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Compression: What is acceptable?
Posted: Thu Jun 12, 2014 1:54 pm
by Snotcycle
Went to go look at a blacktop and externally looked ok.
Pulled off the valve cover and looked fairly low mileage although looked like it had been sitting in the warehouse for a bit.
Compression test number (1-2-3-4):
180-150-200-200
On a JDM 20v Blacktop do you think its just a head gasket failure or something more dire?
Re: Compression: What is acceptable?
Posted: Thu Jun 12, 2014 1:59 pm
by oldeskewltoy
Snotcycle wrote:
Compression test number (1-2-3-4):
180-150-200-200
On a JDM 20v Blacktop do you think its just a head gasket failure or something more dire?
can't really tell...... likely just a head gasket... but could also be more serious (broken ring? bad, or burnt valve, or seat?)
Re: Compression: What is acceptable?
Posted: Thu Jun 12, 2014 2:12 pm
by Snotcycle
Re: Compression: What is acceptable?
Posted: Fri Jun 13, 2014 1:32 am
by totta crolla
That difference is not really acceptable.
It might be worth checking the valve clearances on the low cylinder.
Was the test done with the engine in the car, at operating temperature and the throttle fully open ?
Re: Compression: What is acceptable?
Posted: Fri Jun 13, 2014 6:51 am
by Jimmee1990
A blown head gasket gicing low compression is generally between two cylinders, being so far down in just one cylinder looks like valve sealing to me. Considering it's a blacktop which is an interference motor, I would say it's possible it has a bent valve or valves in that cylinder. Does the reading change with oil squirted in the cylinders?
Re: Compression: What is acceptable?
Posted: Fri Jun 13, 2014 2:37 pm
by Snotcycle
Those compression numbers were on a cold motor. They put some oil in it (the engine not the cylinders) and ran it on this bench thing with an electric motor EDIT: with the throttles open and a belt around the crank pulley.
When they were cranking the motor nothing sounded out of order inside it. I feel like I could've heared a bent valve or something if it was there.
EDIT: Do you guys think those headbolts look rounded or it that just from some sediment in the bolt head (there definitely was SOME sediment in the bolt heads)
Re: Compression: What is acceptable?
Posted: Fri Jun 13, 2014 3:58 pm
by oldeskewltoy
sediment... btw... that is a CLEAN blacktop
frequently they come like this......
Re: Compression: What is acceptable?
Posted: Fri Jun 13, 2014 10:14 pm
by yoshimitsuspeed
Jimmee1990 wrote:A blown head gasket gicing low compression is generally between two cylinders,
There are a crazy number of ways a HG can fail.
You have the cylinders, oil passages, coolant passages and outside the motor. A failure can be between any combination or multiple combinations of those.
Re: Compression: What is acceptable?
Posted: Sat Jun 14, 2014 1:48 am
by totta crolla
What does the person selling the engine say about this ?
Personally I would walk away unless the seller was prepared to allow for some repairs their price.
Unless the valve clearances are incorrect the head is going to have to come off so allow enough money for a head gasket change.
Re: Compression: What is acceptable?
Posted: Sat Jun 14, 2014 2:42 am
by allencr
It's been washed down with something. Sediment? No such thing, only dried/burnt oil that covers most things without direct flow or contact equally, or not at all. NO engine looks like that. Wonder if it was even elec. motored with gasoline/kerosene/mineral spirits/solvent in it?
****
Always check the compression on a low one twice, so it will have pounded out any carbon/smegma that may have stuck on the seat & for your own curiosity/satisfaction.
Re: Compression: What is acceptable?
Posted: Sat Jun 14, 2014 2:23 pm
by Snotcycle
We did retest compression after turning the thing over for a bit. The low cylinder came up into the 175-180ish range and the others stayed more or less the same.
Billzilla: could you elaborate on "Sediment? No such thing, only dried/burnt oil that covers most things without direct flow or contact equally, or not at all."
I think i am going to buy this motor as the price is right so that even if there is some problem that requires disassembly and a machine shop i will be able to pay for it with the money saved from not buying one of those 1300-1600 dollar motors online. Oh and by the way, this one comes shipped to my door free for $812
Re: Compression: What is acceptable?
Posted: Sun Jun 15, 2014 2:10 am
by totta crolla
Snotcycle wrote:after turning the thing over for a bit. The low cylinder came up into the 175-180ish range
http://www.gregsengine.com/cylinder-com ... sting.html
Re: Compression: What is acceptable?
Posted: Sun Jun 15, 2014 3:48 am
by jondee86
From the above link...
When you were observing the gauge during the cranking of the engine, you
should have noticed the way each cylinder pumped up. Usually, a cylinder will
produce about 40 PSI on the first stroke and about 35 PSI on each additional
stroke until the maximum compression is reached.
Why is that then ? Valves are opening and closing, so each compression stroke
should start with atmospheric pressure in the cylinder. Something to do with
oil getting onto the cylinder walls... or rings ?
Where are you
allencr 
Cheers... jondee86
Re: Compression: What is acceptable?
Posted: Tue Jun 17, 2014 1:03 am
by allencr
Dumb, I shouldn't have written that.
Combustion and its by-products + baked/burnt oil = sludge, normal with abuse from infrequent oil changes and/or frequent short trips. It accumulates everywhere, vertical & horizontal, but a bit less then in a low spot/depression, unlike yours that has a few bolt heads that've been cleaned up.
**
The gauge's internals have to get filled - pumped up.
Re: Compression: What is acceptable?
Posted: Sat Jun 21, 2014 4:26 pm
by jondee86
allencr wrote:The gauge's internals have to get filled - pumped up.
Makes sense... kind if like pumping up a bicycle tire with a hand pump.
Everyone should try to learn at least one new thing every day, and that was mine
Cheers... jondee86
