Which way is correct? (compression testing)

[Waternut]

I always thought I was testing the compression on my engine properly until one day I decided to play with the fittings provided... I used to just attach the hose to my engine and crank away with the throttle pinned. That would give me 195psi. Then I added the long reach fitting because it fit and looks similar to the spark plugs. I thought that adding the fitting would increase the pressure slightly but no. With the long reach fitting attached, the numbers measured 175psi.

That's a pretty big difference...Which way is correct?

 

[Scraggly]

use the one thats the same size as your plugs

 

[WFO Speedracer]

x2

 

[LBE]

I have heard different things about holding the throtle open. Don't mena to thread jack, but does anyone have more info on this? I don't think it would matter opened or closed.

 

[Scraggly]

I have heard different things about holding the throtle open. Don't mena to thread jack, but does anyone have more info on this? I don't think it would matter opened or closed.

i believe thats more for lubrication while cranking the engine over.

 

[Waternut]

Wow that's horrible news... That means the SJ I recently bought with a westcoast head produces a whopping 155psi instead of 175psi which I originally thought.

 

[rollin5150]

My guess is that you have a leak when attaching the longer fitting.

 

[restosud]

throttle wide open always.usually 4 to 5 strokes as more will give you a higher reading that doesn't reflect reality with engine running
i doubt the small difference in thread lenght changed by 20lbs.
i would be suspicious of the gauge or different testing method.

 

[2lick]

155 is not bad at all on a stock ski.... im guessing leak with attachment also....

And don't forget all gauges read different and usually are wrong.... just look for them to be even and figure the actual is +-10psi

 

[WFO Speedracer]

To get a truly accurate reading you need either a Snapon or MAC gaugeand you have to use the proper hose,I cannot stress this point enough.

 

[nightrider05]

I am failing to see a 1 inch extension would make a 20 PSI difference?

I have seen various lengths of compression tester hoses...and they all usually read the same. I also think maybe you have a leak of some type.....Maybe im wrong and those other longer gauges are setup differently though

 

[Waternut]

Yeah it's definately a cheap tester and I'm sure it's not that accurate.

So you guys are thinking that the attachment should not be giving such a significant difference? I can't find any leaks in the tester anywhere. Possibly just a tester that has crapped out?

 

[Mark Gomez]

Less resistance when cranking wide open IMO.
But I still dont know to this day whether or not it makes a difference in the accuracy of the final reading.

On customers sit downs I service I leave the throttle alone because I only have so many hands.

 

[WFO Speedracer]

If anything the longer attachment should take up more space making the combustion chamber in effect smaller and the reading should be higher,you got something going on there for sure.

 

[Waternut]

If anything the longer attachment should take up more space making the combustion chamber in effect smaller and the reading should be higher,you got something going on there for sure.

That was my original thought process... Maybe I can find a snap on or mac tester without breaking the bank. :dunno:

 

[nightrider05]

I just know I have the same attachment on mine and its gave me accurate readings with it on...I have compared it to the Snapon ones in school.

I have always been taught to hold the throttle wide open...it wont make alot of difference but wont hurt either. You want to get as much air into the cylinders as possible.

 

[Soul Man]

With my Snap-On gauge I get exactly the same reading throttle open or closed. I now test closed just because it is easier.
The Snap-On may break the bank: I bought the motorcyle kit, but it doesn't have the long extension. The extension ALONE is almost $60 (http://buy1.snapon.com/catalog/item...&group_ID=1423&store=snapon-store&dir=catalog).
The regular set has standard and long reach adapters that should work, and is $199 (http://buy1.snapon.com/catalog/item...&group_ID=1424&store=snapon-store&dir=catalog)

The cheap ones I think are OK if you are just looking for CHANGES rather than absolute numbers. If you want to follow you cylinders to see when the pistons wear, or look for a difference in one cylinder compared to the other I think they work fine.
Do several back-to-back tests and if you get the same # everytime then you know the gauge is consistent, which may be all that you need.
ie: it may read 165 when it really is 175 or 155, but if you find 3 months from now you are 145 you still know rings are going bad.

 

[#ZERO]

It's amazing what a little adapter with a few CCs of volume will do to the compression numbers.

It's also makes a difference if you're holding the throttle open, doing it with fully charged battery or having all the plugs out.

A good compression tester will come with quick release hoses for each spark plug type with equal amounts of volume.

 

[Waternut]

The cheap ones I think are OK if you are just looking for CHANGES rather than absolute numbers. If you want to follow you cylinders to see when the pistons wear, or look for a difference in one cylinder compared to the other I think they work fine.
Do several back-to-back tests and if you get the same # everytime then you know the gauge is consistent, which may be all that you need.
ie: it may read 165 when it really is 175 or 155, but if you find 3 months from now you are 145 you still know rings are going bad.

Well I agree and this gauge is very consistent in terms of its max readings for my engines. I'm just kinda curious to know where I stand in terms of compression and fuel requirements. 175 is fine for pump gas...195 is pushing it.

 

[750SX]

You could always forget about the gauge and figure out your true compression ratio.