cranking compression question

Posted By: gch

cranking compression question - 08/09/13 01:50 AM

On a low comp 440(75 model 8.2-1 advertised)what would be a good cranking compression number?
The motor was cold and the cylinders dry.

Posted By: RapidRobert

Re: cranking compression question - 08/09/13 02:07 AM

I'd be comfortable with 140 give or take 10 either way & happy with 150 or more. Critical is that there are no low ones

Posted By: gch

Re: cranking compression question - 08/09/13 02:55 AM

The cylinders are all within 10 psi but the numbers are a LOT lower.I was thinking 120 at best with that low of a compression but all are between 80-90.

Posted By: ireland383

Re: cranking compression question - 08/09/13 03:03 AM

I was thinking around 100ish. Have the head gaskets been changed anytime that you know of?

Posted By: RapidRobert

Re: cranking compression question - 08/09/13 03:41 AM

Quote:

but all are between 80-90.

It's in need of a rebuild

Posted By: TJP

Re: cranking compression question - 08/09/13 03:41 AM

Quote:

The cylinders are all within 10 psi but the numbers are a LOT lower.I was thinking 120 at best with that low of a compression but all are between 80-90.

80 to 90???? Ouch!!!! it will run but will have its tail between its legs,

Posted By: AndyF

Re: cranking compression question - 08/09/13 04:31 AM

I checked the cranking compression on my stroker motor the other day and it was 235 psi. And that is with a 280 at 0.050 camshaft. I can barely turn the engine over with a cheater bar.

Posted By: ek3

Re: cranking compression question - 08/09/13 04:56 AM

Quote:

On a low comp 440(75 model 8.2-1 advertised)what would be a good cranking compression number?
The motor was cold and the cylinders dry.

100 lbs. will work fine. you would be surprised what just a good valve job or a new timing chain would do. warm it up, pull all the plugs out, and spin it over 3-4 times to test. when you warm it up , run a little marvel mystery oil in it also then re-check it. a good fresh 440 with 8 -1 compression making 130-40 lbs. would run fine. if you are looking for real power , 9:5:1 or better and 175 + is a good starting point. if the engine has set up for a long time, while running it, pour a little diesel fuel [ about a quart untill gone] in the carb when it is hot and revved up some . this will free the rings / valves up .

Posted By: Kern Dog

Re: cranking compression question - 08/09/13 05:27 AM

Quote:

I checked the cranking compression on my stroker motor the other day and it was 235 psi. And that is with a 280 at 0.050 camshaft. I can barely turn the engine over with a cheater bar.

Holy crap! Thats a RACE fuel engine for sure!
I was at 190 in my 440/493. 10.7 static with the MP '509 cam.

Posted By: gch

Re: cranking compression question - 08/09/13 01:30 PM

Motor/car has been sitting for 10 plus years but runs surprisingly smooth.Still has the cat converter lol. I was hoping for a little higher number so I could justify throwing some spare performance parts on it and making a rat rod/beater for fun.Pulling the heads is more $$$ than I want to put in to it.
I drove it home on 13 year old tires and then found out the idler arm was about to fall off of it.lol

Posted By: MoparforLife

Re: cranking compression question - 08/09/13 01:36 PM

Tell you what - It will run fine. All the cylinders are even showing me that it has pretty equal wear pattern. Doing a valve job (unless you have a valve leak) will do nothing for you. an even test like you have does not represent a leaky valve. A cylinder with a leaky valve most generally will have a pretty noticeable compression drop from the rest. Not an even reading like you have.

Posted By: jlatessa

Re: cranking compression question - 08/09/13 02:01 PM

Just a thought, are you cranking with the carb at WOT?

Joe

Posted By: gch

Re: cranking compression question - 08/09/13 02:27 PM

I had my mechanic do it because I am lazy. I will have to ask him.

Posted By: 360view

Re: cranking compression question - 08/09/13 03:15 PM

I have never understood it
but
100 psi minimum
is the cranking compression listed
in the official
Dodge Ram 1995
Field Service Manual
specs page for the
488 ci/8L iron V10
which is a spec of
8.4 to 1 static compression
and has a quite modest duration camshaft.

