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Discussion Starter #1
Hi all,


I am thinking about going from my stock heads to used GT40P heads. My car has the California smog T/E system which is very restrictive (dyno showed 122 horse power). I want to get rid of the T/E system but keep everything in case I want to go back at one day. Apparently the GT40P heads might have issues with the exhaust manifold. Can anybody comment if it will fit? I am ok with some grinding work if needed to make it fit. I also want to change from stock manifold to HiPo (SD C5ZZ-9430/1-B https://www.summitracing.com/parts/sdk-c5zz94301b?seid=srese1&gclid=EAIaIQobChMIof2XzfWU4wIVC7jACh0bBABrEAQYAyABEgLd1vD_BwE).


Anything what need to be changed on the heads (e.g valve springs)? Or is it a bolt on job? Any clearance issues with the power steering, clutch, ....?



In the near future I also want to change the carb to four barrel and the intake manifold like a Weiand Warrior. A mild cam would also be nice if it doesn't require any piston replacement. I am interested in low end torque, not high end horse power.

I am not planning to pull the engine because of garage space. The engine has to stay in the car.



Car specs:
289 with 47k miles (or 147k miles). Most likely never rebuild. Compression test (dry) 150 PSI and one cylinder with 130 PSI.

Power steering
A/C
3 speed manual
No power brakes

California T/E smog system
Autolite 2100 carb
HiPo air cleaner

Stock intake manifold
Stock exhaust manifold
Dual exhaust with glass packs (no H or X), 2 or 2 1/4 inch diameter (I have to measure)
2.8:1 differential


Thanks for the help
 

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@patrickstapler neither of us are using the Kcode manifolds. I’m using MAC longtubes and @2nd 66 is running Edelbrock aluminum heads on his GT40P shortblock with try y headers. The K manifold should fit just fine with the P head. It’s very similar to the 71/73 Mustang 302 image I posted. If any grinding is needed it should be very minor
 

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If I were you, I'd skip the Warrior, and go right to the Stealth. If someone *gave* you the Warrior, sure, it's fine for a mild street build. But if you're going to spend the money, find a Stealth. It'll support anything and everything, with really no downside.

Since you have to modify your exhaust to bolt up to the K manifolds, why not just go to some good Tri-Y headers instead?

With GT40P heads, Weiand Stealth, a good 5-600 CFM Summit carb, and better exhaust, I would not be surprised to see you pick up 100 horsepower even without changing the cam. Your gas mileage will probably improve too.
 

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I do run the hipo exhaust manifolds on my GT40P motor. They fit the heads without any modification. The #8 sparkplug is a little difficult to get to, but can be done. I believe I used plug wires with 135 degree boots to ensure that they'd fit without melting a wire on the exhaust.

If you use the hipo manifolds it would be a good idea to port match them to the heads.
 

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If I were you, I'd skip the Warrior, and go right to the Stealth. If someone *gave* you the Warrior, sure, it's fine for a mild street build. But if you're going to spend the money, find a Stealth. It'll support anything and everything, with really no downside.

Since you have to modify your exhaust to bolt up to the K manifolds, why not just go to some good Tri-Y headers instead?

With GT40P heads, Weiand Stealth, a good 5-600 CFM Summit carb, and better exhaust, I would not be surprised to see you pick up 100 horsepower even without changing the cam. Your gas mileage will probably improve too.
The TRI Y's are No Bueno With P heads ,without doing a cut and repaste anyway as shone here https://mustangforums.com/forum/classic-mustangs-tech/401566-solution-for-headers-on-gt40p.html
Part of the reason I went with the Edelbrock Performer 5.0 heads ,I had JUST installed stainless TRI Y's. Plus I got a smokin' deal.also got a deal on a used B303
As far as picking up a 100 ponies (I have no dyno proof) but by the #'s and seat o' the pants I feel I must of gained close to that vs the old 289 with stock heads and a 448/472 cam
 

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Discussion Starter #8
If I were you, I'd skip the Warrior, and go right to the Stealth. If someone *gave* you the Warrior, sure, it's fine for a mild street build. But if you're going to spend the money, find a Stealth. It'll support anything and everything, with really no downside.

