max dynamic compression for 93 octane gas??
Thread Starter
Drifting
Joined: Apr 2003
Posts: 1,379
Likes: 4
From: rome ny
max dynamic compression for 93 octane gas??
im in the process of building a 396 ci for my car.. the cam im going to with will determine what static compession i need. does anyone know the max dynamic compession you can run on pump gas? static compession is really not the final factor on what fuel you need to run. i know cranking is important also.
Burning Brakes
Joined: Feb 2003
Posts: 1,246
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From: SouthBend IN
I've heard and read that 8.6 is MAX DCR with good combustion chambers and a tight quench. Mine is at 8.2 and i can run 89 octane with 33 total. I have to run 92 with 35 deg timing. Error a little on the safe side, as all gas is not equall and you might have to get gas at an unfilimar station
Thread Starter
Drifting
Joined: Apr 2003
Posts: 1,379
Likes: 4
From: rome ny
my current 350ci engine has a static compression of 10.7/1 but with my cam closing the intake at 48.2* my dynamic cr is about 9.4 /1 . to me it sounds a little high..
with my new engine with static cr around 11.1/1 and a cam that closes around 50, my dynamic would be 9.8/1.. that worries me..
with my new engine with static cr around 11.1/1 and a cam that closes around 50, my dynamic would be 9.8/1.. that worries me..
Le Mans Master
Joined: Feb 2000
Posts: 9,715
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From: Kempner Texas
From a lot of simulations I've run and from what I've read, 8.5 is the max you should see on a pump gas engine.
More than that and you'd have to start pulling timing which can hurt over-all power and fuel economy.
Jake
More than that and you'd have to start pulling timing which can hurt over-all power and fuel economy.
Jake
Race Director
Joined: May 2001
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Just curious, what are you guys using to calculate dynamic compression ratio? Obviously the static ratio will play some role as well as the cam profile, chambers, etc. I'd like to run mine through out of curiosity.
Race Director
Joined: Sep 2000
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From: The Top of Utah
Originally Posted by mos90
my current 350ci engine has a static compression of 10.7/1 but with my cam closing the intake at 48.2* my dynamic cr is about 9.4 /1 . to me it sounds a little high..
with my new engine with static cr around 11.1/1 and a cam that closes around 50, my dynamic would be 9.8/1.. that worries me..
with my new engine with static cr around 11.1/1 and a cam that closes around 50, my dynamic would be 9.8/1.. that worries me..
RACE ON!!!
Thread Starter
Drifting
Joined: Apr 2003
Posts: 1,379
Likes: 4
From: rome ny
http://www.smokemup.com/
i am told that dynamic cr is only part of the equation.. many factors play a role. i know im not seeing any detonation in my car, i run 93 octane and 34* timing.. if i had a smaller cam my drc would be even higher.. and 10.8/1 cr is not uncommon at all..
you need to factor in crank psi, combustion chamber efficiency, drc cam selection reguarding overlap..ect ect...
i am told that dynamic cr is only part of the equation.. many factors play a role. i know im not seeing any detonation in my car, i run 93 octane and 34* timing.. if i had a smaller cam my drc would be even higher.. and 10.8/1 cr is not uncommon at all..
you need to factor in crank psi, combustion chamber efficiency, drc cam selection reguarding overlap..ect ect...
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Thread Starter
Drifting
Joined: Apr 2003
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From: rome ny
Originally Posted by CFI-EFI
Your static and dynamic compression ratios don't square up. I think you are entering a .050" lift duration, rather than a seat to seat (or advertised) duration. My cam has a .050" intake closing point of 50* ABDC. I can duplicate the numbers you've posted, but they are incorrect. The .050" "closing point isn't a closing point. There is plenty of air flow at .050" lift, therefore it is an invalid point to use for figuring the dynamic compression ratio. My cam with the 50* ABDC closing @ .050" lift, has an advertised closing point of 87* ABDC. That number generates a dynamic CR of 7.044:1. Double check your arithmetic. I think you will see what I'm talking about.
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my cam card just says valve close 48.2 abdc. im not sure if its at .050..
Race Director
Joined: Sep 2000
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From: The Top of Utah
I am quite sure I'm right. I took the time to "duplicate" the numbers you posted. Your numbers "work out", only if the intake closing of 50* ABDC is used. That would be a VERY mild cam, indeed. Look at the instructions for the "Smokem Up" dynamic CR calculator. It states to use the advertised number. If you think about it, the .050" number really has nothing to do with sealing the cylinder. It is MY recollection, that a DCR of 7.8:1 is pretty high for pump gas for street use, but I'd like to see more discussion on that. Take a closer look at your cam card. It HAS to indicate advertised or .050" duration.
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Melting Slicks
Joined: Oct 2002
Posts: 3,193
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From: Cincinnati Oh
http://members.uia.net/pkelley2/DynamicCR.html
I've used this and it's excellent for Dynamic, Static and quench info. It can be a little daunting at first, but just go through and punch in your info and it will give you what you need.
edit - it will calculate your seat-to-seat degrees using the info from your cam card. Just make sure you plug in the right numbers. You'll wind up in the mid 7.XX range most likely.
the help instructions are also excellent. I consider 8.4 as a max DC for street - but I know nothing of the subject and am simply repeating what I remember from my internet searches. I'll personally use 8.2 as a max just to be safe.
I've used this and it's excellent for Dynamic, Static and quench info. It can be a little daunting at first, but just go through and punch in your info and it will give you what you need.
edit - it will calculate your seat-to-seat degrees using the info from your cam card. Just make sure you plug in the right numbers. You'll wind up in the mid 7.XX range most likely.
the help instructions are also excellent. I consider 8.4 as a max DC for street - but I know nothing of the subject and am simply repeating what I remember from my internet searches. I'll personally use 8.2 as a max just to be safe.
Last edited by Ramanstud; May 27, 2005 at 05:10 PM.
Race Director
Joined: Sep 2000
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From: The Top of Utah
Originally Posted by mos90
duration is at .050... how do i convert?
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