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Old 08-23-2011, 10:37 AM   #1
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Cumbustion chamber size

I have an 89 5.0 with a stock 302 block and gt 40 p heads on them I'm trying to figure out what size my combustion is I'm looking to bore and stroke my motor to a 347 and have plans for a turbo later so im trying to run 9 to 9.5 to 1

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Old 08-23-2011, 11:33 AM   #2
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The P heads have a 59-61cc combustion chamber.


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Old 08-23-2011, 11:51 AM   #3
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Re: Cumbustion chamber size

MORE INFO - GT40/GT40P Heads
Courtesy of Reinhold Racing

GT40 and GT40P heads are probably the best factory production small block Ford heads. GT40 and GT40P heads have line designations on the end of each casting. 3 lines on the GT-40 and four lines on the GT-40P. GT40 heads are found on 96 and early 97 V8 Explorer/Mountaineers as well as 5.8L Lightnings and 93-95 Cobras. GT-40P heads are found on the 97½ and newer 5 liter Explorer/Mountaineer V8s.
  • The GT40P replaced the standard GT40 on the V8 Explorers and Mountaineers in early 1997. Prior Explorers and Mountaineers had Cobra GT40 heads that were milled to bring it's larger combustion chamber down to the size of the E7 heads to keep compression up.
  • The 5.0 explorer intake was the same from 96-01. These intakes are spin offs of the Cobra Intake.
  • The 5.0 explorer throttle body is 65mm and will fit a Mustang but you must swap out the throttle shaft and arm from the Mustang throttle body.
  • GT40 and GT40P share similar intake runner volumes at about 145cc compared to E7's 124cc. All accessory bolt locations and valve train geometry are the same between the GT40P and GT40.
  • 1.85/1.46 valves compared to 1.85/1.54 for the standard GT-40. The GT40P exhaust port is redesigned and is more efficient than the standard GT40 which is the reason for the smaller exhaust valve. There is room for larger valves but keep in mind going larger on the intake side will cause clearance problems with the spark plug boss.
  • GT40P combustion chambers range from 59cc to 61cc based on factory machining tolerances. GT40 combustion chambers are roughly 65cc.
  • Small emissions-type bump in exhaust port similar to E7TE casting but not as large. This bump is in the center on the port roof right at the exit and can be easily removed.
  • No thermactor holes in the GT40P heads
  • Intake port dimensions are close to a Felpro 1250 gasket.
  • GT40P can use stock pedestal mount rockers.
  • On GT40P heads the spark plug hole is moved down and rotated out which puts the electrode closer to the center of the combustion chamber. providing a better burn, reducing emissions and increasing efficiency. The GT40P does not use standard 5.0L spark plugs. GT40P uses Motorcraft AWSF-32C.
  • Standard unequal length shorty headers won't work because of the GT40P spark plug location. MAC offers a coated GT40P-specific shorty and Ford Racing offers a set of stainless shorties that work. Stock Mustang headers work also. Most equal length shorties will not work. BBK equal length shorty headers have been known to fit. MAC, Hedman and BBK full length 1 5/8" headers fit. Some full length headers require minor clearancing on some primary tubes but is not a big issue. Spark plug wires with 90 degree boots are recommended with all headers.
  • TFS-2500100 valve spring upgrade kit comes with 1.47 springs, retainers, locks and valve seals. A dual spring will NOT fit a GT40 head w/o machining to the inner part of the cup but a basic 1.46/1.47" single spring will fit, There is an inner "lip" that is about .060" tall that most likely will have to be machined flat. You can also go a step further machine the guide too and use the 1.300" LSX dual springs.
  • Gasket matching does not net any significant increases in airflow on these heads. These heads are sensitive to the work in the bowl area (including the guide boss) and the short turn. A good 5 angle valve job is a good idea too. If you are not a head porter, don't try to be one. P heads are easy to screw up so leave the work to a professional. If you want to get carried away, a 1.94 intake valve and 1.50 exhaust valve will help flow a bit. Typical peak flow numbers on a well done set of P heads is about 230 cfm on the intake and 160 cfm on the exhaust at .500 lift.
  • GT40P heads are similar to GT-40 heads with the biggest difference being the P-heads feature a revised spark plug angle which extends the plug into the combustion chamber, putting the flame front close to the absolute center of the chamber, resulting in a smoother combustion process. P-heads have a smaller combustion chamber (approximately 59cc), which will increase the compression ratio over E7TE heads.
Head intake port combustion chamber intake valve exhaust valve
E7TE 124cc 61cc 1.78 1.46
GT40P 145cc 59cc 1.85 1.46
GT40-Explorer 145cc 65cc 1.85 1.54
GT40-Cobra 145cc 61cc 1.85 1.54
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