I was driving my bike and I was having trouble going over 52-55 mph on a very flat road (which might be normal for the bike but I am a bit of a perfectionist) so I thought I would check my compression. I have only about 100 miles from a top end rebuild but thought I would double check everything.
I did the compression test cold and after 2 kicks of the kickstart I read about 61-62 psi. Now at 14.1 atmosphere shouldn't I be getting closer to 69 psi, or is this normal for the bike. Thanks again
email (option): Jeff@infinitymedspa.com
1st, for most accurate results the engine should be at full operating temperature, and the throttle held completely open. Several kicks will be needed to get the gauge needle as high as it will go.
As to the reading, I assume your local atmospheric pressure is 14.1 (above sea level)?
The results won't ever be your static compression ratio of 4.9:1 × atmospheric pressure, except by coincidence.
There are 2 other factors at work in all engines which tend to move the results in opposite directions, but don't cancel each other out.
#1. Compression does not begin until the intake valve closes, which is (nominally) at 65° ABDC. With the M20's rod geometry, when the valve closes only about 75.8% of the stroke (2.805") is captured. This factor alone greatly reduced the effective compression ratio to about 3.96:1.
#2. When the cylinder volume is compressed its temperature goes up, which adds to the cylinder pressure. The effect is an exponent to the CR, the "ratio of variable heats" for gasses, about 1.25 in this case (just a guess).
For the M20 (and assuming all dimensions and data are nominal) my Excel program's rough approximation predicts 59 psi.
A note: a high compression engine with a later-closing intake valve may have the same cranking pressure, but of course will perform quite differently.
A more useful test is for leak-down: how quickly does pressure decay to zero (on the gauge), and from where? In an M20 the leaks are the rings, and/or either valve. Listening for air in the crankcase or either port detects the source.
The leak rate is determined by applying external air pressure through a metering orifice to the spark plug hole, and tracking how much is captured as a percentage with a gauge. 100% is impossible, 98% is a new race engine, anything over 90% should be considered a success.
email (option): sales@victorylibrary.com
There is a description of a leak-down tester on the Technical Info part of the website
email (option): viacons [at] planet'dot'nl
As to the reading, I assume your local atmospheric pressure is 14.1 (above sea level)?
The results won't ever be your static compression ratio of 4.9:1 × atmospheric pressure, except by coincidence.
There are 2 other factors at work in all engines which tend to move the results in opposite directions, but don't cancel each other out.
#1. Compression does not begin until the intake valve closes, which is (nominally) at 65° ABDC. With the M20's rod geometry, when the valve closes only about 75.8% of the stroke (2.805") is captured. This factor alone greatly reduced the effective compression ratio to about 3.96:1.
#2. When the cylinder volume is compressed its temperature goes up, which adds to the cylinder pressure. The effect is an exponent to the CR, the "ratio of variable heats" for gasses, about 1.25 in this case (just a guess).
For the M20 (and assuming all dimensions and data are nominal) my Excel program's rough approximation predicts 59 psi.
A note: a high compression engine with a later-closing intake valve may have the same cranking pressure, but of course will perform quite differently.
A more useful test is for leak-down: how quickly does pressure decay to zero (on the gauge), and from where? In an M20 the leaks are the rings, and/or either valve. Listening for air in the crankcase or either port detects the source.
The leak rate is determined by applying external air pressure through a metering orifice to the spark plug hole, and tracking how much is captured as a percentage with a gauge. 100% is impossible, 98% is a new race engine, anything over 90% should be considered a success.
Ok I will perform a leak down test but sounds as if my bikes compression is probably okay. With normal gearing is 52-56mph all that I should expect n a flat surface. I weigh 200 lb and live in California so most of the gas is either blended or oxygenated so it reduces the oomph u can get. :(
email (option): Jeff@infinitymedspa.com
Assuming the usual sprockets (18 & 19) = 5.28:1 overall gearing, with a 26" rear tire the power peak of 4,100 RPM is about 60 mph.
You can slow the motor down a bit by changing either sprocket but the performance in traffic will be worse.
Is it important to retain original appearance?
email (option): sales@victorylibrary.com
Not really, I like to ride it more than anything. Do u have a suggestion?
email (option): Jeff@infinitymedspa.com
1. very long completely open (no inserts, no muffler, no baffle) exhaust pipe. Yes, it's very loud. The length produces a return wave that assists intake flow into the chamber during overlap at (and close to) a specific RPM. Just a guess: to bump the existing power (max = 4,100) slightly make the pipe 45-50" long port face to end measured along the mid-point of its curve. This length assumes the stock M20 cam. Shifting lower needs slightly more length, etc.
A reverse mega will both add power, and increase the RPM range of the boost, but it's pretty big. I can make a rough guess from the stock exhaust pipe OD, is it 1-1/2"?
2. move the carburetor back 6-12" using a section of straight tube. Telescoping sections will permit length adjustments between tests, etc.This will affect response and may require jet changes.
email (option): sales@victorylibrary.com