Checked your compression lately?

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Checked your compression lately?

Postby lasitter » Mon May 16, 2011 12:56 pm

I'm wondering what the average compression and compression range would be for one of these engines that has not been rebuilt / modified and has 80,000 - 100,000 miles on it.

I think the max range is something like 25 percent between the highest and lowest cylinder before you have an "official Ford" problem.

I understand that compression changes in the low end (like changing from 7 to 8 vs 13 to 14) makes more of a difference in power gain than the higher compression range.

If the average engine has a 10-12 percent low-high variance, I wonder what the biggest benefit would be to evening things out.

If it turned out that most of the variation for an engine had to do with valves / valve seating, I can imagine that a valve job would make sense because it doesn't have to kill you financially, but I'm not sure that trying to improve things in the bottom end would make much economic sense unless you had a serious problem.
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Re: Checked your compression lately?

Postby 1981ford » Mon May 16, 2011 3:12 pm

A used 300 should have about cylinder compression of 100-155 and the cylinders within 20% of each other. The more even they are the better. The compression ratio is 8.8:1. Also the higher the compression, the better.
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Re: Checked your compression lately?

Postby THE FRENCHTOWN FLYER » Mon May 16, 2011 5:27 pm

First of all, I think you are on the right track by thinking about cylinder-to-cylinder variances as opposed to absolute compression pressure, which can vary considerably due to engine cranking speed and atmospheric conditions.

It is a good idea to make all cylinders the same so that octane tolerance and cylinder temps will be fairly equal. Many racing engine builders go to great lengths to make sure all the cylinders have the same amount of cc's above each piston so each makes the same (maximum) amount of power and there are no cylinders prone to detonation.

You can "even things out" by reducing the highest ones or by increasing the lowest ones. The highest ones can have their chambers ground out or their valves sunk (a practice I don't like to do). Or the lowest ones can be brought up to spec by reconditioning the engine - rings and valve job - or by milling the block and/or head faces.

The benefit to evening things out will be a smoother running, more powerful and efficient engine. But I would hesitate to tear down a good running engine just because one or more cylinders is a little low on compression. For a street driver, I just don't see the cost/benefit being there.

I had a V6 Escape that lost compression in one cylinder at 28,000 miles - only 40 psi. Because the car wasn't driven much it was outside the 36 month warranty and Ford wouldn't cover it [believe me - I tried]. I determined that the cause was probably a burned / receeded exhaust valve in that cylinder due to too agressive of a spark calibration on the first batch of Escapes built. Every time that car idled that cylinder would go out and the engine ran on five cylinders. Every time I accellerated that cylinder would re-light and run on six again. Very annoying. (SIDE NOTE: you need at least 60 psi to make a gasoline engine run.) When I checked the proceedure for pulling the cylinder head in the service manual it was 17 pages long, involving dropping the entire drivetrain while raising the entire vehicle on a hoist to get at the cylinder head. That did not seem to be a worthwhile use of my time and resources so I lived with it for a couple of years as the fuel mileage was upper twenties on the highway and mid 20s around town.
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Re: Checked your compression lately?

Postby Buddy Rawls » Mon May 16, 2011 5:51 pm

I checked mine when I got the truck, which is about 3000 miles ago. The motor is quite tired, early 80's, and pretty positive never been rebuilt. I cant guess the mileage, but I assume 70K-100L at the very least. It ranged from 120-140psi, falling right in the middle of the 100-150 psi region assumed target for overall sealing capability, but there is 15% variation from lowest to highest which is not the greatest. Regardless none are glaringly low, so I was happy as a clam, and 15% (given the circumstances) was great.
It doesn't run quick at the track, but it gets long stares wherever it goes.
Never in a million years did I think I would have an antique hot rod truck, much less with its own name emblazoned on the front fender and a freakin' six cylinder for power;
but it is a Ford, it is old, and it is definitely one of the funnest vehicles I have driven.
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Re: Checked your compression lately?

Postby lasitter » Tue May 17, 2011 1:14 pm

So I saw this as a rough conversion rule for converting a known compression ratio to PSI and I want to know if it's right or not:

"A 1:1 ratio is equal to 0 PSI. 14.7 PSI is equal to a 2:1 ratio. Just multiply your ratio by 14.7 to get PSI, or divide PSI by 14.7 to get ratio."

If so, then 14.7*8.8=129.36.

That fits with other post info here, and I guess that's a relief.

Had trouble finding a PSI/kPa number for the 4.9L in the shop manual, but there was a table on 03-00-9 of the Powertrain, Drivetrain Service Manual, but I don't exactly know what to make of it for this engine.

I also don't know why the chart starts off with a max/min PSI of 134/101 and goes to 250/187, but I think it is showing the 25 percent spread all along the way.

http://en.wikipedia.org/wiki/Compression_ratio

After reading the article I understand that there are lots of factors to determining the compression ration / cylinder pressure / etc. at any given moment on a running engine, but I'm still feeling pretty stupid about how you take a known CR provided by a manufacturer and convert it to a PSI/kPa -- unless the quote at the top was correct.

