Compression test dry or wet?

[samo_ott]

A question for folks. When one does a compression check on an engine can it be done out of the water? As I always submerge the water pump when I do a compression test but it would be a lot easier if I could test it dry and on the saw horse. Do others do this or do they do it wet only (submerged)? Would it cause too much stress on the dry impeller? Maybe... Comments? thx...

 

[HighTrim]

Re: Compression test dry or wet?

I wouldnt call it dry, but you can do it in your driveway with muffs on or submerged in a tank.

Never want to run your impeller dry for any reason.

 

[dyzz1128]

Re: Compression test dry or wet?

I am not sure if you are supposed to do it that way or not, but i checked the compression in my boat yesterday evening dry and i got accurate readings. Just dont let is sit there and run while it is dry, let it turn over a few times to get that pressure and turn it off. I would take at least 3-4 readings on each cylinder and take the average to make sure it is accurate.

 

[Molaker]

Re: Compression test dry or wet?

Personally, I test compression with ALL sparkplugs removed. I don't find it necessary to "run" the engine while testing compression. Consequently, there is no concern about damaging the impeller.

 

[walleyehed]

Re: Compression test dry or wet?

That's not the whole truth...It takes only a few seconds to heat and damage the impeller...you can and WILL damage the waterpump everytime you turn it over dry with the starter...if the starter is spinning the engine at 400-500RPM, that's the speed of the impeller too...that hard rubber against the housing gets melting hot in only a couple of seconds.
I've got impellers laying in piles that I've changed out this year that have been started dry...they get hot, take a new set, then lose there effective pressure on the housing.

 

[Scaaty]

Re: Compression test dry or wet?

That's not the whole truth...It takes only a few seconds to heat and damage the impeller...you can and WILL damage the waterpump everytime you turn it over dry with the starter...if the starter is spinning the engine at 400-500RPM, that's the speed of the impeller too...that hard rubber against the housing gets melting hot in only a couple of seconds.
I've got impellers laying in piles that I've changed out this year that have been started dry...they get hot, take a new set, then lose there effective pressure on the housing.

Wrong...do it dry. Pull all the plugs..ground the ING...one cylinder at a time, and ya only need 5..6 revolutions at most to get a reading. Do all the cylinders, write them dowm, then do it again after squirting a bunch of oil (WD will work) and do it again. See if an inprovement. Starter AINT gonna spin the impeller enough to get it hot at 5-6 rev's..(50 years of doing it..)

 

[jtexas]

Re: Compression test dry or wet?

Personally, I test compression with ALL sparkplugs removed. I don't find it necessary to "run" the engine while testing compression. Consequently, there is no concern about damaging the impeller.

You don't want the engine to run during a compression test anyway. You really should ground the spark plug leads, whether the plugs are out or just disconnected.

Myself, I disable spark by throwing the emergency kill switch; alternatively, you could just leave the key turned "off" and jump the solenoid.

Here's a follow-up question: warm engine or cold?

 

[HighTrim]

Re: Compression test dry or wet?

I always bring my motor to operating temp b4 testing....

 

[Molaker]

Re: Compression test dry or wet?

Bringing up to temp is a good idea if you are trying to set a benchmark. But, if you are testing to find out why it misses or otherwise doesn't run right, cold should adequately tell the story.

 

[walleyehed]

Re: Compression test dry or wet?

Wrong...do it dry. Pull all the plugs..ground the ING...one cylinder at a time, and ya only need 5..6 revolutions at most to get a reading. Do all the cylinders, write them dowm, then do it again after squirting a bunch of oil (WD will work) and do it again. See if an inprovement. Starter AINT gonna spin the impeller enough to get it hot at 5-6 rev's..(50 years of doing it..)

I don't give a rat's rear how long you've been doing it wrong...give good, accurate info from proper training and experience then customers don't come back wondering what happened.
In the last month I've changed out about 10-12 housings and impellers that were in need of pumps (some only a year old) and about 5 of these guys admitted to dry-starting to check batt.
BTW, to properly check compression, the throttle cable must be disconnected and plates held wide open...get it right.:redface:

 

[Scaaty]

Re: Compression test dry or wet?

