Diff. between GS750 and GS850 cams

[gssuzukixxx]

Can anyone tell me the difference between the cams from a '79/80 GS850 and the cams from a '77 GS750.

I am upgrading my '77 GS750 to 850 and have the choice of either cams. Which would work best?

Thanks
Brian

[GabrielGoes]

both have the same motor, i'd just stick with OEM parts or as close as you can get

[Suzuki_Don]

GS750: Intake opens 30 degrees BTDC and closes 70 degrees ABDC (280 degrees duration).
Exhaust opens 70 degrees BBDC and closes 30 degrees ATDC (280 degrees duration).

GS850: Intake opens 28 degrees BTDC and closes 68 degrees ABDC (276 degrees duration).
Exhaust opens 66 degrees BBDC and closes 26 degrees ATDC (272 degrees duration).

It depends what you want from the motor. If more torque from lower rpm then the 850 cams would be best. If you want the valves open longer and more power in the upper ranges of the motor then go for the 750 cams.

[gssuzukixxx]

GS750: Intake opens 30 degrees BTDC and closes 70 degrees ABDC (280 degrees duration).
Exhaust opens 70 degrees BBDC and closes 30 degrees ATDC (280 degrees duration).

GS850: Intake opens 28 degrees BTDC and closes 68 degrees ABDC (276 degrees duration).
Exhaust opens 66 degrees BBDC and closes 26 degrees ATDC (272 degrees duration).

It depends what you want from the motor. If more torque from lower rpm then the 850 cams would be best. If you want the valves open longer and more power in the upper ranges of the motor then go for the 750 cams.

Thanks Don, that is exactly what I was looking for.

Brian

[mlinder]

Or, intake cam from the 850 and exhaust cam from the 750.. gives a little more overlap and duration, gives you a bit of both worlds there.

[gssuzukixxx]

GS750: Intake opens 30 degrees BTDC and closes 70 degrees ABDC (280 degrees duration).
Exhaust opens 70 degrees BBDC and closes 30 degrees ATDC (280 degrees duration).

GS850: Intake opens 28 degrees BTDC and closes 68 degrees ABDC (276 degrees duration).
Exhaust opens 66 degrees BBDC and closes 26 degrees ATDC (272 degrees duration).

It depends what you want from the motor. If more torque from lower rpm then the 850 cams would be best. If you want the valves open longer and more power in the upper ranges of the motor then go for the 750 cams.

Are these degree measurements made at a standard valve lift such as .050" or 1mm?

[Nessism]

Are these degree measurements made at a standard valve lift such as .050" or 1mm?

It's from the Suzuki factory service manual(s). Suzuki doesn't reference how the cam duration is measured but I think it has to be at valve lash considering the numbers.

[gssuzukixxx]

I have a degree wheel mounted on my GS750 and I have TDC established using a positive stop. When I advance the crank it is hard to tell exactly when the exhaust valve starts to open but it appears to be between 100º and 110º ATDC (110º is the spec) so it is in the ball park.

But when I keep rotating until the exhaust valve closes it looks like about 50º ATDC (30º is spec). Which is quite a ways off.

I have a feeling it has something to do with my valve clearance.

Any thoughts?

Haven't even gotten to intake.

Brian

[Nessism]

I thought degreeing the cams involved setting the lobe centers, not trying to set the open and close points?

[Steve]

GS850: Intake opens 28 degrees BTDC and closes 68 degrees ABDC (276 degrees duration).
Exhaust opens 66 degrees BBDC and closes 26 degrees ATDC (272 degrees duration).

Those are numbers for the '79 cams. The '80 850s had the intake cam advanced 10 degrees.

Here is a clip from the factory 850 manual:


.

[gssuzukixxx]

I thought degreeing the cams involved setting the lobe centers, not trying to set the open and close points?

Yes, you are right. The openning and closing points are only used to calculate the lobe centres. It is easier to pick the exact point when a certain lift is reached vs when the lift starts.

So my next attempts will be picking say .02" lift on the way up and on the way down and then figuring out lobe centres.

Interesting stuff.

Brian

[gssuzukixxx]

OK here are my lobe centre readings. I used .020" lift as my reference.

Intake lobe centre 122.5º ATDC (supposed to be 110º)
Exhaust lobe centre 104º BTDC (supposed to be 110º)

I took out the intake cam and moved it to 19 links instead of 20 from the #2 mark but that makes too big a difference. I was thinking that with 30 teeth on a cam sprocket that it would make a difference of 360º/30=12º but it actually make twice that difference because the crank sprocket moves twice as far as a cam sprocket.

So now what? I figure 104º on the exhaust wouldn't be too bad because I have seen it suggested that reducing the lobe centre angle will actually improve low and midrange torque. But the intake at 122.5º needs to be addressed. But how do I do this?

Brian

[Suzuki_Don]

Brian, you need to slot your camshaft sprockets so you can adjust the cams to each other and the crank. I have done it on my bike and can give helpful hints if required.

[gssuzukixxx]

Brian, you need to slot your camshaft sprockets so you can adjust the cams to each other and the crank. I have done it on my bike and can give helpful hints if required.

Thanks Don, I will definately contact you if I get in over my head.