compression
Cargo Master
Joined: Sep 2004
Posts: 2,691
Likes: 6
I asked this same question a year ago for my 72 360. My numbers ranged from 111 to 118. I never got an answer. The responses were things like they should be close to one another and so forth. But nobody seems to know what actual compression values should be.
Postmaster
Joined: Jul 2005
Posts: 3,248
Likes: 0
Originally Posted by Mike G
Welcome aboard!
Acceptable is a steady reading across all cylinders, within a limit of 10% from one to the other. Now I recall 110 psi as being a "good" number to work with.
--Mike
Acceptable is a steady reading across all cylinders, within a limit of 10% from one to the other. Now I recall 110 psi as being a "good" number to work with.
--Mike
If the spark plugs look good and no blue smoke from the exhaust. Then cylinders are good.
Fleet Owner
Joined: Jun 1999
Posts: 24,552
Likes: 74
From: Creedmoor, North Carolina
Originally Posted by qman
I asked this same question a year ago for my 72 360. My numbers ranged from 111 to 118. I never got an answer. The responses were things like they should be close to one another and so forth. But nobody seems to know what actual compression values should be.
That is the answer that I get, the amount is not as critical as eveness.
John
Cargo Master
Joined: Sep 2004
Posts: 2,691
Likes: 6
There has to be a range the numbers have to be in. I don't think the engine would be looked at as a strong engine if all the cylinders were 60 psi. In this case they would all be pretty even but I would think they would be too low. Am I right?
I looked over my numbers again, they ranged from 108 to 120. I had one cylinder at 108 and another at 120 and all the others were in the 111-118 range. This is in a 72 360.
I looked over my numbers again, they ranged from 108 to 120. I had one cylinder at 108 and another at 120 and all the others were in the 111-118 range. This is in a 72 360.
Trending Topics
Laughing Gas
Joined: Jan 2002
Posts: 1,008
Likes: 1
From: Tucson
Here's a quote from Steve Christ's FE book:
" The range of pressures should be 100-250 psi. They should definitely be above 100 psi, but there can be a large difference between engines. An older 352 in excellent condition will show about 180 psi per cylinder. With 75% considered acceptable, the lowest reading of any other cylinder should be 135 psi. An engine with this wide range of readings is not in excellent condition."
jor
" The range of pressures should be 100-250 psi. They should definitely be above 100 psi, but there can be a large difference between engines. An older 352 in excellent condition will show about 180 psi per cylinder. With 75% considered acceptable, the lowest reading of any other cylinder should be 135 psi. An engine with this wide range of readings is not in excellent condition."
jor
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
3 Best / 3 Worst Parts of Modern Ford Ownership
Brett Foote
10 Amazing Upgrades That Solve Common Ford Truck Owner Headaches
Pouria Savadkouei
Every 2026 Ford Engine Explained
Brett Foote
10 Ugly Ford Trucks That We Still Kinda Love
Joe Kucinski
10 Things Every Truck Owner NEEDS (2026 Edition)
Michael S. Palmer
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Verdad Gallardo
Top 10 Most Expensive Ford Trucks Ever Sold on Bring a Trailer
Joe Kucinski
2027 Ford Super Duty Buyer's Guide (Every Model, Engine, & Package)
Brett Foote
Top 10 Ford Truck Tragedies
Joe Kucinski
Cargo Master
Joined: Jul 2003
Posts: 2,735
Likes: 20
From: Four Oaks, NC
It depends on a lot, especially the cam. A high performance cam with more valve overlap for example bleeds off pressure at low RPM but builds more at high RPM. So your compression ratio + your cam are two main things that affect your cylinder pressure, that are different on everyone else's vehicle. That's why having them be close to them same from cylinder to cylinder matters more than what the actual number itself is.
Cargo Master
Joined: Sep 2004
Posts: 2,691
Likes: 6
Thanks jor, finally someone that has definitive data. And jor, is that range (100-250) applicable to all engines or does the range for a 300 added to the ranges for all other engines (302, 351, 360, 390, etc) cover that total. In other words, is it OK for one 360 to be like mine in the 108-120 and another 360 in the 180-190 and yet even another 360 in the 230-240.
Last edited by qman; Jul 29, 2005 at 11:55 AM.
Cargo Master
Joined: Jul 2003
Posts: 2,735
Likes: 20
From: Four Oaks, NC
Thats a pretty wide range to be definitive data!
As for your 360 compared to other 360's, it would depend on the cam, and the compression ratio, and whether or not the cam is advanced or retarded. If the engine was made during the smog years (73-newer) there is a good chance the cam has 4 degrees retarded ground into the profile.
As for your 360 compared to other 360's, it would depend on the cam, and the compression ratio, and whether or not the cam is advanced or retarded. If the engine was made during the smog years (73-newer) there is a good chance the cam has 4 degrees retarded ground into the profile.
Cargo Master
Joined: Sep 2004
Posts: 2,691
Likes: 6
Forget about the 10% for a minute. Are the numbers, 108-120 acceptable for a stock 72 360 engine or is my engine shot? Like I mentioned earlier, if all my cylinders read in the 60-65 psi range they too would be within 10% of one another but I'm gonna guess that 60 psi isn't considered acceptable. I would just like to know if the values themselves (i.e. 108, 111 x 3, 114 x 3, and 120) are pressures that would be considered a strong engine. Or is it that my engine should have higher or lower pressures? Or is it like jor reported above that since my cylinder pressures are in the 100-250 range, then it is OK?
Cargo Master
Joined: Jul 2003
Posts: 2,735
Likes: 20
From: Four Oaks, NC
Ok well usually you wouldn't expect all the cylinders to wear evenly, the fact that all 8 are within 10% of each others generally leads you to believe that they are relatively close to what they are supposed to be.
But you want a mathematical answer, so here goes. The atomspheric air pressure is 14.7 pounds. If your engine has 9:1 compression ratio, and the cylinder is perfectly filled with 14.7 pounds of atmospheric air, then you would have 132 psi once the piston reaches TDC.
However most engines do not have 100% effeciency, in fact 85% is considered pretty good, which would put you at 112 psi.
So I think your 108 to 120 range seems to fit pretty well.
But you want a mathematical answer, so here goes. The atomspheric air pressure is 14.7 pounds. If your engine has 9:1 compression ratio, and the cylinder is perfectly filled with 14.7 pounds of atmospheric air, then you would have 132 psi once the piston reaches TDC.
However most engines do not have 100% effeciency, in fact 85% is considered pretty good, which would put you at 112 psi.
So I think your 108 to 120 range seems to fit pretty well.