781 head ? - Chevelle Tech
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post #1 of 9 (permalink) Old Dec 27th, 14, 3:12 PM Thread Starter
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781 head ?

Im planning an 8.5:1 to 9:1 454 with a 5000 to 5500 red line for street only. I will use new springs to match the cam but is there anything to gain with a bigger intake and exhaust valve in the 781's? The calculators say the stock size valves are good to 6000 rpm but thats on paper.

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post #2 of 9 (permalink) Old Dec 27th, 14, 6:25 PM
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Re: 781 head ?

My 781's are stock I gave them a mild port and with my cam it's pulling like bear at 5800 which is were I shift. 1 7/8" headers. I think it'll pull to 6500 easy I'm just don' feel comfortable going there.

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post #3 of 9 (permalink) Old Dec 27th, 14, 7:54 PM
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Re: 781 head ?

If you cut the seats and put 2.19's in, cut the throats out, and blend the short side radius, there's a bunch of power to be gained here. Most builders don't see much improvements by going 1.88 on the exhaust however.

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post #4 of 9 (permalink) Old Dec 27th, 14, 8:14 PM
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Re: 781 head ?

For such a mild engine the stock size valves will work fine.

I have bone stock 781s on my 10.4:1 454, with the exception of cutting down the valve guides for more lift, and they flow good enough to get me 11.9s in the 1/4.

However, you can do whatever your budget allows for upgrades beyond a basic rebuild with new springs.

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post #5 of 9 (permalink) Old Dec 27th, 14, 9:16 PM
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Re: 781 head ?

2.19"/1.88" is the standard prescription.

The large exh valve matters ALWAYS, in a BIG way, because that's the weak side to begin with. The weeeeeeeeker the rest of the exh side is (cam, manifolds/headers, head pipes, and so on) the more difference the larger valve makes. Then, once the rest of the system is improved to the point that the valve becomes The Bottleneck, .... you do the math.

In short, ALWAYS upgrade the exh valve. It's the single biggest bang for the buck you can do. Bigger even than the intake most of the time.
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post #6 of 9 (permalink) Old Dec 28th, 14, 5:41 PM Thread Starter
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Re: 781 head ?

Thanks for the info. Can this mild combo make use of an RPM intake down low? I plan to use the stock cast iron intake and and buy an SMI qjet or use the 650 vac Holley I have and get an RPM intake if the RPM is a match.

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post #7 of 9 (permalink) Old Dec 28th, 14, 8:03 PM
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Re: 781 head ?

Quote:
Originally Posted by RB69SS396Conv View Post
2.19"/1.88" is the standard prescription.

The large exh valve matters ALWAYS, in a BIG way, because that's the weak side to begin with. The weeeeeeeeker the rest of the exh side is (cam, manifolds/headers, head pipes, and so on) the more difference the larger valve makes. Then, once the rest of the system is improved to the point that the valve becomes The Bottleneck, .... you do the math.

In short, ALWAYS upgrade the exh valve. It's the single biggest bang for the buck you can do. Bigger even than the intake most of the time.
Every time I install 1.88/2.190 valves I notice back flow before .500 lift in most cases, and that can't be good, be careful. Gotago brought his 2.07/1.720 valved heads in for flow testing the other day, with porting they went 300 CFM on the intake and 215 on the exhaust, and no back flow.

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post #8 of 9 (permalink) Old Dec 28th, 14, 11:18 PM
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Re: 781 head ?

Mark
have you tried doing the 2.19" and leaving the 1.72"

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post #9 of 9 (permalink) Old Dec 29th, 14, 7:35 AM
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Re: 781 head ?

Quote:
Originally Posted by 67RS502 View Post
Mark
have you tried doing the 2.19" and leaving the 1.72"
Yes on a towing application.

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