gt40 heads vs stock heads
 

whats the different with gt40 heads? i have seen alot of ppl run them and just curious to see what the difference is from the stock heads say on a 91 lx coupe.

If your reffering to the gt-40 iron, like the heads that came factory on the 93-95 cobra, and early ford exploders, they arent a whole lot better, the valves are bigger 1.84 intake, 1.60 exhaust, i could be a little off on the exhaust size, the stock e7's have a 1.78 intake 1.46 exhaust, again i could be off on that, but it's close, i run the gt-40 heads, i had a mahcine shop port/polish, surface, and 3 angle valve job them, they say mine came off a 95 cobra, but the guy that sold them to me said they came off an explorer, but anyway, i can't tell you the performance they have since my car is still apart, i can let you know maybe monday, i should be threw.
I also hear the 40's respond well to porting, and to notice a big difference from e7's you need to do at least a mild port job.

For starters GT40 heads have larger valves 1.84 intake and 1.54" exhaust valves. Also the port casting is much cleaner and smoother, plus the HUGE thermactor air injection bump is gone in the GT40 heads. Also they're faster burn heads because of a revised combustion chamber. I've heard that the aftermarket GT40 heads (cast iron) are supposed to add 30 horsepower to even a stock mustang GT. You can get used GT40 or GT40P heads for VERY cheap (the P heads were used on '96-'99? 5.0L explorers). GT40 heads were originally used on Cobra Mustangs and the 5.8L lightning and also sold through Ford Motorsport.

do they just bolt right up? Any modifcation?

Maybe i should do this since i am tearing my car down to the heads to put on my cobra intake anyway.

Nope, they bolt right up no problem. Mine are ported to a 1250 intake gasket, and I dont remember what size exhaust. I forgot to get pics of them lined up against stock heads, but there is a big improvement in the exhaust ports. They till dont compare with most of the other aftermarket heads, but they are a decent improvement for cheap if you find a used pair.

My car ran a high of 104mph last year wit a ported set of GT-40's and a worn out Carter carb. They will be a big improvement over stock.

If you are going to buy a set of GT40s it is best to go private sale, all of the junk yards I have found know about the desire for GT40's for mustangers and are starting to charge an arm and a leg...