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#92894 - 11/22/17 08:12 PM What cam grind do I need for a blown GMC 302?
Art Offline
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Registered: 09/23/17
Posts: 3
Loc: Washington State
I have a 471 blower on a 302 GMC. Previous owner says it could use a custom grind on the cam. Looking for suggestions. Engine will be street driven 4500 MAX RPM. Static compression ratio is 7.8-1. Blower will run 1 : 1 or even 5% under giving max boost at 4 to 5 lbs. Should be about 10.25 to one compression ratio at max rpm. Any ideas?

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#92895 - 11/23/17 07:01 AM Re: What cam grind do I need for a blown GMC 302? [Re: Art]
panic Offline
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Registered: 01/11/05
Posts: 1496
Loc: Hempstead, Long Island, NY
"about 10.25 to one compression ratio at max rpm"

It's always going to be 7.8:1 compression at every RPM.

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#92897 - 11/23/17 10:51 AM Re: What cam grind do I need for a blown GMC 302? [Re: panic]
mdonohue05 Offline
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Registered: 10/31/09
Posts: 318
Loc: pennsylvania
One of the better old school cam guys is Jerry Cantrell, long time owner of Schneider Cams in San Diego. He has ground my last two Cams and they have been terrific. You get to talk to the boss, tell him what you have what you want to do and he will help you figure out an appropriate grind and grind you a cam. Costs a speck more then one from an order taker and a catelogue but with your combo, you will want good advice instead of guess work.

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#92898 - 11/23/17 12:00 PM Re: What cam grind do I need for a blown GMC 302? [Re: panic]
Art Offline
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Registered: 09/23/17
Posts: 3
Loc: Washington State
My bad, should have said "Final Compression Ratio" FCR

FCR=[(boost/14.7)+1] x CR

CR + static compression ratio
14.7= air pressure at sea level

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#92899 - 11/23/17 12:01 PM Re: What cam grind do I need for a blown GMC 302? [Re: mdonohue05]
Art Offline
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Registered: 09/23/17
Posts: 3
Loc: Washington State
mdonohue05: Thanks !

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#92900 - 11/24/17 07:32 AM Re: What cam grind do I need for a blown GMC 302? [Re: Art]
panic Offline
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Registered: 01/11/05
Posts: 1496
Loc: Hempstead, Long Island, NY
I understand how you calculated it, but the compression ratio does not change unless one of the two volumes (cylinder, combustion chamber) changes.
The answer you got doesn't tell you anything, don't bother with "dynamic compression".
The actual cranking pressure is only distant related to that calculation.
More information in my article here: http://victorylibrary.com/mopar/blower/blower-engine5.htm
Look for "Compression ratio"

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#92901 - 11/24/17 08:46 AM Re: What cam grind do I need for a blown GMC 302? [Re: Art]
70Nova Offline
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Registered: 05/06/11
Posts: 374
Loc: Va
there are too many variances and variables at play, just from poor tolerances inherent to mass production ( with 60's tooling?) and possible repairs done in the past like milling the deck or head etc....or head came from a different engine.. there are head gaskets with different thicknesses, crush ratios... and maybe biggest of all, worn or aftermarket cams with unknown values for LSA, timing and duration. I have built a "few" engines, I don't guess, or take anyone's word for anything. I measure everything so nothing will bite me back later. Still often takes longer to "figure it out" than to actually build it. Seems like there are a lot of (*ahhem*american*cough cough*) builders out there that just drop parts into an engine and make it bigger, and they are happy because it (feels like it) is making more power. At least more noise. laugh Then they proudly preach how its the best way to do it.

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