I was bored today so I did a compression test.
#1 143
#2 144
#3 147
#4 145
#5 150
#6 148
I did the test twice and these are the averages. I drove 7 miles to my folks house and let the engine idle for around 5min to get it good and warmed up. Look good? I have no idea how many miles are on this engine (came from '73 Dart). Oddly, on the second test the readings where higher, except for #5 at 150. These are better readings than the one I did a year or so ago by afew pounds. I added that Restore to my last oil change. Maybe there is something to the stuff!

Looks good to me, I got 150 +/- maybe 5 psi on my freshly built 30over w/everything else stock block.

Open up the throttle plates to WOT when you do the test, you may get higher results.

Crap. I knew I forgot something!

Those are very good numbers for a stock motor. You will only get higher numbers with WOT if you didn't crank it over enough times - maybe 3-5 cam revolutions will give max psi even with the throttle shut.

Lou

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Home of Slant6-powered fun machines since 1988

Pierre, you are right, it's best to open the throttle blades fully before cranking the motor over. I usually pull the air cleaner,, then stick a skinny long screwdriver or dental-type pick tool down against the throttle blade while holding the throttle lever at WOT. You release the throttle lever gently and the tool wedges back against the booster venturi. Keeping the choke and throttle blades at WOT reduces/eliminates fuel being pulled out of the idle and main system (because there's no real pressure drop seen in the carb if the throttle/choke are wide open at cranking speed). This way raw fuel doesn't get drawn into your engine, avoiding potential flooding/fuel dilution of the oil. I usually block open the choke/throttle blades for both a compression or a leak-down test Hope this helps. Thanks, Tracy

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Dare to be Different...

In a word, hellyes. Most of the slants I've owned have been in the 120-130 range. At the time, those cars were all between 12 and 20 years old with between 70k and 220k miles on 'em. None of the mileage or age ranges seemed to matter much, regarding psi of compression readings.

My current slant is not doing nearly as well.

- Erik

PS. Theoretical question: if the static air pressure is 14.7 psi at sea level, and you put a quantity of that in a chamber and compress it by, let's say, a factor of 10, how could the result possibly be any higher than 147, even if the valves sealed perfectly before the piston stroke started, which doesn't ever happen in real life? Your results above look pretty good, by that measure. How does one explain compression results in the range of 150-180, in a passenger car? I've seen it... but I can't explain it. Hmmmm.

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Lots of early Valiants and Barracudas have crossed my path.
Also a handful of other toys for variety now and then.

My '84 D100 has 115,000 miles, did a test a while back, 150-152 on all 6 holes. That really suprised me as that was with the original timing chain and the motor not fully warmed. The quality of the gauges in some of the testers may not be very well calibrated. I was just happy the variance was just 2lb between all 6 holes.

Cecil

I'm still shocked I got those readings without (read: I forgot ) opening the throttle plate....or even taking the stock air cleaner off!!! I think I'll leave it be for now....

Hey mpgFanatic, I think I've got a plausible explanation for you.

There is also heat energy in the air being drawn in. If one compresses that air, the heat energy is concentrated into a smaller area, thus "feeling" hotter. What happens to air as it's heated......expands, right? In this case, it's enclosed in a (relatively) sealed container, so, rather than expanding, the pressure must increase.

I could be way off the mark, but it seems reasonable to my mental capabilities.

Roger
(If you can't dazzle them with brilliance, baffle them with bullshirt.)

Carbon buildup. THink about it, the swept volumn of the piston doesn't change so theoretically you couldn't get a compression of more than 147. However, by decreasing the size of the area into which the same volume of air is compressed the compression ratio nwill rise (thats why milling your head = higher compression) Since Eric is running (I think) a stock 73 head, I would suspect some mild carbon buildup.

Thats my lame-o guess. I don't really know if it is true or not, but it makes sense.

Reed

P.S. Good readings Eric!

Eric, I'm not shocked. When I did a test on my freshly assembled engine all with readings at 150 or maybe even a smidg above, I never opened the throttle plates. I only learned that trick afterwords. But, I kept cranking a bunch of crank revolutions.

With all the plugs out, and the gauge hose in the cylinder under test, when that cylinder reaches compression stroke while cranking, it will make a distinct sound and the crank will pause momentarily. I keep cranking untill I hear this sound at least 2 or 3 times, then read the number. And yes, the final reading is always more then the first or second time the noise mentioned above occurs. So judging solely by expereince, I am going to have to side with Lou on this one and say it probably doesn't matter if the plates are open as long as you keep cranking a few times.

Yes, I did crank it over about 5 or 6 times to get the max reading. Its really hard to detect the "pause" with a mini starter though! It turned the engine over so fast I actually blew a couple of the spark plug tubes out...

Eric, woo.... reminds me of the time I took my head off and pulled the pushrods out, but left the lifters in the block. I wanted to crank it over for a second to observe for a noise (everything else in the engine was still assembled, and the oilpan was still full). When I did my brother screamed at me tellin me not to do that. I shrugged it off and didn't bother to ask why. Then I realized I could have launched a lifter.....

Btw, that was with the Powermaster version of the ministarter, a pound or two lighter because it has no snout at the end and is rated to crank over engines with 18:1 compression.