Compression ratio

[Chris Launders]

I've just measured my Norvin compression as it's always been very easy to turn over and I was retiming it anyway, I got 90cc of oil in, I make this 6.5:1
no wonder it turns over easy, best save up for some different pistons me thinks.
Chris

 

[davidd]

I just recently did some combustion chamber and port molds. I also CC'd the heads. One of the heads was an unused stock Terry Prince 500 head. It measured 86.878 cc. I did the entire head placing the plexiglass on the joint surface, which was 110 cc. I then subtracted the spigot area for the cylinder which was 23.122 cc. Assuming my method and math are good the number should be right.

David

 

[Martyn Goodwin]

Bill, May I suggest that you include Terry Prince in this conversation - he designed, manufactured and supplied the parts and may well have the answer at hand.

But from what I have read so far the underlying issue is not what is the compression ratio but rather that the bike is impossible to start without resort to the decompression lever.

My Comet has a standard head and crank, piston is a CP alleged to give 8.2:1 compression and I have a piston clearance, when built around 4,0000 miles back, of just 0.003". If I do not use the decompression lever I cannot start the motor with the kick starter. I can put my portly 15 stone on the kich start lever without overcoming the compression pressure. BUT if I use the decompression lever its a very easy Comet to start. I will confess that it did take me a couple of weeks of frustration in learning how to use the decomp and kick start levers correctly. As Bill Thomas said earlier its a matter of a measured swing on the kick starter and releasing the decomp lever smartly around 60% through the swing.

Is there someone nearby who could demonstrate the technique for you?

Oh of course I assume that the other 2 things required for an easy start are in tip top condition , being a supply of the correct air/fuel mixture and a healthy and correctly timed spark.

Martyn

 

[Monkeypants]

Terry Prince heads areca totally different animal than standard hemi heads or the hemi heads Godet has been using until recently in his 1330 Eglis.
Terry and I had a conversation about this a couple of years ago at the Vincent Rally in Kanab, Utah. He explained that with the large squish area, engines with his heads are much harder to turn over than a Vincent engine of the same size and compression ratio with conventional hemi Vincent heads.
Dag Riise was unable to kickstart his 1200 Prince head Egli. He was fitting a Grosset estart to the bike a fe months ago, haven't heard if that worked out , but perhaps if the compression release is used along with the estart, it might.
My 1360 with10.5 to one cr. Prince heads does kick start, but if for some reason it doesn't catch in one or two kicks, I am done. It will only roll thru with kickstart on the rear cylinder ( big space) and the moment a little leg strength is lost, it just bounces of compression. Body weight alone will not ge th

 

[Monkeypants]

get the job done. The method is to first position the engine in the right spot, that is just thru compression on the front cylinder, the leap on the kickstart. As you are coming down, the knee bends a little, then the leg muscles ( hopefully) straighten the leg out to give a little more rotational speed before htting compression on the rear. I also have a slightly longer than stock kickstart.
Glen

 

[Monkeypants]

BTW, the Cometic base gaskets sold by the Chicago section are quite thick and fit the oversized Prince 92 mm bore liners perfectly, I don't know why. I used two of these plus a shopmade 12 thou paper gasket in middle to get the desired squish clearance. They seal completely.
The combustion chambre of my Prince heads measured 77ccs.
Glen

 

[Bill Cannon]

Thanks for the new input guys.
I have been in contact with Terry and he confirmed that the pistons are not 9:1 as expected.
I have just picked up some 3mm compression plates which should be fitted during the coming week. These should bring it down to about 8.3:1 compression.
Terry supplied one new liner to replace the one I damaged by dropping the cylinder on the workshop floor! This should be back from boring/honing tomorrow.
I haven't currently got the kickstart mechanism fitted so I am expecting the Grosset starter to do the business.
I'll post if it starts and slit my wrists if it doesn't!
Regards Bill

 

[Monkeypants]

Bill, what will the squish band clearance be with the 3 mm compression plate?
Glen

 

[Bill Thomas]

Who cares, As long as it starts !! Sorry, Bill Thomas.

 

[Monkeypants]

If the squish clearance is too great, it might as well not be there, somewhere around 80 thou it becomes completely ineffective.