high compression
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Joined: Aug 2005
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From: Sweetwater, TN, USA
high compression
I was wondering what is the highest possible compression that is obtainable on 93 octane fuel? Im looking at getting a honda and building a all motor CRX. I would like to use a 14-15:1 CR pistons. Or is this too high of CR for 93 octane?
-Thomas
-Thomas
Honda-Tech Member
Joined: May 2003
Posts: 1,543
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From: Ben Lomond, Ca, USA
Re: high compression (YF-19)
What are you planning to do with this engine? Race only I would hope.
How high do you plan to rev this engine? Compression ratio is directly related to the RPM range the engine runs in. The higher the compression ratio you run the more duration you will need to run on the cam. This is because the static compression ratio is not the one the engine sees. There are three types of compression ratios. Static, effective, and dynamic.
Static compression ratio: chamber volume + swept volume / chamber volume. In your case you are looking at 14-15:1
Effective compression ratio (this is the important one): The compression ratio that is measure begining at the point when the intake valve closes. If you think about it you can't begin building compression in the cylinder until you close the intake valve, otherwise it will just blow back into the intake. On a modern 4-valve engine running 91-93 AKI (octane) fuel the effective compression ratio can be somewhere around 8 to 8.5:1. Two valve heads can run about 7.5-8:1.
Dynamic compression ratio: Effective compression ratio x volumetric efficiency. There is 1 point in the rpm band (<U>peak torque</U>) where volumetric efficiency is highest. At lower rpm the incoming air/fuel charge does not have enough inertia to overcome the force of the piston coming back up on the compression stroke and some is forced back out the intake valve, but as rpm builds and the velocity of the A/F charge increases it is able to resist this tendancy and can almost fill the cylinder to 100% capacity (but it rarely does except on extremely well prepared engines, contrary to popular belief).
So for an engine that runs a compression ratio that high you are going to have to spin the engine pretty damn fast (say 10,000 minimum) to make any power. At those speeds you better have a valvetrain that can handle that RPM, not to mention having a stout bottom end.
-Scott Tucker
How high do you plan to rev this engine? Compression ratio is directly related to the RPM range the engine runs in. The higher the compression ratio you run the more duration you will need to run on the cam. This is because the static compression ratio is not the one the engine sees. There are three types of compression ratios. Static, effective, and dynamic.
Static compression ratio: chamber volume + swept volume / chamber volume. In your case you are looking at 14-15:1
Effective compression ratio (this is the important one): The compression ratio that is measure begining at the point when the intake valve closes. If you think about it you can't begin building compression in the cylinder until you close the intake valve, otherwise it will just blow back into the intake. On a modern 4-valve engine running 91-93 AKI (octane) fuel the effective compression ratio can be somewhere around 8 to 8.5:1. Two valve heads can run about 7.5-8:1.
Dynamic compression ratio: Effective compression ratio x volumetric efficiency. There is 1 point in the rpm band (<U>peak torque</U>) where volumetric efficiency is highest. At lower rpm the incoming air/fuel charge does not have enough inertia to overcome the force of the piston coming back up on the compression stroke and some is forced back out the intake valve, but as rpm builds and the velocity of the A/F charge increases it is able to resist this tendancy and can almost fill the cylinder to 100% capacity (but it rarely does except on extremely well prepared engines, contrary to popular belief).
So for an engine that runs a compression ratio that high you are going to have to spin the engine pretty damn fast (say 10,000 minimum) to make any power. At those speeds you better have a valvetrain that can handle that RPM, not to mention having a stout bottom end.
-Scott Tucker
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