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Old 12-19-2007, 01:02 AM   #1
SlmLrd
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Default Flathead compression test- the results are in!

First of all, Im new to this, so bare with me, one day I will have this knowledge to share.

So after some reading about my smoke issues on my flathead, I decided to run a compression test. My numbers are as follows:

1- 90
2- 90
3- 80
4- 90
5- 75
6- 80
7- 90
8- 90

I understand that 3,5,6 compression is low. For the numbers that Ive read in posts, they all seem a bit low, but in range of each other. What else, if anything does this tell me about my engine? If your numbers looked like this, where would you start? My engine fires right up, and runs well, but smokes here and there under acceleration.

Thanks!
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Old 12-19-2007, 01:06 AM   #2
Minewithnoshine
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Default Re: Flathead compression test- the results are in!

Did you do the test hot? Throttle open? Altitude? Those can have a huge effect on the numbers. You need to look for consistency most and foremost.
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Old 12-19-2007, 01:16 AM   #3
SlmLrd
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Default Re: Flathead compression test- the results are in!

I did the test cold, throttle closed, and at sea level(ish)
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Old 12-19-2007, 02:06 AM   #4
Destralo Roach
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Default Re: Flathead compression test- the results are in!

Open the throtle all the way to get better #'s but I'd say the one with 75lbs your culprit...Roach.
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Old 12-19-2007, 02:15 AM   #5
SlmLrd
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Default Re: Flathead compression test- the results are in!

Quote:
Originally Posted by Destralo Roach View Post
Open the throtle all the way to get better #'s but I'd say the one with 75lbs your culprit...Roach.
So what can I do to the culprit? Should I pour a bit of MMO into it and run her, hoping to seal it up? Could someone break down the science of low compression in one cylinder to me, or direct me to a link? Im close to understanding whats going on here, but the loose ends have me a bit confused still.

I understand that when the piston is moving in the cylinder, and there is a gap somewhere, you will lose compression. Is there a fix for this, assuming I dont have a crack somewhere?

Haha, Its midnight and Im revving the shit out of my engine in the driveway, yeah, the neighbors love me.
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Old 12-19-2007, 04:03 AM   #6
Mercmad
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Default Re: Flathead compression test- the results are in!

it's a flattie, pull the head,drop the sump,undo the rod and push the piston out.
If it's OK and the broken ring ends haven't destroyed the ring lands,replace the rings, reassemble,and get back to driving.

Compression tests are just for quicky diagnosis,a leak down test is the only way to determine engine condition.
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Old 12-19-2007, 04:16 AM   #7
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Default Re: Flathead compression test- the results are in!

Originally Posted by Fortyfordguy
a compression test is two parts. One dry and one wet (oil sprayed into the cylinder). You do the wet test AFTER the dry test of course. This will tell you if the rings were at fault (the readings will be slightly higher because the oil tends to help seal them up temporarily). If the reading stays the same on the wet test, you can figure that there is a leaking/cracked valve.
I found this thread after typing in better keywords in the search, and helped me a bunch. Every cylinder rose in compression to this:

1- 105
2- 110
3- 105
4- 110
5- 95
6- 100
7- 100
8- 110

So, from this I conclude a bad ring in #5. If what I have read and did is correct, I learned something, and understand it now. Now for those rings......
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Old 12-19-2007, 08:42 AM   #8
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Default Re: Flathead compression test- the results are in!

I'd look at valves first...if you're lucky, maybe just a spec of crud on a valve and you'll be home almost for free...
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Old 12-19-2007, 10:40 AM   #9
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Default Re: Flathead compression test- the results are in!

When you did your compression test did you squirt oil into the cylinders first or did you do it dry? Also, as Bruce says, could just be a valve problem. Maybe just a little sticky. I would run it for a while and see what happens. Those numbers aren't that far off.
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Old 12-19-2007, 11:15 AM   #10
Bruce Lancaster
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Default Re: Flathead compression test- the results are in!

It's already been implied here, but in the valve area you are looking at TWO areas: Valves tend to stick in guides after loong sitting (and the open springs may decline in health), and seats may have corrosion or crud preventing seal on a basically good seat surface.
I might just try a shiftless no-work two step preliminary...reach through plug hole and give each valve a good smack down onto seat, using something bent to reach center of valve...and run the thing a while with Marvel Mystery oil/diesel.kero type solvent in gas and oil to treat the guides.
Remember, valves are a lot easier to bend than seems possible.
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Old 12-19-2007, 01:39 PM   #11
man-a-fre
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Default Re: Flathead compression test- the results are in!

