Compression: What is acceptable?

[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?

[oldeskewltoy]

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?)

[Snotcycle]

Here are some photos taken of the motor

https://www.flickr.com/gp/124601068@N07/2766f8

20v Blacktop pics by nick_postma, on Flickr

[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 ?

[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?

[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)

[oldeskewltoy]

sediment... btw... that is a CLEAN blacktop



frequently they come like this......

[yoshimitsuspeed]

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.

[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.

[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.

[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

[totta crolla]

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

[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

[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.

[jondee86]

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