View Full Version : What should cylinder pressure be?
03-03-2003, 12:49 PM
Is there a place to get the information, or can someone tell me what the cylinder pressure on my engine be?
I have a '77 440 truck motor in a Ramcharger. It is completely stock at this time. I understand the compression ratio is 8.2:1. The heads are 452.
Thanks for your help.
03-03-2003, 12:54 PM
I'm thinking 145. It's been a long time since I checked for my '76 Ramcharger with a 400.
Do you have a Chiton's book? I think it's in there.
03-03-2003, 02:46 PM
All I have is the Haynes manual. It states that my cylinder pressure should be a minimum of 100psi with no more than a 25psi variance between cylinders.
03-03-2003, 02:54 PM
Interestingly enough, referencing the Haynes Manual on my 3.9L v6, I get the same thing. 100 psi with a maximum variation of 25%. I can't find the compression ratio though and was wondering if there was any way to find this without getting into it. Can it be decided looking at the compression pressure and some other factors? Or am I just to assume that it being a 3.9, should have the same compression as a 318? Its a magnum. Pretty much stock. Any guesses anyway?
03-03-2003, 02:55 PM
That helps a lot doesn't it? I'll look in my Chilton's when I get home and see if it's in there. Unless someone knows and posts before then.
The guy at the speed shop said about 170ish.
03-03-2003, 03:00 PM
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03-03-2003, 04:03 PM
There is a formula I read in Car Craft years ago, which states you take your ratio multiply by 14.7+5+14.7 should equal your cylinder pressure. In your case 140 psi. This formula seems to be accurate on stock motors.
03-03-2003, 04:09 PM
My '73 400 has 160 psi cylinder pressure.
'92dak- your compression ratio is 9:1
03-03-2003, 04:13 PM
it isnt that critical to know what is recommended but to check all and see if any cylinder varies more than 10 percent. if you have one way down from the rest that would be a problem. as long as they are close,you are ok. several things affect cylinder such as sparks plugs being removed while cranking,throttle open,aftermarket valve overlap also plays a major part in this. consistency is far more important than general rating.good luck!
03-03-2003, 04:45 PM
Ok, a hypothetical questian, [sota]
At what point is the cylinder pressure too low for comfortable "hot rodding"? Ok, towing a 3000 lb. trailer? I know the engine will be weak and have a lack of power, but what do you think the pressure reading would be when the power loss is substancial?
At what point do you think the engine would need rebuilding? Based on cylinder pressure only.
03-03-2003, 05:52 PM
My '74 2bbl 400 had 80 psi in all holes. Since I'm re-rebuilding my 440 for some mid 11 sec quarter, this is all I've got to work with. I did the 'nickel' rebuild (std. size rings, bearing, timing chain, etc) to it. At the same time I put in a cam and lifters that was in my buddies '69 440 Cuda when he bought it. He was told it was a little over stock (who knows what that means). I also had the heads milled .040.
The 400 knows has a slight idle, and comp is now up to about 95 psi with throttle open, only one plug out (tested cylinder), battery fully charged. The 'beast' has bested many 5.0's and other Brand-X junk w/ thousands of dollars into their motors.....runs low 14's....so 95 psi can't be that bad!
One thing to note that hasn't been said yet....hows many cranks does it take to get up to your max level. A couple idicates a engine that is OK.
03-03-2003, 06:23 PM
3 rotations of that cylinder being tested is what I learned.
My Chilton's truck and van repair manual, 1970-1976 reads the "cranking comp. press. to be 100, no max and no min. for 440 engine, '70 thru '76 light and heavy duty.
With compression ratio of 7.5-8.2 1973-1976
03-03-2003, 07:05 PM
Thats what to look for! Take the highest pressure reading and compare the others. make sure the differance is minimal.
03-03-2003, 10:32 PM
I will have to check but im prety shor my haynes manuel gave the same specs for my \6.
03-04-2003, 02:06 AM
I've read somewhere on this site that altitude above sea level
also plays a significant role in your compression readings.
If memory serves, compression results will increase as you get
closer to sea level.
Anybody else heard this?
My 440 has a computed ratio of 8.32:1.
I performed compression test (operating temp., plugs pulled,
carb @ WOT) that yielded 155-160# in all cylinders.
Elevation approx. 800 ft.
03-04-2003, 08:40 AM
Charger Dan is correct..all plugs out, WOT and a warm engine.
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