96 XP Compression Specs

[JoelS]

Does anyone know what the compression should be on a 96XP? Both cylinders are exactly at 120psi.

[Bryan Glynn]

If it's running ok, you have a bad gauge or are using it incorrectly. Spec is 150 with a proper gauge under proper conditions.

[JoelS]

I believe my gauge is okay but maybe not. It is running fine. This is how I test. I take both plugs out, remove the rave valve cap and hold down the valve on the cylinder I am testing. Is that correct or should I do it another way?

[Bryan Glynn]

You need to have both plugs out and make sure your battery is fully charged to spin the starter at full speed. lso make sure you are using the proper 3/4" reach adaptor not a 1/2".

[JoelS]

Thanks for the info. I will give it a try.

[cameron da river rat]

i read somewhere to recieve accurate compression readings to leave the spark plug(s) in the other cylinder(s) that is not being tested to show actual cylinder compression for that cylinder. and by taking the other spark plug(s) out would actuall show/read a higher compression reading then it should because of a load factor difference there is when removing the other spark plug(s).

that is how i check mine...............i'm not sure on this but it is somthing i read (i think at group k) and thought it made sense to me. [img]graemlins/uts.gif[/img]

[Bryan Glynn]

It effects the crankover speed, which effects the reading a little bit, yes. I went through all my manuals and cannot find one factory endorsed method, nor have I seen anything but opinions on it on other webpages. I look at it this way - I can measure a new OEM engine both ways, and I get a reading closer to factory spec or accepted factory spec with both plugs out. It's all about matching the test procedures set by the OEM, and most people agreee that means all plugs out. I won't argue with someone saying plugs in, because there is no proof either way which is technically correct. In most engines it only makes a few psi difference anyway.

[Bill O'Neal]

I typically place a battery charger onto the battery, crank with the compression gague in one hole, and the sparkplug in the other. I do not hold the throttle open, nor do I touch the rave valves. I count to eight each time I test. In a very good oem stock 781 motor, I see 155 psi on my Snap-On compression tester.

[96xper]

So you push down the rave valve? Why is that?
96xper

[Bryan Glynn]

The whole purpose of the RAVE valves is to raise effective compression at low RPMs. Keeping the valve down gives a *slightly* higher reading. If the valve raises compression is lowered. At starter speed it's hardly noticeable, however.

[ November 14, 2002, 10:44 AM: Message edited by: Bryan Glynn ]

[JoelS]

Thanks for all the info. I tested and got it to hold at 140psi. Will that 10psi make a difference in performance?

[Bryan Glynn]

An actual 10 psi makes a difference, but if you are getting equal readings I would say your gauge is just a bit off - you're fine.

[JoelS]

I took this reading with a gauge that was accurate. So now I have to figure out where I am losing that 10psi.