I don't give a rat's rear how long you've been doing it wrong...give good, accurate info from proper training and experience then customers don't come back wondering what happened.
In the last month I've changed out about 10-12 housings and impellers that were in need of pumps (some only a year old) and about 5 of these guys admitted to dry-starting to check batt.
BTW, to properly check compression, the throttle cable must be disconnected and plates held wide open...get it right.:redface:

dry-starting to check batt.
Its a comp check, not a dry start..

to properly check compression, the throttle cable must be disconnected and plates held wide open
not on a two stroke

 

[Scaaty]

Re: Compression test dry or wet?

Bump (and will continue 'till proved wrong)

 

[walleyehed]

Re: Compression test dry or wet?

I consider my information accurate and helpful to most.
Why would you want the plates closed on an engine with reeds???
The CUSTOMER done the dry-starts, not me...if you understand that, you hit the key and see if the batt has enough juice to turn the engine over...sometimes it starts but the key is quickly turned off....very short time from spin to off...the same thing you are doing by cranking dry on your comp test.
I've made no idiot of myself and I have about 90% of the members here to cover my "6".....If that fails, I have authorization to edit, delete any post (or Member) here if personal attacks are made.
I would suggest you call any of the major manufacturers and ask the proper procedure.

 

[Scaaty]

Re: Compression test dry or wet?

Dude, I build motors (50 years now) for Harleys..own probably 25 outboards and way too many 2 wheelers (want pix?). On a compression check, its a couple spins (4 plenty, 2 will do really). Been a Machinist for 21 years, and I know damn well what ya need to heat up an impeller, and it a aint a short spin for a comp check.
Two strokes will be open to the atmosphere on every downstroke...through the exhaust port..throttle not needed for a comp check ..4 cycle a different story...

 

[walleyehed]

Re: Compression test dry or wet?

This would be a good time to hand this thread back to the original poster and maybe you and I stay out.
I could tell you the countless years we were OMC dealers and the fact I build precision powerheads...more and more every year. I could tell you I'm a Master tech on several brands of outboards...it don't matter. The key would be to give accurate info...truth shows in the end.
Let's leave this ON topic now and continue on.

 

[Scaaty]

Re: Compression test dry or wet?

I have authorization to edit, delete any post (or Member) here if personal attacks are made

You called me a fool first, and I know I'm right...

 

[walleyehed]

Re: Compression test dry or wet?

I don't give a rat's rear how you do it is hardly calling you a fool.
Once again, I will ask that we leave this thread to the main topic poster...you and I don't need to stir this up.
And, BTW, I wonder if "my" method IS wrong, why does it always fix the over-heat problem when the pump may be only 1 or 2 years old? Why is this "young" impeller shot with 30-50hrs a year?

 

[Scaaty]

Re: Compression test dry or wet?

My post has been edited...
But...
I build precision powerheads...more and more every year. I could tell you I'm a Master tech on several brands of outboards
And you don't know this?
Again..
Two strokes will be open to the atmosphere on every downstroke...through the exhaust port..throttle not needed for a comp check ..the crankcase does NOT compress anything, only pushes whats in it up to the intake port when it opens..block it off, and the exhaust port will fill the vacumm.....4 cycle a different story...
EOS..seya

 

[walleyehed]

Re: Compression test dry or wet?

You seem to be a bit confused as to how a 2-stroke scavenges...ever wonder why there is vacume on the front of a carb throat or how a choke plate works? ever hear of port-timing? "Max" flow will not travel back through the exhaust port on a comp check....ever wonder why that deflector is part of the piston top?
If we decide we want to get technical, let's do it right and do a proper leak-down test at TDC.
If you want to post my quotes...finish them so they make sense.
You and I are done on this thread unless asked to be included...we've waisted enough space...

 

[Scaaty]

Re: Compression test dry or wet?

Your post are well quoted by you...I just took a chunk of it...
Look, the SIMPLEST thing in the world is a Comp Check. A two cycle Pressurizes the mixture in the crankcase, and when the port opens, in it comes. The COMPRESSION takes place when the piston CLOSES the intake port ,,,yep, extra just got scavanged OUT THE EXHAUST..but AT THE SLOW SPEED OF A COMPRESSION CHECK.. IT DON"T MATTER. Darnit...a Comp check can be taken with the intake completly blocked off..all ya needs is the the damn exhaust port..(and yep, raced 2S Dirtbikes for longer that I care to admit..on dirt AND ice..Goodnight..

(And you explain to me how a leakdown test will tell you the C/R...)
Put it this way..NEVER have I tourched a pump doing a CT