E: Engine Compression pressure

[john998]

What pressure would you expect only using the kick start on a Rapide with 7.3 pistons? John.

 

[davidd]

The Mk1 cam lowers the dynamic pressure about .5, so your 7.3:1 piston would be making about 6.9:1 compression ratio when turning in your engine. I used 7:1 in the formula and it came out at about 118 lbs.

David

 

[vibrac]

And I guess pressure for a mk2 cam would be less?

 

[john998]

Hello, both and thanks for that. Almost 120psi is what I got, once I remembered to open the throttle. This with Mk 1 cams.
I had convinced my self that I had a leaky valve, but it looks like an easy winter. This motor, for many years has felt wooley but pleasant enough to ride. I have checked everything even change the cams all to no avail.
Will have to by satisfied that it is reliable. John.

 

[davidd]

And I guess pressure for a mk2 cam would be less?

Tim,

Yes, less pressure because the intake is closing later. This is why the Mk2 is usually used with high compression pistons. Of course, starting a bike with the lower cylinder pressure of the Mk2 may contribute to starting problems, particularly if your rings or valves are a little soft.

David

 

[vibrac]

Tim,

Yes, less pressure because the intake is closing later. This is why the Mk2 is usually used with high compression pistons. Of course, starting a bike with the lower cylinder pressure of the Mk2 may contribute to starting problems, particularly if your rings or valves are a little soft.

David

The only thing i am missing from calculating dynamic ratios is standard con rod length .....

 

[roy the mechanic]

According to Carillo rod centre 7.250. Generally anything over 100 pounds will run ok.

 

[vibrac]

SO.. using a calculator it looks like a big bore 8:1 piston on MK2 cam will give me a dynamic CR lower than a 7.5 on a Mk1 and it would need an 8.5:1 to be a little higher.
and at 8:1 the crank pressure dynamic is 136 lbs almost the same as a standard comet on 7:5
Very interesting.... especially to my right leg
now who makes a 90 bore lowish compression piston or is it back to Venolia?

 

[Vincent Brake]

SO.. using a calculator it looks like a big bore 8:1 piston on MK2 cam will give me a dynamic CR lower than a 7.5 on a Mk1 and it would need an 8.5:1 to be a little higher.
and at 8:1 the crank pressure dynamic is 136 lbs almost the same as a standard comet on 7:5
Very interesting.... especially to my right leg
now who makes a 90 bore lowish compression piston or is it back to Venolia?

or lower a CP a bit they have a very strong (thick) top deck