Compression #

[Don Jordan]

I was looking in the repair manual and while it is probably in plain sight I could not find what the compression should be on a 218. I did see the compression ratio is 7:1.

My compression ranges from a high of 65 to a low of 55. While the span does not seem to indicate anything to me I don't know if those numbers are even in the ball park. My next step is to take off the head.

[jd52cranbrook]

I think compression is relative to rpm speed the starter turns your engine.

I am running 12 volt through a six volt starter. I do have a 230 crank in a original 218 block.

I measure 150 in all cylinders plus minus 3 lbs. That is with all the plugs out of the head.

I think whats more important is variation between cylinders.

[Big_John]

The next thing you should do is squirt a litttle oil in each cylinder and test it again. If the values come up a lot, your problem is in the rings. If not, its elsewhere.

[RobertKB]

55-65 is very low. I am kinda surprised it runs. Is your compression tester accurate? Maybe the old girl needs some hard miles to free some rings up or loosen sticky valves. Those numbers just sound very low to me.

[busycoupe]

Normal atmospheric pressure is just under 15 psi. 7 x 15 = 105. I would think that engine compression should be near 105 psi. Your 55-65psi sounds low.

Dave

[TodFitch]

55-65 is very low. I am kinda surprised it runs. Is your compression tester accurate? Maybe the old girl needs some hard miles to free some rings up or loosen sticky valves. Those numbers just sound very low to me.

I'm not surprised it runs with 55 to 65 pounds compression. It might be hard to start, but it should run.

When I got my 1933 it was hard to start and had a very rough idle. After a year or so and who knows how many thousand miles I did a compression check: Three cylinders had 50 to 60 pounds. Those were the good cylinders, the other three had zero.

[Don Jordan]

Thank you for your input. There is a slight hill by my house which I have to take in second gear (with much smoke) while my 48 will scoot up the hill at 55 in 3rd.

I guess I'll take off the head and see what surprises await me. Thanks again for your help.

[garbagestate 44]

You could do a leak down test before pulling the head. That might help narrow down where the bulk of the problem is.

[suntennis]

About 500 miles after my engine was completely rebuilt, the compression was 115 to 120 psi. The good news is that your compression differences are not that bad and it probably runs smooth. The other news is that you do not have a new engine. Unless it's broken, why fix it.

[jd52cranbrook]

To clarify my original reply. The rpm is relevant to compression numbers. To get a accurate reading all the plugs should be removed, and some say even to open the carb throttle fully.

Hence, the fast the engine turns the higher the compression reading.

anyway, your probably already done that,,,,,

[greg g]

I believe a stock 218 was 110 psi when newat 6.7 to 1 CR. Your compression is quite low as evidenced by your lack of power. Sounds like time for rings and pistons.

[Frank Elder]

About 500 miles after my engine was completely rebuilt, the compression was 115 to 120 psi. The good news is that your compression differences are not that bad and it probably runs smooth. The other news is that you do not have a new engine. Unless it's broken, why fix it.

Because he doesn't want to go 20 mph uphill.......a small hill:)

[Don Coatney]

To clarify my original reply. The rpm is relevant to compression numbers. To get a accurate reading all the plugs should be removed, and some say even to open the carb throttle fully.

Hence, the fast the engine turns the higher the compression reading.

Think about this for a minute.

If the faster your engine turns the higher the compression reading then what compression reading would you have at say 3000 RPM's?

I believe with 7-8 engine revoloutions at any RPM speed the engine compression readings will top out and go no higher.

Reason for removing all the plugs is to lower the amp load on the starter and battery and to insure there is a direct path to atmospheric pressure in the intake manifold with open intake valves.

[jd52cranbrook]

hmmm you could be right. But think of this for a moment.

I would be willing to bet a coke you will get a higher reading from the same engine with;

one with a six volt system versus with one with a 12 volt set up going through a six volt starter.

Theory being slower turning crank leaves more time for air to escape before reaching tdc.

[Don Coatney]

hmmm you could be right. But think of this for a moment.

I would be willing to bet a coke you will get a higher reading from the same engine with;

one with a six volt system versus with one with a 12 volt set up going through a six volt starter.

Theory being slower turning crank leaves more time for air to escape before reaching tdc.

OK I buy it but only after you quantify it:D

When I first converted to 12 volts I used my original 6 volt starter. What I found was yes the starter motor spins 2X faster but also the bendix engages 2X faster. With my original P-15 (worn) flywheel ring gear this faster engagement caused my starter motor to bind a couple of times and lock itself into the ring gear. To get out of this I had to put the transmission in gear and rock the car forewards and back until the starter popped free. Greg G sent me a 12 volt starter and once I installed it the binding situation never occured again. And with the 12 volt starter my engine spins (at least it sounds like it) as fast as it did with the 6 volt starter.

I wil go for a Pepsi or better yet unsweetned ice tea

Edited October 3, 2010 by Don Coatney

[greg g]

What air escapes where???

[jd52cranbrook]

I like Raspberry Tea

Give me a minute while I look up quantify.

I am still running the 6 volt starter. But my engine starts almost instantaneously... So it hardly labors. I was going to change it out as soon as the six volt starter quit on me, but never has for 5 years now. And, I have a spare six volt-er somewhere around here.

But,,, having said this about the rpm of cranking;

1. Air escapes where there is a leak, if any.

2. Not only that, I have yet to see a engine not get better compression, be it only one pound, when oil, or fluid is not shot down a hole prior to turning.

3. If a engine shuts down when on a compression stroke on any random cylinder, you can bet if you get back in the car the next day the compression is gone. Correct?

Granted there will be a speed when no measurable leakage occurs, hence no more increase in pressure on the gauge no matter how much faster it turns. And that is when you get your maximum reading.

Also, I would also bet all the plugs should be removed to achieve this number.

Edited October 3, 2010 by jd52cranbrook

[martybose]

You should also have the carb blocked open so that you aren't pulling a vacuum; you want free air flow.

Marty