Head / Carb combination question
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Instructor
Joined: Aug 2008
Posts: 115
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From: Denison Texas
Head / Carb combination question
I have a 68 coupe (427/390) 4-speed numbers matching engine/tranny. When I acquired the car, It had a Holley 650cfm carb and the original heads were replaced with 454 peanut port...
It actually ran ok and had lots of power, but this mismatch kinda made me throw up in my mouth just a little every time I thought about it and I wanted to go back with 427 heads and a Rochester carb...
So here's what I found and I'd like to hear any thoughts on how you think it will perform.
I found an 800 CFM Rochester from Jet TEchnologies :
Rochester Big Block Q JET
36003
JET Performance now offers a Quadrajet carburetor tuned for Chevy & GMC cars and trucks with 396 ci and larger modified big block engines. The carburetor is extensively reworked by increasing main well size and is balanced to assure top end high RPM fuel flow. Several circuit modifications are made to keep the fuel curve correct throughout the entire RPM range and to keep consistent idle quality. Includes a .149 needle and seat to ensure adequate fuel flow. The carburetor is designed for modified big block motors with a dual plane intake manifold, headers, 8.5-9.5:1 compression ratio and mild ported or aftermarket heads. Recommended cam specs: Duration @ .050 min 210 degrees, max 240 degrees and a lobe separation angle of 110-114 degrees.
800 CFM Quadrajet custom built for big block engines 396 ci and up.
And as for the 427 heads, I purchased a date correct set of L71 (3919840) heads and I'm having hardened seats installed...
Not sure about the cam....IT's not too radical, but I believe the engine was rebuilt a "little" more aggressive than stock... (I know this is no help concerning lift/duration).
Any thoughts on the combination of carb/heads ?
Amu I correct in assuming that it should (at least) run better than the 650 HOlley/454 heads ?
Thanks & Any feedback would be appreciated.
Mike
It actually ran ok and had lots of power, but this mismatch kinda made me throw up in my mouth just a little every time I thought about it and I wanted to go back with 427 heads and a Rochester carb...
So here's what I found and I'd like to hear any thoughts on how you think it will perform.
I found an 800 CFM Rochester from Jet TEchnologies :
Rochester Big Block Q JET
36003
JET Performance now offers a Quadrajet carburetor tuned for Chevy & GMC cars and trucks with 396 ci and larger modified big block engines. The carburetor is extensively reworked by increasing main well size and is balanced to assure top end high RPM fuel flow. Several circuit modifications are made to keep the fuel curve correct throughout the entire RPM range and to keep consistent idle quality. Includes a .149 needle and seat to ensure adequate fuel flow. The carburetor is designed for modified big block motors with a dual plane intake manifold, headers, 8.5-9.5:1 compression ratio and mild ported or aftermarket heads. Recommended cam specs: Duration @ .050 min 210 degrees, max 240 degrees and a lobe separation angle of 110-114 degrees.
800 CFM Quadrajet custom built for big block engines 396 ci and up.
And as for the 427 heads, I purchased a date correct set of L71 (3919840) heads and I'm having hardened seats installed...
Not sure about the cam....IT's not too radical, but I believe the engine was rebuilt a "little" more aggressive than stock... (I know this is no help concerning lift/duration).
Any thoughts on the combination of carb/heads ?
Amu I correct in assuming that it should (at least) run better than the 650 HOlley/454 heads ?
Thanks & Any feedback would be appreciated.
Mike
Racer
Joined: Aug 2005
Posts: 347
Likes: 0
From: Pace FL
on top end it will run better if it does not run out of cam. It may detonate if it does not have enough cam since you are going from 119cc to 107 cc. This will increase your compression about 1.2 points. If the pistons are original then you will have about 9.7:1. Otherwise who knows... Check dome height when you pull it apart, if ~0.200" then probably like originals.
