Can of worms, gt40p related (p for pain)

[DJCarbine]

After spending countless hours trying to find out where to best spend my budget right now, stock manifolds and exhaust will do for now simply because they "work"

However, while the motor is somewhat apart I have decided it would be a good time to think about my compression ratio.

Apparently gt40p heads are 60cc combustion chamber, and my old 66 289 heads were 54cc. I have a good oppritunity to take the heads off and have them milled.

Would this require the need for longer pushrods? I am using non-adjustable roller rockers right now, I suppose I could just get shim kits, but are they really necessary? I think I ended up using pushrods from some ford truck with the roller engine to have the valve geometry correct, and I was under the impression that with hydralic lifters it doesn't need to be "perfect" in a sense.

That being said, while I am getting them milled I an open for suggestions. Should I go ahead and get them milled down even more for higher compression? Valve clearance issues? Intake manifold issues?

Anyone want to buy a set of gt40p heads?

For reference, the cam in the car is an edelbrock performer RPM. It says it likes high compression (it told me when I went to install it), so maybe the low compression really is hurting the power.

 

[bnickel]

IIRC the P heads are more like 58cc but either way the lower compression may be hurting power a little but it probably makes the engine more pump gas friendly. you can mill the heads and shim the rockers, the shim kits are cheap so it's really no big deal. the only thing is if you mill the heads you'll need to have the intake mnifold flange on the heads milled too or you'll have problems getting the intake to seal.

the P head is a good head for stock or mild engines like yours so i wouldn't worry about changing them for something else since not many aftermaket heads will have a smaller chamber that the P heads do and they definitely flow better than your old 289 heads.

also, i don't how much timing you're running on the 289 but the P head seems to like less timing than other heads because of the better chambers so it could be that you're just running too much timing.

anyway, i'd have the heads milled about .020 to help with compression some and not have to worry about havingl the intake face of the head milled too, if you have them angle milled you might be able to go a little more than that but i wouldn't go more than .030 even if you have them angle milled. just be sure to get the rocker shims.

one more thing, if you want to improve the performance a little more after the heads have been milled you can get some of the 93 cobra 1.7 roller rockers (made by crane, BTW) that will give you a little more lift without having to change the cam

 

[CraigMBA]

I caught a bunch of flack for writing this before. I think it needs mentioning again.

There is no reason to use P heads if you plan on doing headers and don't have them because the difference between what the total package (P heads and the funky headers you have to buy) and (GT-40x heads and any headers) isn't enough to make it worth your hassle. It can't be more than a couple of hundred bucks once you figure it out, PLUS you get a set of aluminum heads. And even there, isn't it almost worth your time to get a set of AFR 165's or Edlebrocks which will make another sixty HP over either the P or the X heads for another $400 or so?

I'm as cheap as they come (believe it) but using P heads doesn't make sense if you've got to buy a set of $500+ headers to use them.

 

[DJCarbine]

I agree, however there are several places offering gt40p headers that fit in early mustangs for 280 a pair, which I think is a fair price.

http://www.melvinsclassicfordparts.com/

I'm just more concerned about the compression loss, but it appears to be no big deal. I have decided not to mill them and just install the weiand stealth manifold that just came in and buy those headers. Going from stock intake/exhaust manifold to those should bring the performance up a bit.

 

[CraigMBA]

I think that's a good plan. I wasn't aware you could get headers that cheap.

There's no reason to bump up the compression on the P heads. They make max power on a wheezing compression limited pickup. If the heads need to be surfaced, fine, else I'm running OTB.

 

[DJCarbine]

I think that's a good plan. I wasn't aware you could get headers that cheap.

I was kicking myself for buying the p heads, sort of still am.... but now that I found decent headers for them at a reasonable price im not kicking myself as much

 

[bnickel]

for the cheap price of the P heads and the $280 headers it's not a bad deal if you're on a really limited budget. they aren't the best heads out there considering all the aftermarket choices but for a stock cast iron head they are about the best you can get without doing exetnsive and expensive port work. if you were to use a ported stock exhaust manifold and the P heads on an otherwise stock or mild motor you'd have a pretty decent "basically stock" motor that will run really well. for what it is.

i guess my point is that for a budget motor built on the cheap using as many off the shelf ford parts as possible you can't realy go wrong with the P heads and if you decide to later on they can still be ported for even better performance.

if you have the budget, though, i would pick another head for a performance buildup, if you don't have the budget the P head is ok in my book.

 

[Fast63]

I think it was a wise choice not messing around with milling and causing other problems. The stealth was also a good choice for your application. Also, if you mill the heads, you need shorter pushrods, not shims, because you are bringing the rocker pivot point closer to the block.

 

[bnickel]

I think it was a wise choice not messing around with milling and causing other problems. The stealth was also a good choice for your application. Also, if you mill the heads, you need shorter pushrods, not shims, because you are bringing the rocker pivot point closer to the block.

huh? that's what the shims are designed for. these are the shims that go between the rocker pivot/pedestal and the head casting on the late model style heads. they effectively shorten the pushrod by raising the rocker arm pivot.

like this

http://store.summitracing.com/partdetail.asp?part=FMS-M-6529-A302&autoview=sku

 

[wicked93gs]

I for one think the P heads are a good deal(I can find them semi-locally for about $100 a pair) and they are what I plan on using, but for my application I wont need maximum airflow, throw on some ported Hi-Po manifolds and it would be a pretty decent performance gain over a stock 289

 

[Helmantel]

Milling the heads should not affect rocker geometry when using pedestal style rockers because position of the valve tip relative to rocker pivot point is not changed.

