Compression test on a 360, result OK or not?

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Stephan D

Stephan D

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Did a compression test on my non restored 1979 360 with 40 000 miles.

Not sure if the the result is within bounds or if it´s time for a rebuild.

How would the enginge behave if the compression is not OK?

Measurements was made with warm engine:

Cyl. PSI
1 150
3 150
5 155
7 140

2 145
4 140
6 142
8 150
 
Stephan D said:
Did a compression test on my non restored 1979 360 with 40 000 miles.

Not sure if the the result is within bounds or if it´s time for a rebuild.

How would the enginge behave if the compression is not OK?

Measurements was made with warm engine:

Cyl. PSI
1 150
3 150
5 155
7 140

2 145
4 140
6 142
8 150
Click to expand...
Your fine.. 10% variance is standard so 15 lb ..
RUN IT.. :thumbsup:..
 
OK, thanks everybody, good to hear!

I'm trying to track down my shaky idle problem, just wanted to make sure it wasn't a compression problem that causes my slight misfires at idle.

Next thing is to measure timing curve and check that the timing is stable at idle, the previous owner had installed softer springs in the distributor he said.
 
try a vacuum guage
how's the timing chain, oem stem seals?, how do the plugs look
good day slanty! I'm cool
 
Being it's that good, yet an issue...
Timing chain, silent stock type could be contributing to the issue..as could burnt or sunk partially sealing exh valve seats.
Can happen with that low mileage and the factory did a marginal job on the induction hardening.

Seems like it would be a good candidate for a re-ring job . With a fresh hone...new rings, new chain n gear... it would be like new, for real.
 
Wyrmrider said:
try a vacuum guage
how's the timing chain, oem stem seals?, how do the plugs look
good day slanty! I'm cool
Click to expand...

I have a vacuum gauge, needle flapping 1 inch at idle in gear.

Vaccum is 18 inch in neutral and around 16 inch(jumping) in gear.

Timing chain is double original on these, low mileage as well, should last longer than 40 000 miles?

New valve stem seals installed recently.

All plugs look like this:
Spark Plug.jpg
 
Last edited:
There is a old timer trick when determining compression pressure on a vehicle's engine especially when the vehicles engine condition is unknown..
1) Every engine has a different compression ratio so that effects the number
+ wear and tear.
2) Test engine with throttle wide open and 10 revolutions (10 needle jumps) hot each cylinder.- compare results should be with in 10% of each other.
Then try again but before you install the compression gauge add about 10 small drops of engine oil ( only in cylinder you are testing so it does not spray out on your manifolds and inner fenders) and see if compression increases.
if it increases across the boards evenly, then it does not have a lot of wear. If it does not increase significantly then your engine is operating a little (or a lot) sloppy.
3) Battery voltage also effects the speed of the starter. Don't over heat the starter but make sure battery voltage is at 12v or higher.
If the battery voltage drops- so does your starter speed. To stay consistent the starter needs to be contestant.
 
I’ve always judged the condition of a engine by using the following, when you are testing compression,remove all plugs, block carb blades open. When testing each cylinder, it should achieve 90% of its maximum in the first 3 compression strokes.
 
This is how I did the compression test:

1. Fully warmed up engine
2. All spark plugs removed
3. Throttle locked in open position
4. Fresh battery

I did not count the strokes, just stopped when max PSI was read.

SGBARRACUDA - Do you think the result was out of bounds?
 
  • sure rough idle isn't the carb ?
  • timing mark jump around when idling? Could be the weights bouncing around depending on what he did in the distributor - I've seen it before
 
318willrun said:
  • sure rough idle isn't the carb ?
  • timing mark jump around when idling? Could be the weights bouncing around depending on what he did in the distributor - I've seen it before
Click to expand...

The Thermoquad is newly restored by a well known carb-guy.

Had to abort the timing test as a rainstorm was getting closer...
 
Stephan D said:
This is how I did the compression test:

1. Fully warmed up engine
2. All spark plugs removed
3. Throttle locked in open position
4. Fresh battery

I did not count the strokes, just stopped when max PSI was read.