Posted By: 360view

Re: cranking compression question - 08/09/13 03:25 PM

167 psi
on a warm engine
with a shot of oil in each cylinder
throttle bores propped fully open
starter turning crank at 300 rpm
75 degrees F
700 feet above sea level
is what the
PerformanceTrends Engine Analyzer 3.2
computer software estimates
the cranking compression
should be on the fourth stroke of the piston
when the camshaft specs for a
1995 488 iron V10 are entered

Posted By: 360view

Re: cranking compression question - 08/09/13 03:35 PM

178 psi
cranking compression
is what the
EA 3.2 estimates for the
1969 Mopar 440 V8

This a an example engine that comes with the program.

Specs are filled in by the author of the software
Kevin Gerten, an ex-Ford engine development engineer.

Static compression is 10 to 1

Intake Valve timing:

centerline 113 degrees
duration at 0.050. 214
open at 0.050 -6 degree btdc
close 40 degrees abdc
gross lift 0.447 inches

SAE conditions HP
on 93 octane AKI gasoline

309 horsepower at 4200 rpm
410 ft-lbs at 3200 rpm
0.497 lbm/hp-hr BSFC at 2200 rpm

Posted By: buildanother

Re: cranking compression question - 08/09/13 04:06 PM

I don't think the o/p stated how many hits of each cylinder were utilized for his findings. I go about 4 cycles which gets you close enough. Some folks have said they just bring it around once and that won't work as the first spin has to fill the gauge-hose etc.

Posted By: Paul_Fancsali

Re: cranking compression question - 08/09/13 04:49 PM

That engine is a 7.5 comp at best 90 psi is plenty after not being run and sitting Drive it some add some sea foam and bet it will be over 100 Those engines new barely cranked 125 psi If they are all even it alone change the timing chain get a cam that will help it out Comp 260 series comes to mind

Posted By: mickm

Re: cranking compression question - 08/09/13 08:45 PM

Quote:

Quote:

I checked the cranking compression on my stroker motor the other day and it was 235 psi. And that is with a 280 at 0.050 camshaft. I can barely turn the engine over with a cheater bar.

Holy crap! Thats a RACE fuel engine for sure!
I was at 190 in my 440/493. 10.7 static with the MP '509 cam.

the new engine is 210 on every cylinder, and runs fine on 91. but i guess 25 more is a lot. same thing for me, tying to turn it over with a breaker bar is an exercise in upper body strength.

Posted By: ademon

Re: cranking compression question - 08/09/13 10:57 PM

205psi on 93 pump, they run so nice with hi cranking pressure !!!

Posted By: ek3

Re: cranking compression question - 08/09/13 11:47 PM

Quote:

Tell you what - It will run fine. All the cylinders are even showing me that it has pretty equal wear pattern. Doing a valve job (unless you have a valve leak) will do nothing for you. an even test like you have does not represent a leaky valve. A cylinder with a leaky valve most generally will have a pretty noticeable compression drop from the rest. Not an even reading like you have.

no one said it had 1 leaky valve . hi mileage on -all- valve seats will flatten them out . worn guides will wear the valve seats and valve faces unevenly thus allowing leakage at "all" the tired worn areas. carbon build up on the valves and seats will further interfere with cylinder sealing . an even reading just means even wear. a burnt valve would have a lower reading on that cylinder. the typical fluid test represents how effective a good valve job is at sealing the combustion chambers. if they are leaking ,they are loosing compression..

Posted By: gch

Re: cranking compression question - 08/10/13 02:57 AM

It was spun over until the gauge reached it's highest reading.Throttle bores were closed but with all plugs out I don't see it making much difference.It is hard to justify spending much money on it when my Polara is languishing under the carport.
I actually have a comp 260 cam setup under the bed(my wife loves me).Now if I hadn't sold the performer intake and those hedman shorty headers.....

Posted By: stumpy

Re: cranking compression question - 08/10/13 03:38 AM

Throttle closed will not allow in enough air necessary to get a correct reading.

Posted By: 360view

Re: cranking compression question - 08/10/13 11:15 AM

119 psi
is what the EA 3.2 predicts
for cranking compression
if the static compression is dropped from
10 to 1
to
7.5 to 1