It will however, affect the lifter preload. The pushrod basically pushes the lifter plunger further down, by the same amount you milled the heads. If the change is large enough to cause problems depends on how much you milled off and the RPMs you plan on running (since lifter pump up issues at higher RPMs can be caused by incorrect lifter preload).

You can bring the lifter preload back where it used to be with the shims under the rocker, but that throws the rocker-valve tip geometry off (assuming it was correct initially). The optimum solution would be push rods that are shorter by the same amount the heads are milled (again, assuming lifter preload was correct before milling).

Shims are useful when installing longer valves (longer valve tips) or probably when installing a cam which has significantly more lift than stock. You can then raise the rocker to compensate.

 

[fede86gt]

I agree, however there are several places offering gt40p headers that fit in early mustangs for 280 a pair, which I think is a fair price.

http://www.melvinsclassicfordparts.com/

Can you please tell me who else sells the headers, I can find all day long low millage gt40p longblocks in my area for $400 dlls, and im very interested...

Thanks in advance.

 

[Fast63]

huh? that's what the shims are designed for. these are the shims that go between the rocker pivot/pedestal and the head casting on the late model style heads. they effectively shorten the pushrod by raising the rocker arm pivot.

like this

http://store.summitracing.com/partdetail.asp?part=FMS-M-6529-A302&autoview=sku

You have it backwards my man. Read the notes for the part

Notes: To attain .090 in. reduced preload, you must stack the shims. Will not fit Ford FE engines.

Adding shims reduces preload, which means you would need longer pushrods to increase preload and shorter pushrods to decrease preload. If you mill the heads, you increase preload, so you need shorter pushrods to compensate and reduce the preload back to where you want it. Make sense?

 

[rbohm]

DJ good call on not milling the heads, the difference in the compression ratio isnt enough to help any. your best bet is to port the heads, and install larger valves.

 

[D.Hearne]

Ford says you can mill stock heads (with the exception of the 93 Cobra's GT40's, these were milled at the factory to bump the comp ratio in these motors) up to .040 without getting into intake fitment problems. As per everyone elses recommendations, you may have to shim the rockers afterward. You definately will not need longer pushrods, shorter rods if any change would be in order. Any time you mill any heads, and BEFORE milling the intake, check the intake fitment after bolting the heads back on.(with the head-side gaskets in place) check both the side fit and the gap under the ends, again, BEFORE milling the intake. You may or may not need to mill the intake. I milled my Canfields .060 and had to mill a Vic Jr to fit on one block, but with the same heads on my 331, no milling was necessary, with my 3x2 intake, only had to wallow the intake bolt holes to accomodate the head holes moving toward the block centerline. Ford's specs on the GT40P's chambers are 58.3 ccs to 61.3 ccs. With that RPM cam, I'd definately mill at least .040 off those heads to increase the comp ratio. With an EFI grind roller cam, you could skip the milling, as the dynamic ratio would get a boost from the EFI grind's wider LSA.

 

[Helmantel]

With that RPM cam, I'd definately mill at least .040 off those heads to increase the comp ratio. With an EFI grind roller cam, you could skip the milling, as the dynamic ratio would get a boost from the EFI grind's wider LSA.

That's not correct:

With a wider LSA, the intake valve is closed later, so the effective (dynamic)compression ratio would be lower, not higher.

 

[D.Hearne]

That's not correct:

With a wider LSA, the intake valve is closed later, so the effective (dynamic)compression ratio would be lower, not higher.

Well it's not, I run em and with the same ratio, the EFI grinds need more octane. The 5.0 in my Ranger is a prime example. F4TE cam, 9 to 1 ratio and it wants 89 octane when the weather is warmer.

 

[Helmantel]

Well it's not, I run em and with the same ratio, the EFI grinds need more octane. The 5.0 in my Ranger is a prime example. F4TE cam, 9 to 1 ratio and it wants 89 octane when the weather is warmer.

That may be true, but that doesn't mean that increasing the LSA increases the effective compression ratio. The effective compression ratio is the ratio between the cylinder volume at intake valve closing, divided by the volume at TDC (I guess you knew that already). Making the LSA wider, retards intake valve closing, so the volume at IVC is reduced and thus the effective compression ratio is lower.

 

[rbohm]

That may be true, but that doesn't mean that increasing the LSA increases the effective compression ratio. The effective compression ratio is the ratio between the cylinder volume at intake valve closing, divided by the volume at TDC (I guess you knew that already). Making the LSA wider, retards intake valve closing, so the volume at IVC is reduced and thus the effective compression ratio is lower.

when you widen the LSA, you reduce the amount of overlap between the valves, and thus increase the dynamic compression ratio. this is an old trick to make an engine think it has more compression than it really does. it was used with the first RV and mileage type cams.

 

[Fast63]

when you widen the LSA, you reduce the amount of overlap between the valves, and thus increase the dynamic compression ratio. this is an old trick to make an engine think it has more compression than it really does. it was used with the first RV and mileage type cams.

I'm going to have to go with Helmantel. Your static compression is increased with a wider LSA and less overlap, but when you're talking 6000rpm, it's the overlap and tight LSA that increases your dynamic compression. Otherwise, what's the point of the overlap if it is bleeding all your compression off all the time?