Compression Test

[marksifrit]

The compression test on my '69 1600 came out as:
1. 160
2. 155
3. 155
4. 160

How does that compare with normal compression readings?

[greydog]

It's probably just fine. You should be within 10% across the board. Most say not to get too hunh up on the numbers, focus mainly on the variaion. As to the number values, it's a little hard to say without knowing more about the circumstances (how many miles, when car last driven, etc) but 160 seems about right.
Dan

[Lee2000]

Looks good IF the engine is not brand new...or if it has a more radical cam than stock. As an example, my freshly honed and re-ringed stock U20 produces 175 psi across the board but it hasn't been fully broken in yet so those numbers may come up a little. The main thing is that your balance is real good so I'd say you are in fine shape. Did you do the test on a warm/hot engine?

[spl310]

Most folks don't do a compression test correctly - they do it with the butterflies on the carbs closed...

[SLOroadster]

Those look a little low actually. The catch here is do you have SUs on the car, and did you have the accelerator floored when you did the test? Either one and or, both can lead to low numbers since you don't get the air flow into the cylinders. I freaked myself out one time when I did a compression test and forgot to open the throttle all the way and ended up with numbers that were way low. Since the SUs have the dash pots, they don't flow well at cranking speed so you end up with really low numbers, especially if you also forgot to open the throttle all the way. Lots of air restriction there. You also want to have the motor at or near full operating temp. A cold engine will result in strange readings. Also, make sure your valves are all adjusted correctly.

That all said, your numbers look very even so I'd do a wet test and see if they come up as well as making sure the carbs are fully open. If the numbers do come up with the wet test and you did have the throttles fully open on the first run, your rings are suspect, if they stay about the same, it might be time for a head refresh. If the throttles weren't open when you did the first test, go back and redo it with the throttles open and see what you get.

When I first got my car on the road, it was showing 190s across the board running a B cam, Mikunis, and it had an unknown number of miles on the motor. Shortly there after, I cracked all 4 pistons with a missed shift (5th to 2nd at about 70 mph. Ouch.)

Will

[RoadsterJon]

I tested my 1600 about a year ago and it tested much lower than that. Still runs great. Maybe a little low on power but if you don't feel like rebuilding your motor I'm sure there are plenty more miles left in your engine.

[mraitch]

I always forget the butterflies needing to be open - thanks for reminder.
@SLO - how on earth do you get from 5th to 2nd! (perhaps you could look at the shifter springs)
I didn't realise until I took my 5-speed apart that there are in fact TWO types of shifter springs - one for 1st & 2nd and another for reverse & 5th (Overdrive) (the latter presumably being stronger)
I put the two 'overdrive' springs in - the shift lever hardly 'wobbles' and makes shifting (or attempting to so do) into reverse almost impossible - it just almost forces the lever into the 3rd/4th position with `1st and 2nd having equal resistance.
FWIW

[dbrick]

I usually test with throttles wide open and all spark plugs removed warm, not hot, engine. Count the same # of pulses on the needle, usually 5 or 6. Valve adjustment can affect the readings. They are even, so that's very good regardless of specific test method.

[SLOroadster]

I always forget the butterflies needing to be open - thanks for reminder.
@SLO - how on earth do you get from 5th to 2nd! (perhaps you could look at the shifter springs)
I didn't realise until I took my 5-speed apart that there are in fact TWO types of shifter springs - one for 1st & 2nd and another for reverse & 5th (Overdrive) (the latter presumably being stronger)
I put the two 'overdrive' springs in - the shift lever hardly 'wobbles' and makes shifting (or attempting to so do) into reverse almost impossible - it just almost forces the lever into the 3rd/4th position with `1st and 2nd having equal resistance.
FWIW

I was looking for 4th, and found second. Mind you, the transmission was totally worn out. It needed to be double clutched on all the up and down shifts. It only happened once. The transmission called it quits the day the first new motor was totally broken in, 5th gear let go (I think it was a Monday). Two days later I broke my first axle leaving a stoplight, (had a spare) then the water pump called it quits, by the end of the week the starter packed it in. Talk about an expensive week, especially when I was only making $8 an hour.

Will

[mraitch]

$8.00 per hour - luxury

[Solex68]

Most folks don't do a compression test correctly - they do it with the butterflies on the carbs closed...

Yikes...I did forget that the last time I did mine. Oops!

[S Allen]

Here is the write-up from the tech wiki-FAQ's:

Question: How do I do a compression check?

Answer: A compression check will give you a good indication of the condition of your motor. It is best to warm the motor up before doing a compression check. If you cannot get the motor started go ahead and do the check any way. Below are the steps for doing a compression check:

1. Remove the high tension lead from the coil to the distributor.
2. Remove all of the spak plugs marking the wires as you remove them.
3. Remove the air cleaner assembly and prop the piston of the carbs wide open.
4. Install the compression checker into spark plug hole #1.
5. Crank the engine over several times and jot down the reading
6. Do this for the other cylinders writng down the reading each time.

All of the readings should be plus or minus 10 PSI of each other. An example would be #1-170,#2-175 #3-165, and #4-170. These would be considered okay. Ideally you want them all even but realistically that rarely happens especially on an older motor. If you have a really low reading that indicates a problem. It could be worn rings, valve not seating, or a bent pushrod to name a few.

Next we will do a second set of readings with a squirt of oil in each cylinder. The procedure is the same as above just squirt some oil into the spark plug hole before you install the compression checker. Write down the readings once again. If the readings stay low out of spec you may have some issues with your head. You can try adusting the valves but if that does not remedy the problem. It is time for a valve job. If the readings go up then your rings are worn out and need replacing.

That pretty much covers doing a compression check.

Tech Wiki is your friend.

Steve

[jake7140]

Well that mentions propping the pistons up but not opening the throttle plates. I imagine both should be done? Certainly no good to prop the pistons with closed plates.

[S Allen]

Updated the tech wiki to have both throttle plates and pistons propped open.

S

[svwilbur]

Updated the tech wiki to have both throttle plates and pistons propped open.

S

I know this is an older thread but....What is a good, safe way to "prop the piston of the carbs wide open"?
We are talking about the Dashpot pistons in the SU carbs.
I don't want to do it with something that will block the flow or with something that would get sucked down into the cylinder.
I would say use my fingers to hold them up but I think they will be turning the ignition key.

Would a shortened clean painters stick for stirring paint work, propped up vertically and with the long length horizontally coming out of carb area?

What else would work better / safer ?