P heads.....

GT-40P
Is the P for Performance, Pathetic, or Pain-in-the-Ass!?

ATTENTION GT-40P USERS!!!! I want to include your stats in my "User Comments" section. My hope is to show real world gains, expectations, results, etc. from everyday Mustangers like yourself. Info I'd like to include are Name (first only is fine), email address, year and model 'Stang, basic list of mods b4 and after the head install, any 1/4 or 1/8 mile data b4 and/or after, and a "user commment". Anything from, "Car hauls ass now!!" to a short paragraph detailing your experience with the head. Send your info to me at motorcitymustang@comcast.net

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Info
There's a lot of talk lately about the latest GT-40 casting from Ford found on the 97½ - current Explorer/Mountaineer V8s. The GT-40P. I've heard it more than once that the GT-40P is a poor choice for a performance application because it came from the truck parts bin. It's a "workhorse" type head and that's where it should stay. Well, the E7TE came from that same truck parts bin and we've all seen some decent NA applictions using those, haven't we?!

When I started on this '84 GT project the decision was made that it was going to be a budget buildup. No fancy aluminum heads. I set a goal for 12s on the motor and it was gonna be using iron Ford castings. I had pretty much resigned myself to the fact that'd I'd be using either ported E7TE's or a set of 289/302 58cc heads with Chevy valves. About that time is when I heard about the GT-40P: The direct decendant of the Cobra GT-40 head. Hmmmm.....this may be a neat alternative.

Well....I decided to do some research. I kept that "these heads aren't for performance" comment in the back of my mind to stay objective. I tempered that, however, with the reasoning that what was going to be important to me was how the head flowed and what it's cost would be. It hasn't been easy finding information but included here is everything I've come to learn about these heads. I'll be documenting the home Stage I/II cleanup and then final assembly of these heads. First....let's cover some general info to make sure there's no false information still clouding judgement......



Seen here is the combustion chamber of the E7TE casting. - photo courtesy of MM&FF & my scanner

Shown above is the combustion chamber of the standard GT-40 casting. - photo courtesy of MM&FF & my scanner

Seen here is the combustion chamber of the GT-40P casting. Notice the relocated plug position. This "intrusion" into the combustion chamber is what gives the GT-40P it's smaller chamber size. - photo courtesy of MM&FF & my scanner

Above you can see the change in plug location from the GT-40 (right) to the GT-40P. - photo courtesy of MM&FF & my scanner

The GT-40P replaced the standard GT-40 on the V8 Explorers and Mountaineers in early 1997. Yes, that's right, the Explorers got Cobra heads (although the Cobra's heads were milled to bring it's larger combustion chamber down to the size of the E7TE to keep compression up).

Similar runner volumes to the standard GT-40 head (around 145-155cc on the intake side, I think, compared to E7TE's 124cc). Also flows similar intake and exhaust cfm numbers to the standard GT-40 head. All accessory bolt locations and valve train geometry are the same as the standard GT-40.

1.85/1.46 valves (compared to 1.85/1.54 for the standard GT-40). The exhaust port was redesigned and is more efficient, thus the ability to use the smaller exhaust valve. This actually works well since to get equal flow numbers the velocity must be higher.....exactly what we want.

There's a little bit of room for larger valves but start going larger on the intake side and you're going to have to grind on that plug boss to clear it. And....don't forget.....a bigger valve isn't necessarily better. The whole reason these heads work so well is the decent flow they provide while still having smaller port dimensions. If you're not careful and hog out the bowls for big valves you could easily hurt flow. Just remember this when planning your mods.

Smaller 59cc (up to 61cc based on factory tolerances) combustion chambers (65.5cc in unmilled GT-40 w/ FRPP valves). Great for NA cars as this equates to a raised compression ratio when switching from E7TE castings. Should get you close to 9.5:1 (w/ stock pistons). Cut 0.030" off the bottom and you're close to 10:1 (w/ stock pistons).

Small emissions-type bump in exhaust port similar to E7TE casting but it's nowhere near as pronounced. This bump is dead center on the port roof right at the exit. Easily removed if you feel it's hurting flow. I removed mine, smoothing the transition from the guide boss to the port exit. I raised the roof of the port here to eliminate the offending bump and open the port to a size a bit closer to the Fel-Pro 1415 gasket.

No thermactor holes in the front/rear face to plug and no hole in the exhaust port to mess up flow.

