A518 verse 727 transmission question

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maca

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Hey guys
I currently have an A518 transmission behind my turbo small block in my 1974 aussie valiant charger. I'm making just over 600hp at the wheels. I'm not sure how much horsepower the A518 can handle and I'm not sure how long it will live at that horse power. I would love to keep the Overdrive for cruising on the highway. I have noticed that a lot of the bigger horsepower cars have the 727 not the A518.
I know they similar to a 727 transmission with an Overdrive on the back but how similar are they. Do they use the same parts to build them and can it handle the same horsepower. Can anyone point me in the right direction to work out what needs to be done to my transmission to handle that horsepower and if it's even worth doing.
 
Lots of opinions there, but your trans is effectively a 727 with overdrive. not similar, same (depending on year).
 
The overdrive itself may or may not be somewhat weaker, but generally, the main part of the A518 is nearly exactly a 727, except for the changes to operate the OD

Has it been built........I'd guess.........to eliminate the lockup converter?
 
maca said:
Hey guys
I currently have an A518 transmission behind my turbo small block in my 1974 aussie valiant charger. I'm making just over 600hp at the wheels. I'm not sure how much horsepower the A518 can handle and I'm not sure how long it will live at that horse power. I would love to keep the Overdrive for cruising on the highway. I have noticed that a lot of the bigger horsepower cars have the 727 not the A518.
I know they similar to a 727 transmission with an Overdrive on the back but how similar are they. Do they use the same parts to build them and can it handle the same horsepower. Can anyone point me in the right direction to work out what needs to be done to my transmission to handle that horsepower and if it's even worth doing.
Click to expand...
The diesel boys have been using the A518 behind lots of modified diesels that produce lots of HP and tons of torque, so the A518 can survive. If you go to www.tcsproducts.com you can see what they are manufacturing in the line of HD parts for the 727 and the A518. These items are not inexpensive, but they have been designed to cure the common problems found when you're asking the transmission to live beyond the power level it was designed for by the factory. E-mail Paul Parkins there and he will be more than pleased to answer any and all questions you have.
 
Thanks guys
I will contact them. At the moment my 518 is a stock box with a non lock up 2800 stall converter.
A guy was telling me last night that because i am getting up in the HP range my 518s 1st gear will be too low with my 3.5 diff gears. He said I may be better to just run a 727 box and drop the diff gears down to 2.5 or 2.9
 
??? They (ratios) very likely are exactly same as a 727
 
He was just saying that because first gear with high horsepower is so short, they end up changing the diff gears and not needing the overdrive because of the new diff gear ratio. So the 727 box is just smaller and lighter if I'm not going to use the overdrive.
 
Well a 355 rear is not that "short" so no I don't agree with that assessment.

Another factor........not being familiar with those "southern" cars........is an A518 OD won't fit up under the A body crossmember without major upper crossmember (the part welded to the floor) AND floor surgery. There are several threads on here about putting A500 (904 based) and A518/46R transmissions into A bodies
 
Yes they require floor and cross member modifications. I did mine several years ago and have been running the 518 ☺
 
maca said:
Thanks guys
I will contact them. At the moment my 518 is a stock box with a non lock up 2800 stall converter.
A guy was telling me last night that because i am getting up in the HP range my 518s 1st gear will be too low with my 3.5 diff gears. He said I may be better to just run a 727 box and drop the diff gears down to 2.5 or 2.9
Click to expand...

I've heard of turbo cars liking taller gears so he may be on to something. The question is which would be more economical for you in the long run... maybe it would be best to find a local performance transmission shop that knows Chryslers and pick their brains, considering your location and all.
 
GeeWhiz I wish I had your problems. lol

I mean how much power are you making at 2800, then? Is this a racecar?
If yes it's a racecar then........I better be quiet,lol

But I'm pretty sure the A518 is a standard 2.45 low tranny.
That would make your current road gear starter 2.45x3.55=9.02
But if it was a 2.74, then it would be 9.73, about 11.8% higher.

The other idea,namely 2.45 x 2.5=6.13 This is a huge drop from 9.73! This is 6.13/9.73=68% . Your turbo is gonna have to make up the torque difference,namely 1/.68=47% more torque, at 2800, just to break even. Then there is the small matter of; will the 2.5 rear end take all that torque.
But if the tires spin all bets are off; you just spent a pile of time and money to see your ET go up in smoke.

How about taking a smaller bite; namely keep the 518 and swap in 2.94s. This, if a 2.74 low,gets you a starter of 8.06,which is kindof up the middle, and now your turbo will have to kick in just 9.73/8.06= plus 21%, more torque to break even.
Although cruising that monster engine with a final drive ratio of just 2.0, might be dicey, as 65mph will be around 1300 to 1600,heehee, depending on your tire diameter.I wonder how the 2800TC will like that..... Or the engine for that matter.

One thing I see, between say, the 3.55s and the 2.94s,is that the difference in mph at any particular rpm after the TC is tight,is also 20.7%; so, every point in the rpm band will be 20.7% further up the speedometer. And this will continue in every gear. In a NA car this is bad. Bad cuz less average HP is being put down per unit of distance.
But,I know nothing about turbo cars.
 
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There's a few questions missing out of this equation. First would be what your usage of the car is. Second will be what the torque curve of your engine looks like. How much boost are you running and when does that boost start to build?
 
My 15 psi of boost is all in by 3000rpm. Its a street cruiser and twice a month to the track. I have a low boost setting of 6 psi and 430hp for lighter duties. I will look into the taller diff gears as 727s are plentiful here.
 
I have a 8 3/4 diff and semi slick tyres. I still cant do a full launch as I just spin but if I give it just enough to stick it takes off like a rocket and then I can go WOT.lol
I am currently working out a few bugs like wastegates sticking open etc but will have it back to the track soon to see what it can do.
 
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