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Discussion Starter #1
Does anyone know where or how to get Compression gauge reading Specs? Not the comparison between cylinders. A maximum reading capable from a stock motor.
For example. If you had a stock fresh build and broken in, ZC SOHC. What is the maximum read you could see on the compression gauge?
 

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It is completely variable based on temperature. Factory specs have a range for this reason.

It is more important to see compression pressure similar across the cylinders rather than specifically a certain rating.


Now if all 4 show 40psi exactly even, that is obviously still a very low value.

I would expect a typical zc soch vtec which is similar to d16z6 or d16y8, to show 90-140 psi at lower temps. Can be as high as 200psi at hotter temps where the rings are sealing a bit better due to expansion.

Test the compression properly, then just go by how they compare to each cylinder. Disable fuel and ignition, hold the throttle fully open, crank for 10 seconds each test.

Make sure the battery is healthy, charged up, and gets topped off prior to each cylinder test, that way you do not get weird results based on the starter lagging after each test
 

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Discussion Starter #3
Thanks. And I understand all that. Let me see if I can make this a little more clear. We are trying to see if a motor is stock or has it been changed to get higher compression with out taking it apart and measuring ever part in the motor.
For example we have some guys claiming 225 psi is normal and stock .We are trying to find a way to check them easily.
 

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There is no easy way to check from compression testing unless you can get the temps and baro from a stock engine during comrpession testing.

heck, incorrect valve lash can mess with compression readings.

You can stick a boroscope down a spark plug hole and see if it is a stock piston. With a good enough bore camera, you can read the factory casting number if the piston is clean.


Otherwise, it is too easy to pop the cylinder head off. If at all worried about the engine, now would be the best time to toss in a new MLS head gasket and timing belt.
 

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You can attempt with compression, but you can never know for sure. You would need a lot of testing on known stock engines, and known modified engines, in both good states, and maybe even bad states? different temp ranges, to get enough data resolution to even come close to attempting to identify in cylinder mods using compression.

You could probably guestimate rod and piston lengths and heights with some kind of stroke gauge (long ass 1/4" extension), coupled with some plasticine shaped into an L shaped straw stuck down in the cylinder through the spark plug hole that would squish to hopefully show if P2V and quench are tighter than typically found when stock.

This MIGHT allow you to figure out if someone has longer rods/higher comp height pistons/thinner HG/shaved head or block compared to stock.

Of course you would need stock measurements with this tooling and noted results, then attempt a test with a modded engine and see how well you can tell if in cylinder components changed.
 

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Discussion Starter #6
I realize all that. Believe me when I say we have debated this topic to death. It is just that someone found that spec for a different motor, a so call min - max. We are hoping to find the same for the d-motor
 

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with correct cam timing on mild and correct lash, 20 psi per point of compression is a general term.

A cold D should be 175-180 cranking.
 

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So you are trying to find the service specs for your engine in question?

Every factory service manual has compression specs, like EFB055 posted above. Are you having trouble finding your manual?

Notice how there is usually only a min spec, with a max deviation, there is no defined max, only a rule of thumb min.

Here's a Y8:

Screenshot_20210222-183040_Chrome.jpg


Here's a Z6:

Screenshot_20210222-183350_Chrome.jpg


Here's a 2014 Ram 1500 5.7 hemi:

Screenshot_20210222-183440_Chrome.jpg
 
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