bajardine
New member
I have a question about figuring compression ratios. From what I have read on this site to calculate the compression ratio you add the displacement of the cylinder plus the compression chamber volume and divide by the chamber volume. For example on the Viper you have two dome sizes 24 cc and 21 cc so here are my figures:
24 cc
696 cc engine divide by 3=232 cc’s per cylinder + 24 cc = 256 cc divide by 24 cc = 10.67
21 cc
696 cc engine divide by 3=232 cc’s per cylinder + 21 cc = 253 cc divide by 21 cc = 12.05
So my calculations show 10.67 to 1 and 12.05 to 1 but according to Yamaha documents, the compression ratios for the Viper are 5.8 to 1 and 6.6 to 1 respectively. I would tend to trust the Yamaha specs. So what am I doing wrong? I know there is a volume I’m not accounting for in the deck height space between the top of the piston and the combustion chamber but I’m sure it can’t be that much. Any help here?
24 cc
696 cc engine divide by 3=232 cc’s per cylinder + 24 cc = 256 cc divide by 24 cc = 10.67
21 cc
696 cc engine divide by 3=232 cc’s per cylinder + 21 cc = 253 cc divide by 21 cc = 12.05
So my calculations show 10.67 to 1 and 12.05 to 1 but according to Yamaha documents, the compression ratios for the Viper are 5.8 to 1 and 6.6 to 1 respectively. I would tend to trust the Yamaha specs. So what am I doing wrong? I know there is a volume I’m not accounting for in the deck height space between the top of the piston and the combustion chamber but I’m sure it can’t be that much. Any help here?
800
New member
The problem is the chamber cc number don't include the top of the piston, you need to install them and cc them on the motor. Generally installed they will be around 2.5 cc tighter. So you would be around 11.3 and 13.5. Again, you should install them and cc them to get actual numbers.
These are what we call Full Stroke Compression ratio numbers, the lower numbers you read in the Yamaha book are actual compression ratio's. A much longer calculation. Most people just use the full stroke numbers because it's easier to figure and generally works well.
As long as you haven't moved your exhaust port.
These are what we call Full Stroke Compression ratio numbers, the lower numbers you read in the Yamaha book are actual compression ratio's. A much longer calculation. Most people just use the full stroke numbers because it's easier to figure and generally works well.
As long as you haven't moved your exhaust port.
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bajardine
New member
Full stroke compression
So which ratio would you use when you are figuring what octane fuel will be safe to run? I would like to set my Viper up to run 130-150 psi but I'm trying to research the octane needed. I don't want to be borderline race fuel or any mixing. We ride in the mountains between 5-10k feet and the stardard premium around here is 91 octane. I understand in higher elevations it takes less octane to prevent detonation.
So which ratio would you use when you are figuring what octane fuel will be safe to run? I would like to set my Viper up to run 130-150 psi but I'm trying to research the octane needed. I don't want to be borderline race fuel or any mixing. We ride in the mountains between 5-10k feet and the stardard premium around here is 91 octane. I understand in higher elevations it takes less octane to prevent detonation.
800
New member
General rule of thumb is 12.5 to 1 for 93 oct. at lower elevations, at 5 to 8K ft the 13.5 would be fine, when we would test at 8500 to 10K we used 15.5. At 85 it was on the edge but at 95 to 10.5 they still ran good. When I did a comparison, here at say 900ft, a motor with say 180 psi had 125 psi at 8500ft, same gauge same sled.
If you want to be more correct, just cc the motor, generally the compression numbers really don't matter, the compression ratio does, however, if you want to set it up for your altitude, and we know what the compression on a Viper is at sea level, you could set your heads up to be the same at your elevation, and use pump gas. 150psi would be too tight, but if you're going up quick it would be OK at higher altitudes.
When we would start out at 8500 you could just feel the sleds get lazier as we went up over 10K, then on the way back as you were decending, you could feel them get crisper, and quick.
If you want to be more correct, just cc the motor, generally the compression numbers really don't matter, the compression ratio does, however, if you want to set it up for your altitude, and we know what the compression on a Viper is at sea level, you could set your heads up to be the same at your elevation, and use pump gas. 150psi would be too tight, but if you're going up quick it would be OK at higher altitudes.
When we would start out at 8500 you could just feel the sleds get lazier as we went up over 10K, then on the way back as you were decending, you could feel them get crisper, and quick.