Intake ports are already very close to a Felpro 1250 gasket...probably no need to gasket match. Mine were only off on one end.....the other three side being almost perfect. I just matched the last side.....no sweat.

They can use stock pedestal mount rockers. I upgraded to stud-mount adjustables. Be sure to get a shim kit to ensure you'll get the rockers to torque down correctly if you're using bolt-downs. You may not need 'em if you don't mill the heads but the kit is only $10.

Spark plug is moved down and rotated out placing the electrode closer to the center of the combustion chamber. This provides a better and more complete burn. Not only does this help performance but it also reduced emissions. Note: The GT-40P's don't use standard 5.0L plugs. You must use plugs for a 98+ Explorer/Mountaineer V8 or for a Cobra/Lightning equipped with iron GT-40 heads!! Forget about the shorty plugs (Accel p/n P526S). I've heard they're close to $10 each for this application! Plug specs are: 14mm thread, 0.708" projection, tapered seat, resistor plug. Thanks to Derek Hyde for the Accel plug specs!!. The 2000 FRPP catalog lists a Motorcraft AWSF-32C. I bought Champion Truck Plugs p/n 4013 (I used the Truck Plugs on a '97 V8 Dakota I had and was happy with them. Thought I'd try 'em again.)

Standard unequal length shorty headers won't work because of this new plug location. The primary pipes make their downward turn towards the collector too soon. Central Coast Mustang offers a P-specific shorty (made by MAC) for $199 and FRPP is offering a set for about $250. I know the FRPP pieces are stainless.....not sure about the MAC....most likely their HTS coating. I thought I read once that stock tubular exhaust manifolds might work. Why bother using those though if you're upgrading the heads?

In starting my research for the header-fit table I've been told that standard FRPP unequal length shorties will work for not only the Fox application but the SN95 shorties will work for the 94-95 cars as well. These are not a perfect fit like the MAC-for-CCM shorties and will most likely require 90° boots and/or heat booties, but they are supposed to fit without any primary dimpling.

Equal length shorties won't work at all. Well....sort of. Again....in researching the header-fit table it looks, via some good installed shots - don't have provider's info handy....credit will be given) that BBK equal length headers may fit. Still studying the pics for a more solid opinion on this.

Some longtubes and 3/4 length headers will work. Their ability to allow the primaries a longer exit straight from the exhaust port b4 turning down towards the collector provides adequate access to the plug. I know of MAC longtubes (1 5/8"), Flowtech longtubes (1 5/8"), and Heddman longtubes (1 1/2") being used.

The Heddmans alledgedly require a small clearance dimple on the #4 primary. The MAC's are snug on the #7 primary but clear fine. CCM offers these as a longtube alternative to their P-specific shorties. I had unknown-brand 3/4 length headers (1 1/2") on my '84 (look like DynoMax) that will clear no sweat. MAC longtubes for the SN95's are a good fit. CCM finallyhas a SN95 P-fit shorty. CCM is also offering a set of Tri-Y headers for GT-40P use in classic Mustangs. This is a huge plus as I've had many questions regarding header fit for P's in classic 'Stangs.

The Flowtechs were originally thought to work and they can be used. The dimple that was said to be required in the #4 primary is more like a crater. You need to crush the primary half closed to clear everything. And you'll need wires with 90° to clear a few of the other primaries as well. Check out the Progress Page for more info on this. There's some great shots of the Flowtechs installed (and "dimpled") as well as the CCM MAC built P-specific shorties.

P heads take a 7/16" headbolt just like other 289/302 heads. No funky stepped bolts or bushings needed. Big plus IMO here. Bad for 351W installs but great for us 302 based users.

Note that in the picture showing the plug locations you can see the GT & GTP casting designators. This is a good way to spot the castings in a boneyard or swap meet. There are also "bars"' on the end of each casting; three on the GT-40 and four on the GT-40P. These can be seen on the images below.

