Pontiac 400 heads

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rebelrouser
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Pontiac 400 heads

Post by rebelrouser »

Helping a guy out with a 400 Pontiac for a 1969 GTO, street car. doing the heads and installed hard seats, going to do some minor porting work. Any Pontiac Guru advice? The intake valves are 30 degree, does back cutting the intake valve to say 15 degrees help?
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Re: Pontiac 400 heads

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Mike R
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Re: Pontiac 400 heads

Post by rebelrouser »

Thanks a lot that was a great article. It would be best to buy a copy of the book, but being out of print it is really expensive. and it looks like they did use a 15 degree back on the intake valves.
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Re: Pontiac 400 heads

Post by ClassAct »

I would make it a 45 degree seat on the intake. I don’t waste time with any 30 degree seat.
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Re: Pontiac 400 heads

Post by Walter R. Malik »

rebelrouser wrote: Wed Jul 14, 2021 10:13 pm Helping a guy out with a 400 Pontiac for a 1969 GTO, street car. doing the heads and installed hard seats, going to do some minor porting work. Any Pontiac Guru advice? The intake valves are 30 degree, does back cutting the intake valve to say 15 degrees help?
Just about anything you can do to help the intake side will have a beneficial effect.
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Re: Pontiac 400 heads

Post by Bryan Maloney »

If you start with new valves, you can grind a 40 degree intake seat angle on them without running out of margin.

On the stock heads I flow tested years ago, the intake port did not flow much more than the exhaust port. You will break into water in the short turn corners before you get decent flow from those intake ports.

Are aftermarket heads not an option?
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Re: Pontiac 400 heads

Post by PRH »

If you follow the porting advice and guidelines in Jim Hands book........ you can make pretty respectable power.
Somewhat handy with a die grinder.
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Re: Pontiac 400 heads

Post by mag2555 »

Do not follow others misguided advice to convert the intakes over to a 45 degree seat unless your planing on running over .550” lift and a big duration roller Cam to hold the valves open longer , and especially not if you are only interested in doing some minor clean up work in the ports and your unfamiliar with where the choke points truly are and where the air mass running through the port naturally wants to go to make its way into the chamber.
You can cut a man's tongue from his mouth, but that does not mean he’s a liar, it just shows that you fear the truth he might speak about you!
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Re: Pontiac 400 heads

Post by rebelrouser »

Bryan Maloney wrote: Thu Jul 15, 2021 12:27 pm If you start with new valves, you can grind a 40 degree intake seat angle on them without running out of margin.

On the stock heads I flow tested years ago, the intake port did not flow much more than the exhaust port. You will break into water in the short turn corners before you get decent flow from those intake ports.

Are aftermarket heads not an option?
It is the original engine in the car, belonged to his dad they are fixing it up. Dad was a hot rodder so it has headers and aluminum intake. Goal is around 450HP with a reasonable idle.
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Re: Pontiac 400 heads

Post by rebelrouser »

mag2555 wrote: Thu Jul 15, 2021 2:45 pm Do not follow others misguided advice to convert the intakes over to a 45 degree seat unless your planing on running over .550” lift and a big duration roller Cam to hold the valves open longer , and especially not if you are only interested in doing some minor clean up work in the ports and your unfamiliar with where the choke points truly are and where the air mass running through the port naturally wants to go to make its way into the chamber.
I agree with staying with the 30 degree intake seats as cam the is small .443 lift intake and .466 exhaust, 214 and 224 duration @ .050, 112 center line and I am going to advance it some.
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Re: Pontiac 400 heads

Post by ClassAct »

mag2555 wrote: Thu Jul 15, 2021 2:45 pm Do not follow others misguided advice to convert the intakes over to a 45 degree seat unless your planing on running over .550” lift and a big duration roller Cam to hold the valves open longer , and especially not if you are only interested in doing some minor clean up work in the ports and your unfamiliar with where the choke points truly are and where the air mass running through the port naturally wants to go to make its way into the chamber.
Valve lift has zero influence on when to use a 45 degree seat. I won’t do a Pontiac with a 30 degree seat and the lift doesn’t matter. It’s all about the shape.
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Re: Pontiac 400 heads

Post by Geoff2 »

Valve lift has everything to do with seat angle. Pontiac used low lift cams, just over 0.400" with most engines, where the 30* gave superior flow up to about 0.450" lift. I know of an Edel head for a Pontiac that had the intake converted to a 30* seat from the as delivered 45* seat. It had substantially more flow at lower lifts [ eg, 164 v 139 cfm @ 0.200"lift ] & was on par up to 0.550" lift
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Re: Pontiac 400 heads

Post by mag2555 »

When the springs get set up on your buddy's motor be sure to set the intakes to 125 to 130 psi.
The intakes with there 30 degree seat tend to bounce more then a 45 degree seat.

Advancing the Cam 2 degrees is a a good idea just to compensate for normal and enveitble chain stretch .
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Re: Pontiac 400 heads

Post by mag2555 »

ClassAct, when you have the time put your flow sheets where your Mouth is please.

I will respond in the same.
You can cut a man's tongue from his mouth, but that does not mean he’s a liar, it just shows that you fear the truth he might speak about you!
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Re: Pontiac 400 heads

Post by Elroy »

mag2555 wrote: Fri Jul 16, 2021 8:17 am ClassAct, when you have the time put your flow sheets where your Mouth is please.

I will respond in the same.
Are you wanting to make the sheets into paper airplanes and try racing them?
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