Compression ratio

[VolvoJoe]

Ok so I bought this Spree and have not had time to work on it so I took it to a shop. The guy said he couldnt get it started so he check the compression ratio and said it was 60. Does anyone know the compression on these engines are? Thank you

[Trafficjamz]

60 is too low 125- 135 is good

[Bear45-70]

The manual says and I quote "8.0 kg/cm3 (114 PSI) Minimum. 60 means a minimum of stuck rings.

Compression ratio for a Spree is 7.2 to 1 thru '86, 6.8 to 1 (7.0 to 1) after 86.

[Trafficjamz]

The manual says and I quote "8.0 kg/cm3 (114 PSI) Minimum. 60 means a minimum of stuck rings.

Compression ratio for a Spree is 7.2 to 1 thru '86, 6.8 to 1 (7.0 to 1) after 86.

Normal atmosphere pressure (1 bar) = 14.5037738 lbs (at sea level)

So your comp ratio is 60/14.503 or 4.137:1 which is way too low (math edit) still way too low






Are 7.2:1 and 6.8:1 minimum ratios or normal specs?

[Bear45-70]

The manual says and I quote "8.0 kg/cm3 (114 PSI) Minimum. 60 means a minimum of stuck rings.

Compression ratio for a Spree is 7.2 to 1 thru '86, 6.8 to 1 (7.0 to 1) after 86.

Normal atmosphere pressure (1 bar) = 14.5037738 lbs (at sea level)

So your comp ratio is (60+14.503/14.503 or 5.137:1 which is way too low






Are 7.2:1 and 6.8:1 minimum ratios or normal specs?

Last word in first sentence.

[Trafficjamz]

The manual says and I quote "8.0 kg/cm3 (114 PSI) Minimum. 60 means a minimum of stuck rings.

Compression ratio for a Spree is 7.2 to 1 thru '86, 6.8 to 1 (7.0 to 1) after 86.

Normal atmosphere pressure (1 bar) = 14.5037738 lbs (at sea level)

So your comp ratio is 60/14.503 or 4.137:1 which is way too low


Are 7.2:1 and 6.8:1 minimum ratios or normal specs?

Last word in first sentence.

The reason I ask. 7.2:1 and 6.8:1 do not equal 114psi. 114/14.503 = 7.86 So which one is it?

[Bear45-70]

Normal atmosphere pressure (1 bar) = 14.5037738 lbs (at sea level)

So your comp ratio is 60/14.503 or 4.137:1 which is way too low


Are 7.2:1 and 6.8:1 minimum ratios or normal specs?

Last word in first sentence.

The reason I ask. 7.2:1 and 6.8:1 do not equal 114psi. 114/14.503 = 7.86 So which one is it?

The thing you are missing is that 2 strokes are super charged by design. The mixture is compressed in the crankcase first, so that when the intake port opens the mixture is forced into the chamber under pressure, there by making the compression ratio higher (called dynamic compression) than the static compression numbers show.

[Trafficjamz]

So 114psi is the lowest allowable dynamic comp ratio?

What should the static comp ratio be on good running spree engine?

[Bear45-70]

So 114psi is the lowest allowable dynamic comp ratio?

What should the static comp ratio be on good running spree engine?

Static compression is done just like you did it earlier. But static is just a number whereas dynamic is what you have to live with with a running engine.

[eliteguy50]

So 114psi is the lowest allowable dynamic comp ratio?

What should the static comp ratio be on good running spree engine?

The lowest recommended pressure, yes. Dynamic is basically a measure of the atomic mass in the cylinder and static is the linear measurement of volume.
I would say 90psi at a minimum. The engine will run below that but you don't want to risk breaking a ring. The other problem is that compression gauges have hoses and the longer the hose, the more volume is offered for diffusion of the 50 cc output of the motor...longer the hose, the lower the reading. I had a gauge that read 56psi and the engine still ran (not well but would go 20mph). I made a new gauge with a shorter hose and it was roughly 20psi higher.

[Bear45-70]

The hose thing is a very valid point. The shorter the better for accuracy.