thefatman Posted April 10, 2013 Report Share Posted April 10, 2013 Right there i've been building up a bigger engine for my little 125 http://pbmagforum.co.uk/index.php?showtopic=41450 So the crank now has a longer stroke, 63mm bore up from 55mm so it should have a bit more umph. I had a choice between two cylinder heads, so I measured them using a syringe, one head gives a cr of 8.9 - 1, the other 14.9 -1. I think it'll need a spacer plate to bring the 14.9 - 1 down to a more reasonable level. What is a good level to aim for? I want to run normal unleaded, max revs is around 9.5k rpm, combustion chamber is 14cc's. So 12-1, 12.5-1, more/less? And is it best to put the spacer plate below the barrel, or between the barrel and the head? Link to comment Share on other sites More sharing options...
porter_jamie Posted April 10, 2013 Report Share Posted April 10, 2013 skim the 8.9 head Link to comment Share on other sites More sharing options...
thefatman Posted April 10, 2013 Author Report Share Posted April 10, 2013 That'd involve tools( i've not got a lathe/milling machine) and making a new exhaust(which I could do) as well it's a twin port vs single port so I was thinking it'd be easier to cut out a spacer plate. Its taking it from 10hp up to maybe 16hp so it's not a high performance machine by any standards. Link to comment Share on other sites More sharing options...
porter_jamie Posted April 10, 2013 Report Share Posted April 10, 2013 do you know how thick the spacer needs to be to make a sensible CR? i would guess at something like 11.5 or 12:1 Link to comment Share on other sites More sharing options...
thefatman Posted April 10, 2013 Author Report Share Posted April 10, 2013 I've just bashed in some number into online calculator and it looks like a 1mm spacer will give 11.9:1 Does that sound reasonable I need to check the measurements as i've not got them to hand. Link to comment Share on other sites More sharing options...
Evilchicken0 Posted April 11, 2013 Report Share Posted April 11, 2013 You need to to make a sandwich of base gasket / spacer / base gasket, so you can cut it out of 0.75mm ally but have a 0.5mm ready in case it's too much. Link to comment Share on other sites More sharing options...
Superdunc Posted April 11, 2013 Report Share Posted April 11, 2013 I would say put the spacer under the barrel and aim for 10.5/1. I don't suppose the cam has a lot of over lap, or the combustion chamber or squish have been optimised, so any higher will result in detionation at lower rpm. Link to comment Share on other sites More sharing options...
thefatman Posted April 11, 2013 Author Report Share Posted April 11, 2013 Thanks i'll go for that then. I wouldn't know an optimised combustion chamber if it jumped up and slapped me in the face so i'll keep it on the safer side of things. I was just hoping to cheat an extra bit of power and economy out of it. Link to comment Share on other sites More sharing options...
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