351M compression readings - FSB Forums
Register Home Forums Active Topics Photo Gallery All Albums Mark Forums Read Auto EscrowInsurance FSB Store
FullsizeBronco.com is the premier Ford Bronco Forum on the internet. Registered Users do not see the above ads.
Reply
 
Thread Tools Display Modes
Old 11-15-2012, 12:00 PM   #1
TNT89Bronco
How did I get 6 Rigs??
 
Join Date: Oct 2011
Location: San Ramon, CA
Posts: 593
iTrader: (0)
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
Check out my '79 Restoration Project...http://www.fullsizebronco.com/forum/...d.php?t=203050
TNT89Bronco is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 11-15-2012, 01:17 PM   #2
UTfball
Registered User
 
UTfball's Avatar
 
Join Date: Sep 2007
Location: North Carolina
Posts: 1,930
iTrader: (4)
Bronco Info: 1979 w/400, 4" lift, 35's, 3.55's
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.
__________________


Ben
UTfball is offline   Reply With Quote
Old 11-15-2012, 01:41 PM   #3
American Thunder
Registered User
 
American Thunder's Avatar
 
Join Date: Aug 2010
Location: Hudson Valley, New York
Posts: 334
iTrader: (0)
Bronco Info: 1978 Bronco 9" lift, 35s, Cummins power
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 9" lift on 35s, with 1991 Cummins 6BT swap, Denny T fuel pin, pump mods, 3200 gov. spring, Trac-Loc front diff, C6 built by Dynamic Transmissions with billet 1200 rpm converter, 4" exhaust, no muffler. GOT TORQUE?
American Thunder is offline   Reply With Quote
Old 11-15-2012, 04:43 PM   #4
TNT89Bronco
How did I get 6 Rigs??
 
Join Date: Oct 2011
Location: San Ramon, CA
Posts: 593
iTrader: (0)
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
TNT89Bronco is offline   Reply With Quote
Old 11-15-2012, 05:18 PM   #5
American Thunder
Registered User
 
American Thunder's Avatar
 
Join Date: Aug 2010
Location: Hudson Valley, New York
Posts: 334
iTrader: (0)
Bronco Info: 1978 Bronco 9" lift, 35s, Cummins power
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 9" lift on 35s, with 1991 Cummins 6BT swap, Denny T fuel pin, pump mods, 3200 gov. spring, Trac-Loc front diff, C6 built by Dynamic Transmissions with billet 1200 rpm converter, 4" exhaust, no muffler. GOT TORQUE?
American Thunder is offline   Reply With Quote
Old 11-15-2012, 05:47 PM   #6
TNT89Bronco
How did I get 6 Rigs??
 
Join Date: Oct 2011
Location: San Ramon, CA
Posts: 593
iTrader: (0)
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
TNT89Bronco is offline   Reply With Quote
Old 12-13-2012, 07:39 PM   #7
78-U10
Registered User
 
Join Date: Dec 2012
Location: Phoenix, Arizona
Posts: 35
iTrader: (0)
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?
78-U10 is offline   Reply With Quote
Old 12-14-2012, 10:45 AM   #8
speedyweasel
Certified Asshat
 
speedyweasel's Avatar
 
Join Date: Jul 2008
Location: Riverside, CA
Posts: 3,625
iTrader: (3)
Bronco Info: Dentside Bronco & '04 Excursion XLT
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.
speedyweasel is offline   Reply With Quote
Old 12-14-2012, 07:17 PM   #9
Alvin in AZ
AKA: Butthead
 
Alvin in AZ's Avatar
 
Join Date: Apr 2009
Location: Gadsden Purchase
Posts: 2,880
iTrader: (0)
Bronco Info: '91 RoadQueen 351w-2wd-ZF camtiming+4* 265/75-16E's dual Ranchos F&R, SuperCab bench seat EEC=F250
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. ;)

Alvin in AZ
Alvin in AZ is offline   Reply With Quote
Old 06-14-2013, 10:42 AM   #10
speedyweasel
Certified Asshat
 
speedyweasel's Avatar
 
Join Date: Jul 2008
Location: Riverside, CA
Posts: 3,625
iTrader: (3)
Bronco Info: Dentside Bronco & '04 Excursion XLT
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?
speedyweasel is offline   Reply With Quote
Old 06-14-2013, 12:08 PM   #11
7LBRONCO
Fullsize Member
 
7LBRONCO's Avatar
 
Join Date: Apr 2007
Location: Gunnison, CO
Posts: 1,737
iTrader: (1)
Bronco Info: 78 XLT, 429 Thunderjet, C6, sh ackle flip in rear, 4" springs in front, 4.56 gears, f& r trac loc
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.
7LBRONCO is offline   Reply With Quote
Old 06-14-2013, 12:11 PM   #12
speedyweasel
Certified Asshat
 
speedyweasel's Avatar
 
Join Date: Jul 2008
Location: Riverside, CA
Posts: 3,625
iTrader: (3)
Bronco Info: Dentside Bronco & '04 Excursion XLT
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.
speedyweasel is offline   Reply With Quote
Old 06-14-2013, 03:24 PM   #13
7LBRONCO
Fullsize Member
 
7LBRONCO's Avatar
 
Join Date: Apr 2007
Location: Gunnison, CO
Posts: 1,737
iTrader: (1)
Bronco Info: 78 XLT, 429 Thunderjet, C6, sh ackle flip in rear, 4" springs in front, 4.56 gears, f& r trac loc
Pull the intake. The lifters get a varnish ring build up on them, so they usually require more "lift" force than a magnet.
7LBRONCO is offline   Reply With Quote
Old 06-14-2013, 04:03 PM   #14
speedyweasel
Certified Asshat
 
speedyweasel's Avatar
 
Join Date: Jul 2008
Location: Riverside, CA
Posts: 3,625
iTrader: (3)
Bronco Info: Dentside Bronco & '04 Excursion XLT
Crap. I knew you were gonna say that.
speedyweasel is offline   Reply With Quote
Old 06-15-2013, 02:58 AM   #15
fol4321
Fullsize Member
 
Join Date: Mar 2006
Location: rosamond,Ca
Posts: 1,225
iTrader: (5)
Bronco Info: 1978 ranger xlt,1995 eddie bauer/1978 custom/1996 xlt
ive used a old speaker magnet
__________________
dont fake the funk on a nasty dunk
fol4321 is offline   Reply With Quote
Old 07-01-2013, 06:49 PM   #16
captbo
FSM Lifetime
 
captbo's Avatar
 
Join Date: Dec 2009
Location: on the shores of the Chesapeake
Posts: 267
iTrader: (0)
Bronco Info: 78, xlt 351M ,1992 EB 5.0, 1995 xlt 5.8
great thread!
__________________
Kingpin of the bronco Mafia
captbo is offline   Reply With Quote
Sponsored Links
Advertisement
 
Reply

  FSB Forums > Bronco Discussions > 1978-'79 Bronco Tech


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -4. The time now is 04:25 AM.


2003-2009 FullSizeBronco.com. All rights reserved