307ci 7A to 5A Head Swap

Gunmetal_Cutlass

Not-quite-so-new-guy
Nov 19, 2023
5
0
1
Hello everyone!

I have read these forums for awhile, but first time posting here, so thank you in advance for any advice.

I have a '86 Cutlass with the vin 'Y' 307 engine. When I first got the car, I put a performance Edelbrock intake and heads on it, because it was cool and I was 16 and had no idea how to properly build an engine at the time.

Years later, I realize that the performance intake and headers are doing nothing for the engine since the heads are still the stock 7A small port heads. In fact, many claim that due to the port size mismatch, the intake and headers actually hurt the engine performance due to turbulence. I have read online that the early 80's 5A heads are a direct replacement for the 7A heads and flow significantly better. I have a donor engine, and I plan to keep everything from the 7A heads (springs, rockers, push rods, lifters, valves, etc).

Is it true that the heads are identical except for the port sizes? I plan to take a ton of measurements and compare the heads, but knowing ahead of time that they can or cannot be swapped would save me some time and energy tearing the donor engine down.

Also, this is a low budget build. I really do not have the money to get the current 7A heads ported, built or swapped with anything high performance. I am working on the 307 this winter anyway, because it has a few fluid leaks and needs some gaskets. I am just looking to help the current engine build breathe better, and I would like to keep the current mods because my dad and I built the engine together, so it is special to me.

Thank you in advance for any help and info you have!
 
Oct 14, 2008
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Melville,Saskatchewan
They are a direct swap but honestly it will only help slightly at higher rpm for power, the stock camshaft limits the gains. Was the timing chain replaced? The nylon coated cam gear falls apart into thr oil pick up screen. Full length headers on an Olds V8 adds 30 HP when added with dual exhaust.
 
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69hurstolds

Geezer
Supporting Member
Jan 2, 2006
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Throw the 7A's away, or sell them to anyone trying to restore an engine with 7A heads with a cracked one or something.

The 5A heads will bolt one without issue, have the nice, larger rectangular ports which will match up way better to your E-brock intake. But if it's still a 307 Y roller pig, there's really no need for a performance intake. Just get a normal aluminum factory A4 intake if you go with the 5A heads. (A5 intake was used with 7A heads)

But whatever. The 5A's are still better for your application.
 

Gunmetal_Cutlass

Not-quite-so-new-guy
Nov 19, 2023
5
0
1
They are a direct swap but honestly it will only help slightly at higher rpm for power, the stock camshaft limits the gains. Was the timing chain replaced? The nylon coated cam gear falls apart into thr oil pick up screen. Full length headers on an Olds V8 adds 30 HP when added with dual exhaust.
That is good to know, and thank you for answering the question.

To answer yours, the timing chain and gear has been replaced when my dad and I went through the engine. I want to say we put a cam in it as well, one that was just above stock performance, but I cannot remember for sure...

Also, we did put true dual exhaust on it when we put the headers on, along with a set of 3:73 posi gears from a Monte Carlo SS. Both mods livened up the car quite a bit honestly.
 
Oct 14, 2008
8,787
7,700
113
Melville,Saskatchewan
I would bet you put a Vin 9 roller cam in it, being a factory roller cam. I would get the 5A heads milled to gain some compression. It will work nicely with your combination.
 

carnutjw

G-Body Guru
Sep 17, 2017
616
1,450
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Throw the 7A's away, or sell them to anyone trying to restore an engine with 7A heads with a cracked one or something.

The 5A heads will bolt one without issue, have the nice, larger rectangular ports which will match up way better to your E-brock intake. But if it's still a 307 Y roller pig, there's really no need for a performance intake. Just get a normal aluminum factory A4 intake if you go with the 5A heads. (A5 intake was used with 7A heads)

But whatever. The 5A's are still better for your application.
 
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Oct 14, 2008
8,787
7,700
113
Melville,Saskatchewan
The Edelbrock Performer intake actually has very small intake runners, smaller than the factory iron intakes. I never measured the A4, worked great for me with 5A heads in a truck application.
 

69hurstolds

Geezer
Supporting Member
Jan 2, 2006
8,026
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But one thing the Performer doesn't have is "Ruffles Ridges" on the intake floor that the 307 has. Which is not a bad thing per se, it just makes the A4 better at low end atomization, and not a higher RPM manifold. But why would you need a high RPM 307 anyway? You can always massage the A4 and get a good flowing intake, but, well, if you already have the performer, then use it.
 
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