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View Full Version : He he...engine compression numbers


subzali
07-26-2007, 08:35 PM
This is pretty awesome. I don't think these are exact, but they're within 5 psi (I left the paper in the truck. This is after warming it up for a few minutes, and cranking it over for about 4 seconds which I'm pretty sure is more than three compression cycles.

(DRY)
#1 - 110
#2 - 110
#3 - 105
#4 - 100
#5 - 105
#6 - 110

(WITH A COUPLE SQUIRTS OF OIL)
#1 - 115
#2 - 115
#3 - 110
#4 - 102
#5 - 110
#6 - 112

I think it's supposed to be around 140-150 right? Well I don't know what to say the stupid thing runs great! Maybe I'll try cranking it for a little longer in the morning to see if that makes a difference.

SteveH
07-27-2007, 03:21 PM
The best 2F I ever measured in Denver was 119 at 5000' of elevation. At my 7000', I see 112 as a good number. So, your numbers are pretty even and good enough. Adjust the valves and drive it and don't worry about it.

Realize that compression testers can be off quite a bit, as they age and get knocked around.

Steve

subzali
07-27-2007, 07:06 PM
That's funny, I guess spec is 113-137 or something like that. I just bought this brand new gauge and this was the first time it's ever been used. But if that's typical for this altitude, then I won't worry about it. The thing pulls hard all the way through the speed range, pulls many hills at 50-60 and can go upwards of 70-75 when it's a little flatter.

Rezarf
07-28-2007, 08:20 PM
Low compression isn't too bad if the numbers are still in the same range as one another. It is more a concern when you have 5 cylinders at 145, and one at 90psi.

I think your numbers aren't awesome, but they are consistant!

FWIW, mine came in at 120ish before I redid the head, then they jumped up a bit. :D

Drew

subzali
07-29-2007, 06:53 AM
Yeah what I meant by awesome was that they were surprisingly low for how good the truck runs! I was actually kinda disappointed that they were so low, but now I know I can look forward to more performance if/when I play around to get those back up!

Rezarf
07-29-2007, 09:50 AM
...If it ain't broke!!.... :D

Red_Chili
07-29-2007, 03:14 PM
fix it till it is!

subzali
07-29-2007, 08:39 PM
:D



Don't worry I have enough junk that's broke to keep me busy for a while...and like I said the truck runs great!

Rezarf
07-29-2007, 09:17 PM
fix it till it is!

LOL! :D :lmao:

subzali
12-17-2010, 08:09 AM
I wanted to resurrect this thread because we never talked about adjusting the compression readings for altitude. Can someone verify with me how this is done? Do you just take the ratio of atmospheric pressure at sea level to the atmospheric pressure where you're taking the reading, and multiply that by the reading the gauge gives you?

So,

Psea/Palt*Calt=Csea --> 14.7/10.6*Calt=Csea --> 1.39*Calt=Csea

Psea=atmospheric pressure at sea level (14.7 psi)
Palt=atmospheric pressure at altitude where reading is taken (10.6 psi)
Calt=compression reading at altitude where reading is taken
Csea=what compression reading would be at sea level

The reason I'm not sure about this is that if I take the numbers from above I get:
#1 - 166
#2 - 166
#3 - 159
#4 - 147
#5 - 159
#6 - 161

which seems to high for me, especially after seeing the condition of my cylinders a couple years ago.

I'm feeling stupid.

RicardoJM
12-17-2010, 08:52 AM
I basically use the information here (http://www.pelicanparts.com/techarticles/mult_Engine_Rebuild/mult_engine_rebuild-2.htm) to determine what the specification (from sea level) becomes at a given altitude. At 5,000 feet (Longmont) the specification range in the F engine FSM (128 to 150) becomes 110 to 129. At 7,000' the range becomes 122 to 104.

Use the chart in that link to figure out what the range becomes at the altitude you did the compression test at, then see how your numbers compare. :thumb:

subzali
12-17-2010, 09:06 AM
So I did the test at my parents house which is at 7000 ft or so. According to that chart the multiplication factor is about .813 (linear between 6500 ft and 7500 ft).

So my numbers, adjusted for altitude, are:
#1 - 141
#2 - 141
#3 - 135
#4 - 125
#5 - 135
#6 - 138

Okay, so that's not too bad.

Even if it's closer to 6500 ft, they are:
#1 - 139
#2 - 139
#3 - 133
#4 - 123
#5 - 133
#6 - 136

I feel much better now.

RicardoJM
12-17-2010, 09:27 AM
So what were your readings directly from the gauge? At 7,000 feet the adjusted range is 122 to 104.

Whether you adjust your readings or adjust the range and compare you actuals shouldn't matter, but I'm used to adjusting the range. It is easier for me to not have to do the math on each reading:D.

subzali
12-17-2010, 09:47 AM
First post; 102-115 wet test.

Air Randy
12-17-2010, 09:59 AM
So what were your readings directly from the gauge? At 7,000 feet the adjusted range is 122 to 104.

Whether you adjust your readings or adjust the range and compare you actuals shouldn't matter, but I'm used to adjusting the range. It is easier for me to not have to do the math on each reading:D.

Guys, don't make this more complicated than it needs to be. The rule of thumb is you adjust 3% for each 1,000 above sea level. So if Matt's location where the test was performed is 7,000 feet it's a 21% adjustment.

Take the gauge reading of 113 and divide it by .79 which equals 143. Thats close to being within 10% of spec which is 150 psi at sea level. If all of the cylinders are roughly within 10% of each other, then your motor is in pretty excellent condition.