273 Compression Test Results

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PocketAces

PocketAces

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Below are the results of a compression test on my 1965 273. The car has a 1969 short block with 1965 heads and manifolds. It's a 2bbl and I doubt the block was a hipo/commando. Since the block does not have a VIN stamp, I'm thinking it was a warranty replacement.

The engine runs fine with no symptoms of oil consumption, or blow by. It's no racer, but then I'm not really sure what to expect from it. Mostly, I'm just trying to decide what I should do with it when I restore the car. If it needs a rebuild, I think I would hunt down a 1965 block. But if it's healthy then maybe I just refresh the heads. The previous owner stated that the engine was rebuilt in the 80's and then driven maybe 30K miles since.

The numbers are after 5-6 compression strokes, basically once the gauge stopped climbing.



Here are my questions:
  1. I forgot to open the throttle plate, should I repeat the test?
  2. What is a healthy reading for this engine?
  3. If the engine was a warranty replacement, what cam would it have come with? I think they were using hydraulic valves by 1969. Not sure if I have adjustable rockers or not. Need to pull the valve covers.
  4. What is a typical increase for the wet test? Does +10 to +15 indicate healthy rings?
  5. What's the next step? I think I should check the valve lash to make sure the valves are closing all the way. I could do a leak down test, or I could just pull the heads and have a look.
 

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I pulled the drivers side valve cover. The engine indeed has adjustable lifters. I verified that the valve lash is correct so that eliminates one possible cause.

But I did notice one thing. One of the adjusting bolts was turning so freely that I could adjust it by hand. That seems like a problem. Can the stock rockers be upgraded with jam nuts to prevent this?
 
Your compression results look about average to me. The wet test to me indicates the rings might not be sealing as good as they should. But in my opinion are not that bad. I probably would not rebuild this engine if its running fine and the blowby is not excessive.

I would be concrened about the valves on the 100 psi reading. Seems something is going on there and will probably only get worse with time and mileage.
 
Thanks for the reply. Valves seem like the most likely culprit to me too.

I couldn't find any info on what readings a healthy 273 should have. What psi would you expect from a fresh 273 and at what point would you say a rebuild is in order?
 
Think my manual says it should be around 120. I got a 67 273 HiPo that was rebuilt and
cam put in by someone, I only get around 110 on all cylinders. Think they put in 60 over
stock pistons instead of the HiPo ones and cam is too big. Compression is so low that I can't get it to ping with regular gas no matter how high I bump up the timing. Needless to say, engine is out right now and going to put in a stock 84 360 I got a year ago. Did a compression test on it and only get 120 in all cylinders.
 
Drive the hound of it. It's fine.
 
I've confirmed that I have a bad exhaust valve. If 120psi indicates healthy rings, then I guess I will get the heads redone, install a gasket set, paint it up nice and pretty and call it done.

Here's what I did, in case anyone can benefit from the experience. Feedback, tips and better techniques welcome.

I rigged up a fitting to put comressed air into the cylinder. I used a fitting that came with my compression gauge and an air hose nipple. Screwed it into the spark plug hole and chucked on the air hose at 110 psi. Lot's of air coming out the tail pipe. Bumped the starter, now lots of air out of the carb. Kept trying to get it to stop at a spot where both valves are closed, but no luck.

Cut the air supply, pulled the valve cover off and bumped the starter until both lifters were loose. Valves definetly should be closed now. Turned the air back on and still blowing out the tail pipe. Repeated this at least 5 times. WTF?

Those of you with more experience already know where this is headed. Myself being significantly less experienced had to do some head scratching. Quit for night, closed up the garage and within 10 minutes it dawned on me. Air pressure was driving the piston down until the exhaust valve opened. After some trial and error with the regulator, found that 30 PSI was not enough to crank the engine.

Set the valve cover back on and repeated the test. I could hear and feel a tiny bit of air coming out of the breather indicating some blow by, but the engine was cold and dry and some air is to be expected past the ring gaps right?

The real smoking gun came when I discovered that if I pressed my cheek against the tail pipe, pressure would build up. That's not supposed to happen if the exhaust valve is closed. Repeated the test on another cylinder and felt and heard similar air at the breather, but nothing at the tail pipe.

Mystery solved.
 
hmmm, might have to try that on mine someday - thanks! Since the 273 doesn't have the hardened seats, I could see that happening. Wonder how much power you would lose
if you just bolted on some good newer 318 heads with the hardened seats? Only thing I hate is that you can't buy 273 head gaskets, have to use 318 which ar bigger so will loose some compression.
 
I found this thread on FABO http://www.forabodiesonly.com/mopar/showthread.php?t=69190

Which provides this link for 273 head gaskets
http://www.summitracing.com/parts/FEL-521SD/

But if you read to the end of the thread, the bore on those gaskets is 4.18" so it looks like you may be right. Another poster computes that this extra volume decreases compression by only 1%.

The OP on that thread also states that 140-150 PSI is "acceptable" and 120-125 PSI is "tolerable". By his standards my engine is tired.
 