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Bare-Casting
In this photo one of the GT-40P's exterior identifying marks can be seen. Pay attention all you boneyard hunters!! This is what you need to look for to ensure you get the right head!! The standard GT-40 head simply says "GT" in this same location.
This photo shows the GT-40P's other exterior identifying mark. The four bars on the side face of the head mark this as a P casting. The standard GT-40 head will have 3 bars located here.
This photo shows the bowl areas of the intake and exhaust ports. I'll attempt to get better shots of this once the porting progresses a bit. It's difficult to get good shots as the camera has a hard time focusing on the cast iron surface finish for some odd reason. You can also see the reason for the smaller combustion chamber size here. Notice how far into the combustion chamber the plug boss extends.
Finally got a better shot of the intake runner. Can't see the bowl area or valve guide boss but you get a good, clear look at the intake runner.
And finally some better shots of the exhaust port. Those that are familiar with the exhaust port of the E7TE will notice the lack of the emissions bump. There is a very small dimple (you can see the flat plateau created by it in the port roof) protruding into the port but it would be easily reduced with a cartridge roll.


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Port Cross Sections
I've got to give a huge thank-you to Fred Meyer for providing these pics. A buddy of mine (thanks for putting me in touch with Fred, Steve) ran into Fred on the FFR Cobra Forum discussing porting GT-40Ps. Fred hit a water jacket during his porting and had to get another clean casting. What to do with the bad one? Cut it in half of course!

When Fred and I spoke I mentioned to him that I had wanted to do this very thing. Well.....he beat me to it and is more than happy to share this info with the rest of us P users. Turns out he and I are pretty local to each other and it looks like I may be able to get the pieces for furthre evaluation. I'd eventually like to provide some key measurements for you guys so can lay out your porting plans and be able to stay clear of any trouble. Fred's whole reason for doing this.......to help others steer clear of the problem he ran into.

Thanks Fred!!
The picture says more than I can really. Hope to eventually get you guys some measurements, and maybe a section thru the pushrod for wall thickess. That seems to be the biggest area of concern when working on the intake side.

This is a tremendously informative shot. Without seeing this section view (those that have closely looked over their bare castings will know what I'm talking about here) the Thermactor boss in the exhaust port doesn't look to be anywhere near the obstruction it really is. I was shocked when I saw this picture. I was also surprised to see the water jacket in the exhaust guide boss. Typically you can wipe this out on a set of stock sbf heads. I know of one GT-40P user (thanks for that tip John!) that found this jacket the hard way. In the water jacket shot you can more easily see the line you'd normally be able to take by eliminating the guide boss. That section shot is thru a port without the crossover runner.

"Ok...", you're saying, "but what kind of power can I expect to make?". Well....let's take a look at Fun Ford's Trophy Stock class and NMRA's Pure Street and Factory Stock classes for some examples. Now.....some may argue that these are purpuse built race cars but they're not. Most of these guys run this class because it's based on real-world street type cars. There's just a bit more attention to detail and the cams (in TS and PS) can get a bit lumpy.

I originally kept track of the current record in TS but it's changing to quickly to keep up. :^) If you look into these classes tho you'll see that the top contenders in each class all have something in common. GT-40P heads. Not all of them (especially in Pure Street since aftermarket heads are allowed) do but you'd be surprised at how many are using them....and with HUGE success. Trophy Stock and Pure Street racers are running solidly in the 11s w/ P's and Factory Stock racers are running solidly into the 12s.

These are very impressive results considering the limitations placed on the competitors in these classes. Following these classes is also an excellent way to get ideas on low-buck combinations that provide deep 12 second/hi 11 second performance.




Below are some preliminary flow numbers that I've come across. The GT-40P information was from the May 1999 issue of Super Ford. Other information was collected from various magazines and internet sources but was found all in one spot on Fletch's Carbureted 5.0 Webpage. Since this data comes from different flow benches the data should be used as a rough comparison only.



All data is for stock, unported castings
Intake Flow @ 28" H20

.100" .200" .300" .400" .500"
GT-40P 61 128 169 195 196
GT-40 54 107 157 183 192
E7TE 59 114 144 156 156
'69 351W 52 103 149 174 183
289 50 97 127 156 165
GT-40 (Y303) 64 114 167 199 213
GT-40 (X303) 53 114 170 207 226
Exhaust Flow @ 28" H20

.100" .200" .300" .400" .500"
GT-40P 52 90 123 135 139
GT-40 47 94 119 128 128
E7TE 42 78 105 115 116
'69 351W 48 77 100 113 118
289 37 71 93 104 107
GT-40 (Y303) 52 98 128 137 134
GT-40 (X303) 50 106 139 153 161


There is possibly a more important, or at least equal, value to consider than the above flow numbers; which most obviouly tend to rate by the peak flow. And I want to say thank you to Aaron Smith for bringing this up to me. I had never even thought of including that comparison here even tho I had already done the calculations. Thanks Aaron!

