The "P"-Heads are suppose to flow really well butt...........I think they are limited if you want something with much more than stock lift.
Ain't nothing sez you can't install a cam with 'plenty' of duration and/or LSA to make up for the limited lift!!
6sal6

The GT40P heads are more lift restricted due to the exhaust valve rotators.

Every post I've read about them with stock manifolds has mentioned grinder for plug clearance.  Personally I'd just get a set of shorty headers that fit, though that is likely going to remove any of the cost savings from the "cheap" iron heads because  think you'll be forced to go with something like those from FPA, unless there are a set for a Foxbody that fit.

GT40 and GT40P heads were used on Ford Explorers and Mercury Mountaineers from '96-'97 and '98-'00 respectively.  I would be real cautious buying used heads out of the junkyard.  Its fairly common for them to be cracked.  Minimally any used heads that aren't from a trusted source should be Magnafluxed to make sure they are sound.

Here's a link with some information I found helpful:  http://classicbroncos.com/forums/archive/index.php/t-170792.html


And a chart that lists different head varieties and GT-40/GT-40P sources

Last edited by John Ha (12/01/2020 6:14 AM)

josh-kebob wrote:

I'm running the P heads with hipo mannys, no issues.

Explorers and mercury mountaineers are your main donor's. I believe the P heads came out about mid 1998.

Just so he knows, Josh, those Hi-PO manifolds did receive quite a bit of grinding for spark plug clearance before you got 'em.

BB1

Regardless of which GT40 heads you go with, make sure your valve springs will match your cam. I installed a set of GT40 heads and ended up having to get stiffer valve springs because I was experiencing valve float at higher rpms. The cam I had installed originally was running with stock 69 351W heads.

DC wrote:

I ran the GT40s on my race car for years pretty much stock with just a little port matching back 1 inch from gasket surface. I ran the BBK long tube headers and had no issuss getting to plugs. Didn't melt any plug wires either.

That was an SN95 though, right DC?

Can you describe valve float?  At higer RPM I'm getting a slight backfire through the carb. It's been a long time since I have dealt with it.

Well, it SHOULD happen a high RPM, but as the valve springs get old they become fatigued and it can happen at much lower RPM.  The big issue you get with the GT40s (especially the Ps) is that there isn't a ton of installed height available.  The stock springs are working only a few coils, so the fatiguing of the material is exacerbated.  IMO I would change the springs just to be safe, because its a lot easier to do it on the bench than in the car. 

GT40's (non P) pop up every so often I'd watch the Corral and if you do FB look on some of the Mustang and Lightning groups.

I have to say though, by the time everything is done to prep those GT-40's you're in the territory of some basic TFS 170's.

Plus, do they Magnaflux them?  Two different machine shops I've talked to over the years told me that 1 out of every 3, or every 5 GT40 heads they see are cracked.  Those bulk rebuilding places also usually use the cheapest parts they can find and buy in bulk.  Anything like that is always going to be buyer beware. 

senzstang wrote:

TKO and Val - both great points.  I'm just bench racing thru catalogs and that is great info to consider.

A set of good aluminums probably cheaper in long run.

Buy once, Cry once.   

Its the conclusion I've always arrived at with both GT40s for Fords and Vortec heads for Chevys.  Mostly also because I try to build an engine once and be done rather than be perpetually underwhelmed with the performance and keep spending money 2 and 3 times on the same part trying to get it where I want it.  If you can leave well enough alone the factory heads are an okay option.  The minute you start talking about a real cam, real RPM, real power, etc. its time to just buy the best aftermarket heads you can afford.  Start at Edelbrock and end at AFR if funds allow.  Otherwise its kind of like how the cheapest car you buy ends up being the most expensive one you own. 

Ah, aluminum vs. cast iron.  First, the weight savings cannot be ignored.  You are taking a solid 50lbs off the front end of a vehicle that has a pretty bad front weight bias.  This is going to be helpful in terms of handling.  It will quicken the car a bit, but not enough to notice.  50lbs equates to a 1/4 mile ET reduction of 0.05 seconds.

Second, aluminum heads will tolerate an additional point of compression without detonation.  So if you have a carb you are maxed at about 9.5:1 compression running pump gas.  With aluminum you can bump that up to 10.5:1 without issue, giving you the ability to run a more aggressive cam and ultimately make about 10% more power.  That's pretty significant.  The difference between a 350HP engine and a 385HP engine you will notice.  That's a 0.35 reduction in ET in the quarter. 

Third, if you look at any engine built for performance that is not limited to iron heads by some kind of racing class rules they all run aluminum heads.  I've used aluminum heads on every engine I've built since the late '90s and have yet to be disappointed.