Compression Test Questions

[hodakamax]

Hey Gang. I've just bolted the Webco head on the Day One project. I squirted some WD-40 in the cylinder, no carb mounted. The engine has not been turned but a few times. Initial readings are only about 105 psi. I suspect they are low because the rings aren't seated. It looks as though the engine will need some run in before an accurate reading can be obtained. I suspect the Webco head was for early high crown pistons and I have a new Wiseco replacement for a model 93 which is low crown. My questions are; what are some good numbers for compression above stock but below race spec (its just a trial/trail bike) and any ballpark guesses or guides on how much to remove from the head. I know I'll probably have to re-cut the squish and I'm just looking for input from all my expert friends.

Thanks Gang!

Max

PS--or to make it real simple, what's the average compression (psi) on say a fresh broken-in Model 93 Super Rat?

[dcooke007]

Hey Max,
I don't have any numbers for you concerning the 93 model....sorry. I can tell you I just completed a Combat Wombat engine with a single ring Strictly Hodaka / Wiseco piston and fresh bore job. Checked cranking compression before starting engine and it was 170 psi. Started engine and allowed it to come up to operating temperature, shut down and allowed to cool. Restarted engine and brought up to operating temperature. Checked cranking compression with engine hot and it measured 160 psi. 160 range was what I was shooting for and very interesting compression dropped after thermally cycling engine twice.

I can only guess but would think somewhere around 140 to 150psi might be close to stock.

Danny

[hodakamax]

Ah, thanks Danny. Kinda a vague question. My ancient compression gauge is not the ideal set-up either. It's hard to hold the rubber tip in while kicking it. 140-150 psi sounds along the lines of my thought too. This is something I can fool with after break-in and performance evaluation. Thanks again for your input.

Max

[dcooke007]

Max,
If you have a model 93 head and a burette you could compare the combustion chamber volume.

Danny