Wiseco big bore compression numbers?

[80GS1000]

The GS1000 has done about 800 miles since a top-end rebuild with new valves and a 1100 cc Wiseco kit. Compression numbers on all 4 cylinders are at about 120 PSI. Is this normal, or should it be higher? And if compression should be higher, is it likely to improve at this point? I've been riding the snot out of it since the rebuild out in the desert to break in the rings, WOT to redline once warm then engine braking to 2K RPM then WOT again. Cylinder #4 shows what appears to be a bit of oil fouling on the plug.

[psyguy]

sorry to say but i think the compression numbers should be higher after a re-bore and new pistons/rings and new valves, and after that break-in mileage

would be interested in hearing from someone WHY your numbers are not better

[rphillips]

I can see your concern, but I've always heard the across the board readings are most important. Not how high the #'s are, but how equal the #1 thru #4 readings are.

[trippivot]

what thickness of base gasket and thickness of head gasket did you use? (deck height)

do you have stock camshafts at stock cam timing? (cranking pressure)

the specifics of those 3 details will have the cylinder psi adjustable up or down.

but like rphillips said equal readings are more important.

I won't even ask what piston to cylinder clearance or top ring end gap settings.

[80GS1000]

what thickness of base gasket and thickness of head gasket did you use? (deck height)

do you have stock camshafts at stock cam timing? (cranking pressure)

the specifics of those 3 details will have the cylinder psi adjustable up or down.

but like rphillips said equal readings are more important.

I won't even ask what piston to cylinder clearance or top ring end gap settings.

Thanks for the help.

Stock Wiseco head gasket, stock Suzuki base gasket.

.395 lift Web cams timed in by the book: http://www.webcamshafts.com/pages/mo...ta/tc_664.html

[Dan Ruddock]

Try a different gage. Dan

[Nessism]

Obvious question but are you holding the throttle wide open when checking compression?

[80GS1000]

Obvious question but are you holding the throttle wide open when checking compression?

Yes, WOT with a full battery. Should the other spark plugs be removed when testing each cylinder? The 120 PSI number is with the other plugs still installed.

[Jim Hansen]

Compression Readings

"The GS1000 has done about 800 miles since a top-end rebuild with new valves and a 1100 cc Wiseco kit. Compression numbers on all 4 cylinders are at about 120 PSI. Is this normal, or should it be higher? And if compression should be higher, is it likely to improve at this point? I've been riding the snot out of it since the rebuild out in the desert to break in the rings, WOT to redline once warm then engine braking to 2K RPM then WOT again. Cylinder #4 shows what appears to be a bit of oil fouling on the plug".


If all of the cylinder compression readings are the same, then everything is probably ok. If a cylinder had a problem with a valve or compression rings, then one cylinder would have a low reading.

If a cylinder has the same compression reading as the other cylinders, but is pumping oil, then there might be a problem with the oil ring or with the valve guides. But this is unlikely in a rebuilt engine. The engine might just need more time to break in the oil rings.

When the engine displacement is increased, this has the effect of increasing cranking pressure (compression ratio) unless metal is removed from the combustion area or thicker head or base gaskets are used. However, with a high-performance cam profile with increased valve overlap, this has the effect of reducing measured compression readings. I wouldn't worry about any of this unless you have pinging, which would be from too much compression, not from for too little.

With constant-velocity (CV) carburetors, you must not only open the throttle butterflies, but also block open the carb. slides during a compression test. Otherwise, you will get erroneous low readings. All sparkplugs should be removed during the test in order to maximize cranking speed and maximize the compression readings.

There are a lot of variables involved when making a compression test, so the results require interpretation. If you are still concerned about the results, you can always do a Leakdown Test. It's not difficult, and the leaddown testers are not expensive (But you need a compressor or some source of air). I think that your engine is fine.

J. Hansen
1982 GS1100G

[gearhead13]

Pretty interesting break in procedure there, bud. Seems to me that by going WOT all the time you are trying your hardest to force air by the rings. I will be keeping my rebuilt engine well below redline and mostly no more than 1/2 throttle.

[Don-lo]

When I put the MTC 1055 piston kit in my 1000 about 10 years ago, I rode it nice and kept it under half throttle. I worked for a performance shop and the owner liked to say "break it in fast if you want it fast". I prefered to trust Joe Minton and Kevin Cameron, so I took it easy. I monitored it pretty closely, wanting to see the improvement, but it stayed low, 120 psi, past 500 miles. I gave up checking and thought maybe the hone job wasn't good. I think I checked again around 800 miles, and still no better. Then I checked around 1200 miles and it was 160-170, where it still is today.

[SqDancerLynn1]

Part of your low #s is most likley due to the cam overlap, not that anything is wrong

[Dan Ruddock]

Part of your low #s is most likley due to the cam overlap, not that anything is wrong

When the intake valve closes @ BDC will have more effect on cranking presure than overlap. For a given duration more overlap will increase cranking presure. This is from webcam website. Dan

VARYING LOBE SEPARATION ANGLE

Widen

Raise Torque to Higher RPM

Reduces Maximum Torque

Broadens Power Band

Reduce Maximum Cylinder Pressure

Decrease Chance of Engine Knock

Decrease Cranking Compression

Decrease Effective Compression

Idle Vacuum is Increased

Idle Quality Improves

Open Valve-Overlap Decreases

Closed Valve-Overlap Decreases

Natural EGR Effect is Reduced

Increases Piston-to-Valve Clearance

Tighten

Moves Torque to Lower RPM

Increases Maximum Torque

Narrow Powerband

Builds Higher Cylinder Pressure

Increase Chance of Engine Knock

Increase Cranking Compression

Increase Effective Compression

Idle Vacuum is Reduced

Idle Quality Suffers

Open Valve-Overlap Increases

Closed Valve-Overlap Increases

Natural EGR Effect Increases

Decreases Piston-to-Valve Clearance

[chef1366]

I had 120 across the board for the first thousand miles. Now with a couple of thousand miles I'm at 130 even. My cams are degreed 110 110.
Our Wiseco pistons are not high compression.
I would worry about the oil on the #4.

That is the proper break in method. At least how I've been taught.

[80GS1000]

I had 120 across the board for the first thousand miles. Now with a couple of thousand miles I'm at 130 even. My cams are degreed 110 110.
Our Wiseco pistons are not high compression.
I would worry about the oil on the #4.

That is the proper break in method. At least how I've been taught.

Need to re-ring the #4 cylinder you think?