Compression Test

[Tyler6357]

Hello Friends:

I got a compression gauge and I want to test my compression. What is the correct procedure for doing a test? Should I just remove the spark plug, screw in the gauge hose, engage the clutch and spin the back wheel once or twice or do I need to peddle it around for awhile? Also, what is a good compression reading for a ported 69cc with a .6 Fredhead?? Should it be different with a reed valve?

Thank you!

 

[crassius]

with a cold motor and throttle held open, you need to turn motor over until gauge stops going up (maybe 25 or 30 turns)

different motors will get different values, but between 120 and 165 is good

 

[Tyler6357]

with a cold motor and throttle held open, you need to turn motor over until gauge stops going up (maybe 25 or 30 turns)

different motors will get different values, but between 120 and 165 is good

Thank you, I will do that!

 

[Tyler6357]

Update: Okay, I have done compression tests on both of my bike engines. They are both China Girl PK80, 69cc/47mm cylinder/40 stroke with the short connection rod and the low wrist pin on the piston. However, the bikes have different set ups.

GT Bike--> Ported by my friend who works at a machine shop. He ported the intake/exhaust/transfer ports and decked the cylinder to .03' squish gap while using a mill. The bike is also set up with a .6cc Clone FredHead, CHD-G2 4 peddle reed valve with windowed piston, 15mm Dellorto Clone Carb (HT/RT), MZ Miami expansion pipe, FMF Powercore 2 sliencer. This engine has about 400 miles on it.

Peugeot Bike----> Purchased from Dax about 6 years ago also Duane from Dax ported the engine for me. I decked the cylinder myself using a piece of glass and sandpaper method attempting to get it to .03" squish gap. The bike has a .6cc Real Fredhead, a 15mm shorty intake, stock piston, 15mm Dellorto Clone Carb (HT/RT), Sick Bike Parts Expansion pipe. This engine has over 2000 miles on it.

The results of the test came out:

GT--------> 158 psi
Peugeot--> 195 psi

I gotta say that the Dax engine on the Peugeot has always been just an awesome engine, once I found the right jet (#60) the thing just took off. Not only did it have awesome throttle response and a steady low idle, it has almost no chain drag, the engine sprocket seems to spin endlessly. It has got to be the best engine I have ever gotten. I had the bike running so well that I didn't want to change anything and, you know, screw it up, haha. But perhaps it's just the luck of the draw? I don't know. But I was very surprised to see a compression of 195 psi, a full 37 psi higher than the GT one and 30 psi higher than the top range that you gave me. Is that unusual?

 

[crassius]

seems reasonable with that many mods - I'm often concerned about the strength of the bearings at that point tho, but you have to pay somewhere for performance

 

[gary55]

I'll throw a wild guess out there and without knowing say the peugeot probably has more low end torque ( I know, no duh, 37 more lbs. comp.) and the gt has more top end. With both engines having same squish and same 6cc head those variables are gone so the gt probably has a higher exhaust port window inside the cylinder allowing slightly less time and volume for pressure to build up after the piston closes it off. Just a guess.

 

[Tyler6357]

I'll throw a wild guess out there and without knowing say the peugeot probably has more low end torque ( I know, no duh, 37 more lbs. comp.) and the gt has more top end. With both engines having same squish and same 6cc head those variables are gone so the gt probably has a higher exhaust port window inside the cylinder allowing slightly less time and volume for pressure to build up after the piston closes it off. Just a guess.

Your guesses are pretty good! Yes, it has awesome low end, much better than the GT had until I put the reed valve on the GT, now it has slightly less low end then the GT because of the reed valve but the difference is negligible, it can almost keep up. As far as top end, I would say the GT is 6-8 mph faster, both bikes have the same 40T rear sprocket (lots of hills around here). I assume that the difference in top end is because of the better pipe on the GT as well as the reed valve and windowed piston. The Peugeot will max out at about 38 mph with a 40T sprocket on a good day. I'm not exactly sure what the GT is hitting at WOT, the speedometer isn't working, but it is faster than the Peugeot for sure.

I was wondering what would happen if I added the reed valve, pipe and silencer to the Peugeot engine. Won't the bigger and better pipe add more compression? Putting it well over 200 psi? And what about the reed valve?? Will all that blow the crank seals or something? And I'm still wondering, why this engine has such a high compression rating in the first place?

 

[Tyler6357]

seems reasonable with that many mods - I'm often concerned about the strength of the bearings at that point tho, but you have to pay somewhere for performance

Yeah, but I have already gone over 2000 miles on the Peugeot without any failures. Well, except once, when I first put it on it wouldn't run at all with the stock 70 jet so I went to a 65. The 65 seemed okay but it was 4 stroking some. I ran it with the 65 jet for about 500 miles until one day I was coming up a very steep hill and it 4 stroked with a heavy load on it and blew the head gasket. It was at this time that I put on the Fredhead and decked the cylinder and put on a new head gasket. I also put on the short intake so the carb would fit better in the frame and changed the jet to 60. Those things really woke it up! That was over 1500 miles ago and I've had no failures since.

 

[gary55]

Your guesses are pretty good! Yes, it has awesome low end, much better than the GT had until I put the reed valve on the GT, now it has slightly less low end then the GT because of the reed valve but the difference is negligible, it can almost keep up. As far as top end, I would say the GT is 6-8 mph faster, both bikes have the same 40T rear sprocket (lots of hills around here). I assume that the difference in top end is because of the better pipe on the GT as well as the reed valve and windowed piston. The Peugeot will max out at about 38 mph with a 40T sprocket on a good day. I'm not exactly sure what the GT is hitting at WOT, the speedometer isn't working, but it is faster than the Peugeot for sure.

I was wondering what would happen if I added the reed valve, pipe and silencer to the Peugeot engine. Won't the bigger and better pipe add more compression? Putting it well over 200 psi? And what about the reed valve?? Will all that blow the crank seals or something? And I'm still wondering, why this engine has such a high compression rating in the first place?

Pipes don't change compression, well at least not substantially any more than a reed might. Both are beneficial in aiding a denser charge, but the effect on compression is negligible. The reed will increase case pressure, and the pipe will help in reducing it through scavenging.

Case pressure and compression ratio of the chamber are two different things.Case pressure has a minimal effect on combustion chamber pressure, and chamber pressure has no effect on case pressure in normal operating conditions. Like I said the probable reason for the diff. in comp.of the two engines with same head volume and squish gap is differences it exhaust port window heights in the cylinder. If the window is higher the compression begins later in the stroke thus the volume being compressed becomes lesser. The combustion period becomes shorter, and the blowdown period becomes longer allowing more time for exhaust gasses to escape before the transfers open to introduce the new charge.

 

[crassius]

Putting it well over 200 psi? And what about the reed valve?? Will all that blow the crank seals or something? And I'm still wondering, why this engine has such a high compression rating in the first place?

don't recall the numbers, but somewhere near that level you'll need a much higher octane fuel - I doubt much of that pressure will get to the seals, but a retainer plate could be added