used statistical test analysis (NOT just "which one makes more power", because there will be a mix from the 2 groups) to see ON AVERAGE which one runs better.

In the engineering world, we call a lot of that stuff, "measure with micrometer, mark with chalk, cut with axe".

Main thing is, DON'T do a Cletus and BillyBob "hog em out" job. Remove as little metal as possible. What you mostly want to do is, clean up all those weird angles and steps and other irregularity where the factory just jammed their cutter down the throat of the port. That's about 80% of the value of the port part of porting stock heads. EVERYTHING ELSE in the port is the other 20%. Out of that 20%, about 15% is in trimming the guide to a "airfoil" or "teardrop" shape. About 1 - 2% lies in the difference between 85% and 90%. To put that in perspective, if you have (let's just pluck some #s out of thin air here) a 300HP motor, and your port job is good for 40 HP, then 32 of those HP are attributable to cleaning up the cutter mess, and 8 are attributable to EVERYTHING ELSE; and out of that 8, 6 are from profiling the guide, and the variation from 85% to 90% of throat area will result in a measured result variation of about ½ HP.

Especially with a 305. *shakes head in disbelief* If you care about "power", you've already left the cheeeeeepest power money can buy sitting on the table unclaimed; those 45 CID you are missing. Which are not just "45 CID"; they're THE MOST IMPORTANT 45 CID, because they not only affect CID, but also, they affect flow.

That said, I usually use an old valve of the appropriate diameter as my gauge when porting stock heads. Conveniently enough, a 1.6" exh valve is a bit over 82% of a 1.94" int valve, and so makes a perfect gauge for the intake port; and the 1.5" exh valves you have there, are 81% of your 1.84" intakes.

On the exh side, all you want to do is smooth out the transitions, as above; clean up the "short side radius" WITHOUT LOWERING THE FLOOR OF THE PORT; and profiling the guide. You don't need to enlarge it at all.

 

thanks for the reply sofakingdom. oops i meant the push rod pinch point area..its about 1.8 csa stock ( i think. heads at shop now getting seats done) what do ya think of expanding that area to 2 CSA. I started grinding down the curved side top to bottom and stopped about halfway to about the bolt holes. after reading more flow occurs at upper port area. im thinking i might have got a little antsy with the grinder i think i would have been better leaving it alone for my application.

 

I would not TOUCH it.

That's not where gains are made in stock heads.

Especially not with a motor that inherently already has a more restrictive flow bottleneck built in, that NO head work in the world will overcome.

Yes more flow occurs at the top of the port (the "roof")... shoot a garden hose into the port and watch where the water goes.

One of the places you will see that it DOES NOT go, is against the outer wall. It goes along the "common" wall, next to the other intake port, on the roof.

But all of that is moot. These heads are going on a 305. None of that matters. They're doomed.

 

Why limit it to a 350, builf a 406 or a 454 or a 540......That being said 400 hp is not impossible to get under 6500 rpm from a 305.

 

so what would be optimum pushrod pinch csa? should i go back to stock size? it appears by playing around on the Wallace acing computer online enlarging the smallest area..the pinch point at the pushrods...the peak torque goes up in RPM.. im building this 305 because its a tribute to my uncle who pasted away , he had a 40 chevy panel truck, he was to poor to afford anything but a 305 givin to him. So i got the 601 heads to raise compression, i ported them. i was tinkering with increasing the pinch point thinking it was so small compared to the valve throat area. its gonna get a 256 isky supercam for torque to cruise through the mountains, any suggestions are welcome. my 39 plymouth sets in the garage with the 350 blower motor waiting for attention also.Snows meltin, time to get busy , thanks guys for the inputs , but im building this for my uncle 'bud" FOR NOW

 

For an application like that, there is nothing to be gained by messing with that.

With a little RV cam like that, the valve isn't going to be open long enough to allow the engine to run at a high enough RPM to where it's going to matter. And the valve is only going to be just barely bumped open a crack. The CSA of the valve opening is DWARFED by everything to do with the port, and the air will have all the time in the world to drift through it at the low RPMs the engine will be limited to. And it only has to feed 305 CID ANYWAY; you're not trying to feed a 434 at 7500 RPM here. Nothing about the port is in control of the flow. You could make the port 100 square inches all the way from throttle plate to valve, and it won't move ONE SINGLE MOECULE MORE of air than it does now, because no more air will fit through the valve regardless of port size.

I'd recommend leaving it alone.

Clean up the bowls, contour the guides, smooth the short side radius on the exh side SLIGHTLY. Leave everything else alone.

That means, no "gasket matching", no "push rod pinch", no "raise the roof", etc.

Only other thing that WILL ACTUALLY HELP YOU would be, if you're putting 1.94" int valves in them, lay back the chamber wall around the valve slightly. Can't lay it back much because the bore is in the way, because it's a 305. Ultimately, that's the hard unalterable inescapable unavoidable flow limit, and you're going to hit that LONG BEFORE the push rod pinch can begin to have any effect whatsoever on overall flow.

If you're leaving the 1.84" valves in it, then the push rod pinch is already MUCH MUCH MUCH MUCH larger than the valve opening. Leave it ALONE. It only becomes an issue AFTER EVERYTHING ELSE IS MAXED OUT. Ya gotta understand that: you accomplish, AT BEST, exactly NOTHING by dinking around with things that aren't The Restriction. and instead are more likely to create problems. In your case, The Restriction(s) are the bore size, the valve size, and the tiny valve opening.