As said id put some marval in the oil and run it around for awhile may decarbon valves and free up rings.
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Old 12-19-2007, 02:37 PM   #12
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Default Re: Flathead compression test- the results are in!

Im gonna pick some MMO up today and see how we do. And yes, she sat for a little while. The PO told me that she smoked alot at first, but over time it got better.
The smoke now isnt horrible, I just hate any smoke blowing out of the back of one of my vehicles, and want to get her running top notch.
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Old 12-19-2007, 02:50 PM   #13
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Default Re: Flathead compression test- the results are in!

Be aware the MMO itself may smoke a bit...but it'll loosen up anything that's capable of loosening up without teardown.
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Old 12-19-2007, 02:58 PM   #14
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Default Re: Flathead compression test- the results are in!

The compression readings were done on a cold engine, so they are a decent indicator but not done under ideal conditions (near operating temp). On a warm engine things may look a bit better, maybe not.

Since the high and low readings are within 20% of one another I'd run it and wouldn't worry about it. You're doing pretty good to see the highest at 110 and the lowest at 95 on an older engine that's been sitting.

This stuff will free up the rings (it's designed to do exactly that) just as well as and likely better than MMO will:http://auto-rx.com/

I just poured a bottle of it into a '74 Pontiac 455 that has the original bottom end/short block still untouched. It's a great-running older engine that might even seal up and run better now with the ring pack cleaned up. I've used it to keep the insides of a couple of may late-model driver engines healthy as well.

In an old flathead that's never seen detergent oil you might not want to use any kind of cleaner, but I thought I'd throw this out there.....
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Old 12-19-2007, 05:36 PM   #15
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Default Re: Flathead compression test- the results are in!

Well now its conclusive!

Maybe Ill just run her for a while and see what happens. Hell, I dunno..
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Old 12-19-2007, 06:14 PM   #16
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Default Re: Flathead compression test- the results are in!

Quote:
Originally Posted by draggin'GTO View Post
In an old flathead that's never seen detergent oil you might not want to use any kind of cleaner, but I thought I'd throw this out there.....
id say this would be so if it was being put into the oil pan... Bruce suggested adding some MMO to the fuel to aid in cleaning up any stubborn carbon buildup that'd prevent proper valve seal, to do this won't require a large amount. trickling water down the carb at operating temp does this as well.

slmlrd - try the MMO... if it doesnt do anything, dont worry about it.
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Old 12-19-2007, 08:01 PM   #17
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Default Re: Flathead compression test- the results are in!

I'd run the MMO down the carb while revving the engine too...makes a helluva smoke screen for the neighbors, but will really help unstick stubborn valves...
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Old 12-19-2007, 11:44 PM   #18
draggin'GTO
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Default Re: Flathead compression test- the results are in!

Just for the sake of spreading some useful info here is what Pontiac told their dealers back in the '60s about engine compression testing. Most of it should apply to just about any automotive engine, even the beloved Ford flathead V8.

Here is what the Dealer Sevice Bulletin number 67-1-27 dated 2-3-67 says:

Low compression pressures obtained on GTO, 421 HO, or 428 HO engines are NOT a valid indication of engine condition. Due to the long valve overlap period with camshafts used on these engines, compression pressure readings (at cranking speeds) as low as 120 PSI are considered normal.

An engine should NOT be condemned as faulty on the basis of these low compression pressures alone. The readings obtained from all 8 cylinders should be taken and recorded. If the lowest cylinder is not less than 80% of the highest, the engine compression is probably okay.

If there is still some doubt, the engine may be tested using a cylinder leak tester. If leakage at TDC on the compression stroke is not over 20%, the cylinder is okay.

Compression pressures should be checked as follows:

1. Run engine until completely normalized - 15 minutes minimum

2. Shut off engine and hook up primary tachometer.

3. Remove all spark plugs.

4. Disconnect coil tower lead at coil and ground to engine.

5. Block choke and throttle open fully.

6. Install compression tester in No. 1 cylinder and crank engine at least 5 compression strokes. Observe cranking speed and record highest pressure obtained.

7. Repeat for other cylinders. Be certain cranking speed is the same for each check (within 10 RPM) and is at least 150 RPM.

8. Take 80% of the highest reading. If no other reading is below this figure, the engine is probably okay.
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