For some reason I couldn't locate a web calculator where you could punch in a known compression ratio and get back a PSI number.
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Re: Checked your compression lately?

Postby THE FRENCHTOWN FLYER » Tue May 17, 2011 5:08 pm

From a thermodynamic standpoint your statement would be true only if you assume something called adiabatic compression / expansion as defined by wikipedia:

Adiabatic - This model assumes that no energy (heat) is transferred to or from the gas during the compression, and all supplied work is added to the internal energy of the gas, resulting in increases of temperature and pressure.

In the real world this isn't the case; heat energy is lost to the engine, there are still pumping losses, parts don't seal perfectly, friction is involved, etc, etc.
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Re: Checked your compression lately?

Postby argo » Sat May 21, 2011 10:37 am

I read this earlier and I had two students who needed to do compression checking for their competency list, so I had them do it on Friday (may 20) and these are the results:

Cylinder 1: 142
Cylinder 2: 140
Cylinder 3: 141
Cylinder 4: 138
Cylinder 5: 141
Cylinder 6: 139

Not bad for 312K.
"The Argo" 321,000 miles and counting
It was a 1996 2wd Ford F-150 300ci Inline 6 NP-435 2.73:1 8.8" Rear;
It is a 1996 4wd Ford F-150 300ci Inline 6, C6 Transmission, 3.55:1 axles (limited slip in rear), BW 1356 manual shift transfer case with manual hubs
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Re: Checked your compression lately?

Postby Luckyman » Sat May 21, 2011 11:00 am

argo wrote:I read this earlier and I had two students who needed to do compression checking for their competency list, so I had them do it on Friday (may 20) and these are the results:

Cylinder 1: 142
Cylinder 2: 140
Cylinder 3: 141
Cylinder 4: 138
Cylinder 5: 141
Cylinder 6: 139

Not bad for 312K.


That is absolutely amazing! Outstanding! Incredible! (but I believe it anyway) :thumbup:
1 "76" F150 RC, LB, 2WD, 300, NP435, 9" open 3.00, special order 2-76/Delivered 4-76. Still "new".

1 "73-79" F150 RC/SS/SB/4WD, "84"-300, T18, NP205, 9" open 3.50, Dana 44 3.50 open, Offy DP, Holley 470, EFI + single 2.5" exhaust. Gathered from 15+ donor/parts trucks. "Fubar". Runs good, safe, still needs details/project continues.
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Re: Checked your compression lately?

Postby Lazy JW » Sat May 21, 2011 2:01 pm

Don't get too hung up on compression test numbers. They are a useful guide, but primarily to see how even they are.

You really need to know the EXACT moment of intake valve closing (completely closed, not the .003"-.006" number on the cam sheets) in order to make decent calculations. Most spec sheets don't even list this number.

Cranking speed has about as much effect on changing the numbers as anything else. The only way that compression numbers are really helpful is if you take compression readings on a regular basis and record the data, then you can follow the trends. Better write down the ambient temperature and atmospheric pressure though.
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Re: Checked your compression lately?

Postby argo » Sat May 21, 2011 2:49 pm

Luckyman wrote:
argo wrote:I read this earlier and I had two students who needed to do compression checking for their competency list, so I had them do it on Friday (may 20) and these are the results:

Cylinder 1: 142
Cylinder 2: 140
Cylinder 3: 141
Cylinder 4: 138
Cylinder 5: 141
Cylinder 6: 139

Not bad for 312K.


That is absolutely amazing! Outstanding! Incredible! (but I believe it anyway) :thumbup:


To be fair, that is with an entire lifetime of Synthetic oil, except the first 6,000 miles when I let it break in on conventional.
"The Argo" 321,000 miles and counting
It was a 1996 2wd Ford F-150 300ci Inline 6 NP-435 2.73:1 8.8" Rear;
It is a 1996 4wd Ford F-150 300ci Inline 6, C6 Transmission, 3.55:1 axles (limited slip in rear), BW 1356 manual shift transfer case with manual hubs
"Ah Ha Ha Ha Ha Ha! I Swear! Tractors is so dumb!" - Mater
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Re: Checked your compression lately?

Postby lasitter » Sat May 21, 2011 3:09 pm

Lazy JW wrote:Don't get too hung up on compression test numbers. They are a useful guide, but primarily to see how even they are.
I'm pleased enough that mine are even, but curious that they are 15-20 PSI lower than argo's.

The barometric pressure was approximately 30.09 at sea level. The dealership is just a half mile from TF Green airport (PVD) so figuring that out is pretty easy.

With my hi-torque starter spinning the engine about 11 percent faster than average I might have expected a little better in terms of readings.

After reading the instructions shop manual instructions, I'll bet you anything they let the engine go cold or didn't open the throttle while testing.
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Re: Checked your compression lately?

