How does cylinder boring affect Compression Rate?

fluke12

Been Around the Block
I know the specific answer, it affects it differently, depending on ALL the variables, but I was looking for more general, larger cylinders = higher compression, or something like that.

My situation, 1972 CL350, went 1mm over due to burned pistons and new exhaust valves. My compression went from like 150psi on one side (and something really low on the other) to 120psi on each.

I know not all things are equal, like the pistons can have different domes, but i'm just trying to make sure everything is fine and i don't have a leak somewhere.

Thanks for any info!
 
Lower PSI is rather normal on a fresh engine.
Could be that not everything has found its seat yet.

How did you hone the cylinders?
Did you gap the rings correctly?
Are the rings installed correctly? Top to top and such?

Why a 1mm overbore? Just seems silly unless the liners needed it.
They sell overbores in .25mm steps. Going straight to the 1mm isn't going to gain you any power, and means you cant overbore again on most bikes.
But then again few see enough mileage to need pistons again.
 
I sure hope it was honed properly, I took it to a machine shop to do the boring and honing.

I gapped, clocked, and installed the rings, I'm pretty sure everything there was set properly.

1mm - No real reason, the shop said it was pretty bad, and I had access to 1mm so I went with it.

It makes me feel a little better that it could be that everything just hasn't found its seat yet. It was literally the first time I've run the engine since its been back together. Maybe i just need to give it a bit more time and check again?
 
As long as the gap is correct I would cycle the engine a few times and break it in properly.
Then recheck
 
1mm over will probably get you a few extra cc's but I would get less than 0.1 ~ 0.2 compression ratio increase?
 
ring gap does not effect compression much at all,you just don't want the gap too small
proper honing is important for long ring life but also does not effect a comp test very much at all
just run it,break it in hard, but be sure to retorque the head twice once after the first decent heat cycle and another after the first longer test ride
 
So what I'm hearing is, don't really worry just yet. Break in the engine properly and check again. I have faith in my machine shop, a few other people here have used them and we're happy with them.

Xb is helping me sort out a carb issue on a different thread at the same time. That coupled with the fact that I live in Indiana in the winter makes breaking it in properly a little more challenging.

It was unseasonably warm over the weekend and running through the gears, I didn't get a sort of pause at 5k rpm like I did before I took it all apart. That has me very hopeful.
 
If the motor hasn't even been run, then all the compression test is telling u that things are sealed. You don't know the real compression until after you do a hot torque and the rings have seated.

But to answer your question, boring has nothing to do with compression. Compression is only relative to piston dome and valve timing. Did you change camshafts or valve timing? Because if you installed bigger cams changing the overlap, that will definitely lower dynamic compression.
 
OK... so CB350 has a 9.5:1 compression ratio, a bore of 64mm, and a stroke of 50.6mm.

Following the math through, this means the displacement for each cylinder is 162.77cc.

The formula for calculating chamber volume is:
cv = displacement / (compression ratio - 1)
or
cv = 162.77 / (9.5 - 1)

cv = 19.15cc

So... when you increase the core, the chamber volume doesn't change, only the swept volume (aka displacement). If you go 1mm over on the bore, each cylinder is now 167.90cc.

Going back to our equation, the variable is now the compression ratio, not the chamber volume. Algebra is your friend.

19.15 = 167.9 / (x - 1)
19.15 * (x - 1) = 167.9
x - 1 = 167.9 / 19.15
x - 1 = 8.77
x = 9.77

Your new compression ratio is 9.77:1
 
Awesome info everyone. I didn't change anything other than the bore (with matching pistons and rings) and some bent exhaust valves. I wasn't really chasing extra power or higher compression, I just wanted to make sure that if I messed something up, I could pull it all apart again without having to buy a whole new set of gaskets.
 
High On Octane said:
Put 10-20 miles on it and do a hot torque and you'll probably be ok. Have you actually fired the bike yet?
NOpe
torque it to specs once it has cooled down a at least a few hours not when HOT
usa a good accurate inchlbs wrench and torque right at the high end of the spec 5-10% over is ok
 
xb33bsa said:
NOpe
torque it to specs once it has cooled down a at least a few hours not when HOT
usa a good accurate inchlbs wrench and torque right at the high end of the spec 5-10% over is ok
I guess I've been doing it wrong. Thanks for the clarification and sorry for the misinformation.
 
I have fired the bike up, let it idle to warm and run through all the gears at maybe 1000-2000 rpm below redline. That's where I checked compression and came up with low numbers.

I have two different types of torque wrenches so I end up double checking anything that has a very specific torque. I used to over tighten everything so I'm kind of overly cautious now.

It might be a little while before I get 10 miles, considering its 18 degrees and snowing outside
 
High On Octane said:
I guess I've been doing it wrong. Thanks for the clarification and sorry for the misinformation.
some motors may benefit from a hot torque(maybe an all iron job ?) i dunno but honda recomends doing it cold for good reason
an aluminum block and head motor like the 350 with steel thru studs would take a lot more force to tighten up when hot
in otherwords you would not see any looseness when trying to torque they will be tight when hot BUT may actually need retorquing you cant tell till it all cools down because the aluminum grow twice that of the steel
 
You need to load motor. Put it in gear and use rear brake. May need a few applications, try not to overheat berake too much (shoes will glaze over)
 
unless it is for a customer i dunno why you need to test compreshun after a rebild anyway,as long as it runs good and don't smoke like the titanic just enjoy the goddam thing and ride it :D
 
Sonreir said:
OK... so CB350 has a 9.5:1 compression ratio, a bore of 64mm, and a stroke of 50.6mm.

Following the math through, this means the displacement for each cylinder is 162.77cc.

The formula for calculating chamber volume is:
cv = displacement / (compression ratio - 1)
or
cv = 162.77 / (9.5 - 1)

cv = 19.15cc

So... when you increase the core, the chamber volume doesn't change, only the swept volume (aka displacement). If you go 1mm over on the bore, each cylinder is now 167.90cc.

Going back to our equation, the variable is now the compression ratio, not the chamber volume. Algebra is your friend.

19.15 = 167.9 / (x - 1)
19.15 * (x - 1) = 167.9
x - 1 = 167.9 / 19.15
x - 1 = 8.77
x = 9.77

Your new compression ratio is 9.77:1

dilbertAllMathy.gif
 
Back
Top Bottom