What should the actual compression psi be? - PeachParts Mercedes-Benz Forum

nofrills · #1 07-11-2006, 01:52 PM

Hi All,

My '72 250 (130.923 I6, auto, carbs) project and I just joined the forum. Looks like a great wealth of info, and I'm greatful because my Haynes manual and crumbling owner's manual leave a lot to be desired...

One piece of info I can't seem to find anyware--okay, two pieces, are what should the actual cylinder compression psi be (versus the ratio, which is all over the web and in the books) and the acceptible psi difference between cylinders for this engine? Just tested them yesterday, and all were 130 with the excpetion of #5, which was 125. This concerns me a bit, because in the car's service records there is a reference to the compression being tested in the late 70's measuring 150 across the board.

Any info here would be tremendiously appreciated.
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Mike D · #2 07-11-2006, 02:23 PM

True compression readings are a tricky thing. In order to get the maximum optimum reading you need to:

a: Drive down to sea level.

b: wait for the ambient temperature to drop to 70 degrees F.

c: wait for relative humidity to stabilize at 50%.

d: pressure test each cylinder simultaneously at operating temperature with all valvetrain components adjusted to exactly the same degree and with zero wear on all compression components.

Not possible? Then you take the rated p.s.i. (which was calculated under exactly those conditions) multiply it by 80% (this actually is a S.W.A.G. designed to compensate for any variation of the above conditions) and you go with that.

150 p.s.i. X 80% = 120 p.s.i. as an acceptable rating.

More critical are variations between cylinders. Use the 80% in this case also.

130 p.s.i. X 80% = 104 p.s.i. so your 125 reading is no cause for worry.

nofrills · #3 07-11-2006, 02:42 PM

I appreciate it.

Now I can carry forward troubleshooting the rough idle with a little more confidence...bent choke valve on rear carb, multiple plugged-off vacuum lines (including the distributor advance lines), old and different plug wires, really rich emissions and fouled plugs. Looking forward to it, actually! It's saving grace right now is that it cold-starts better than my '01 Subaru and actually runs very smoothly at higher RPMs.
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t walgamuth · #4 07-11-2006, 07:52 PM

i agree pretty much with everything stated above except i believe that generally 10% is the desired max diff between cyl. so you are well below that. also my old chiltons lists for the 250/8 which i think you have as you stated about 150 with 125 as min. if you are fairly high altitude it will run less. i am not sure how to compensate for altitude except to say that it is noticable at 700' above sea level, here in lafayette indiana.

tom w

ctaylor738 · #5 07-11-2006, 09:03 PM

At those readings, it's not going to be a barn burner, but it should run decently.

My friend Bob, who buys and sells this stuff for a living, told me that a perfect new M130 engine should show 160, and 140 was the line where he would actually sell an engine to a customer.

If you need carb parts send me an email - I have a bunch of parts carbs that I have been slowly giving away.

Good luck and welcome to the Forum.

Mike D · #6 07-11-2006, 09:19 PM

Tom,
Actually his difference of 5 p.s.i. is well within those limits (130 p.s.i. X 90% = 117 p.s.i.) so he's good to go on that. 10% difference could just be a piece of dirt in the sealing area. I always use 20% to compensate for cylinders which might be running rich and fouling the ring or valve sealing surfaces.

You're right about the 700 ft. elevation difference affecting the p.s.i. Each 1000 feet above sea level is considered as a 5% drop in diesel power.

I grew up in Indy and I was a drag racer. My motorcycle lost (gained in E.T.) almost a full second in the quarter when I moved to Tucson (the track is about 2100 feet higher than at IRP). I had to re-jet and advance my cam timing to compensate.

t walgamuth · #7 07-11-2006, 10:04 PM

interesting.

yeah, if that car hasnt been run for a while just putting in clean oil and driving it could bring up the compression a good bit.

i always wondered what the formula was...5% for 1000 feet, so at 700 i would be 3.5% difference so if it should be 150 that would make it about 5 psi. difference.