Le Mans Master
Joined: Apr 2006
Posts: 5,403
Likes: 95
I have a 68 coupe (427/390) 4-speed numbers matching engine/tranny. When I acquired the car, It had a Holley 650cfm carb and the original heads were replaced with 454 peanut port...
It actually ran ok and had lots of power, but this mismatch kinda made me throw up in my mouth just a little every time I thought about it and I wanted to go back with 427 heads and a Rochester carb...
So here's what I found and I'd like to hear any thoughts on how you think it will perform.
I found an 800 CFM Rochester from Jet TEchnologies :
Rochester Big Block Q JET
36003
JET Performance now offers a Quadrajet carburetor tuned for Chevy & GMC cars and trucks with 396 ci and larger modified big block engines. The carburetor is extensively reworked by increasing main well size and is balanced to assure top end high RPM fuel flow. Several circuit modifications are made to keep the fuel curve correct throughout the entire RPM range and to keep consistent idle quality. Includes a .149 needle and seat to ensure adequate fuel flow. The carburetor is designed for modified big block motors with a dual plane intake manifold, headers, 8.5-9.5:1 compression ratio and mild ported or aftermarket heads. Recommended cam specs: Duration @ .050 min 210 degrees, max 240 degrees and a lobe separation angle of 110-114 degrees.
800 CFM Quadrajet custom built for big block engines 396 ci and up.
And as for the 427 heads, I purchased a date correct set of L71 (3919840) heads and I'm having hardened seats installed...
Not sure about the cam....IT's not too radical, but I believe the engine was rebuilt a "little" more aggressive than stock... (I know this is no help concerning lift/duration).
Any thoughts on the combination of carb/heads ?
Amu I correct in assuming that it should (at least) run better than the 650 HOlley/454 heads ?
Thanks & Any feedback would be appreciated.
Mike
It actually ran ok and had lots of power, but this mismatch kinda made me throw up in my mouth just a little every time I thought about it and I wanted to go back with 427 heads and a Rochester carb...
So here's what I found and I'd like to hear any thoughts on how you think it will perform.
I found an 800 CFM Rochester from Jet TEchnologies :
Rochester Big Block Q JET
36003
JET Performance now offers a Quadrajet carburetor tuned for Chevy & GMC cars and trucks with 396 ci and larger modified big block engines. The carburetor is extensively reworked by increasing main well size and is balanced to assure top end high RPM fuel flow. Several circuit modifications are made to keep the fuel curve correct throughout the entire RPM range and to keep consistent idle quality. Includes a .149 needle and seat to ensure adequate fuel flow. The carburetor is designed for modified big block motors with a dual plane intake manifold, headers, 8.5-9.5:1 compression ratio and mild ported or aftermarket heads. Recommended cam specs: Duration @ .050 min 210 degrees, max 240 degrees and a lobe separation angle of 110-114 degrees.
800 CFM Quadrajet custom built for big block engines 396 ci and up.
And as for the 427 heads, I purchased a date correct set of L71 (3919840) heads and I'm having hardened seats installed...
Not sure about the cam....IT's not too radical, but I believe the engine was rebuilt a "little" more aggressive than stock... (I know this is no help concerning lift/duration).
Any thoughts on the combination of carb/heads ?
Amu I correct in assuming that it should (at least) run better than the 650 HOlley/454 heads ?
Thanks & Any feedback would be appreciated.
Mike
Last edited by Little Mouse; Nov 20, 2008 at 11:44 PM.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
When the 'gas crunch' happened in the early/mid 70's, this was a common practice for making a big block "economical" {
"economical" "big block"....that's an oxymoron if I ever heard one}. Anyway, most owner who did that did not bother to change the cam; those who did installed a mild, early 70's low-CR cam to match the low-CR heads.
"economical" "big block"....that's an oxymoron if I ever heard one}. Anyway, most owner who did that did not bother to change the cam; those who did installed a mild, early 70's low-CR cam to match the low-CR heads.