Maybe that 140-150 number is for the HiPo 273, believe my manual at home says around
120 for both motors is OK. If I had to guess, since you said you can hear air coming out of the valve is that when you pull your heads the valves will be razor blade thin on the
edges. Thought you said earlier that you had a stock 273 and not the HiPo. Too bad you
don't live up around Mass, I also have a 318 out of a 84 Dodge truck that supposedly has
10k on a rebuild that I would let you have for cheap. Did a compression test on it and it's around 150 on all 8. I'd be using it but picked up a 360 afterwards and from what I read, everyone likes that one better. Best thing about a 318 is that nobody can tell it from a 273 unless the look at the number on the block.
 
Just FYI

I pulled out my repair manual tonight. Regarding the compression test says to do it on a warm engine. Carburetor butter flies open and the range given is 150-120 psi for a 9:1 273 engine
 
These techie posts are very helpful.

The FSM for my 65 Dart says 120-150 psi, 20 psi variation for 2-barrel (9:1) and 130-165 psi, 25 psi variation 4-barrel (10.3:1 HiPo), ditto zhandfull's post. Of course, the compression is greatly affected by the cam. Even if the factory cam, lobe wear can change the readings.

I have never gotten higher readings by propping the throttle and choke plates (multiple cars, usually when I forgot), but that may make the pressure come up quicker. One rule I have read is that it should reach within ~5 psi of final pressure after the 3rd stroke. If it comes up slowly that may indicate bad rings. Not sure I believe that because the compression gage tube volume probably has a lot to do with that. BTW, if you ever change the Schraeder valve in the tip, don't use a regular auto/bike one (been there). The gage requires a special one with a very weak spring, usually has a white plastic seal.

Readings I got in my 1965 273 2-barrel in 2009:
140, 155, 125, 145, 148, 150, 120, 150 psi
very similar to 65DartGTconv. It took 6-8 strokes to reach max pressure in all cylinders.

I recall the engine was warm. I ran it for 30 sec several times with no radiator. Haven't run it since, since busy de-rusting, painting, and rebuilding other parts. Like 65DartGTconv, I needed to know if I needed to re-ring it early on. The engine sat for years before that I think, so maybe some rings need to free up.

Don't fret over the one 100 psi reading yet. I had a 95 psi reading on #7 initially, but later got 120 psi. I had added a little oil to check the rings (don't add too much), but recall getting the same a few days later after the oil should have drained, so maybe the oil helped free the rings. I decided to leave the heads on and test it again after some driving.

I did the same "apply air to cylinder" trick. We must have the same Harbor Freight compression gage. I had the oil pan, intake, and valve covers off so could tell when each piston was at TDC. I could hear air hissing in all cylinders, which is normal since you always get some ring leakage. Sometimes you can feel it flowing out the breather, intake, or exhaust to pinpoint the culprit. I can tell if the exhaust pipe, but the other places are more tentative.

Perhaps a better test of the valves and rings is to turn the engine over by hand, with a socket wrench. In my newer cars, you can feel each cylinder like a spring, especially the 4 cyl, and they hold pressure ~15 sec. I recall that my slant six cylinders would leak off almost as fast as the piston came up, 1 or 2 sec, even though the engine ran fine. I re-ringed my 2.4L and it uses regular rings with an end-gap (not the special overlapping type), so don't know how it holds the pressure so well (even 70K mi after rebuild). Ditto for my 3.8L.
 
Unlike some others who have posted on this subject, this engine has not been sitting around for years. I drove it from Sacramento to Los Angeles 2 years ago and I try to drive it once a month since then.

Judging by the various numbers I'm hearing, and also by the service manual quotes, I'm thinking that 150 is a good healthy number for a newer engine, and 120 is down near the low end of the range for an older engine. My one really low cylinder is clearly due to a bad exhaust valve and the rest of them are right around 120.

So what to do? My goal for the car is to have a correct and original 1965 dart. My dad bought the same car with the same color and options the week I was born so it's a bit of a nostalgic thing for me. It's never going to be a sports car no matter how much money I spend on it, and when I want to go fast, I have a Yamaha FJR1300 that will do 0-60 in first gear in 2.8 seconds (so I'm told).

I read some engine rebuild threads from ToolManMike and others and it sounds like a quality engine rebuild can cost $3000. I think the thing to do is rebuild the heads, bolt them on and see if the compression comes up. If I decide later to find a 1965 block and rebuild it, I can reuse these heads.

By the way BillGrissom, it is a harbor freight compression gauge. I used that spare fitting they give you. This weekend, I'll try putting known pressure into the guage to make sure it's calibrated correctly.
 
If it were mine I would get the valve job and run it, especially if its already running fine now and it will never be a race car, you could spend a couple grand for it two run exactly the same.
 
It was warm during the first test. Also I retested two cylinders with the throttle plates open and didn't see a difference. It might have pumped up faster, but the final numbers were about the same.
 
"I think the thing to do is rebuild the heads, bolt them on and see if the compression comes up. If I decide later to find a 1965 block and rebuild it, I can reuse these heads."

I think this is a very sound and logical plan
 
You guys are awesome. I learned a lot. I have a 66 dart gt convertible with the 273. And I have never done a compression test or leak down. It's so dang simple. Except for the part I forgot who wrote that pulled the oil pan..... just pull the oil fill cap.. but if you are getting leak down there... not good.. anyways. Awesome thread. I'm new here and this thread is awesome it's old but awesome
 
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