Grtechguy
Grtechguy SuperDork
1/19/09 7:55 p.m.

If I have an engine with the following specs, how many PSI should my guage read? or am I not providing enough information?

Displacement: 171.00 ccm (10.43 cubic inches)

Engine type: Single cylinder

Stroke: 2

Power: 15.00 HP (10.9 kW)) @ 7500 RPM

Compression: 6.8:1

Bore x stroke: 66.0 x 50.0 mm (2.6 x 2.0 inches)

RobL
RobL New Reader
1/19/09 8:02 p.m.

If your compression is 6.8:1 - a compression test should read about 100PSI on a 4 stroke. But it's a 2 stroke so I'm not sure.

Nashco
Nashco SuperDork
1/19/09 9:11 p.m.
Grtechguy wrote: If I have an engine with the following specs, how many PSI should my guage read? or am I not providing enough information?

Which gauge?

chaparral
chaparral New Reader
1/19/09 9:13 p.m.

6.8:1 is the ratio of the volume of the cylinder when the last port closes to the combustion chamber volume.

You're looking for at least 14.7 PSI x 6.8, so 100. You should get somewhat higher numbers if it's a reed or rotary valve engine due to crankcase compression.

chaparral
chaparral New Reader
1/19/09 9:14 p.m.

What engine is this?

Grtechguy
Grtechguy SuperDork
1/19/09 9:15 p.m.

It's a Yamaha 175cc still mounted in the '80 DT175mx

Grtechguy
Grtechguy SuperDork
1/19/09 9:16 p.m.
Nashco wrote:
Grtechguy wrote: If I have an engine with the following specs, how many PSI should my guage read? or am I not providing enough information?
Which gauge?

The one I use to get compression readings. reads 0-250psi. screws in the spark plug hole

chaparral
chaparral New Reader
1/19/09 9:16 p.m.

100 PSI.

If you want more, check the crankcase seals and put in some new reeds.

RXBeetle
RXBeetle New Reader
1/19/09 11:04 p.m.

dynamic vs static compression ratio is the curve ball here. This may help: http://www.dirtrider.net/forums3/showthread.php?t=152854

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