displacement

Is there an easy way to see if an engine has been bored or modified?

A compression check would be my first guess, although if the engine has considerable wear even that might not be a tell-tale sign of any engine mods.

thats what I was thinking, but if its even across all four and not 8.5:1 it would suggest if has been modified?

pop the carbs or header off and look inside of ports,pop valve cover off see if head has been clearanced for cams,look at front of cylinder does it say 903 or 1015............

RedneckSamurai wrote:

thats what I was thinking, but if its even across all four and not 8.5:1 it would suggest if has been modified?

You can not tell compression ratio by doing a compression test. Compression test only measures cylinder pressure.

Compression ratio is a mathematical equation involving cylinder displacement and combustion chamber volume.

Jay

Right, but if the compression is the same across all 4 and it's higher than my stock motor, wouldn't it suggest it has been modified?

high compression just means good rings it wont tell you the ratio

Your stock motor could have low compression. Mine has around 110 in each cylinder so just about any bike will have higher compression than mine. The only real way to check is pull the head off and measure the bore.

Well I guess there is no easy way. So I just stopped caring about it, until I have to get in there for another reason. Thanks guys.

I picked up a GPz750 motor, and when I did a compression test it showed 180 psi on all cylinders. My first thought was "Wow! I got lucky! This thing must have an 810 kit in it!", so I pulled the head to verify it.

I discovered that it had a stock bore and carbon build up in all cylinders was causing the high readings

Carbon build up is common when the mixture is rich. My 900 had 224 from that. Compression does not mean more power. Once you have more comp. than than the amount of air the heads can flow your done. Ive seen this with auto as well. Anyone with a 12 or higher ratio should definetly have over sized valves. Then cams. Then ..... yeah..... riding across the country, just rained, fresh air, plenty of power, if only i had a sixth gear..... hammer that sucker and listen to it wind up, when the pitch is high and it sounds like its gonna blow, ok here we go rrrrrrrrrrrrrRRRRRRRRREEEEEEEEE,EEEEEEEEEEE,eeeeeee,thats third gear to fifth. pull in the clutch, coast for miles, wow.

RedneckSamurai wrote:

Right, but if the compression is the same across all 4 and it's higher than my stock motor, wouldn't it suggest it has been modified?

Not necessarily. If a motor had aftermarket pistons with 10.25:1, but also had longer duration cams which will bleed off cylinder pressure, you might get it to only pump what a stocker does.

On the other hand, some one who is playing around with cam timing on a stock motor, and spread the overlap out, which can cause cylinder pressure to raise even though the motor is stock.

One has to really measure and examine components to see if the engine has been modified.

Jay

Post edited by: APE Jay, at: 2006/07/16 21:15