Explain: Too much port volume head on 408, and effect of increased piston speed.
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Explain: Too much port volume head on 408, and effect of increased piston speed. I have been told over and over that a 270 port head on a small stroker motor like a 408 is too much NA, I assume that with all that head the only things that can be done to compromise is getting more air in NA, (like a bigger cam) and also demanding more air via increased piston speed (rpm and stroke).
So, what else will help optimize a poorly optimized head choice for NA?
-Larger Camshaft
-increased RPM
what else? and is there a theory with choosing head port sizes with cubic inch???
So, what else will help optimize a poorly optimized head choice for NA?
-Larger Camshaft
-increased RPM
what else? and is there a theory with choosing head port sizes with cubic inch???
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Is 11.35:1 and 7200rpm enough?
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If you are trying to make huge HP I am a huge fan of big volume heads. As long as your bottom end was put together well and your valvetrain is built for it you should have no problem at 8500+rpm. My 421 (4.125 stroke) has seen 8500, talk about huge piston speeds 
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maybe for a SR setup... but Hydrolic... yeah right! Valvetrain stability is nowhere near as efficient with stock rockers, lifters, and pushrod setup. I was thinking more like 7k-7200.... is this enough rpm and piston speed?
ALot of people that say and talk about port volume are from the OLD SCHOOL where carbs had to have a good velocity to carry FUEL into and down the length of the port.
EFI engines are not bound by this old rule.
Port volume also has to take into account port LENGTH. You got a 2.5 inch 270cc port...prolly too big....a 4.5 inch 270cc port...seems fine to me.
I got a 250 cc port on a 23* headed 383 LT1 making good power on pumpgas.
Low speed driveability is AWESOME, it is making 400 ft lbs of tq by 3000 rpm.Through a 9 inch and a 4L60E.
Do some more research...dont damn the heads just yet. Camming them right is the ticket.
David
EFI engines are not bound by this old rule.
Port volume also has to take into account port LENGTH. You got a 2.5 inch 270cc port...prolly too big....a 4.5 inch 270cc port...seems fine to me.
I got a 250 cc port on a 23* headed 383 LT1 making good power on pumpgas.
Low speed driveability is AWESOME, it is making 400 ft lbs of tq by 3000 rpm.Through a 9 inch and a 4L60E.
Do some more research...dont damn the heads just yet. Camming them right is the ticket.
David
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ALot of people that say and talk about port volume are from the OLD SCHOOL where carbs had to have a good velocity to carry FUEL into and down the length of the port.
EFI engines are not bound by this old rule.
Port volume also has to take into account port LENGTH. You got a 2.5 inch 270cc port...prolly too big....a 4.5 inch 270cc port...seems fine to me.
I got a 250 cc port on a 23* headed 383 LT1 making good power on pumpgas.
Low speed driveability is AWESOME, it is making 400 ft lbs of tq by 3000 rpm.Through a 9 inch and a 4L60E.
Do some more research...dont damn the heads just yet. Camming them right is the ticket.
David
EFI engines are not bound by this old rule.
Port volume also has to take into account port LENGTH. You got a 2.5 inch 270cc port...prolly too big....a 4.5 inch 270cc port...seems fine to me.
I got a 250 cc port on a 23* headed 383 LT1 making good power on pumpgas.
Low speed driveability is AWESOME, it is making 400 ft lbs of tq by 3000 rpm.Through a 9 inch and a 4L60E.
Do some more research...dont damn the heads just yet. Camming them right is the ticket.
David
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You would run a shorter duration and later opening intake lobe with the bigger L92 intake port on the same engine to generate more port velocity but you still need the right exhaust timing since the exhaust port isn't really any different on an L92 at all and you used to have some fairly large NOS too at one point. I think you just now have a smaller version of your old cam which will help at low rpm but not at higher rpm or with NOS at all. I think you had other stuff going on holding you back more than that cam you ran at first.
+1
While it is true that you can get away with a slower port velocity with MPFI, to discount it completely would be foolish.
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You would run a shorter duration and later opening intake lobe with the bigger L92 intake port on the same engine to generate more port velocity but you still need the right exhaust timing since the exhaust port isn't really any different on an L92 at all and you used to have some fairly large NOS too at one point. I think you just now have a smaller version of your old cam which will help at low rpm but not at higher rpm or with NOS at all. I think you had other stuff going on holding you back more than that cam you ran at first.
If it makes a 50 more hp under the curve I will be happy with the smaller cam..... but if not chalk it up to another step backwards
You would run a shorter duration and later opening intake lobe with the bigger L92 intake port on the same engine to generate more port velocity but you still need the right exhaust timing since the exhaust port isn't really any different on an L92 at all and you used to have some fairly large NOS too at one point. I think you just now have a smaller version of your old cam which will help at low rpm but not at higher rpm or with NOS at all. I think you had other stuff going on holding you back more than that cam you ran at first.
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no normally or earlier depending on things. The exhaust would be treated normally by me. Greg's exhaust port he did for Mike though is a very good port so he won't be hurt as much by the shorter exhaust timing as would a normal L92 deal.
FormerVendor
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I actually do agree with you though that the port velocity being slow isn't good for tq or power so thats why the cam has to change when an intake port is bigger than normal vs the cubes in question.