Looking for some understanding here fellas- I have read that the 518 is an overdrive version of the 727. To me then I would assume that the 518 is just as strong as a 727. Is this true? If not, why?

The front part is as strong if not stronger.. the
rear(the OD section) wont take a ton of torque being
that a 800# spring pressure applies the OD.. but if
you are racing it the OD would be turned off... I keep
the OD switch turned on all the time.. lacking any track
time... the 518 is 47# heavier than the 727 and it has a
bit larger case in front and is longer
EDIT
The coolant lines are 3/8 on the 518.. 5/16 on the 727

Mr P Body,

Thank you for the concise information. You bring a lot to Moparts!

So if I have this right, I can put an overdrive switch in the car, and when it is turned off it will act like a normal 727- no worries on strength.

When the switch is turned on, Overdrive will be active and I would be good to go on the street.

Do you have to manually toggle the overdrive each time, or will leaving the switch turned on allow it to work like the overdrive in my daily driver?

Sorry for all the questions just want to be informed.

Here is a circuit I made to activate mine.

Gary

First you would need to use the RH version of the
518 instead of the RE(the E is the electronic version)..
you can find 2 types of the RH.. if you look at the OD
plug located behind the shift lever on the case.. there
is a 2 pin or a 3 pin plug... some early ones had a single
pin... but you dont want the 7 pin plug... that uses the
computer to operate the trans... I have both a 2 pin and a
3 pin... the 3 pin is a OD and lock up... I changed out the
input shaft and the pump to allow the use of a 727 conv.. I
have a 727 5000 stall in mine... this will eliminate the lock
up.... I use a manual toggle switch to run the OD.. if you
leave it on, when it hits 3rd gear it will go into OD.. turn
it off and its a 727... without the lock up in it I leave the
switch on unless I'm gonna race it
EDIT
If you run a lock up conv then you have to turn that off
when coming to a stop or it will try to stall or lug... but
the lock up conv isnt strong(the lock up clutch is small) and
to get a lock up conv built with any decent stall is pricey

It's the other way around, the spring applies the direct clutch which provides through-drive when not in OD so the torque capacity in direct is limited by the spring. The OD clutch is applied hydraulically so the OD clutch apply pressure can be manipulated.

Ok... I wondered if I wrote it backwards... but
either way that clutch cant take a ton of torque..
its small

i think the clutches in the overdrive are the same ones as used in the main transmission--diesel ones have only one more clutch in the OD and everything else is the same--wonder how they hold up so well.....

When you roll into the power it kicks out of OD..
in my parts book it calls out different P/N for the
OD clutch

PATC sells many upgrade parts for the 518 such as extra capacity clutch packs.

I got some of my parts from them... a lot of the
stuff I get at the local Transtar(which has bought
out most everyone)

How much extra power does a 518 suck up? (Or how much does it slow you down?)

I have the 518 in my 92 D150 which I bracket race on occasion, when racing I always push the o/d off button on the dash if I don`t it goes into o/d which slows the truck quite a bit. I have a Mopar Performance/ Turbo-Action shift kit , the 1st to 2nd shift is quite hard even under normal driving but it works well. I was going to add an off the shelf convertor to try and get the truck to leave better, instead Lenny at Ultimate said he could rework my stock lockup convertor which he did and the truck responded well although it only runs low 15s my 60ft times dropped from high 2 teens- low 2 twenties to low/ mid 2.0s a little over a tenth.

I have the 518 in my 92 D150 which I bracket race on occasion, when racing I always push the o/d off button on the dash if I don`t it goes into o/d which slows the truck quite a bit. I have a Mopar Performance/ Turbo-Action shift kit , the 1st to 2nd shift is quite hard even under normal driving but it works well. I was going to add an off the shelf convertor to try and get the truck to leave better, instead Lenny at Ultimate said he could rework my stock lockup convertor which he did and the truck responded well although it only runs low 15s my 60ft times dropped from high 2 teens- low 2 twenties to low/ mid 2.0s a little over a tenth.

So you have just a bit over stock stall... I checked into
a lock up high stall conv.. 5000 stall and it was
$1800 for a conv.. and he didnt know if the LU clutch
would live... he wanted to make it about a 1" thicker
so I would have needed a 1" spacer between the engine
and trans... most of the conv places that I talked to
wanted to go thicker if I wanted the stall I was looking
for... right now I'm using the old J-conv from T/A and
when I talked to Paul about the slippage it didnt know
for fact what it would be in OD... he said 10%... he was
close... its 9% at 75 mph.. I havent seen any issue with
trans temp.. I normally see about 180* on a long run(75 miles
or more).. but I run a nice size cooler.. and if I needed more
I have another with a fan that I would add to it also
EDIT
This slippage data was when I was still having issues
with the trans.. it was eating itself up due to no
torrington bearing in the OD.. so I will need to do
a new set of numbers since this last time of going
through the trans.. I expect it to be better now

Yes Mr P a little better than stock and it was only a few hundred dollars to convert my stock convertor. Off course I don't have much power just a 5.2/ 318 with a few bolt ons and 4.10 gears. Is the 518 in your trick Rampage?

Yes it is... I finally have it working right.. I had issues
with the rebuilds( 3 times before... so I finally did it myself)
and it seems to be fine... it has all the gears now but I havent
put it on the road as of yet on the build.. now it has injection
on it also which is working nicely(Holley multi point)

You have a sweet looking '92 and 15s are respectable for what you have and in bracket racing, if your vehicle can repeat and you cut the light, you win.

My '87 D100 will have the Dana 60-2 in place by the spring time with the 4.10 PowerLock. I have the original 318/727 with an E-brock Performer and a #1406 E-Brock on top and it feels strong though it has never been to the strip...yet. It was planned to be my tow vehicle for the Ramcharger, but I may be using it in next years Car Club Challenge while I work on the Ramcharger and I may want to upgrade to a 518 like you have for the gas mileage, but I only have to drive 20 miles to Pacific Raceway and a little farther to Bremerton Raceway, so with a real tall tire it probably won't make any real difference. I have a fresh 3,000 stall 727 converter and a used Hemi converter that I can experiment with. I just love working with junk.

Like John K said the large 800 lb spring applies the presure to the direct clutch in the O/D unit and all power goes throught the direct clutches in first , second and third. It does also have an overrunning clutch in the overdrive that holds if the direct clutches slip. It actually has that overrunning clutch (sprag) to prevent shift flare going from third to overdrive but it would hold if the direct clutches slip. I just dont really know how much power it can handle. Ron

PACT sells a heavy duty overdrive spring for $70. That and some good OD clutches along with the oiling mods to the shaft should support quite a bit of power? Not sure how much though, the 518's I have built were all stock replacements.