Cranking compression..Pros/Cons
Thread Starter
Melting Slicks
Joined: Feb 2000
Posts: 3,299
Likes: 0
From: Beltsville Maryland
Cranking compression..Pros/Cons
What are the Pros and Cons of having a high cranking compression? I have a 14.5 to 1 SBC NA motor.
I'm going to try and lower the compression by adding more duration to the cam and installing a thicker head gasket.
Does anyone have any experience with having 300+ psi readings?
I'm going to try and lower the compression by adding more duration to the cam and installing a thicker head gasket.
Does anyone have any experience with having 300+ psi readings?
Burning Brakes
Joined: May 1999
Posts: 760
Likes: 1
From: Greenwich, CT
In my experience it's important to consider the Dynamic CR limit rather than just the
Static CR.
There's a nifty tool here for the DCR calculation (along with some design discussion):
http://cochise.uia.net/pkelley2/
Most of the discussion seems to be taken from Dave Vizard, so I figure it has a fairly
solid experimental basis. It also matches with what I've been able to measure on a
limited number of cams in my SB.
Static CR.
There's a nifty tool here for the DCR calculation (along with some design discussion):
http://cochise.uia.net/pkelley2/
Most of the discussion seems to be taken from Dave Vizard, so I figure it has a fairly
solid experimental basis. It also matches with what I've been able to measure on a
limited number of cams in my SB.
Thread Starter
Melting Slicks
Joined: Feb 2000
Posts: 3,299
Likes: 0
From: Beltsville Maryland
SG4206, many are telling me that the 300+ cylinder pressure is too high and should be more like 230-260psi.
Dr.J , thanks for the link. The DCR is what I'am concerned about. I'm in the process of installing a larger duration cam with a later closing intake valve.
Dr.J , thanks for the link. The DCR is what I'am concerned about. I'm in the process of installing a larger duration cam with a later closing intake valve.
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Pro
Joined: Oct 2004
Posts: 510
Likes: 5
From: Pipe Creek TX
I would dyno the engine as is ; then lower compression and re- dyno. This is the only way to find out if your theory on the psi for this combination is correct.
The biggest mistake I see people make; is assuming what works on Mr. Go Fast engine ,will work on my combination too. The dyno is the best tool to compare combinations and standardize the results.
ET's only indicate how efficiently you put the power to the ground. MPH is how much power your combination is making.
Good luck with your new toy regardless.
The biggest mistake I see people make; is assuming what works on Mr. Go Fast engine ,will work on my combination too. The dyno is the best tool to compare combinations and standardize the results.
ET's only indicate how efficiently you put the power to the ground. MPH is how much power your combination is making.
Good luck with your new toy regardless.
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
To elaborate a bit on the quench comments. A close collision between the piston and the flat portion of the head creates turbulence in the combustion chamber which promotes a faster burn that suppresses detonation, and makes more power with less spark lead. This is the "quench area". "Chevrolet Power" recommends .035" -.040", minimum. Some people go tighter than that. The benefits of a tight quench area diminishes as the clearance increases. At about .060" all benefits are gone. If your piston to head clearance is already near .060", a thicker head gasket won't hurt much. If you DO have a good tight quench area, a thicker head gasket is likely to aggravate a detonation condition, through the loss of the benefits of quench, more that the decrease in compression ratio will help it.
RACE ON!!!
RACE ON!!!
Last edited by CFI-EFI; Oct 23, 2004 at 12:12 PM.
Thread Starter
Melting Slicks
Joined: Feb 2000
Posts: 3,299
Likes: 0
From: Beltsville Maryland
Dyno numbers would definitely be the way to handle this situation. I also meant to mention that looking at the MPH was an omportant factor for track testing. Thanks SG (Super Gas ????)
CFI, thanks for your input. I'm debating whether or not to installing the thinner head gaskets (.041) with the newly installed larger (duration) cam.
CFI, thanks for your input. I'm debating whether or not to installing the thinner head gaskets (.041) with the newly installed larger (duration) cam.
Thread Starter
Melting Slicks
Joined: Feb 2000
Posts: 3,299
Likes: 0
From: Beltsville Maryland
I put the heads back on with the same thickness gasket. Checked the cyl. pressure and it dropped from ~340 to 255 psi. 
I will break the new cam in with the other spring only. Then it's back to the track next weekend.
Thanks
I will break the new cam in with the other spring only. Then it's back to the track next weekend.Thanks
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by wheelsup
CFI, thanks for your input. I'm debating whether or not to installing the thinner head gaskets (.041) with the newly installed larger (duration) cam.
RACE ON!!!
Pro
Joined: Oct 2004
Posts: 510
Likes: 5
From: Pipe Creek TX
With a zero deck , I would use the .041 gasket to minimize detonation. Like Comp and CFI has said ; too much total clearance promotes problems.
Yeh , SG4206 is my NHRA comp number. Like most of the door slammers in this class; we can run SC also. Actually my '82 is more consistent at 8.90 than 9.90 I just don't like being chased all the time ; but it does feel real good to take out the dragsters. If your a Super Pro racer with your vette ; you know what I mean.
Yeh , SG4206 is my NHRA comp number. Like most of the door slammers in this class; we can run SC also. Actually my '82 is more consistent at 8.90 than 9.90 I just don't like being chased all the time ; but it does feel real good to take out the dragsters. If your a Super Pro racer with your vette ; you know what I mean.
Thread Starter
Melting Slicks
Joined: Feb 2000
Posts: 3,299
Likes: 0
From: Beltsville Maryland
CFI, you understood correctly. I WAS thinking of installing a thicker gasket, because it looked like the bigger cam was not bleeding off enough cyl. pressure with the original thin gasket. . It turned out that a lubicate used to retard cyl rust while the motor was apart was giving us a false reading with the new cam.
Burning Brakes
Joined: Feb 2003
Posts: 1,246
Likes: 1
From: SouthBend IN
Originally Posted by wheelsup
I put the heads back on with the same thickness gasket. Checked the cyl. pressure and it dropped from ~340 to 255 psi. I will break the new cam in with the other spring only. Then it's back to the track next weekend.
Thanks
I will break the new cam in with the other spring only. Then it's back to the track next weekend.Thanks
Mike
Drifting
Joined: Dec 1999
Posts: 1,462
Likes: 1
From: Reno nevada
SG4206 - I went to the Vegas NHRA just a few days ago and they had 200 entries in S/C. You get so tired of dragster after dragster.
Wheelups - Personaly from what I've seen in race motors they seem to shift over to Methanol with the very high compression motors like yours because the pressure wave of alki is less abusive to the bearings and the motors run so much cooler.
Wheelups - Personaly from what I've seen in race motors they seem to shift over to Methanol with the very high compression motors like yours because the pressure wave of alki is less abusive to the bearings and the motors run so much cooler.