351M compression readings - Ford Bronco Forum
 
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post #1 of 16 (permalink) Old 11-15-2012, 12:00 PM Thread Starter
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Bronco Info: '79 351M NP435 and '78 Custom 400M C6 w/sliding windows
351M compression readings

I tried to find the values dong a search, but couldn't find anything. I wanted to check the "health" of my engine and did a compression test on each cylinder. Almost all of them came in around 90...one came in at 110.

Does anyone have their engine readings to compare to?

'79 Bronco 351M 4 Speed Custom.
'79 Bronco. 400 C6 Custom w/AC.
'78 F250 400 Custom 4x4
'78 F250 460 C6 Camper Special
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post #2 of 16 (permalink) Old 11-15-2012, 01:17 PM
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Don't have my 'official' notes, but I wanna say on fresh 400's I'm 140ish. I wouldn't call 90psi bad though. Typical rule of thumb is you want all cylinders to be within 10% of eachother...but that's usually when you have 1 cylinder significantly LESS than the other 7.



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post #3 of 16 (permalink) Old 11-15-2012, 01:41 PM
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With a stock/mild cam and 9:1 compression ratio, you'll get 140-150 psi. If I recall, I think a stock '75 302 with 8:1 CR I tested awhile back had 118psi. Like UT mentioned, consistency is more important than peak psi. It's an indicator of a healthy engine; that all cylinders, rings and pistons as well as valves/seats are wearing fairly evenly.
90 psi is on the low side, especially considering the other one is 20 psi higher.. But it may be leaky valves causing it, rather than rings.

Are the spark plugs oily when you took them out? If so, that's an indicator of ring issues.
Have you wet tested the compression yet? (squirt a couple pumps from an oil can into each cylinder before checking compression) And then compare the results to the dry tests. If there is a drastic difference, the rings are shot, or the bore is out of round or just worn too large for the rings to seal properly.
If the wet and dry tests are faily close in psi, you can check the valves and seats by bringing each cylinder to TDC of compression stroke, and then applying an air compressor hose to the cylinder, with a special fitting that threads into the spark plug hole. Once the cylinder is pressured up, you can listen for air leaking from the valves.

1978 Bronco Custom, 6" skyjacker & 3" body lift, 37x12.50R16.5s, posi front diff, 3.50 gears, Dynamic Racing C6 w/billet 1200 rpm converter, upgraded brakes, '91 Cummins 12V, Denny T fuel pin, injection pump maxed out & timing advanced, 3800 gov. spring, 60# valve springs, ARP head studs, 4" exhaust.
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post #4 of 16 (permalink) Old 11-15-2012, 04:43 PM Thread Starter
How did I get 6 Rigs??
 
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Bronco Info: '79 351M NP435 and '78 Custom 400M C6 w/sliding windows
I don't remember the spark plugs being oily. They were dark, but I don't think wet. I will go and see. I will try the "wet" test and see. I also need to go back and try the test again, because I know my battery was low, and I had to recharge my battery to continue the test.

The engine runs fine and it passed smog, but I was just curious more than anything. I am guessing that you take standard air pressure and multiply it by the compression ratio to get the target compression value?

'79 Bronco 351M 4 Speed Custom.
'79 Bronco. 400 C6 Custom w/AC.
'78 F250 400 Custom 4x4
'78 F250 460 C6 Camper Special
Check out my '79 Restoration Project...http://www.fullsizebronco.com/forum/...d.php?t=203050
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post #5 of 16 (permalink) Old 11-15-2012, 05:18 PM
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It's difficult to calculate the compression psi mathmatically, since there are variables like cylinder filling %(volumetric efficiency) and all that fun stuff.

p.s. remove all the spark plugs before testing the compression too

1978 Bronco Custom, 6" skyjacker & 3" body lift, 37x12.50R16.5s, posi front diff, 3.50 gears, Dynamic Racing C6 w/billet 1200 rpm converter, upgraded brakes, '91 Cummins 12V, Denny T fuel pin, injection pump maxed out & timing advanced, 3800 gov. spring, 60# valve springs, ARP head studs, 4" exhaust.
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post #6 of 16 (permalink) Old 11-15-2012, 05:47 PM Thread Starter
How did I get 6 Rigs??
 
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Bronco Info: '79 351M NP435 and '78 Custom 400M C6 w/sliding windows
Oh. I guess I was doing it wrong then. I will remove al of them and give that a shot. Thanks!

'79 Bronco 351M 4 Speed Custom.
'79 Bronco. 400 C6 Custom w/AC.
'78 F250 400 Custom 4x4
'78 F250 460 C6 Camper Special
Check out my '79 Restoration Project...http://www.fullsizebronco.com/forum/...d.php?t=203050
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post #7 of 16 (permalink) Old 12-13-2012, 07:39 PM
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is the 351M even worth putting money into? Mine seems to run fine -- haven 't driven it yet to doing suspension and other mods. If it should die-- how hard is it to just put a windsor based engine in it?
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post #8 of 16 (permalink) Old 12-14-2012, 10:45 AM
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Good thread and better timing. I should've done a compression test two weeks ago but haven't managed to get it done yet. These figures and suggestions are useful... I love this forum.

Quote:
Originally Posted by 78-U10 View Post
how hard is it to just put a windsor based engine in it?
Ew.
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post #9 of 16 (permalink) Old 12-14-2012, 07:17 PM
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Quote:
Originally Posted by 78-U10 View Post
is the 351M even worth putting money into?
...how hard is it to just put a windsor based engine in it?
Bellhousings don't match between the two engine types.
The 429/460 bellhousing pattern is the same as 351m/400 tho. :)

A 351m can be woke-up with a cam/cam-timing change. :)
That plus a crank from a 400 would make it more than worth while. ;)

Did the cam-change-thing with a 360FE, makes a whoppin' difference!
More power over a wider band and better gas mileage too, beat that. ;)

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post #10 of 16 (permalink) Old 06-14-2013, 10:42 AM
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Thread revival!



What say you guys? Left bank is lower than the right. Head gasket leak? Negligible difference? Worth worrying about?

Cyl 1 has a distinct click, but can't tell with the stethoscope whether it's intake or exhaust. Really don't want to pull the head but may have to. And if I do it, should I pull both for decking/valves/springs?
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post #11 of 16 (permalink) Old 06-14-2013, 12:08 PM
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10% variation, + or -, not worth worrying about, IMO.

I would pull the valve cover, check for a loose rocker arm, on #1. Could be a sticky/dirty lifter making the noise.
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post #12 of 16 (permalink) Old 06-14-2013, 12:11 PM
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Quote:
Originally Posted by 7LBRONCO View Post
10% variation, + or -, not worth worrying about, IMO.

I would pull the valve cover, check for a loose rocker arm, on #1. Could be a sticky/dirty lifter making the noise.
Rockers are all secure. I pulled the valve cover on 11/29/12 and inspected the pushrods and rockers. Snug and with no more play that the others...

I haven't pulled a lifter before -- just stick a magnet wand down there or what? How do you get it out?







Pushrods were clean and bright, as were the rockers.
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post #13 of 16 (permalink) Old 06-14-2013, 03:24 PM
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Pull the intake. The lifters get a varnish ring build up on them, so they usually require more "lift" force than a magnet.
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post #14 of 16 (permalink) Old 06-14-2013, 04:03 PM
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Crap. I knew you were gonna say that.
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post #15 of 16 (permalink) Old 06-15-2013, 02:58 AM
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ive used a old speaker magnet

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post #16 of 16 (permalink) Old 07-01-2013, 06:49 PM
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great thread!

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