Compression Ratio and Octane
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Joined: May 2003
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From: Ben Lomond, Ca, USA
Re: Compression Ratio and Octane (Mi2age)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Mi2age »</TD></TR><TR><TD CLASS="quote">I was wondering if anyone has a table or a chart the lists what octane (preferrably PON) must be used with different compression ratios.
Thanks in advance</TD></TR></TABLE>
No chart can be made because the static compression ratio has no effect on what Octane fuel the engine must run.
No pressure can be built in the cylinder until both valves are closed. The static compression ratio is calculated assuming that at (BDC) Bottom Dead Center both the intake and the exhaust valves are closed. This is the ratio you get when you 'CC' the combustion chambers with a burette or, less accurately, what the piston maufacturer has labeled it's pistons (i.e.: 10:1, 10.5:1, etc.).
In reality the engine can not begin building pressure in the cylinder until all the valves are closed. On the start of the compression stroke the intake valves are still open allowing air to escape and not let the pressure in the cylinder to build above atmospheric. The intake valves don't close for a while after BDC, say 60 degrees for example. The engine can not begin compressing the air until the intake valves have closed at that point. The compression ratio that is measured from the point the intake valves close and then up to (TDC) Top Dead Center is called the effective compression ratio.
For a well designed 4-valve aluminum block and head engine that ratio will be about 8.5:1. An iron block with aluminum head is a little lower at approximately 8.0:1. A 2-valve cast iron block and head engine is even lower at 7- 7.5:1. These numbers are approximate but it should show you that engines like Hondas are able to run higher effective ratios than the old small block Chevys and such because the aluminum helps draw away heat and prevent detonation.
When you have a higher compression ratio you need a cam with more duration. The purpose of increasing the duration is to increase the RPM range at which the engine operates (as well as to give it more overlap and generally make it breath better but I'm not going into depth about that). To compensate for the higher static compression ratio, the intake valves are kept open longer, say 75 degrees after BDC. This keeps the effective compression ratio in that desired ~8.0:1 range. If it exceeds this the engine will detonate. If it is less, the engine is not being used to its full potentional.
A good cam grinder will work with you to choose the right cam to suit your engine. Blindly picking a cam out of a catalog may or may not cause you big headaches so it is always recommended to go with your grinders recommendation and don't listen to marketing hype. Bigger is not better. If your effective compression ratio is low your HP and Torque will be down from the engines potential period.
The myth that compression ratio is directly related to Octane number requirement has been around for a long time ago and is probably mostly to blame on the magazine editors from the 70's after gas quality took a dive and they couldn't explain why cars now pinged. From what little information they had they knew it had something to do with compression ratio but they didn't know exactly what so this myth developed. The editors of magazines usually have journalism degrees and were hobbyist who got into the business so unfortunately there is a lot of bad info (theories) being spread around. And because magazines are read so widely it gets spread around a lot.
Thanks in advance</TD></TR></TABLE>
No chart can be made because the static compression ratio has no effect on what Octane fuel the engine must run.
No pressure can be built in the cylinder until both valves are closed. The static compression ratio is calculated assuming that at (BDC) Bottom Dead Center both the intake and the exhaust valves are closed. This is the ratio you get when you 'CC' the combustion chambers with a burette or, less accurately, what the piston maufacturer has labeled it's pistons (i.e.: 10:1, 10.5:1, etc.).
In reality the engine can not begin building pressure in the cylinder until all the valves are closed. On the start of the compression stroke the intake valves are still open allowing air to escape and not let the pressure in the cylinder to build above atmospheric. The intake valves don't close for a while after BDC, say 60 degrees for example. The engine can not begin compressing the air until the intake valves have closed at that point. The compression ratio that is measured from the point the intake valves close and then up to (TDC) Top Dead Center is called the effective compression ratio.
For a well designed 4-valve aluminum block and head engine that ratio will be about 8.5:1. An iron block with aluminum head is a little lower at approximately 8.0:1. A 2-valve cast iron block and head engine is even lower at 7- 7.5:1. These numbers are approximate but it should show you that engines like Hondas are able to run higher effective ratios than the old small block Chevys and such because the aluminum helps draw away heat and prevent detonation.
When you have a higher compression ratio you need a cam with more duration. The purpose of increasing the duration is to increase the RPM range at which the engine operates (as well as to give it more overlap and generally make it breath better but I'm not going into depth about that). To compensate for the higher static compression ratio, the intake valves are kept open longer, say 75 degrees after BDC. This keeps the effective compression ratio in that desired ~8.0:1 range. If it exceeds this the engine will detonate. If it is less, the engine is not being used to its full potentional.
A good cam grinder will work with you to choose the right cam to suit your engine. Blindly picking a cam out of a catalog may or may not cause you big headaches so it is always recommended to go with your grinders recommendation and don't listen to marketing hype. Bigger is not better. If your effective compression ratio is low your HP and Torque will be down from the engines potential period.
The myth that compression ratio is directly related to Octane number requirement has been around for a long time ago and is probably mostly to blame on the magazine editors from the 70's after gas quality took a dive and they couldn't explain why cars now pinged. From what little information they had they knew it had something to do with compression ratio but they didn't know exactly what so this myth developed. The editors of magazines usually have journalism degrees and were hobbyist who got into the business so unfortunately there is a lot of bad info (theories) being spread around. And because magazines are read so widely it gets spread around a lot.
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ndlyng
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Jul 24, 2006 07:27 AM
