P-heads!
  Not sure of the CC size (so have them shaved if necessary)
  IF they have-a-hump...grind it out butt.....don't try to change ports because they're suppose to flow well.
THEY'RE SUPPOSE wear HiPo mani's really well.(instead of headerz)
6sal6

Not sure where I got the 65mm MAF from.  Stock Mustang TB was 58mm, Ford Racing used to sell 65mm ones, but they were discontinued years ago.

Anyway, my point is that you never want a TB that is larger than the MAF, AND you do not want a larger TB on a stockish engine.  The TB is not the restriction to flow in the intake tract; the MAF is.  Furthermore, if you do not need the flow what you get is odd throttle response, especially without tuning, because the opening area of the larger TB is so much bigger than stock.  The ECU cannot "see that though, all it "sees" is the rate of opening as read at the TPS.  This causes a mismatch with fueling and a lack of smoothness during transition throttling. 

The solution is also not to just throw a bigger MAF on it, because without tuning the MAF transfer function will not be correct for the larger MAF and you will get poor running.  "Calibrated" MAFs are BS; they try to trick the ECU into accepting the larger MAF and/or different size injectors, but it never scales right because its still all running through a stock MAF transfer table. 

Before I really understood how the EECIV worked we used to throw parts at the 5.0s and they would always seem to pick up on the top end.  We always took the finicky running everywhere else as the price we had to pay for running a modified engine.  The truth is that the ECU did a great job of allowing it to run at all, but was hindered by stock programming.  Its a great system, but it only knows so much.  When changes are made it needs to be tuned to optimize how it runs everywhere, not just at WOT.  If the goal is a stock engine you're better off with stock parts.  If the goal is a modified engine that makes more power and is still enjoyable to drive I'd advise tuning.  There are shops that can do it, or you can buy something like the Quarter Horse and learn to do it yourself.  I absolutely NEVER recommend "chips".  Its a one size fits all fix that isn't tailored to your vehicle and where its operated. 

Last edited by TKOPerformance (8/12/2019 4:07 PM)

Here's the straight dope on the GT40Ps:

1.) Valvesprings are too weak for what most people would call a performance cam
2.) Exhaust valves use a rotator that drops the lock location on the valve reducing the available installed height for the exhaust valve spring to about 0.180" less than the intake valves.  This doesn't give a better spring wound from heavier wire much space to move while avoiding coil bind.  For real performance cam use the exhaust valves need to be changed and the rotators eliminated.
3.) Net lash valve adjustment, meaning no way to account for the heads or block being decked without shims or shaving parts down.  Not a good system for performance use, and the 5/16" bolts used are relatively weak. 

Changing all these parts is going to add up quick. 

The E7TE heads have a lot of the same issues though.  They also use net lash valve adjustment.  The springs are also going to need to be swapped to match the cam, unless its just a stock HO cam. 

The 289 heads if they need a complete rebuild are a loser.  You'll have too much in them and they aren't a great flowing head.  They were designed to build low end torque on a small engine and be done at 5,000RPM. 

The 351 heads might be your best choice cost wise, but you're dealing with really dated technology.

Here's my advice: sell everything you have now and buy a set of Edlebrocks.  By the time you weigh the cost to upgrade any of the heads you have with what you could get for the them as is vs. the cost of a set of Edelbrocks its going to be a wash.  The Edelbrocks will flow better than any of them and can handle cams up to 0.575" lift on the stock springs.  They are on special at Summit now for less than $900/set. 

Last edited by TKOPerformance (8/14/2019 10:13 AM)

If you can find or modify manifolds/headers that fit, you can do a spring upgrade on the gt40p's for a reasonable cost.  My engine guy says that the exhaust valves may need some clean-up as they tend to get a little rough in the Explorer applications but that should be pretty straight forward.  Alex Parts also offers new valves and retainers to get rid of the uneven valve length and odd retainers on the stock GT40P.  Of the heads you listed, assuming that you get your water passages and head gaskets sorted out I would go with the GT40P heads as long as you are in target range for compression.  Steve is absolutely correct - what are you going to do with the motor?

One example of drop-in springs - they offer variations depending on lift and cam type - can't beat the price.

https://www.alexsparts.com/sb-ford-gt40p-drop-in-valve-springs-kit-hyd-roller-580-lift/

The car has a T5, or will based on a previous post. 

Let's be realistic.  If the car is ever going to see a track you aren't going to want to stop revving an engine at 5,000 RPM that stock was rev limited at 6,250.  Play or not I know how this goes.  You're going to want to get more out of it, and one of the best ways is to avoid having to shift into 4th at the end of the track.  The only way to do that is to rev it past 5,000.  So my advice is don't build something that in that circumstance is essentially a grenade just waiting for you to pull the pin by trying to convince yourself it'll hold together. 

The glaring weakness in the 5.0 was the heads.  I clearly remember dyno tests where AFR got an additional 80HP out of an otherwise stock 5.0 just by swapping on a set of their heads, and that was at the rear wheels.  For the sake of fairness that car was a '93 Cobra and already had the Cobra intake which was essentially a cast version of the GT40, 1.7:1 roller rockers, 24 lbs/hr injectors, Cobra spec MAF, Cobra ECU, etc.  Still, it made all of 220HP at the rear wheels stock, and with just the AFRs it made over 300.  This was also without ECU tuning and an observed mixture that was still too rich on the max power pull.  Check it out if you want:

https://www.fordmuscle.com/archives/2003/06/afrinstall/

Now the AFRs are pricey, but they are the best.  That said, the Edelbrocks I'm confident would net at least an additional 50HP over stock, and if you get rid of the pinched down factory headers and open up the exhaust it will be even more.

Plus, from a handling standpoint you're taking weight off the front of a nose heavy car.  Never a bad thing. 

So, bat it around, but I just ask you to be realistic with what you want and might want in the future.  Speed and power get addictive quick.  If you have multiple cars one can probably be a parts chaser that isn't terribly quick.  If this is your toy, then it probably should be quick, especially if you see yourself hitting the track, even just for fun.  No one wants to be the slowest Mustang as the strip.  I've done it.  It was fun, but blowing the doors off a scoffing new Mustang owner with an 8,000 lb. Diesel truck was a LOT more fun, and that's what I drive every day...