yeah, i used to have an anemic 80 datsun wagon with a little 1300 engine and a fiver. when i put new tires on, in a stroke of genius i decided to go up one size from 155 to 165 i think. so then it would only go about 65 flat out in fifth. it would go faster in fourth, so if i had someone to draft i could accelerate up to 75 in fourth and shift to fifth and hold it there. i once drafted a 240d from north of indy to lafayette. with so little power you noticed everything. i could actually feel the draft of a motorcycle in that car. and when down by new orleans running across the causway from east texas to new orleans, if running solo i could barely go 60 from the heavier air, but in a group of cars i could go 70 with ease.

as i said if you have a very underpowered car you can feel the difference in air viscosity.

i eventually sold that car after about six months of ownership even though it was in perfect condition. i couldnt stand it. i once was trying to meet my ex to exchange my daughter and goind south on 31 i had a stiff headwind and could only go about 45. grrrrrrrr.

guess i kindof veered off there.

tom w

nofrills · #8 07-11-2006, 10:22 PM

Thanks, all. Wonderful info.

No barn-burner for sure, but if I can squeeze off a few more years before a rebuild it woudl be hugely beneficial. Sounds like I'll be alright there.

Mike D · #9 07-12-2006, 08:14 AM

When you get your carb and vacuum line situation straightened out I believe you will be quite surprised with your M130. A rocket off the line it isn't but it will be a strong, smooth puller.

Tomguy · #10 07-12-2006, 09:44 AM

You can calculate your compression ratio (approximatley) if you have a barometer to measure the pressure where you're at (or if you live, say, close to the news station and they do the weather at noon and say the pressure is 30.21 and steady). Let's say your outdoor pressure is 30.0 (inches of mercury) - that's 14.73 PSI (quick chart for reference: 28.8/14.15, 29.1/14.29, 29.4/14.44, 29.7/14.59, 30.0/14.73, 30.3/14.88, 30.6/15.03, 30.9/15.18, 31.2/15.32, 31.5/15.47, 31.8/15.62, 32.1/15.77 - and the air pressure here never really goes above 31.5 or so but I did higher for you at or below sea level).

Now, if it's 30.0 and steady when you read your compression reading of 130PSI you simply divide 130.0 by the PSI (NOT the inches of mercury): 130PSI compressed / 14.73 non-compressed = 8.83:1 compression ratio. This info is more useful than hard pressure numbers because weather AND elevation can both affect them. If you get 145PSI, what's your barometric pressure? if it's 31.5 inches, your compression ratio is 9.37:1; if it's 29.4 inches, it's 10.04:1 - and that is a HUGE difference with the same "resulting PSI" and why everyone is saying numbers themselves don't mean crap.

nofrills · #11 07-12-2006, 01:33 PM

You know, I was wondering if it was really that simple. That is, multiplying atmospheric pressure by the stated compression ratio to get what the psi should be (ex: At 1:9, if atmospheric pressure is 14.7psi, then 14.7x9=132.30psi). Just figured other factors would be at work.
For the record, I'm about 150ft above sea level, on a plateau just a few miles from the sea, really, in San Diego, so sea level-specific figures are easy to calculate for me.

Thanks again.

Tomguy · #12 07-12-2006, 03:57 PM

Well, in theory yes, but you should account for the fact that even on a perfect engine, some loss is inevitable from the rings and valves. It could be really low on a high-tolerance race engine, or on an engine like say a 40's era Ford, could be like a 5-10% loss! If you check it on a day before, during or after a storm, it might appear be low because of barometric pressure. Tons of stuff can result in lower readings. 130 seems alright for now. It might indicate tight valves.

t walgamuth · #13 07-12-2006, 07:08 PM

well, yeah.

all true. that is why the book says 147 to 160 psi is normal.

but i am very glad to hear all those good facts. it is good to know that barometric pressure can make so much difference.

again, the books stress the main thing is that there is evenness across the varous cylinders.

tom w

nofrills · #14 07-12-2006, 09:42 PM

Okay, I have to ask (TomGuy), why would a 40's Ford be less susceptible to loosing compression? I'm guessing extra rings or really long piston skirts?

Just curious.

t walgamuth · #15 07-12-2006, 09:48 PM

i thought he meant more susecptable. a race engine is set up loose to allow for heat expansion. also if equipped with an aggressive cam it might have a mechanical compression of say 12 to one but due to valve overlap it might be an actual 10 to one since the intake is held open while the compression stroke is starting...(or is it the exhaust?).

tom w