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Old 04-04-2010, 11:29 AM
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Machining requirements for cam with GT40 heads

I'm building a 302 roller cam block with GT40 heads with the ports cleaned up and am looking at CompCams for cam choices. Looking at this from 2 points of view, how much lift can stock (originally 96 Explorer) GT40 heads and rockers take without machine work, and then, exactly what machine work is required for the larger lifts? Shortening the guides only, or is there any spring pocket cutting required?

XE266HR and 270 or 274 numbers all look good on the high side and require machining - I'm just not sure "what" machining. No power brakes and 3.73 gears behind an AOD in a 65 Mustang is where this is all taking place.

Lastly, I'm tinkering with CamQuest software. I say the HP numbers are ridiculously high. Similarly with Desktop Dyno. Comments?

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Old 04-05-2010, 07:22 AM
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35 reads with no response? I know some of you had to have used GT40 heads with different cams. Simply put, what were some things you had to do to make the heads and cam compatible?
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Old 04-05-2010, 11:32 AM
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Quote:
Originally Posted by KMatch
I'm building a 302 roller cam block with GT40 heads with the ports cleaned up and am looking at CompCams for cam choices. Looking at this from 2 points of view, how much lift can stock (originally 96 Explorer) GT40 heads and rockers take without machine work, and then, exactly what machine work is required for the larger lifts? Shortening the guides only, or is there any spring pocket cutting required?

XE266HR and 270 or 274 numbers all look good on the high side and require machining - I'm just not sure "what" machining. No power brakes and 3.73 gears behind an AOD in a 65 Mustang is where this is all taking place.

Lastly, I'm tinkering with CamQuest software. I say the HP numbers are ridiculously high. Similarly with Desktop Dyno. Comments?
The Explorer cast iron GT40 heads are good to about .5 to .51 inch. But you always have to check as engines are subject to normal production line tolerances if they haven't been reworked by the factory or aftermarket. Factory rework comes in new production engines, these are parts that failed inspection but are not so far off spec that they can't be reworked (fixed) and released to assembly. So for example often the deck height standard is compromised, crankshafts come with undersize journals, bores can be oversized, and host of other things can be done at the factory to recover rather than scrap parts with errors. So you never know for sure what any build up is dimensionally, this then requires you check critical dimensions which also means you potentially assemble the engine at least twice. A quality gasket set will tolerate this.

What your checking for with a non standard cam are several critical clearances:

- Valve to piston, this requires a lump of clay on the piston and a total rotation of the engine to stamp the valve into the clay. Then pull the test head back off and measure the clay thickness at the point of depression by the valve, .050 is the minimum acceptable. Lack of clearance is usually accommodated by increasing the depth of the valve relief in the piston. This, however, requires that you measure the thickness of the piston crown under the relief, this shouldn't be less than .1 inch.

- Rocker slot to stud clearance, at max lift this needs also to be a minimum of .050 inch. Lack of clearance here means the rocker slot needs to be lengthened.

- Valve spring retainer bottom to the top of the valve stem seal at max lift, again .050 is the absolute minimum, more is better. Lack of clearance here requires the top of the guide be machined lower.

- Pushrod to guide clearance, the pushrod can not be allowed to bind in its clearance hole thru the head this should be at least .070 inch as push rods bend a bit, especially with "racing" valve springs.

- Valve spring and retainer clearance to the rocker. These parts cannot be allowed to contact the rocker, I like .070 as these parts are rather mobile when in action and .050 often just isn't enough.

- Rocker to retainer, the Ford uses a pretty deep retaining ridge if you're running the OEM style rocker. Give these at least .050 to the retainer, more if you can safely get it by grinding on the rail not the retainer.

- Valve spring coil to coil clearance, each coil at max lift needs to be at least .050 inch from its neighbor.

Those are pretty much the dimensions you need to check for clearance. In addition you need to check the interface of the rocker's tip on the valve stem as it sweeps thru its arc of movement, this establishes the length of push rod that will be best with the cam you select. The Comp Cams site has a good description of this process. Go here http://www.compcams.com/Technical/FA...inGeometry.asp

Like I said, most engines fit into the production dimensions but many do not resulting from factory production salvage and repairs to aftermarket rebuilds, therefore it isn't a simple task to tell you what works and what doesn't, you'll have to put the parts together, take measurements and make adjustments as necessary.

This is my quick and dirty hp formula:
Crankshaft HP = {(bore area)x(Static Comp Ratio x 14.7psi)x(stroke inches/12 inches)x(cam power peak RPM/2)x(number of cylinders)x("choose one factor" .9 stock street, or .95 performance street, or 1.0 mild race, or 1.1 pro race)}/33,000


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Old 04-05-2010, 06:33 PM
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Can't help you with the specifics on the GT40 head beyond what Bogie has given you, but I have found that Camquest is about 10-12% high, I don't use it to find an actual hp # but use it to compare several cams in the same engine to find the one the gives the best #'s for the rpm range I am looking to run. I feel Desktop Dyno is a little high also, not as bad as the Camquest is, but I don't have a copy of DD, this is just observing #'s that others have put up.
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