compression numbers, how bad are these Options

[mobymutt]

Just did a compression check, here's what I measured:

108, 120, 96, 85

I've got a stock 1.8L, but would like to build a 1911 or 2056 in the relatively near future. Hopefully I'll be ok for another year or so with the current setup.

[914Sixer]

Looks like time to rebuild. Go for the 2056.

[DM_2000]

Do a leak down test, that wil let you know where the losses are. Are the valves adjusted properly? ( RE too tight to the point a valve is staying open will lower compression )

[rgalla9146]

A leakdown test should be done as well. It will give more specific info.
The compression test should be done after a proper valve adjustment on a cold engine.
For the compression test the engine should be at operating temp, battery fully charged, fuel pump disabled, wide open throttle and uniform number of compression strokes on each cylinder.
Your engine may be healthier than you think

[mobymutt]

Full disclosure: I did just have the engine apart, but (for a number of reasons) I didn't replace the pistons, cylinders or rings.

There was actually a pretty big 'pocket' in one of the cylinder walls (looked like a casting void), so having at least one cylinder with low compression doesn't surprise me.

Is there an actual spec for what the compression numbers should be on a good engine?

[rgalla9146]

Full disclosure: I did just have the engine apart, but (for a number of reasons) I didn't replace the pistons, cylinders or rings.

There was actually a pretty big 'pocket' in one of the cylinder walls (looked like a casting void), so having at least one cylinder with low compression doesn't surprise me.

Is there an actual spec for what the compression numbers should be on a good engine?

Uniform numbers is the goal.
The actual number is dependent on engine configuration.

[Morrie]

Full disclosure: I did just have the engine apart, but (for a number of reasons) I didn't replace the pistons, cylinders or rings.

There was actually a pretty big 'pocket' in one of the cylinder walls (looked like a casting void), so having at least one cylinder with low compression doesn't surprise me.

Is there an actual spec for what the compression numbers should be on a good engine?

Uniform numbers is the goal.
The actual number is dependent on engine configuration.

Exactly. I have run engines down below 100 PSI compression numbers so long as the numbers do not vary dramatically. 10% of highest compression subtracted from highest number is a reasonable rule of thumb. For example, if your highest compression cylinder is 100, lowest should be 90. This is a quick real world rule of thumb, not a textbook number. Problem with large variation is it places uneven stresses on the lower end of the engine, and can cause problems in the bottom end sooner than later.

[porschetub]

Full disclosure: I did just have the engine apart, but (for a number of reasons) I didn't replace the pistons, cylinders or rings.

There was actually a pretty big 'pocket' in one of the cylinder walls (looked like a casting void), so having at least one cylinder with low compression doesn't surprise me.

Is there an actual spec for what the compression numbers should be on a good engine?

Did you remove the pistons from the barrels and refit the old rings ?.
I always do the compression test as much as 3 times,surprizingly you can get a fair amount of difference,on the 3rd test put a little engine oil down the plug holes if you see a noticeable increase if your rings likely bad.
It really pays to check/reset the valve clearances (cold engine) first ,can make a real difference,also make sure your engine is up to normal running temp when testing.
I've never done a leak down test but it sure seems to be the best check to do.
Compression pressure depends on your compression ratio ,you could have dished low compression ,flat top,or raised top high compression pistons,deck height could have been changed if the heads have been flycut,these are all factors to take into account.
I would say 120 - 150psi would be a healthy engine.
Good luck.

[iankarr]

If you separated the pistons and jugs and didn't replace the rings and hone the cylinders to like-new (fresh crosshatch and no scoring) that could be a major cause of reduced compression numbers. The rings are "mated" to the cylinders and, once apart, that close fit is gone until you hone (or cut), re-ring and go through a seating break-in.

[DM_2000]

I don't do the often repeated "add oil to the cylinder to check ring sealing" Also see below about rings being matched to cylinders.

Oil or any other non compressible liquid added to a cylinder will raise compression ratio because cylinder / head volume has been reduced.

As rings wear, oil will naturally be in the cylinders so adding more 'aint gonna make things seal better.

For a leak down test you don't need a fancy gauge setup. Many compression gauges have the same quick connect fitting as a air compressor hose. Remove the check valve from the tip of the compression gauge hose with a tire valve tool. ( don't lose the check valve, it has a way lighter spring than a tire valve. )

Thread the hose into a cylinder, turn the engine to piston at top , both valves closed. Apply air ( 100PSI is an good even number but anything from 75 to 125 would be fine ) in a quick burst while watching the crank pulley / flywheel. If it turns, rotate in the opposite direction and try again.

When you are able to keep air applied without the engine turning on it's own, listen for air leaking out.

Slight exhaust valve leakage is common on mid mile motors.

Ring leakage is always present and gets worse at the engine ages.

Leaking out the intake will really reduce power.

Leaking between the head and cylinder needs to be addressed.
~~~

As for rings being matched to cylinders, it is possible to remove pistons from cylinders and reinstall without fear of leakage.

Piston rings slowly rotate as the engine it running so the perfect gap stagger of a new engine is soon lost. I've worked on countless engines across most brands and none retain the factory gap stagger.

The only rings that don't spin are 2 stroke piston port engines as if the gap made it's way to a port it would snag.

I'm pretty sure Chrysler did tests in the 50's / 60's with a radioactive pellet in a piston ring to determine ring rotation.

[malcolm2]

The compression test should be done after a proper valve adjustment on a cold engine.

Totally agree. Double check your valve clearances. I thought I had a problem with 1 cylinder. Adjusted the valves and rechecked the compression. I got a better balance of comp #s.

[mobymutt]

Thanks to all for the responses (and patience, as I'm sure this topic has been covered many times..).

When I 'rebuilt' the engine, I did keep the pistons/rings/cylinders as a matching set. The cylinder ID's had no cross-hatching and, I said above, one cylinder one had a big void. Valve clearances have been set post-rebuild.

Basically, I was doing a super cheap rebuild, just to get my car back on the road ASAP. Since I'm still learning how to tune etc, I figure it's better to keep driving it as is for another year, rather than spend a lot of money doing the upgrade and then destroying it because I don't know what I'm doing.

On another note, I tried to sync my carbs today, found out that was impossible because my linkage bushing are all worn. Seriously considering buying the CSP bell-crank linkage now.

[Mark Henry]

Start saving some coin Matt, likely there's going to be a Dr Evil transmission clinic sometime in the late spring, at my place.
Details TBA, but you can do a search on some of the past clinics.

[injunmort]

consider the tangine racing cable set up rather than bellcrank. i replaced my bellcrank with cable setup. superior in every way. quick install, a real throttle pedal that you can modulate rather being a light switch and easy adjustment and syncronizing.

[mobymutt]

consider the tangine racing cable set up rather than bellcrank. i replaced my bellcrank with cable setup. superior in every way. quick install, a real throttle pedal that you can modulate rather being a light switch and easy adjustment and syncronizing.

I did look at those but, unfortunately, the price was almost as much as my whole car. Unfortunate for my car, that is!

[mobymutt]

Start saving some coin Matt, likely there's going to be a Dr Evil transmission clinic sometime in the late spring, at my place.
Details TBA, but you can do a search on some of the past clinics.

I saw that. Definitely want to go. I guess it will depend on how much coin. My car does change gears currently, so the transmission is probably the best thing on it!