GT40 heads on a 68 J-code 302 - Vintage Mustang Forums
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Old 12-04-2009, 09:00 AM   #1 (permalink)
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Default GT40 heads on a 68 J-code 302

As I go through the rebuild plan for my motor I have one question that I can't seem to find.

Recently a forum member suggested I switch my original heads on my 302-4V over to GT40 heads for better flow.

Since the original heads had small combustion chambers 52 to 55 cc's and a compression ration of 10:1, will moving to the GT40 heads, and a lower compression ratio, be better? Or will keeping the original heads do what I am looking to do?

My goal for this engine is to keep it close to stock looking but some mild performance and reliability gains. I want to be able to get in the car after it sits for a week in the garage and turn the key and start it up and drive away. I want a nice throaty idle that sounds strong.

In addition to the heads I plan on reusing the Carter AFB 625 CFM carb, switching to a Comp Cam roller upgrade unit (with applicable head upgrades) Better internals (pistons, rods, bearings etc) Changing to a Pertronix or other electronic distributor.

Also, if I go to the GT40 heads I may want to upgrade to shorty headers. Will they fit with PS, A/C and Power disc brakes?

Thanks for the help
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Old 12-04-2009, 10:09 AM   #2 (permalink)
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Since you are changing your pistons anyway then you can maintain the existing 10:1 cr with the GT40 heads.

You really need to add headers to take advantage of either head at the top end. The stock manifolds are way to choking.

Any of the sbf heads will work with power steering and a/c.

The roller cam is a great way to go.
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Old 12-04-2009, 10:14 AM   #3 (permalink)
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Quote:
Originally Posted by Stillboardin View Post
My goal for this engine is to keep it close to stock looking but some mild performance and reliability gains. I want to be able to get in the car after it sits for a week in the garage and turn the key and start it up and drive away. I want a nice throaty idle that sounds strong.

Also, if I go to the GT40 heads I may want to upgrade to shorty headers. Will they fit with PS, A/C and Power disc brakes?
Shorties are usually no problem. If you don't want to spend the serious coin required to change to GT40 heads, you can port-match the J code heads to the exhaust.

I can't overstate the difference this makes. I have done it to my 289HP, which had better exhaust ports than the J code, and the difference was immediately apparent even still in the driveway.

Closer to your application, I once saw a 68 Shelby GT350, which of course had the same heads and crappy manifolds as your J code, and had been restored to fantastic show condition, except it had the C9OZ-C cam, and 289HP manifolds port-matched to the heads. Result was, it ran FAR better than stock, yet looked completely stock unless you were savvy enough to spot the manifold substitution.
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Old 12-04-2009, 02:08 PM   #4 (permalink)
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I'm currently doing the same modification to my 289. For the compression reasons you stated I now have gone to a 302 rotating assembly in my 289 block. The GT40 heads I have are 63cc so by the time i'm done (new flat top pistons), I'll be right about 9:1, maybe just a little shy. The 289 would have been too low on compression with the GT40 heads, no matter what I did.

Tom
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