Could My Heads Be Milled Or Something???

BowTie Eater 5 Liter · 01-31-2002, 01:30 AM

I did a compression test just a couple months ago, and my compression was at a 148psi low, and 158 high.

I have been told by lots of people a stock 5.0 is around 140 stock.

Oh, BTW I have 110000 miles on the clock, so its well worn.

Jeb_Bush_2000 · 01-31-2002, 01:37 AM

That's, what, a low of 10:1 and a high of 11:1? Hope you're running 93 octane.

PKRWUD · 01-31-2002, 03:17 AM

Quote:

Originally posted by Jeb_Bush_2000
That's, what, a low of 10:1 and a high of 11:1?

?????????????????

Anyway, what was the difference between wet and dry? There is no set answer for a stock 5.0. I've seen all kinds of readings. That's why they stopped listing a recommended number, and instead gave a range that was acceptable. Several things could affect your readings, too. On an engine with high miles, carbon build up often occurs, and that can both raise compression and encourage pinging.

You should run a test again, both wet and dry, and with all of the plugs removed during the test. Post the results.

Take care,
-Chris

Jeff Chambers · 01-31-2002, 07:59 AM

Doesn't quite work like that Jeb. Static compression tests are a measure of the pumping ability of the cylinder. Many things affect it such as ring seal, ring gap, piston clearances, engine temperature, throttle (open/close), exhaust restriction, etc. You just can't take the static compression test pressure and divide by 14.7 to get the compression ratio. Remember that you've also got the volume of the test guage to consider in the process. That's why you have to crank it 5 to 10 times before the pressure reading peaks.

For example, my motor is above 12:1 so by your calculations I should see about 176psi on a static tests. My normal readings are more on the neighborhood of 250 to 260 psi per cylinder. That would say that I've got a compression ratio of 17:1! God, I wish!!!! The extra pressure comes from many factors, including the fact that there's no exhaust scavenging present to help empty the cylinders, among others. Bow Tie is right in the normal range for a stock motor. There's only 7% difference between what he saw and what he was expecting. Hot/cold, adiabatic heating, inaccuracy in the gauge, and many more things could stack up to that 7% real quick. Static compression test should be used as more of a qualitative test rather than a quantitative test. As long as all cylinders were within a reasonable range of each other, he's OK.

fiveohpatrol · 01-31-2002, 10:09 AM

Quote:

adiabatic heating

Jeff, I thought the term "adiabatic" meant there was no heat tranfser in the system? So what exactly is adiabatic heating? Just wantin to learn. Maybe my Thermo Proffessor doesnt know what he's talking about

Oh yah, how'd those pedal's work out for ya?

jimberg · 01-31-2002, 10:42 AM

Yes, adiabatic heating would be an oxymoron.

My compression ratio is 9.5 to 1 and I read about 181 psi in all my cyls.

Jeff Chambers · 01-31-2002, 12:36 PM

Maybe what I was reaching for was adiabatic compression. What I was thinking was that there's work put into the system in compressing the volume which would increase the temperature of the volume to some extent. Would it mean much to the system? Probably not, since there's enough thermal mass in the cylinder wall, heads and pistons to probably wick that heat away pretty quickly. But in the theoretical world, it'd be easiest to consider the compression cycle as adiabatic and ignore the heat lost to the wall. Conversely, if done when the motor is warm, the heat input would probably be something to consider. Check with your professor and see whether it'd be an adiabatic, isobaric, isometric, isothermal, isentropic or polytropic process. Gawd how I hated thermo! I'd much rather deal with 'lefty loosey, righty tighty'.

Point is though that'd just be one of the many tiny little variables that all stack up in the variability.

chris91LX · 01-31-2002, 01:20 PM

I was told that compression tests held way too many variables to be truly acurate and that the best way to check the cylinders was by doing a leak down test. Supposed to be more accurate anyway.

PKRWUD · 01-31-2002, 10:33 PM

Chris-
Compression tests are very usefull. They aren't as good a diagnostic tool as a vacuum gauge, but it's still an important test. It tells you where to look for the problem, but won't tell you what the problem is.

