Topic: Just cann't get my head around this

[clarkjh]

Here goes

Went 2nd oversize pistons on my 550, that shouldn't chance CR right?  Now I had the cylinders flattened when it was bored, 1 to 1.5mm maybe, stupid I know I didn't ask.  I also plan on having the head flattened as well, probably be .5 to 1.5mm to clean it up.  Would this bring the CR over 10.5 to 1?  Were will the higher CR show up in the power band?  Am I doing all this work to this engine just to have the thing blowup?  I didn't plan on doing any performance mods.  I want to have her running this year but after 2 years one more to get it right doesn't matter.

James

[mlinder]

Overbore shouldn't affect CR, but any change in deck height, piston height or combustion chamber size will.

[paulages]

if the cylinder diameter (and therefore displacement) has increased, but the amount of space in the head part of the combustion chamber hasn't, wouldn't this slightly impact the CR? if the volume at TDC is still the same (assuming the piston-top geometry is the same), but the volume at BDC is now greater than it was before wouldn't this have a small effect?

[mlinder]

if the cylinder diameter (and therefore displacement) has increased, but the amount of space in the head part of the combustion chamber hasn't, wouldn't this slightly impact the CR? if the volume at TDC is still the same (assuming the piston-top geometry is the same), but the volume at BDC is now greater than it was before wouldn't this have a small effect?

A small one, yes, but without doing the math, I believe the difference is negligible at best.

[paulages]

yeah probably so, but you gotta figure that at 85-184 cc's per cylinder (depending on the bike, obviously), any small change in the chamber can have a big impact.

[clarkjh]

I'm starting to think I'm putting together a time bomb.

Big thing is that I don't want to go past (or even get near) the max on this engine.

James

[MRieck]

if the cylinder diameter (and therefore displacement) has increased, but the amount of space in the head part of the combustion chamber hasn't, wouldn't this slightly impact the CR? if the volume at TDC is still the same (assuming the piston-top geometry is the same), but the volume at BDC is now greater than it was before wouldn't this have a small effect?

Yes..increasing bore will increase compression in a stock chamber. It will do other things too.

[Sam Green Racing]

Skimming the head and block will also throw your valve timing out a tad.

Sam.

[Soos]

To compensate could you add a head gasket extra thick?
I know Cometic will do a .063 (1.6mm) head gasket.(if you got the$$$)
May make up for some or all of the lost deck height.


l8r

[clarkjh]

Sam

when you say a tad, do you mean opening early and closing late or am I going to get valve to piston contact.

James

[Bodi]

It wouldn't be enough to crash valves and pistons, unless you radically decked the head and cylinders reducing clearance a lot.
Because you've made the cylinder head and block stack shorter, the chain is shorter between the crank and cam sprockets. That makes the cam timing slightly late since the chain's pulling down the front of the engine. If it worries you can get slotted cam sprockets and degree your cam exactly to specifications (or alter it for different performance).

[Soos]

Went 2nd oversize pistons on my 550, that shouldn't chance CR right?  Now I had the cylinders flattened when it was bored, 1 to 1.5mm maybe, stupid I know I didn't ask.  I also plan on having the head flattened as well, probably be .5 to 1.5mm to clean it up.

Having worked at a engine repair shop at one time.... 1 to 1.5mm is EXTREME (IMO) for an amount to be removed from block deck.
Most engine repair shops will take the MIN. to clean up the surface, plus about 0.05mm.
If you take your head to a good shop, unless your head is warped severly, they should not have to remove more than 0.4mm.
And that should be TOTAL amount removed from the thickness of your head.

At the shop I previously worked at, If we had to remove more than .3mm then it was company policy to contact the customer to inform him/her, and explain possible effects, and get his/her ok.
You may want to contact the place that did your surface grinding on your cylinder block (was it surface ground, or was it cut?) and find out their policy, and ask if you can find out how much was removed.
Possibly talk to the employee that did the work, he may rember if it was recently done. Especially if the shop does not specialize in motorcycle work.

Damn... I wish the U.S. would just convert to the metric system... converting back and forth sucks....


l8r

[clarkjh]

Soos

It's been over a year since I had the work done, and I asked them to just clean up the deck.  These guys do allot of car blocks and the machinist knows what he is doing and never called to warn me about having to take allot off.  I'm still going to have the head cleaned up so might have to go with a thicker custom head gasket for the overbore.

James 

[Soos]

Do you have thge cylinder block loose right now?

Perhaps you could measure your block's height (or have measured by someone) and compare to a stock cylinder block height.
Since it was so long ago... not too likely you could find out like you said...

Perhaps loose assemble it, and rotate your crankshaft, and check to see how far out your pistons stick out(if any).
Since it was done at a engine machine shop, I would bet they just removed enough to clean up like you asked them to.

If it was mine, I would do some grinding/polishing on the head to compensate for the larger cylinders,and get your compression closer to stock.
The area just above the pistons can be enlarged to match the OD of the pistons, and while your are at it remove anything that would impede the flow of gasses in and out.

just my .02...


l8r

[turboguzzi]

I am dropping the head by 1.5 in my racing cb500, that should yeld about 11.5:1-12:1 according to my math, which is indeed quite extreme for an air cooled stret bike. the track and 30 minutes long races with special gas is something else of course.

you could very easily compensate for this by using extra block-cranckase gaskets. each is 0.5mm

best do a dry assembly and measure CR volume and squish. I am machining the pistons to match the 15 degree angle of the small squish ring in the head, otherwise they'll hit the head with 1.5mm removed

[clarkjh]

Soos
My whole engine is boxed at the moment so I should be able to get some measurement.  Just getting over the flue so, not allot of gumpshum at the moment.

turbo
Never thought of adding an extra gasket on the bottom to make up the difference.  With such a high CR, are you still running stock cylinder studs?

James