Postby argo » Sat May 21, 2011 4:34 pm

That's possible. I had my kids do it (under supervision) with the throttle wide open and the engine at operating temperature. I also have a big marine battery and double ought cables, so it cranks fast even with a normal starter.
"The Argo" 321,000 miles and counting
It was a 1996 2wd Ford F-150 300ci Inline 6 NP-435 2.73:1 8.8" Rear;
It is a 1996 4wd Ford F-150 300ci Inline 6, C6 Transmission, 3.55:1 axles (limited slip in rear), BW 1356 manual shift transfer case with manual hubs
"Ah Ha Ha Ha Ha Ha! I Swear! Tractors is so dumb!" - Mater
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Re: Checked your compression lately?

Postby AbandonedBronco » Fri May 27, 2011 3:19 pm

Ran into some issues with my engine a month or so ago, so I found a used engine on craigslist. The guy's brother pulled it out of his 78 pickup because it was the old original engine with an untold amount of miles on it. It'd been sitting for 5 years in the shed, so he sold it to me for $50.

I cleaned it up and swapped mine out and put this one in its place so I could drive it around for a few while I fixed mine up. Clattery and noisy with a worn out valve train, but I did a compression test on it last week:

1: 155
2: 150
3: 153
4: 150
5: 155
6: 153

I love these old 300s!
1981 Ford Bronco 300. 3.00 final drive, 4 speed OD manual. 4bbl Holley 390 w/Offenhauser DP Intake. EFI manifolds and 2.5" exhaust.

Sixes owned:
1957 Chevrolet 150 Sedan 235ci I6
1987 Toyota Supra Turbo 183ci I6 (3.0L)
1981 Ford Bronco 300ci I6
1984 Ford Bronco 300ci I6

Never owned a V!
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Re: Checked your compression lately?

Postby Luckyman » Thu Jun 09, 2011 1:08 pm

AbandonedBronco wrote:............ Clattery and noisy with a worn out valve train, but I did a compression test on it last week:.........


With that kind of compression, the noise is probably just noisy/gummy lifters. A quart of Rislone replacing a quart of oil at every oil change should quiet things up in short order. (IMHO)
Last edited by Luckyman on Thu Jun 09, 2011 2:07 pm, edited 1 time in total.
1 "76" F150 RC, LB, 2WD, 300, NP435, 9" open 3.00, special order 2-76/Delivered 4-76. Still "new".

1 "73-79" F150 RC/SS/SB/4WD, "84"-300, T18, NP205, 9" open 3.50, Dana 44 3.50 open, Offy DP, Holley 470, EFI + single 2.5" exhaust. Gathered from 15+ donor/parts trucks. "Fubar". Runs good, safe, still needs details/project continues.
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Re: Checked your compression lately?

Postby AbandonedBronco » Thu Jun 09, 2011 1:41 pm

Thanks for the suggestion. Is that kinda the same as Marvel's Mystery Oil?

The clatter is really bad. Not the usual minor lifter tick.
I'm thinking that if it is lifters, and they're still salvageable, I may need to remove them and clean them by hand.

http://www.youtube.com/watch?v=wzFMDvWc7MI


I loosened and retorqued all the rocker arm nuts and it reduced the noise a little bit.
1981 Ford Bronco 300. 3.00 final drive, 4 speed OD manual. 4bbl Holley 390 w/Offenhauser DP Intake. EFI manifolds and 2.5" exhaust.

Sixes owned:
1957 Chevrolet 150 Sedan 235ci I6
1987 Toyota Supra Turbo 183ci I6 (3.0L)
1981 Ford Bronco 300ci I6
1984 Ford Bronco 300ci I6

Never owned a V!
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Re: Checked your compression lately?

Postby Luckyman » Thu Jun 09, 2011 5:18 pm

AbandonedBronco wrote:......Thanks for the suggestion. Is that kinda the same as Marvel's Mystery Oil?........


I have never used "Marvel Mystery oil" But I have used Rislone for 35+ years. It works to de-sludge and keep things cleaned up, but not so quickly as to risk plugging pick up screens or clog oil passages if you watch the oil and keep it changed as necessary. I believe that for the most part it holds the crap in suspension for the next change if you dont go too long. Once the engine is cleaned up, normal oil change intervals work just fine. It does quiet lifters and keep them quiet if the lifters are mechanically ok. It may also have some friction modifiers and or zinc in it but am not positive about that. Will do a little more research on that.

More research done, discussion and links here http://www.bobistheoilguy.com/forums/ub ... er=1076592 seem to support what I reported above
1 "76" F150 RC, LB, 2WD, 300, NP435, 9" open 3.00, special order 2-76/Delivered 4-76. Still "new".

1 "73-79" F150 RC/SS/SB/4WD, "84"-300, T18, NP205, 9" open 3.50, Dana 44 3.50 open, Offy DP, Holley 470, EFI + single 2.5" exhaust. Gathered from 15+ donor/parts trucks. "Fubar". Runs good, safe, still needs details/project continues.
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