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How much is sensible to a compression ratio of gs550 ?

  • Thread starter Thread starter SuziGS_project
  • Start date Start date
S

SuziGS_project

Guest
Considering increased heat production & other things.

How much do you have increased it ? Can it be 10.5:1 for example
in normal street GS bike ?

How much must be then removed material from cylinder head ?

Model is -81 gs500 E

Thanks,
Mikko
 
GS550 has a compression ratio of 8.6 to 1 which is quite low - even when compared to other GS engines. That said, my opinion of course, but I'd leave the engine alone. The old 2 valve GS engines have low compression because the combustion chamber design is dated. Increasing the CR will make the engine more detonation sensitive, which will in turn force the useage of more expensive gas. At the end of the day it's just more trouble than it's worth considering the small possible power gain.
 
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I raised my combustion ratio quite a bit, and am paying for it now...

I filled up with 100 octane this weekend, and it'll knock just slightly at idle with 91-92...

But it's so worth it.

:)
 
brveagle said:
I raised my combustion ratio quite a bit, and am paying for it now...

I filled up with 100 octane this weekend, and it'll knock just slightly at idle with 91-92...

But it's so worth it.

:)
Click to expand...

did you clean up the head at all? i have heard the heads on these old bikes are kinda crusty/edgy which gives the chambers hot spots, which could cause knock. You may want to take the head off and see about cleaning it up.

BTW, what psi are you now getting with those 750 pistons? and where are your PICTURES!



Mikko, if you want to up the compression you will need custom pistons or do what brveagle did. (search for his posts) and you will certianly need a oil cooler if you want to go big.
 
Last edited:
first timer said:
did you clean up the head at all? i have heard the heads on these old bikes are kinda crusty/edgy which gives the chambers hot spots, which could cause knock. You may want to take the head off and see about cleaning it up.

BTW, what psi are you now getting with those 750 pistons? and where are your PICTURES!



Mikko, if you want to up the compression you will need custom pistons or do what brveagle did. (search for his posts) and you will certianly need a oil cooler if you want to go big.
Click to expand...

I did clean the heads up nice... So that's not a problem. As it's breaking in, it's definently running alot smoother and gaining power every ride as the rings seat in.

I'm slowly making progress on my bike.. PSI is unknown... Gonna try and get that checked this weekend.. Still working all the bugs out, but all i can say is hello torque.

And i 2nd the oil cooler. I'm running one with Royal purple motorcycle oil and i feel that the cooler is what is keeping my bike form overheating.
 
brveagle said:
I did clean the heads up nice... So that's not a problem. As it's breaking in, it's definently running alot smoother and gaining power every ride as the rings seat in.

I'm slowly making progress on my bike.. PSI is unknown... Gonna try and get that checked this weekend.. Still working all the bugs out, but all i can say is hello torque.

And i 2nd the oil cooler. I'm running one with Royal purple motorcycle oil and i feel that the cooler is what is keeping my bike form overheating.
Click to expand...

Why you have lost significantly torque? shouldn't average pressures be higher now and have more power within the whole RPM range ?
Have you been forced to retard ignition too much because of pinging ? :?
 
brveagle said:
I raised my combustion ratio quite a bit, and am paying for it now...

I filled up with 100 octane this weekend, and it'll knock just slightly at idle with 91-92...

But it's so worth it.

:)
Click to expand...

Here in Finland we have only choices 95/98/100+ octanes. I am shamed to even mention how expensive it
is to drive hereby.. :cry:
 
isn't material removing from cylinder head an alternative to creplacing pistons ?

isn't material removing from cylinder head an alternative to creplacing pistons ?

I used to thinking that it might be even more accurate way to modify CR than pistons when only a moderate increase to it is a target..
let's say 0,5mm off from head -->
CR no greater than 10:1 ??? :-s

There are no thinner head gaskets available to these bikes aren't there ?

brveagle, how much more speed have you got doing this ?
 
yet, how much it should be on your's now after new pistons in, brveagle ?
 
SuziGS_project said:
I used to thinking that it might be even more accurate way to modify CR than pistons when only a moderate increase to it is a target..
let's say 0,5mm off from head -->
CR no greater than 10:1 ??? :-s

There are no thinner head gaskets available to these bikes aren't there ?

brveagle, how much more speed have you got doing this ?
Click to expand...


Shaving the head is a great way to gain compression and yes they do make thinner head gaskets for our bikes. Cometic does. I ordered a block spacer through them.

I haven't be able to take the bike to the track yet, but i can feel the difference greatly. The track will let me know just how much

SuziGS_project said:
Here in Finland we have only choices 95/98/100+ octanes. I am shamed to even mention how expensive it
is to drive hereby.. :cry:
Click to expand...

European octane is different than the American scale.

SuziGS_project said:
Why you have lost significantly torque? shouldn't average pressures be higher now and have more power within the whole RPM range ?
Have you been forced to retard ignition too much because of pinging ? :?
Click to expand...

I have gained significant amounts of torque through a greater displacement. It really wakes up around 5k...

And yes, my compression figures will be higher, i just haven't measured them since the break in.

Haven't touched the ignition yet, that'll be my last resort.

SuziGS_project said:
yet, how much it should be on your's now after new pistons in, brveagle ?
Click to expand...

I'm guessing compression is in the high 11's, low 12's.
 
I really want to hear your results after bench tests, brveagle :-D
 
American Octane is usually the average of the Research and Motor octane rating (R+M)/2. IIRC, research is usually about 5 points higher than the motor rating.
 
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