metri wrote on 01/25/08 at 19:49:04:LANCER wrote on 01/21/08 at 18:57:03:
Quote:HAD THE HEAD SHAVED 0.015" (limited due to head/cyl bolts)
Is there anyway around this to be able to shave the head enough to get the compression to 9.5:1 ?
LANCER wrote on 01/21/08 at 18:57:03:
Quote:CYLINDER SHAVED 0.035"
That's about 1mm overbore I guess... Do you think it's safe to overbore 2mm? How much wall thickness would be left with .080" bored out? ( think 1mm is 39.5 thou, but I may be off. I found 2 sizes of oversized pistons for sale, 1mm over and 2mm over...)Thanks
The mm conversion figure is 39.4 (– actually 39.37 but we’re not to less than a tenth of a thou with bike work!!).
Skimming 35 thou off the cylinder (without touching the head) will take you to 8.99:1.
Upping the bore to 95mm would give you 8.61:1 if you change nothing else. Remember that there’s 4mm of free cylinder above the piston, so if you up the bore you also up the unswept cylinder volume and therefore the total compressed volume (chamber + gasket thickness + unswept cylinder volume).
Up the bore to 95mm
and skim 40 thou, and you’ll be up to 9.29:1.
Taking metal off the top of the cylinder has greater effect than taking metal off the head, because part of the head at the sides of the chamber is metal to provide squish – the effect of skimming the head is reduced by the percentage of the bore area which faces solid metal rather than open chamber.
To get to 9.5:1 you’ll need to remove 58 thou through a combination of head skim, cylinder skim and gasket thickness.
I haven’t confirmed the valve to piston clearance on the Savage. Being a low compression engine (and low revving ie little cam overlap) it may not be a great problem, but don’t bet on that when you’re potentially bringing the valves a full 1.5mm closer!
58 thou is a lot of metal to remove – 1.5mm of slack on both sides of the timing chain – so you’d have an extra 3mm of chain to deal with through the Savage’s already struggling cam chain tensioner. Re-profiling the tensioner guides will help a little, as will slotting the cam wheel so you can dial in the correct cam lobe timing to compensate for so much chain adjustment, but I think of you go more than about 1mm of skim you’re opening up a real Pandora’s box of problems unless you have a workshop in your own back garden, plenty of machinery in it and a lot of spare time!!
Skimming is a great answer on pushrod engines – just shorten the rods to keep the valvetrain geometry as it should be – but with overhead cams it can get ugly. If you want to go for anything more than a mild CR increase and thin skim with the Savage you’re going to need a high compression piston. I’ve looked around and can’t see anybody listing them – I considered ordering up a one-off, but is it worth the cost in terms of dollars per horsepower?