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3.9l -> /6 parts
https://www.slantsix.org/forum/viewtopic.php?t=30924		 Page 1 of 2
Author:  Middy [ Thu Sep 04, 2008 7:08 pm ]
Post subject:  3.9l -> /6 parts
Anyone know if a 3.9 liter flexplate will bolt up to a slant?

Anyone know if a Magnum 3.9 flexplate will bolt up? :twisted:

Thanks as always.
Author:  Joshie225 [ Thu Sep 04, 2008 9:19 pm ]
Post subject: 
If you leave one bolt out it will fit. Those 3.9 flex plates have the V8 bolt pattern which only differs on one hole from a slant. More important is the fact that 3.9s are semi-odd fire (the crank pins are only split 22° instead of the 30° needed for even firing) and use either a crank trigger or a hall effect trigger in the distributor to help run the engine management system.
Author:  Middy [ Fri Sep 05, 2008 8:29 am ]
Post subject: 
I'm not sure I understand the term "Semi-Odd fire"

I should explain here that I'm long on imagination and short on practice.

On theory I'd put myself somewhere in the middle.
Author:  Joshie225 [ Fri Sep 05, 2008 8:39 am ]
Post subject: 
An inline 6 fires the cylinders at 120° intervals. The 3.9 V6 fires the cylinders at 112° and 128° intervals. The 90° between banks of the V6 would normally mean 90° and 150° firing intervals (the Buick 196, 225 and odd-fire 231 were this way), but Chrysler split the crank pins of the 3.9 by 22° to reduce the difference between bank-to-bank firing intervals making the engine smoother. If they had split the crank pins 30° the firing intervals would have been an even 120° like an inline 6 or 60° V6, but the crank would have been weaker.

What this really means is 3.9L V6 engine management is no good for the slant. If you time the ignition for #1 properly then 2, 4 and 6 will be 8° late. I once modified a slant six reluctor to properly run a 3.9 with a conventional distributor, but sold the engine and related parts before I ever tried it.
Author:  SlantSixDan [ Fri Sep 05, 2008 9:44 am ]
Post subject: 
Quote:
Chrysler split the crank pins of the 3.9 by 22° to reduce the difference between bank-to-bank firing intervals making the engine smoother.
Which sorta succeeded, kinda...but couldn't do anything about the ugly, blatting exhaust note.
Author:  Middy [ Fri Sep 05, 2008 9:52 am ]
Post subject: 
Interesting,

Thanks for your input joshie, I also found this website with some useful info on odd fire v-6's.

http://home.off-road.com/~merls_garage/oddfire.html
Author:  Joshie225 [ Fri Sep 05, 2008 10:55 am ]
Post subject: 
Quote:
Quote:
Chrysler split the crank pins of the 3.9 by 22° to reduce the difference between bank-to-bank firing intervals making the engine smoother.
Which sorta succeeded, kinda...but couldn't do anything about the ugly, blatting exhaust note.
Right. Not a great compromise if you ask me. I do believe the OHC 3.7L V6 is even fire, but haven't been able to confirm it. I considered buying a 3.7 from a Jeep Liberty at one time.
Author:  Middy [ Fri Sep 05, 2008 11:16 am ]
Post subject: 
Do you know if the computer uses the Crank Pos. Sensor to fire the injectors or does the computer have the offset built in to it's programing?

What I mean is, if you took the +/- 8deg. out of holes in the flexplate would the computer just use the new points for reference or does it know they should be 112 or 128 respectively apart.

Thanks again for your input.
Author:  Joshie225 [ Fri Sep 05, 2008 11:30 am ]
Post subject: 
My Magnum 3.9 flywheel has 3 pair of notches for the crank position sensor so my best guess is the computer requires 6 pulses per revolution from the crank position sensor and likely correlates that to cam position with a sensor in the distributor.

MegaSquirt is cheap and programmable. The 3.9 stuff may be cheaper if you already own it, but it's not programmable and I'd put up $100 that says you'll never get the ignition to fire at 120° intervals.
Author:  Middy [ Fri Sep 05, 2008 6:53 pm ]
Post subject: 
Is MS sequential or batchfire?
Author:  SlantSixDan [ Fri Sep 05, 2008 10:18 pm ]
Post subject: 
Quote:
My Magnum 3.9 flywheel has 3 pair of notches for the crank position sensor so my best guess is the computer requires 6 pulses per revolution from the crank position sensor and likely correlates that to cam position with a sensor in the distributor.
I doubt that, since Magnum engines haven't got distributors. ;-)
Author:  Joshie225 [ Fri Sep 05, 2008 10:29 pm ]
Post subject: 
Dan,

Magnum 3.9s, 5.2s and 5.9s all have distributors as does my 2.5 'Magnum'. The V10s, OHC 3.7/4.7 and new hemi engines are distributorless.

Middy,

Old Megasquirt was batch fire, but the newer ones can do sequential. Sequential isn't a big deal unless you are working on emissions performance.
Author:  DusterIdiot [ Fri Sep 05, 2008 10:32 pm ]
Post subject:  Not in the traditional sense...
Quote:
I doubt that, since Magnum engines haven't got distributors.
I know my 1992 Ramcharger with the 5.2 M had a hall effect rotor distributor, but was definate-ly computer timed, so it's not a dizzy in the
strictest sense of the word, but it did use a rotor to pass the spark to the plugs like it's earlier LA bretheren... Now the 4.7 in my mom's Durango... that's a different story...

-D.Idiot
Author:  Matt Cramer [ Sat Sep 06, 2008 6:21 am ]
Post subject: 
It appears the offset is both built into the crank trigger and the computer; many of the Magnum V6s had a weird pattern with nine teeth (three on their own, six in pairs of two - actually not quite as weird as the tone ring on a 4.7 camshaft) plus a one tooth Hall effect sensor in the distributor. (This may be different from year to year, as Josh mentioned one with three notches.) I don't think you could take the odd fire out of a Magnum PCM.

The current Megasquirts are still batch fire - there's a sequential one that's about to go into beta testing, although it's a bit more expensive.
Author:  Middy [ Sat Sep 06, 2008 8:39 am ]
Post subject: 
God I love this board.

Thanks everyone for your input and patience. Hopefully I will get some money to actually do something on the car, rather than set the board off on wild goose chases all the time. :D
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