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Checking compression with engine out - (No Bay Window Content)
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Abscate Premium Member
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PostPosted: Wed May 04, 2016 6:53 am    Post subject: Checking compression with engine out - (No Bay Window Content) Reply with quote

I have an engine for sale which I would like to test and document compression with the engine out, rather than installing.

Whats the best way to do this?

I have motors, sockets, tools etc and could easily spin it with the flywheel nut and a motor, but am interested in what rpm a starter typically turns over at. Im guessing its low, like 50-100 rpm at best.

Anyone give guidance here?

Or, could I get a reasonable test of compression with using a lawnmower type strap on the generator pulley??
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PostPosted: Wed May 04, 2016 7:32 am    Post subject: Re: Checking compression with engine out - (No Bay Window Content) Reply with quote

You need to have a bellhousing you can bolt up to the engine if you want to generate compression numbers, plus you would have to connect up a starter, battery, and rig up the ignition. A leakdown test can be done without spinning the engine though and in many ways is preferable.

If you just rotate the engine a couple of turns by hand you can learn a lot but will get no numbers.
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PostPosted: Wed May 04, 2016 7:52 am    Post subject: Re: Checking compression with engine out - (No Bay Window Content) Reply with quote

typical starters are between 250-350 rpm if my memory serves me
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PostPosted: Wed May 04, 2016 8:00 am    Post subject: Re: Checking compression with engine out - (No Bay Window Content) Reply with quote

skills@eurocarsplus wrote:
typical starters are between 250-350 rpm if my memory serves me


Is that starter rpm skills? That would make engine rpm something like 9/109 or 25-30 rom under crank...
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PostPosted: Wed May 04, 2016 8:05 am    Post subject: Re: Checking compression with engine out - (No Bay Window Content) Reply with quote

You're going to need a dc motor to turn that engine. I doubt something ac has enough start up power for the job. Plus I think just using a regular starter, 12V battery, and some kind of ring to mount the starter to the flywheel would simply work the best. As stated, a differential compression test could be done without needing to spin the engine. You could never pull the engine through with a rope like you do your lawnmower. Well, maybe Arnold could have done at one time.
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PostPosted: Wed May 04, 2016 8:12 am    Post subject: Re: Checking compression with engine out - (No Bay Window Content) Reply with quote

Abscate wrote:
skills@eurocarsplus wrote:
typical starters are between 250-350 rpm if my memory serves me


Is that starter rpm skills? That would make engine rpm something like 9/109 or 25-30 rom under crank...


that's just what I know from service info...not sure of the ratio per se
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PostPosted: Wed May 04, 2016 8:34 am    Post subject: Re: Checking compression with engine out - (No Bay Window Content) Reply with quote

aeromech wrote:
You're going to need a dc motor to turn that engine. I doubt something ac has enough start up power for the job. Plus I think just using a regular starter, 12V battery, and some kind of ring to mount the starter to the flywheel would simply work the best. As stated, a differential compression test could be done without needing to spin the engine. You could never pull the engine through with a rope like you do your lawnmower. Well, maybe Arnold could have done at one time.


I grew up watching my mother start her Beetle with a leather purse strap on the generator pulley whenever she left the lights on. Years later, you could buy that as an accessory from the JCW catalog.

With the plugs out for a comp test, I think a small motor would do it. I have big motors too, though,
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PostPosted: Wed May 04, 2016 8:36 am    Post subject: Re: Checking compression with engine out - (No Bay Window Content) Reply with quote

aeromech wrote:
You're going to need a dc motor to turn that engine. I doubt something ac has enough start up power for the job. Plus I think just using a regular starter, 12V battery, and some kind of ring to mount the starter to the flywheel would simply work the best. As stated, a differential compression test could be done without needing to spin the engine. You could never pull the engine through with a rope like you do your lawnmower. Well, maybe Arnold could have done at one time.


i dunno....


Link

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PostPosted: Wed May 04, 2016 8:42 am    Post subject: Re: Checking compression with engine out - (No Bay Window Content) Reply with quote

Okay, but why don't you try it for us and let us all know how it goes? It might just work. I've been wrong before.
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PostPosted: Wed May 04, 2016 8:46 am    Post subject: Re: Checking compression with engine out - (No Bay Window Content) Reply with quote

you need a consistent RPM to get even compression. You can use compressed air at TDC to check rings and valves for leakage. That is just as good.
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PostPosted: Wed May 04, 2016 9:10 am    Post subject: Re: Checking compression with engine out - (No Bay Window Content) Reply with quote

aeromech wrote:
Okay, but why don't you try it for us and let us all know how it goes? It might just work. I've been wrong before.


haha, not today good sir! I like my engines where they are, in the cars! But if you wanted to get real ghetto you could strap it to a pallet, hook up a battery and give it a pull. I dunno...was more of a tongue in cheek response. I mean, no one's gonna pay real money for a used engine they can't see run, so Steve's suggestion of compressed air test is just as good as any and doesn't require nearly as much effort as building a test stand or jury-rigging it somehow.
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PostPosted: Wed May 04, 2016 10:17 am    Post subject: Re: Checking compression with engine out - (No Bay Window Content) Reply with quote

You need a minimum of 120 engine rpm for compression testing.

Image may have been reduced in size. Click image to view fullscreen.


Good luck
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PostPosted: Wed May 04, 2016 3:19 pm    Post subject: Re: Checking compression with engine out - (No Bay Window Content) Reply with quote

Thanks TCash - on the spot again, as usual.

I can do that easily with available parts..
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