compression check

KaZooCruiser wrote:
[ .[/quote]

I have a question:

If atmospheric pressure is 14.7 (at sea level?), and the compression ratio is 9.5:1 (stock factory spec),

anything above @ 140psi seems indicative of a problem.[/quote]

comp. ratio is just a calculated ratio. says nothing about actual compression so forget those figure:laugh:
I measurd it with stock cams in my kz900 was 165 psi, then with hotter cams it was 135 psi (cold engine)

If valves are closing why would hotter cam make any
difference?

Because Harry, The physics of gasses at engine speed and idle cranking speeds are not well connected. Different cams have different amounts of valve overlap. This overlap may add power at speed and reduce compression when the speed is limited to what the starter can crank out.

kzcsi

With 175 pounds at test. What fuels are you able to run?
kzcsi

TODDZ wrote:

Thanks I did the check 175,175,175,175 psi

That bike must run like a rapped date:blink: :woohoo:

comp. ratio is just a calculated ratio. says nothing about actual compression so forget those figure:laugh:
I measurd it with stock cams in my kz900 was 165 psi, then with hotter cams it was 135 psi (cold engine)[/quote]

The hotter cams have a later intake closing thats why you have less cranking compression.

KZCSI wrote:

With 175 pounds at test. What fuels are you able to run?
kzcsi

Has to be running 93 octane.

KZCSI wrote:

Because Harry, The physics of gasses at engine speed and idle cranking speeds are not well connected. Different cams have different amounts of valve overlap. This overlap may add power at speed and reduce compression when the speed is limited to what the starter can crank out.

kzcsi

A common misconception is that camshaft overlap reduces cranking cylinder psi. NOT SO Over lap indeed reduces peak combustion psi by diluting the incoming intake charge with exhaust,and by sending some of the combud-stible mixture out past the exhaust valve before ignition, but this does not effect compression presure.
The intake valve closing event is the only related factor that noticably effeccs compression psi.

JMKZHI wrote:

What steel said.

Warm engine (preferably), Wide open throttle (WOT), spark plugs removed, compression tester installed, press & hold the starter button.

The following is what I got with my compression check on an 81 KZ650 & using a $10 screw-in type tester:

Pressing & holding the starter button for a few seconds (4-5?), the gauge needle fluttered a bit immediately upon pressing the button, then the first pulse went up to ~50 psi (or thereabouts), the next pulse went on up to ~145 psi (or thereabouts), and the psi's maxed to about 180 (or thereabouts).

JMK.

YOu need to crank over the egine and count the times it takes to get maximum compression ( when the needle stop climbing). The reason for the first crank being so low is that you added a hose that has to fill with air before it moves the gauge.

A gauge that has shrader valve at the gauge reads lower compression then a gauge with the shrader vavle at the bottom of the hose. Roughly 10 psi difference.

A bike that has under 130 on cranking compression will have a low running efficiency.You will experience slugish power and low throttle responce. A bike with 130-160 will have better efficiency. Decent power and snappier throttle.A bike with 160-180 will have optimum efficincy. Instant throttle responce and you will get the most power per cc out of your engine.


Bigger is not better when it comes to cams. You need to look at the timing of the lobes. Most (bigger cams) will lower cranking compression, thats why it is good to get higher compression pistons or lower cc head. You might have 12:1 static compression(sweep volume), But dynamic compression (actuall cylinder psi) can be 130 without the right combination.

Adjusting valves can play a big roll in determining this too. If the vavles are set at .003 it will close the intake valve later(lowering the compression) If the valve is set at .008 it will close the intake valve sooner (raising the dynamic compression) 4-8 psi.
Shaving the head or lowering the deck height can change valve timing too. Shaving can retard vavle timing and lower dynamic compression. Thats why shaving on overhead valve engines you wont see a big jump in compression. Thats why you should degree cams.