Compression test results/accuracy

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MilesnMiles
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Compression test results/accuracy

Post by MilesnMiles »

Decided to come test my 273 hi po motor today. Comments and flaws pointed out welcome. Motor has solid cam and adjustable rockers.
Ran the motor for ten minutes to get it warm, but not hot by any means. Left all plugs in and span motor for about 7-8 seconds.
Cylinders all showed 115-130 psi.
Motor has nice oil pressure and no knocks.
Waddya think?
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Pete
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Re: Compression test results/accuracy

Post by Pete »

Did you open the Butterflies fully each tine you spun it over?
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MilesnMiles
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Re: Compression test results/accuracy

Post by MilesnMiles »

Yes I did Pete, not sure how long it should be spun for, haven’t done a comp test in many years.
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Pete
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Re: Compression test results/accuracy

Post by Pete »

That is plenty long enough in my opinion.

My 580ci was 180psi on all cylinders with virtually no spread at all.
BIG cam but nominally built for 11:1 Static compression.

Jack's engine was (I think) 13:1 and was 195 psi when cranking.

What are you looking for?
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MilesnMiles
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Re: Compression test results/accuracy

Post by MilesnMiles »

I’m not actually sure Pete. I’m currently looking around the net to find what the figure should be. Read that a decent high miler 318 will show 130 so maybe not that far out.
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Blue
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Re: Compression test results/accuracy

Post by Blue »

More important than the actual figure is the variance between cylinders. Squirting in a bit of engine oil and trying again to see if the pressure goes up will give you an idea of how good the ring seal is.
“This ain’t no party, this ain’t no disco, this ain’t no foolin’ around”
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Dave999
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Re: Compression test results/accuracy

Post by Dave999 »

[Ah what blue said]




id worry if it was substantially higher or lower on some than others
...believe an expensive gauge with a certificate in the box, don't believe (as much) a machine mart/clarke "special" i have had the pointer fall off...!
comparison across the 8 and a close match is more important here i think

your static CR will be 8 or 9:1

you could do it all again with a small squirt of oil and the engine HOT. and maybe see all at 130+ any truly low ones that come in line with the rest potentially have worn bore or rings or both

you don't have any really low so i presume you have no busted rings no badly seated valves no badly adjusted rockers etc etc
you might have the kind of ware that an engine that has worked for a living, has.

no further action required...if its running on all 8 nothing to worry about.

if it was less than 100 across the board i'd be suggesting its just worn out


if you did have a problem a leak down test would show you which problem you had. 2 gauges + funky valve in 1 unit, and a compressor necessary

hissing in the valve covers....rings/bore air is escaping in some reasonable volume past the piston into the crank case
hissing up the carb. inlet valve not seated
hissing at the exhaust, or it blows up a rubber glove.... exhaust valve not seated
bubbling in the radiator or it blows up a rubber glove.... cylinder head gasket or cracked head

but would not be running well at all....

what it can't show is a leak from cylinder, via gasket, into oil, if you have a rocker shaft feed up through the block to the head...although you might get a rocker blowing oily bubbles i have not seen or heard of that, would depend on where the feed comes from, i'm assuming "open to the air rocker/shaft" would be easier escape than blowing back past a tight clearance at the lifters, crank or cam, and it would have to be a trench through the gasket for you to notice .

Dave
Last edited by Dave999 on Tue Oct 06, 20 5:34 pm, edited 1 time in total.
Reason: Beat me to it
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MilesnMiles
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Re: Compression test results/accuracy

Post by MilesnMiles »

Wow, ok chaps. Car is ‘down’ at the moment for a complete brake rebuild, so didn’t drive the car to fully warm up before testing. Also, compression tester wasn’t a pricey unit, so accuracy may be questionable. I’ve since found that factory spec for 273 varied from 120-150 and none of my cylinders are down by huge variance. I thought I’d check as when driving I’ve had trouble getting the motor to perform much after 3,000 rpm even with a new carb and more timing pushed in.
At the back of my mind is, ‘do I refresh this original 273 or go down the 360 route’. The latter is tempting if I can find a spare motor.
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Dave999
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Re: Compression test results/accuracy

Post by Dave999 »

yeah but the 273 probably has a nice steel crank and adjustable rockers..

rebuilding the 273 into a performance motor will probably cost more and you'll be marked as bonkers by the "why would you build a performance 225, poly, hemi 6, 273, 318, when you could have a stroker 360" crowd.

But hey ive stuck with my 6 cylinder, resisted the temptation to go to the biggest motor fitted to a charger (360)... and i can scare myself with it maybe i'm just a chicken.... but my passengers get a sore neck and grab at the dash if i drive it like an arse.

in theory you could fit
high compression pistons
slight port of the heads
bigger valves
headers
cam

its 4.47 liters, ever so slightly bigger (0.02 liters) in size as my bored hemi 6. would go really well with 4, 44 IDFs on it you could put in a mental cam have 11:1 cr and have a torque monster up to 4500 rpm with enough to spin up to 5000+ which you won;t need

you'd need to tig weld the side rails of a 4bbl manifold onto the centre weber section off a ford small block... :) nobody does an IDF or IDA manifold for mopar but it would go well with the webers

you can't compare cfm rateing of a holley to a weber set up properly but 4 IDFs in 48 mm is general considerd to be 1000CFM+ of carb if you bend the maths and take some liberties to make yer point.... http://jiminglese.com/ look at the treasure here just don't buy it there are much cheaper ways



so some 44s are not really going to be a limiting factor.... or even 40s

small motor big heart

Dave
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Dave999
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Re: Compression test results/accuracy

Post by Dave999 »

full rebuild kit with high comp pistons

https://egge.com/kit/pl273m64-67w-4brl/

or bore it and use chevy 305 pistons expands the range considerably

https://www.performanceinjectionequipme ... job-part-1



jolly 273 v8 chat http://www.moparchat.com/forums/perform ... mance.html

dave
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MilesnMiles
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Re: Compression test results/accuracy

Post by MilesnMiles »

Hi. Dave, cheers for info. I did research the Egge stuff and multiple 273 build. As it stands I’m picking a built 360 layer this week, that gives me the option on what to do long term. Biggest cost for a rebuild seems to be the head work and I’m sure that the 273 heads would need sorting. 360 already has fully built J heads and is a drop,in replacement.
Now got to hope the trans holds up...and off we go again in Mopar madness world!
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