Since you have to modify your exhaust to bolt up to the K manifolds, why not just go to some good Tri-Y headers instead?

With GT40P heads, Weiand Stealth, a good 5-600 CFM Summit carb, and better exhaust, I would not be surprised to see you pick up 100 horsepower even without changing the cam. Your gas mileage will probably improve too.

My understanding was the Stealth is more for high RPM power and the Warrior more for low end torque.



I am afraid the headers will lead to many issues like clearance, heat and leakage. This is why I want to go with the K manifold.
 

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Discussion Starter #9
I do run the hipo exhaust manifolds on my GT40P motor. They fit the heads without any modification. The #8 sparkplug is a little difficult to get to, but can be done. I believe I used plug wires with 135 degree boots to ensure that they'd fit without melting a wire on the exhaust.

If you use the hipo manifolds it would be a good idea to port match them to the heads.

Sounds great!



Yes, I want to do the port matching. Seems like a good idea without increasing the costs. Did you do any modifications on the heads?
 

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No mods to the heads, but I'm using the stock explorer cam because I don't plan on this motor ever seeing more than 5k rpm. If you plan to use a more performance oriented cam, you'll probably want to go with stiffer valve springs since the stock GT40P springs tend to float at high rpms.

Even the stock explorer engine is noticeably more powerful than the stock D code 289 it replaced.
 

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Discussion Starter #11
No mods to the heads, but I'm using the stock explorer cam because I don't plan on this motor ever seeing more than 5k rpm. If you plan to use a more performance oriented cam, you'll probably want to go with stiffer valve springs since the stock GT40P springs tend to float at high rpms.

Even the stock explorer engine is noticeably more powerful than the stock D code 289 it replaced.



Thanks @New2me


Sounds like right now I can keep the GT40P springs.
 

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Thanks @New2me


Sounds like right now I can keep the GT40P springs.
The Explorer cam should work very well. I put the spring kit in from Alex’s Parts primarily because I was going to install a E cam that was given to me. Somewhere in my possession it got wet and was unusable. So I decided a 5.0 cam would still give good low end and extend the rpm range a little bit. I think the stock Explorer springs would probably be fine with the 5.0 cam
 

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Discussion Starter #13
The Explorer cam should work very well. I put the spring kit in from Alex’s Parts primarily because I was going to install a E cam that was given to me. Somewhere in my possession it got wet and was unusable. So I decided a 5.0 cam would still give good low end and extend the rpm range a little bit. I think the stock Explorer springs would probably be fine with the 5.0 cam

Will it work with the stock pistons? Or do you see pistons to valve clearance issues?
 

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Will it work with the stock pistons? Or do you see pistons to valve clearance issues?
The stock pistons will be fine, no worries. Generally about .500” lift is safe and sometimes you can go more depending on cam timing. The stock Explorer with the stock cam as mentioned will be a night and day difference from the typical 289/302. I was caught off guard the first time I drove mine and how quickly it rev’d especially since I didn’t have a tach at the time. The better breathing heads make the cam more effective
 

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That's my hope as well...

The stock pistons will be fine, no worries. Generally about .500” lift is safe and sometimes you can go more depending on cam timing. The stock Explorer with the stock cam as mentioned will be a night and day difference from the typical 289/302. I was caught off guard the first time I drove mine and how quickly it rev’d especially since I didn’t have a tach at the time. The better breathing heads make the cam more effective
Seems the budget build where they eventually got 503HP out of a used Explorer 5.0 got almost 300HP out of the stock GT40P Explorer setup with just a set of headers, RPM Air Gap and some porting on the Ps. That's pretty impressive.

https://www.hotrod.com/articles/make-503hp-with-a-350-junkyard-302/

Probably won't do anything to my heads other than gasket matching, but hoping to get something decent with the Sniper, RPM, long tubes and the little Voodoo cam.
 
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