GT40 head upgrade with or without new cam

[JimC25]

So I was actually able to find some GT40 heads at a junkyard in Oklahoma to replace my stock heads. I dropped them off at the machine shop earlier today to get them checked out. I am hoping to get a little more power on the low end, but I don't want to sacrifice gas mileage. I have a 1989 XLT with the speed density 5.0 and AOD transmission. I'm definitely going to swap the heads if they check out all right, since that is supposed to give me some more horsepower, but I'm on the fence about a new cam. I'm planning to call Comp Cam on Monday to get their recommendation, but I wanted to see what others who have done this experienced. Is it pretty standard to lose gas mileage from a mild cam upgrade or does it really depend on which cam you go with? Also, I've seen alot of people say not to waste your time changing heads if you don't also do the cam swap. Has anyone done the GT40 upgrade only? If so, did it give you any noticeable gains?

 

[miesk5]

Yo Jim,
Until others join in;
Consider Ford Fuel Injection » EFI Performance Upgrades by FireGuy50 (Ryan M)

And his How to pick a CAMSHAFT for EFI

 

[JimC25]

Thanks Miesk, great info! Comp Cams and FireGuy both recommended the 35-255-5 for Flat Tappet, Speed Density, Fuel injected 302's.

I'm on the fence now though about what to do because Comp Cams only recommended the cam if I kept the stock heads. The guy I talked to didn't think that the cam with my Speed Density computer would "fill" the GT40 heads properly, so I might even lose performance with that combo as a result. To use the GT40 heads, he suggested I convert to a roller cam and change to MAF so the computer could be programmed to get the most out of the GT40 heads. I don't doubt that would give me the best performance, but it's way more than I am willing to tackle at this point. If the GT40 heads check out, I might just throw them on and hope for the best...

If anyone has used the 35-255-5 cam with GT40 heads on a Speed Density, fuel injected 302, I'd love to hear how it worked for you. The only real upgrades I have at the moment are shorty headers and a 6-liter tuneup. Thanks!

 

[L\Bronco]

Do you have the casting number from the orig heads?
It should be near the location in the pict.
(You will get a more accurate response if we know where you are starting from.)
Id throw in the explorer head numbers too as they made a few versions of them.
Its going to change for sure, we just need to make sure the compression doesn't go too high on you.
Be careful on the cam, if the lobe separation (overlap) gets too high, the idle goes lumpy, (sounds great, but wreaks havoc on MAP sensors, which are the soul of the speed density calculation.
Hope that helps
Cheers

 

[JimC25]

Thanks, I looked but don't think any of the casting numbers for my heads are visible without pulling one of the valve covers. I'll pull one this weekend to grab them. Also, the GT40 heads I'm looking to use are at the machine shop getting checked out, so I'll have to get those later as well. I was pretty sure the GT40 heads would be a direct bolt on to my 302 and would give me at least a little increased performance, but I started to second guess everything after talking to the Cam rep and hearing his reservations. I did another deep dive into old threads and found a few specific to SD 302's where they used the GT40 heads with no problems and one or two with mild cams as well. I'm probably just talking myself into it, but I'm back to thinking the GT40 heads and the 35-255-5 cam that is recommended for flat tappet SD 302's should be fine. Maybe it's just wishful thinking, but I'm pretty good at talking myself into projects. LOL

 

[L\Bronco]

The Gt40 heads will bolt on for sure.
The performance increase comes frome increased compression ratio, improved flow (intake and exh), and increased turbulence in the combustion chamber (swirl) If the compression ratio gets too high the fuel charge will detonate before the spark. We need to know the combustion chamber size of the old vs new heads to see how much it will go up. If you keep the cam mild it should be fine, providing the compression ratio remains below 10:1 unless you plan to run at least 91-93 octane fuel.
Hope that helps
Cheers