Compression

mustangman65_79 · 01-10-2002, 08:22 AM

what kind of compression is a 351C running when the #'s are 195lb per cylinder?

PKRWUD · 01-10-2002, 09:37 AM

It doesn't work that way. Those are very high numbers, though. Break 200psi per cylinder and you'll need race gas.

Take care,
-Chris

Gearhead999 · 01-13-2002, 09:32 AM

They don't mix. Static compression ratio, ie 10 to 1, 9.6 to 1, have no relation to the cranking compression that the engine has. The cam timing can completely change that.

But, 195 lbs sounds a little on the high side. Is the engine new?? Maybe carbon build up. Also, are you checking it dry or wet??

7DMACH1 · 01-13-2002, 02:25 PM

A 351C 4v was 11-1. RAY

MrWesson22 · 01-13-2002, 04:55 PM

As already said, static and cranking compression are independent. A cam with a later intake valve closing will bleed off more compression and have a lower cranking compression even with the same static CR. That said, all you really need to worry about is cranking in determining what octane fuel you need. Witht 195, you can still use 92 octane as long as you watch your timing and listen for detonation. A 351C's stock compression varied as much as 2 or more full points depending on the year and heads. Without knowing the CC of the pistons and heads, there's no way of calculating static CR.

mustangman65_79 · 01-14-2002, 05:50 AM

well, thanks anyhow

About the only thing I can say was that they were brandnew enegines, never used, dry commpression test. They were only being sold for $800.

I wish I had the money.

diablopony · 01-14-2002, 08:53 PM

There is an interesting article in the February Car Craft on this subject. For a small block they are saying that 175 - 185 psi is it for 92-octane gas. On big blocks, lower the psi by 15 or 20.

As noted previously, the cam can change all this and how the cam is degreed in (i.e. retarded for later intake closing).

They also provided a way to calculate static CR if you know your engine's bore, stroke, head gasket volume, cylinder head combustion chamber volume, piston deck height and piston dome volume. I think I'll just use the gauge.

Rev · 01-15-2002, 08:04 PM

My '66 tests at 195 psi or there abouts with a Comp Cams 268 HE. that's 218/218 at .050" lift. It was set up with a 9.5/1 CR. I run a total 45 deg. of advance, all in by 3000.

The reason it needs that much is because I'm using 351W heads which are not really very efficient. When I get my new TFTW heads in (as I type), I'll start with 36 geg. total advance.

So far have had no detonation problems using 92 octane pump gas. Crossing my fingers with these new heads.

Rev

01-10-2002, 08:22 AM	#1
mustangman65_79 My poor 79 RIP Join Date: Oct 2001 Location: Suisun City, Posts: 2,320	Compression what kind of compression is a 351C running when the #'s are 195lb per cylinder? __________________ '84 mustang GT turbo '85 LTD '89 Camaro Irocz z28 '94 Lightning '96 Trans am

01-10-2002, 09:37 AM	#2
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	It doesn't work that way. Those are very high numbers, though. Break 200psi per cylinder and you'll need race gas. Take care, -Chris __________________ Webmaster: Rice Haters Club Jim Porter Racing Peckerwoods Pit Stop Support Your Local RED & WHITE!

01-13-2002, 09:32 AM	#3
Gearhead999 Registered Member Join Date: Sep 1998 Location: Oviedo, FL Posts: 992	They don't mix. Static compression ratio, ie 10 to 1, 9.6 to 1, have no relation to the cranking compression that the engine has. The cam timing can completely change that. But, 195 lbs sounds a little on the high side. Is the engine new?? Maybe carbon build up. Also, are you checking it dry or wet?? __________________ "Support our Troops" Dave 1968 Cougar 2004 Thunderbird 2007 F150 Harley-Davidson, SuperCrew 1986 LTD 1997 Ranger 1992 Honda Civic

01-13-2002, 02:25 PM	#4
7DMACH1 Registered Member Join Date: Nov 2001 Location: PHILA. PA USA Posts: 469	A 351C 4v was 11-1. RAY __________________ Your Way Network VIDEOS www.yourwayvideo.com

01-13-2002, 04:55 PM	#5
MrWesson22 Registered Member Join Date: Sep 2000 Location: Dacula, GA Posts: 73	As already said, static and cranking compression are independent. A cam with a later intake valve closing will bleed off more compression and have a lower cranking compression even with the same static CR. That said, all you really need to worry about is cranking in determining what octane fuel you need. Witht 195, you can still use 92 octane as long as you watch your timing and listen for detonation. A 351C's stock compression varied as much as 2 or more full points depending on the year and heads. Without knowing the CC of the pistons and heads, there's no way of calculating static CR. __________________ Neal 69 stang 351C/4sp

01-14-2002, 05:50 AM	#6
mustangman65_79 My poor 79 RIP Join Date: Oct 2001 Location: Suisun City, Posts: 2,320	well, thanks anyhow About the only thing I can say was that they were brandnew enegines, never used, dry commpression test. They were only being sold for $800. I wish I had the money. __________________ '84 mustang GT turbo '85 LTD '89 Camaro Irocz z28 '94 Lightning '96 Trans am

01-14-2002, 08:53 PM	#7
diablopony Registered Member Join Date: Jan 2001 Location: Clayton, CA USA Posts: 116	There is an interesting article in the February Car Craft on this subject. For a small block they are saying that 175 - 185 psi is it for 92-octane gas. On big blocks, lower the psi by 15 or 20. As noted previously, the cam can change all this and how the cam is degreed in (i.e. retarded for later intake closing). They also provided a way to calculate static CR if you know your engine's bore, stroke, head gasket volume, cylinder head combustion chamber volume, piston deck height and piston dome volume. I think I'll just use the gauge. __________________ '68 convert, 5.0 GT-40 Crate motor, C-4, Tri-Y headers, 3.25 rear gears

01-15-2002, 08:04 PM	#8
Rev Registered Member Join Date: Nov 1998 Location: Houston, Tx. Posts: 3,887	My '66 tests at 195 psi or there abouts with a Comp Cams 268 HE. that's 218/218 at .050" lift. It was set up with a 9.5/1 CR. I run a total 45 deg. of advance, all in by 3000. The reason it needs that much is because I'm using 351W heads which are not really very efficient. When I get my new TFTW heads in (as I type), I'll start with 36 geg. total advance. So far have had no detonation problems using 92 octane pump gas. Crossing my fingers with these new heads. Rev __________________ '66 Coupe, 306, 350-375 HP, C-4, 13.07 e.t., 104.8 mph, 1/4 mi. O.B.C. #2 '66 coupe

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