Can someone explain Compression Ratios???
Thread Starter
Melting Slicks
Joined: Jan 2002
Posts: 3,419
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From: Arlington TX
Can someone explain Compression Ratios???
I see everybody talking about the different compression ratios they're running and I have no idea what they mean...from what I can tell if you are running all engine apps some ratios are better than others and if you are running power adders another ratio is better....can someone explain this to me? Thanks.
Le Mans Master
Joined: Apr 1999
Posts: 6,129
Likes: 14
From: Vette
Re: Can someone explain Compression Ratios??? (los7883)
The higher the CR the more power/tq you make.
However, the higher the CR the higher octane gas you will be required to run (assuming same or similar tuning)
This is true for both NA and FI applications.
The thing with FI applications is - you’ll make A LOT more power with lower CR and more boost then with more CR and less boost. :yesnod: There’s only “that” much boost you can run on a given CR, given octane.
Because of that you’ll see people always wanting to lower CR when running boost.
Hope this clears it up a bit.
However, the higher the CR the higher octane gas you will be required to run (assuming same or similar tuning)
This is true for both NA and FI applications.
The thing with FI applications is - you’ll make A LOT more power with lower CR and more boost then with more CR and less boost. :yesnod: There’s only “that” much boost you can run on a given CR, given octane.
Because of that you’ll see people always wanting to lower CR when running boost.
Hope this clears it up a bit.
Thread Starter
Melting Slicks
Joined: Jan 2002
Posts: 3,419
Likes: 0
From: Arlington TX
Re: Can someone explain Compression Ratios??? (EuG)
The higher the CR the more power/tq you make.
However, the higher the CR the higher octane gas you will be required to run (assuming same or similar tuning)
This is true for both NA and FI applications.
The thing with FI applications is - you’ll make A LOT more power with lower CR and more boost then with more CR and less boost. :yesnod: There’s only “that” much boost you can run on a given CR, given octane.
Because of that you’ll see people always wanting to lower CR when running boost.
Hope this clears it up a bit.
However, the higher the CR the higher octane gas you will be required to run (assuming same or similar tuning)
This is true for both NA and FI applications.
The thing with FI applications is - you’ll make A LOT more power with lower CR and more boost then with more CR and less boost. :yesnod: There’s only “that” much boost you can run on a given CR, given octane.
Because of that you’ll see people always wanting to lower CR when running boost.
Hope this clears it up a bit.
Le Mans Master
Joined: Apr 1999
Posts: 6,129
Likes: 14
From: Vette
Re: Can someone explain Compression Ratios??? (los7883)
So the higher the compression ratio the better for a NA motor?
And the lower the CR the more boost you can use?
Le Mans Master
Joined: Apr 1999
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From: Burlington ON
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Re: Can someone explain Compression Ratios??? (TexasRedY2K)
Compression ratio is the ratio of the volume of air your piston's movement from top dead center to bottom dead center displaces versus the volume of air above the piston when it is at top dead center.
The less room above the piston, the higher the compression ratio, the more power the car makes but the higher octane fuel it requires to not predetonate.
The less room above the piston, the higher the compression ratio, the more power the car makes but the higher octane fuel it requires to not predetonate.
Thread Starter
Melting Slicks
Joined: Jan 2002
Posts: 3,419
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From: Arlington TX
Re: Can someone explain Compression Ratios??? (los7883)
So what is the highest compression ratio you could run on a stocked or cammed car? How would you be able to check this...is there some kind of equipment you would have to use or is it all math...??? Thanks for the answers so far...learning a lot...keep 'em coming.
Le Mans Master
Joined: Apr 1999
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From: Burlington ON
Cruise-In I, II, III, IV & V Veteran
CI-I & II Auto-X Champ
Re: Can someone explain Compression Ratios??? (los7883)
The highest compression ratio I would recommend running in a street car where only 92 or 93 octane is available is 10.8 to 1... If you want to ALWAYS run 94 octane and maybe mix in some race gas when at the track, you can go to 11.0 or 11.5 to 1.. 10.8 is about the highest you can go by shaving LS1 head castings.. 11.0 is about the highest you can go by shaving LS6 castings... To get higher than 11.0 you have to go with 5.3L truck head castings which have a smaller chamber.. If you are changing out pistons then you can pretty much get whatever desired compression ratio you want.
There are tools that are used to figure out compression ratio but it is usually done by mathematics using the deck height of the block, the chamber size of the heads and the compression height of the pistons...
My motor is 11.5 and it is right on the edge with 94 octane.. When I race it, I make sure to mix in some 105 or 110 octane to make sure detonation doesnt occur.. If it is a pure street car, I would recommend keeping it under 11.0..
There are tools that are used to figure out compression ratio but it is usually done by mathematics using the deck height of the block, the chamber size of the heads and the compression height of the pistons...
My motor is 11.5 and it is right on the edge with 94 octane.. When I race it, I make sure to mix in some 105 or 110 octane to make sure detonation doesnt occur.. If it is a pure street car, I would recommend keeping it under 11.0..
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Safety Car
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St. Jude Donor '09
Re: Can someone explain Compression Ratios??? (los7883)
LOS7883-thanks for a fun post. It always amazes me how many different ways (angles) Forum members can look at a subject and provide GREAT examples.
Thanks to ALL! :cool: :cool: :cheers:
Thanks to ALL! :cool: :cool: :cheers:
Burning Brakes
Joined: Aug 2002
Posts: 1,045
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From: KC, MO Missouri
Re: Can someone explain Compression Ratios??? (MattG)
CR takes a lot of factors to calculate the max. Cam LSA is very important. If you have a 106 lsa like the ASA cam you can run more compresion. You have to for the cam to perform properly and make good power. The lobe seperation bleeds off cylinder pressure. Cylinder pressure is also felt as torque. A 116lsa will detontate sooner at 11 to 1 than 112 lsa will. Hope that keeps it simple. The Z06 cam is 117.5 lsa so you really have to watch high compression there and take some timing out. There is a lot of cylinder pressure present there!
Advanced
Joined: Sep 2002
Posts: 65
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From: Effingham IL
Re: Can someone explain Compression Ratios??? (MattG)
Going with GTP heads/ TR224 112. I believe the compression ratio is 10.8:1. Has anyone had detonation issues with these heads running on pump gas. This is a week end driver.
Greg
Greg
Thread Starter
Melting Slicks
Joined: Jan 2002
Posts: 3,419
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From: Arlington TX
Re: Can someone explain Compression Ratios??? (Villain)
I enjoyed reading this thread. Lots of good information. You cats seem to know your stuff. :cheers:
Well If I have a stock block with a 220/.561 114 cam, more than likely what would my CR be? Or is this not enough information? (ballpark figure)
Drifting
Joined: Jul 2002
Posts: 1,613
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From: Spring Tx
Re: Can someone explain Compression Ratios??? (los7883)
Your Cr doesn't change when you change cams. You gain power on the intake side by keeping the valve open longer allowing more air /fuel into the cylinder. On the exhaust side more duration get the exhaust out more comepletely.a lower Lobe separation angle , which we oldtimers used to call overlap promotes better exhaust gas scavaging, thus allowing a purer fuel/air mixture which makes more power. Lift is the height the valve moves when open, of course higher is better, also you have to take into consideration ramp rates which is how quickly the valve reaches its full open height. Have you got this straight? There will be a quiz...
