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Old 09-04-2011, 10:36 PM
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5C or 670 Heads??

Hello everybody! Been lurking around here for a couple of weeks and finally have a good question. Im just starting my pontiac build and the engine in it is a 400 75" block and 5C heads. I am trying to build a raunchy weekend driver with a CR of about 9.5. to run on pump gas. Would milling the 5C's and a .042 gasket give me the cr and performance im looking for. Or i could buy some 670 heads for $600.00. and get dished pistons to lower the CR. The 670's also have 1.77 valvles instead of the 5c's 1.66. Which option would give me the better performance?
Any feedback is appreciated! Thanks in advance!

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Old 09-05-2011, 03:45 PM
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The head charts and CR calculators will be a good place to start. Don't rely on the charts for exact chamber cc measurements to get exact volume you will have to perform a chamber measurement. You can fill the chamber with water or clay to get the ccs. Be sure you do this after a valve job since a valve job will add chamber volume.

If possible try to use a head with screw-in rocker studs. Use the CR calculator to determine compression ratio. You can also factor in head milling or having the block decked. Stock deck height has always been 0.023" throughout the production of the Pontiac engine. You can have the deck height milled to 0.0". This allows the head gasket to control quench. Just something else to consider.

This should give you a good start on head selection. The last link shows head flow numbers.



Last edited by beertracker; 09-05-2011 at 03:54 PM. Reason: added head flow chart
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Old 09-05-2011, 04:00 PM
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X2 what BT said there.
I used 7H1 heads with port work and 1.77 exhausts installed, and milled to 84 CC on my 389/400
static is at 9.3 to 1, oh yeah, my block is zero decked as BT also mentioned.
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