What do you think ...................

[jetta90]

of these compression numbers for a 1984 GS 750 E......................

135 - 130 - 130 - 135

above numbers were obtained with adjusted valves, throttle wide open and engine was nicely warm.
I would have expected slightly stonger numbers, really.
Engine has about 41,000 miles on it.
I don't know the bike at all as i just bought it but it doesn't appear to have been through a tough life.
I am having trouble finding a manual for this beast so any suggestions on where to find would be appreciated.
Maybe with the above numbers it is due for a top end rebuild or do i just part it out?
What do you folks think?
thank you.

1984 Suzuki GS 750 E
1981 Ducati Pantah 500

 

[focus frenzy]

sounds fine to me, ride it!!

if I did my math correct it works out to 9.3-1 and 8.9-1 compression ratio.

 

[tfb]

Those figures indicate an engine in very good condition, and the fact that they are all within 5 psi of each other is also excellent.

By way of contrast, I only got about 100psi per cylinder out of my old GS1000S today... which is basically telling me it's top-end rebuild time.

Mike.

 

[Deesel]

sounds fine to me, ride it!!

if I did my math correct it works out to 9.3-1 and 8.9-1 compression ratio.

Can you share your math equation. I didnt know you could figure actual compression ratio with static compression readings. Do you take into account the cam timing, duration and things like that?
Thanks
ECR

 

[Guest]

those numbers are fine... the closeness in the PSI is all that matter as long as they are above 110.

I've even read some posts on here where the bike is running with all the compression numbers under 100.

 

[bwringer]

Those are excellent compression numbers.

 

[focus frenzy]

Can you share your math equation. I didn't know you could figure actual compression ratio with static compression readings. Do you take into account the cam timing, duration and things like that?
Thanks
ECR

compression ratio is how many atmospheres, also known as BAR are compressed in the combustion chamber.

a compression ratio of 10.0:1 would be 10 BAR, 1 BAR = 14.504 PSI, so a engine with 10:1 ratio will make 145.04 PSI of pressure.

135 psi / 14.504 = 9.307777etc

 

[jetta90]

Interesting feedback on the compression numbers. Thanks you for taking the time to respond.

I don't have a manual as i indicated previously but even looking at other GS models i see that compression should be somewhere between 145 to 170 or so...........................so, thats where i kinda got scared.
I appreciate your feedback and any others who wish to chime in.

The more info we can exchange the better off we all are!!

1984 Suzuki GS 750E
1981 Ducati Pantah 500

 

[Thomas Kenny]

Hey Jetta90, I ride a 1984 GS550ES, and the book calls for compression pressure to be between 142 and 199PSI. I checked my compression recently, good valve adjustment, hot engine and WOT. I got 123 to 125 across all four cylinders. I put a teaspoon of oil in each bore and checked them again and they all rose to just under 200PSI. My bike seems to run fine at these pressures, however longer term I plan to do a bore job, new pistons and rings. It just seems to be a bit down on power to me. You should be fine until you get down to 110 or less. Tom

 

[Nessism]

compression ratio is how many atmospheres, also known as BAR are compressed in the combustion chamber.

a compression ratio of 10.0:1 would be 10 BAR, 1 BAR = 14.504 PSI, so a engine with 10:1 ratio will make 145.04 PSI of pressure.

135 psi / 14.504 = 9.307777etc

I don't think compression ratio is calculated that way. I think it based on air volumes.

And combustion pressure has a lot to do with how much valve overlap there is so it's not always proportional to compression ratio anyway.

 

[Deesel]

I don't think compression ratio is calculated that way. I think it based on air volumes.

And combustion pressure has a lot to do with how much valve overlap there is so it's not always proportional to compression ratio anyway.

I agree, its the ratio of the cylinder volume into combustion chamber volume.