Re: Compression

[farna@xxxxxxx]

Good tips Matt! One thing on "excesive oil consumption" though -- that varies with engine. The older engines will typically use more. I know a quart in 1,500-2,000 miles is "normal" for a 196 that is run hard, and a quart in 1,000 miles isn't excessive for that engine either. The newer engines should, of course, do a lot better! More than one quart added between changes is usually considered excessive by most people. Some manufacturers won't look at a new engine as "excessive oil use" unless it's more than a quart every 1,000 miles.

On February 12, 2006 Matt Haas wrote:

> Assuming a stock engine, a jumped or severely stretched timing chain can
> cause low compression readings across all cylinders since it alters when
> the valves open and close in relation to the pistons. A leak down test will
> help pinpoint what's causing the low readings.
> 
> A quick way to check for a stretched timing chain is to pull the spark
> plugs and remove the distributor cap. Slowly turn the engine by the
> harmonic balancer bolt until the rotor starts moving. Then, slowly turn the
> engine in the other direction and see how much you have to turn it until
> the rotor starts moving again. On a new timing chain, the rotor will start
> moving almost right away. Any more than about 10 degrees is bad. Note that
> if you jerk the wrench when you turn the engine counter-clockwise, you may
> loosen the balancer bolt. If that happens, make sue you torque the bolt
> back to specifications.
> 
> A jumped timing chain is a little more difficult to check and the best way
> is to pull the timing cover and make sure the marks on the gears still line up.
> 
> Also, if your engine has a bunch of miles on it, even, low compression is
> normal and is a sign that the engine wasn't abused. You'll get better fuel
> economy and more power with a rebuild but as long as you aren't using an
> excessive amount of oil (more than about a quart very 3,000 miles) and the
> engine isn't smoking, it would be okay to leave as-is.
> 
> Matt
> 
> At 05:23 PM 2/12/2006 +0000, you wrote:
> >"Normal" compression varies with the engine. A 1978 232 should have 140
> >psi, a 258 150 psi... NEW. The 82 TSM states 120-150 for the 258. I
> >believe that's pressure measured with the engine warmed up. Before you get
> >scared, 95-100 psi is still good. Should be no more than 10 psi variance
> >between adjacent cylinders, 30 psi from highest to lowest.
> >
> >If you read 85 psi across the board, you're probably still good, and
> >likely measured cold. You can also put a squirt of oil in the plug hole
> >and measure compression. That should increase it just about the same as if
> >the engine is warm. As it warms up some oil gets to the rings and helps
> >sealing, dry rings don't seal good.
> >
> >--
> >Frank Swygert
> >Publisher, "American Independent
> >Magazine" (AIM)
> >For all AMC enthusiasts
> >http://farna.home.att.net/AIM.html
> >(free download available!)
> >
> >original message----------------------------------------------
> >
> >Date: Sat, 11 Feb 2006 16:58:34 -0600
> >From: Todd Tomason <jayscore@xxxxxxxxxxxxx>
> >Subject: Compression
> >
> >What is normal for compression, anyway?  I did a compression test on my
> >258 in
> >my Spirit a while back, and it was about 95 lb. on each cylinder.  Is that
> >normal or low?
> >
> >Todd
> 
> mhaas@xxxxxxx
> Cincinnati, OH
> http://www.mattsoldcars.com
> 1967 Rambler American wagon
> 1968 Rambler American sedan
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