SGBARRACUDA - Do you think the result was out of bounds?
Click to expand...
The quicker it hits max tells you the condition of ring seal, valves sealing, head gasket seal.
 
Stephan D said:
I have a vacuum gauge, needle flapping 1 inch at idle in gear.

Vaccum is 18 inch in neutral and around 16 inch(jumping) in gear.

Timing chain is double original on these, low mileage as well, should last longer than 40 000 miles?

New valve stem seals installed recently.

All plugs look like this:
View attachment 1715526667
Click to expand...
Everything's questionable now if we gaug your intelligence based upon you setting a spark plug on that nice shiny painted fender there.
Go hire a mechanic before you destroy the car!
 
MOPAROFFICIAL said:
Being it's that good, yet an issue...
Timing chain, silent stock type could be contributing to the issue..as could burnt or sunk partially sealing exh valve seats.
Can happen with that low mileage and the factory did a marginal job on the induction hardening.

Seems like it would be a good candidate for a re-ring job . With a fresh hone...new rings, new chain n gear... it would be like new, for real.
Click to expand...

Always one in every crowd. Killjoy. lol
 
RustyRatRod said:
Always one in every crowd. Killjoy. lol
Click to expand...
40k on stock timing, mystery gas and unknown usage. Numbers vary by 10psi on a few cylinders.
So in my mind there's two options..
option 1 continue driving it down hill to a costly rebuild.
Option 2 Spend 300.00 on rings, gaskets, oil pump, timing chain, bearings and make it fresh with a hone/glaze break.

It could run okay, though he says hes chasing some issue.. but then again... the bearings could be disintegrating from corrosion and oil contaminants...from sitting with condensation over nite ...for years.. idk where the motor has been to only have 40 k from 1979...that's like 900 miles a year...or 2-3 years driving and 38-39 yrs sitting.
Maybe it was climate controlled...yeah.
I've had old motors that sat for decades... lose rod bearings after some lead footing .

Food for thought.
 
MOPAROFFICIAL said:
40k on stock timing, mystery gas and unknown usage. Numbers vary by 10psi on a few cylinders.
So in my mind there's two options..
option 1 continue driving it down hill to a costly rebuild.
Option 2 Spend 300.00 on rings, gaskets, oil pump, timing chain, bearings and make it fresh with a hone/glaze break.

It could run okay, though he says hes chasing some issue.. but then again... the bearings could be disintegrating from corrosion and oil contaminants...from sitting with condensation over nite ...for years.. idk where the motor has been to only have 40 k from 1979...that's like 900 miles a year...or 2-3 years driving and 38-39 yrs sitting.
Maybe it was climate controlled...yeah.
I've had old motors that sat for decades... lose rod bearings after some lead footing .

Food for thought.
Click to expand...

Well, I imported the truck from US three years ago, it´s a one owner vehicle, the previous owner was a Mopar fan, having several garaged in a dry and warm garage, completely rust free (in New Jersey), it has been used occasionally over the years, newer in rain or snow.

But time takes it´s toll of course, spending 300 USD will not help me much, more like 3000 USD as I don´t have a garage to make this kind of job myself, that´s the reason I wanted to try to figure out the condition of the engine with the compression test.

It runs really well as soon as revs is just a tad over idle, not a hint of misfires at all.

Putting the plug on the fender was for photographing, not to worry, I'm caring a lot for this little truck that I have longed for since 1978...
 
Don't jump right to a rebuild. You will find the problem probably fuel or electrical related
 
SGBARRACUDA said:
Sounds like it could be a vacuum leak at idle.
Click to expand...

Yes, that was my first tought as well, I have sprayed brake cleaner around the intake manifold gaskets, hoses etc. with the engine running at idle, but not a reaction.

Somtimes after driving, shutting the engine of for approx. 15 minutes and then starting again, the misfires at idle goes away, with me coming to the conclusion that the problem is solved, but no, the next day, it´s the same again.
 
Easy way to tell if it’s lean run situation is to manually shut the choke a little and see if it smooths out.
 
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