Scooterotica

Funny one this, took an AF 175 ally iron liner kit off of a running scooter,AF`s rebored it and supplied a new piston, I then built up another engine using this kit, ended up with so much compression difficult to kickstart it,so took it apart and did some measuring, the head volume was 31cc another head I had lying around had 34cc, so put that on plus a .75mm ally base gasket in place of the .5mm base gasket and .5mm head gasket ended up with a squish of 1.7. Now I measured bore, stroke, exhaust port to TDC and came to a corrected C.R of 5.37 does that sound OK. funny thing was on the other engine case it was ok.

corected? ya numbrs dont add up

What's the raw calculation?

Right ... bore 64,3 stroke 58mm exhaust port height 39mm, compression chamber volume (measured with engine at TDC and plug hole vertical) 29cc. I think the main problem was the cylinder head, which strangely seemed ok on the other engine, could be down to deck height differences.

sounds not far off as the 34cc head is big for such a small cc barrel but remember that if using a decent exhaust that the corrected ratio ill be wrong because it cant take into account of how much the exhaust stuffs the port.

that's some low compression... 7.5-1 uncorrected !! and 3.13-1 corrected allowing for no exhaust stuffing. and you say if feels like its got a lot of compression? are you sure about the head volume? did you grease around the top edge of the piston before you put what ever fluid in when you measured it?

Yes Eden I did grease around the piston, but its not easy to accurately, I was was as accurate as I could be, I still get to 5.37 corrected, how have I got this wrong.

I'm presuming what you've done is

Swept volume = 3.14 * 32.1 x 32.1 x 39 / 1000 = 126.2

Swept volume 126.2 + compressed volume 29 = 155.2/ compressed volume 29 = 5.35

This seems to be a fairly usual way to calculate this but edens calculator gives lower figures, but not sure if he has a more complicated formula??

Edit - I never get the teminology right but I think the calculation is the same...

Those head volumes for a small cc kit seem very large!? Are you sure? How are you measuring them?

Don't worry about corrected compression for now, I'm trying to get my head around the geometric first.

Adam

Well Adam, it all started when a friend asked me to fit an RT195/200 kit to his special, he had the aforementioned AF Ally/ iron liner 175 kit on it, which is basically a tidied up SR / Ebay type kit which ran ok, until he holed a piston, I had the cylinder rebored and a new piston supplied, I then fitted this to a completely rebuilt engine which a friend bought from me, after fitting he said he could not kick it over, to much compression, it was quite free with the plug removed so eliminating any other issues,
I took the head off and measured the volume with the spark plug in and got 31cc, I had a standard head around which I measured the same way an. got 34cc ok I thought this could explain the high compression and also the holed piston possibly, although it did kickover ok.
the squish was 1.55mm. I then fitted the spare head which had a 34cc volume the squish then measured 1.2, removed the cylinder, replaced the std.base gasket with an ally gasket .75mm which did help the piston position in relation to the exhaust port. fitted the other head, and then took measurements as described above. the trapped cc in the head with piston crown then measured 29cc. stroke 58mm ex.port height 39mm with a bore of 64.3, which I calculated to end up with a cr of 5.37 which I thought would be ok.Am I wrong ? there is scope to machine say, .2mm off of the head or fit a thinner head gasket, if this compression now seems to low. my fingers ache now