Take care,
-Chris

01-31-2002, 01:30 AM	#1
BowTie Eater 5 Liter Registered Member Join Date: Nov 2000 Location: Maple Ridge B.C. Canada, The Best Place In The World Posts: 871	Could My Heads Be Milled Or Something??? I did a compression test just a couple months ago, and my compression was at a 148psi low, and 158 high. I have been told by lots of people a stock 5.0 is around 140 stock. Oh, BTW I have 110000 miles on the clock, so its well worn. __________________ 1984 Bronco II, 36" Michelin Military Tires, 95 EFI 5.0, NP435 Tranny, NP208 Doubler, NP205 Transfer Case, Fill Size Solid Dana 44 Front End, WH Progressive Rate Rock Crawling Coils, Bronco 8.8 Rear, Skyjacker Softride Leafs, 5.13 Gears, Detroit Locker. Final Crawl Ratio Of 175:1 88 GT Best 60ft. 2.181 Best ET 14.350 Best MPH 98.01 ~WRECKED!!!!~

01-31-2002, 01:37 AM	#2
Jeb_Bush_2000 Registered Member Join Date: Jul 2001 Location: Taxachusetts Posts: 374	That's, what, a low of 10:1 and a high of 11:1? Hope you're running 93 octane. __________________ Stripped-down '93 LX Coupe -- "Danielle" R.I.P. 05/30/02 "I've seen some crazy stuff in my life, but that...was...AWESOME!!! Oh, sorry about your car, man."

01-31-2002, 07:59 AM	#4
Jeff Chambers Moderator Join Date: Aug 2000 Location: Milan, OH Posts: 2,699	Doesn't quite work like that Jeb. Static compression tests are a measure of the pumping ability of the cylinder. Many things affect it such as ring seal, ring gap, piston clearances, engine temperature, throttle (open/close), exhaust restriction, etc. You just can't take the static compression test pressure and divide by 14.7 to get the compression ratio. Remember that you've also got the volume of the test guage to consider in the process. That's why you have to crank it 5 to 10 times before the pressure reading peaks. For example, my motor is above 12:1 so by your calculations I should see about 176psi on a static tests. My normal readings are more on the neighborhood of 250 to 260 psi per cylinder. That would say that I've got a compression ratio of 17:1! God, I wish!!!! The extra pressure comes from many factors, including the fact that there's no exhaust scavenging present to help empty the cylinders, among others. Bow Tie is right in the normal range for a stock motor. There's only 7% difference between what he saw and what he was expecting. Hot/cold, adiabatic heating, inaccuracy in the gauge, and many more things could stack up to that 7% real quick. Static compression test should be used as more of a qualitative test rather than a quantitative test. As long as all cylinders were within a reasonable range of each other, he's OK. __________________ Jeff Chambers 1990 Mustang GT 10.032 Seconds / 137.5 MPH 14-time Street Warrior World Record Setter CRT Performance 2001 Tropic Green Mustang GT - 12.181 / 113.2 MPH 2002 Ford F-250 Crew Cab 7.3l Power Stroke - 17.41@77.2 "There's nothing boring about a small block automatic shifting gears at 9400 rpm!"

01-31-2002, 10:42 AM	#6
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	Yes, adiabatic heating would be an oxymoron. My compression ratio is 9.5 to 1 and I read about 181 psi in all my cyls. __________________ 351W 89 Mustang GT Convertible

01-31-2002, 12:36 PM	#7
Jeff Chambers Moderator Join Date: Aug 2000 Location: Milan, OH Posts: 2,699	Maybe what I was reaching for was adiabatic compression. What I was thinking was that there's work put into the system in compressing the volume which would increase the temperature of the volume to some extent. Would it mean much to the system? Probably not, since there's enough thermal mass in the cylinder wall, heads and pistons to probably wick that heat away pretty quickly. But in the theoretical world, it'd be easiest to consider the compression cycle as adiabatic and ignore the heat lost to the wall. Conversely, if done when the motor is warm, the heat input would probably be something to consider. Check with your professor and see whether it'd be an adiabatic, isobaric, isometric, isothermal, isentropic or polytropic process. Gawd how I hated thermo! I'd much rather deal with 'lefty loosey, righty tighty'. Point is though that'd just be one of the many tiny little variables that all stack up in the variability. __________________ Jeff Chambers 1990 Mustang GT 10.032 Seconds / 137.5 MPH 14-time Street Warrior World Record Setter CRT Performance 2001 Tropic Green Mustang GT - 12.181 / 113.2 MPH 2002 Ford F-250 Crew Cab 7.3l Power Stroke - 17.41@77.2 "There's nothing boring about a small block automatic shifting gears at 9400 rpm!"

01-31-2002, 01:20 PM	#8
chris91LX Registered Member Join Date: Oct 1998 Location: Suburban Chicago, Hanover Park Posts: 695	I was told that compression tests held way too many variables to be truly acurate and that the best way to check the cylinders was by doing a leak down test. Supposed to be more accurate anyway.

01-31-2002, 10:33 PM	#9
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	Chris- Compression tests are very usefull. They aren't as good a diagnostic tool as a vacuum gauge, but it's still an important test. It tells you where to look for the problem, but won't tell you what the problem is. Take care, -Chris __________________ Webmaster: Rice Haters Club Jim Porter Racing Peckerwoods Pit Stop Support Your Local RED & WHITE!

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