E: Engine compression ratios

[vibrac]

what I am looking for is a table giving (say) Lbs per sq inch on one side and compression ratio on the other so that by quickly measuring the pressure you could get a rough idea of the static CR
Perhaps its a pipe dream and varies too much by engine size
I have looked on the web but generally get the age old formula for calculating the compression ratio by volume or into some calculator allowing for blowers

 

[orwollma]

A very approximate estimate is given by CR ~ (PSI/14.7) - 1

 

[davidd]

Tim,

I am not sure where you are going. The static CR is not of much use because it only exists on the piston box, not the engine. Measuring the pressure in the engine will determined by the cam, whereas the static CR assumes that there are no valves or cam in the engine, just a pressure gauge in the spark plug hole. You could make your own chart. The Wallace DCR Calculator can be used. Just don't fill in anything for the boost.

http://www.wallaceracing.com/mobile/dynamic-cr.php

So, the fill-in looks like this:

When you press "Calculate" the following will display:

I think the numbers are correct for a stock Vincent, but I did not double check them. You can't use seat to seat specs for most Vincent cams as the valves are open most of the time and close only momentarily due to the irregularities on the base circle. Megacycle cams are the exception.

You can run the numbers with different static CR's and note the pressure, then work backwards from the pressure numbers. It will be different with a Mk1.

David

 

[greg brillus]

There is some figures given in Tuning for speed that give info that you want. I just recently read it when looking for something else.

 

[chankly bore]

Also, the Dennis Minett notebook gives combustion chamber volumes for equivalent C.R.'s on standard sized engines.

 

[Martyn Goodwin]

Tim,
Lucky for you I had a clean out today and remembered seeing this (attached).

 

[vibrac]

Having exhausted all the obvious carburetor and ignition reasons for not kick starting I come the the forum remark about kickstarting a Comet and being a pussy so I decided that that could be the problem confirmed when I fitted what I call a Harley or chain saw decompressor and bingo the valve was shutting without firing a sure sympton of high compression so it was time to bite the bullet and off with the head, and low and behold what I had always assumed was a 600 cc motor is a 673! (I lost all my race notes years ago) and its compression rato is way up at 9:1! Now having lost 2 stone and not in the flush of youth I can see the problem- no wonder the Foxley could start it and I could not! And no wonder it went so well! its still is a race motor( back then I think it was running in races at 11:1)even though I'detuned' it and thought I was on 8:1 because of the extra 75cc it was at 9:1 well its a big motor for even that so I will go even lower and see if the B***d will start then. If not its a top hat bush and back to standard (boo ho)
Over confidence and not enough checking I should know better

 

[Pete Appleton]

Before you emasculate an interesting sounding engine have you tried coil ignition?
It sounds to me that even in your dilapidated condition you are getting it over TDC, just not quick enough for a magneto system to give a decent spark. I understand the concept of not relying on 'electrickery' but with a Walkernator you should have no problem.

 

[Robert Watson]

Tony Cording's Comet is a 636 (90 X 100 I seem to recall) running 11:1 (well that was before Texas) and it was a pretty easy starter. Did it just like a Thruxton Velo. Ease it just past compression and give it a boot. It is on a BT-H for spark.

 

[vibrac]

Canadian boots? I eased it past compression gave it a hefty kick it would hit the next compression and bounce back ignition at 4 degrees BTDC (but not lets go to ignition again ) anyway its having a 2mm compression plate now and like it