This value is a calculation of the "area under the curve". Once the flow numbers are plotted in a graph there is a calculation that will closely approximate this area. What this value gives is the curve that provides the most flow over the entire range of lift. For example.....let's say you are comparing two cylinder head's flow data. One head peaks 10 cfm higher than the other but the other has much better mid-lift numbers. The resultant "area under the curve" will be higher for the lower peak head. This head would most likely outperform the hi-peak head as you'd have more power across the entire range of lift. Remember....your cam has to pass mid-lift twice for every trip to peak.

The formula is really simple. For the sake of simplifying the formula let's call the value for the flow in cfm at a particular lift as the letter "F" followed by the lift (we'll call it "L") value numbering from lowest to highest. i.e. FLmin, FL2, FL3, etc. ending with FLmax. So...using that as our variable the formula reads like this:


0.05 * (FLmin + (FLmin + FL2) + (FL2 + FL3) + (FL3 + FL4) + (FL4 + FLmax))

Ok....now that we have the formula....let's look at a couple of the examples in the above flow data chart and a couple other head choices on the market.
Intake
A.U.C. Exhaust
A.U.C.
GT-40P 65.1 46.95
GT-40 59.7 45.2
E7TE 55.1 39.8
'69 351W 56.95 39.7
289 51.45 35.85
GT-40 Al. 65.1 48.26
GT-40X Al. 65.7 52.9
Twisted Wedge 76.65 57.3
"Power heads" 67.05 57.9
A couple notable observations from this table. Notice the increased peak flow of the '69 351W head compared to the E7TE. Then look at the "area under the curve" values. In stock form they're not that great of an improvement. Now look at the A.U.C. for the GT-40P compared to the standard GT-40 iron. We all know the gains to be had from the standard GT-40 head. The P head really shows it's strength when you compare them this way. Now look at the A.U.C. of the GT-40P compared to the standard GT-40 aluminum head. Still want to spend all that money on those aluminum heads? Maybe not, eh? Just to be fair.....compare the A.U.C. values of the GT-40P to the "Power Head" (for those that aren't familiar with this head.....it's a CNC ported E7TE with bigger Chevy valves). Not bad from a E7TE casting. My only complaint with the numbers for the Power Head is the intake to exhaust ratio. The average is about 88%. Way too high IMO. Those of you running a ?303 cam with single duration and lift values will probably be ok with this situation although that ratio is still a bit too high. The rest of the castings that have a more typical 70ish% I2E ratio would benefit from a split duration cam that favors the exhaust side.

One last item I'd like to cover regarding flow numbers is the intake to exhaust ratio. The GT-40P exhaust port has been improved so much that it's easy to raise the I2E ratio above the typical 70-75% that you would typically see in a cylinder head. This isn't necessarily a problem provided the ratio doesn't get too high. Get close to the 80% range and you really should be looking at a single pattern cam (ala the FRPP 303 line) as the exhaust doesn't need the help of a split pattern cam. The table below shows the I2E ratios of the heads we've looked at above.

Now none of the above or below flow data is saying that you can't make power with other stock castings. It's all a matter of matching parts. What I am trying to show here is that the P head should definitely not be ruled out simply because it's current use is a truck.

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Intake To Exhaust Ratio
.100" .200" .300" .400" .500" avg.
GT-40P 85% 70% 73% 69% 71% 74%
GT-40 87% 88% 76% 70% 67% 77%
E7TE 71% 68% 73% 74% 74% 72%
'69 351W 92% 75% 67% 65% 64% 73%
289 74% 73% 73% 67% 65% 70%
GT-40 (Y303) 81% 86% 77% 69% 63% 75%
GT-40 (X303) 94% 93% 82% 74% 71% 83%
Power Heads 98% 89% 872% 83% 81% 88%

"Ok....", you're saying, "I see the potential here but what about the everyday driver that isn't raced regularly and run at WOT a lot?". Well...this following section will cater directly to that question. I'm going to include here comments and examples from your average Mustang owners that are already enjoying the GT-40P heads on their own cars. I'll try to include a short vehicle breakdown, some comments from the owner, and an email address if you'd care to ask them questions directly about their combinations and experiences with the heads. The few that I've been able to talk to that are already using the heads are great guys. More than willing to help with questions and clear up any confusion with this install.



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