Torque Converters…

MNT65

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Hey y’all, I’ve got a 1965 Plymouth Fury convertible that we are doing a restomod on. I’m in the process of building a big block 383 (450-500 HP) fuel injected, and a 727 trans with a gear vendor. I’m not sure what rear end I have, but I plan on running a .355 gear ratio and no smaller than an 8 3/4 differential. What torque converter would you recommend for this set up? I don’t plan on drag racing, but would like moderate/clean performance out of what I have here! Your participation is very welcomed…
 

Davea Lux

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Hey y’all, I’ve got a 1965 Plymouth Fury convertible that we are doing a restomod on. I’m in the process of building a big block 383 (450-500 HP) fuel injected, and a 727 trans with a gear vendor. I’m not sure what rear end I have, but I plan on running a .355 gear ratio and no smaller than an 8 3/4 differential. What torque converter would you recommend for this set up? I don’t plan on drag racing, but would like moderate/clean performance out of what I have here! Your participation is very welcomed…
Look at the RPM range on your camshaft specs. Usually they will give a range, as an example, say 2500-5500 RPMs. You would want for street use to spec a convertor with a stall speed at the lower number range, in the case of this example, 2500RPMs. Be advised that a 3.55 gear ratio will be ok for running stop light to stop light but will beat the engine to death at open highway speeds. Also, the higher the stall speed, the poorer the fuel mileage,a consideration with today's high fuel prices.

Dave
 

MNT65

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Look at the RPM range on your camshaft specs. Usually they will give a range, as an example, say 2500-5500 RPMs. You would want for street use to spec a convertor with a stall speed at the lower number range, in the case of this example, 2500RPMs. Be advised that a 3.55 gear ratio will be ok for running stop light to stop light but will beat the engine to death at open highway speeds. Also, the higher the stall speed, the poorer the fuel mileage,a consideration with today's high fuel prices.

Dave
Thanks Dave for the reply. I am running the gear vendor (OD), but if .355 is too low of a gear ratio, what would you suggest would be appropriate for decent take off and also highway speeds? Maybe .308’s?
 

Mudeblue

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Look at the RPM range on your camshaft specs. Usually they will give a range, as an example, say 2500-5500 RPMs. You would want for street use to spec a convertor with a stall speed at the lower number range, in the case of this example, 2500RPMs. Be advised that a 3.55 gear ratio will be ok for running stop light to stop light but will beat the engine to death at open highway speeds. Also, the higher the stall speed, the poorer the fuel mileage,a consideration with today's high fuel prices.

Dave
I was sitting in front of Arby's with my GTO and a woman walked up and asked "I curious, what kinda gas mileage do you get with that car?" My response, "if your concerned with gas mileage, maybe you shouldn't own a classic car!"
 

Justin Plant

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I was sitting in front of Arby's with my GTO and a woman walked up and asked "I curious, what kinda gas mileage do you get with that car?" My response, "if your concerned with gas mileage, maybe you shouldn't own a classic car!"
I've had that question asked by some one driving a Suburban at a gas station, My response was " bout the same as your school bus" the conversation ended right there!
 

mikedrini

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Thanks Dave for the reply. I am running the gear vendor (OD), but if .355 is too low of a gear ratio, what would you suggest would be appropriate for decent take off and also highway speeds? Maybe .308’s?
3.23's are the perfect compromise according to most. That's what I have.
 

57fury440

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With a stock converter and 27 inch tall tires my E body Barracuda is doing about 2,700 RPM at 60 MPH with 3:55 gears. At 65 MPH it is around 2,900 RPM. If you have an OD I think they would be okay for all around use but if you do lots of highway miles then 3:23 would be better.
 

CBODY67

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To me, "stall speed" is a variable "black science" of sorts. Reason? A 727 with an appropriate torquc converter for a Slant 6 will stall at one rpm level. Yet when that same torque converter is put behind a 383/335, it will stall in the 2300rpm range, and a bit higher when behind a 426HEMI. Just because there is more power in front of the converter.

A side dynamic is that you WANT the engine cruise rpm to be above the actual stall speed, so that the converter "slip" is minimized at those highway rpm levels (which means less heat in the ATF, too!). Note: unless the torque converter is a lock-up design, there will always be a small bit of slip at cruise rpms (like up to 5% or so?), which is normal. AND this tends to relate to your rear axle choice with the GV OD unit. As with the 3.55 in OD, the effective rear axle ratio is approx 2.50. You can tell when the torque converter is "tight" as throttle response will get very accurate when that happens, whereas just a few mph lower and the throttle response will be more soggy. For example . . . on my '70 Monaco Brougham with the factory 383 "N" 4bbl V-8 (3.23 axle ratio, P225/75R-15 tires, 25mph/1000rpm, factory cast iron intake, stock 4734S (replaced the orig 4732S) AVS, stock distributor, smaller diameter factory torque converter, stock 256/260 cam, factory dual exhaust), at 60mph on the highway, throttle response is a bit soggy, but at and past 62mph, the response to small inputs is very responsive, by comparison.

Another side dynamic is how the torque converter "loads" the engine at 700rpm hot base idle. How the intake manifold vacuum changes between "P", "N", and "D", for example. A too tight of a converter will put too much load on the engine and lower the idle intake manifold vacuum to less than 10"Hg, which can trigger initial power valve circuits for additional enrichment. Not good!

So, IF you "size" the converter to match the cam and such, that can be good. BUT with the GV OD unit engaged on the highway, the converter might not be past the lowest rpm level for good operation. In which case going to a 3.91 or 4.10 axle ratio might be advised.

Just some thoughts,
CBODY67
 

sprice

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To me, "stall speed" is a variable "black science" of sorts. Reason? A 727 with an appropriate torquc converter for a Slant 6 will stall at one rpm level. Yet when that same torque converter is put behind a 383/335, it will stall in the 2300rpm range, and a bit higher when behind a 426HEMI. Just because there is more power in front of the converter.

A side dynamic is that you WANT the engine cruise rpm to be above the actual stall speed, so that the converter "slip" is minimized at those highway rpm levels (which means less heat in the ATF, too!). Note: unless the torque converter is a lock-up design, there will always be a small bit of slip at cruise rpms (like up to 5% or so?), which is normal. AND this tends to relate to your rear axle choice with the GV OD unit. As with the 3.55 in OD, the effective rear axle ratio is approx 2.50. You can tell when the torque converter is "tight" as throttle response will get very accurate when that happens, whereas just a few mph lower and the throttle response will be more soggy. For example . . . on my '70 Monaco Brougham with the factory 383 "N" 4bbl V-8 (3.23 axle ratio, P225/75R-15 tires, 25mph/1000rpm, factory cast iron intake, stock 4734S (replaced the orig 4732S) AVS, stock distributor, smaller diameter factory torque converter, stock 256/260 cam, factory dual exhaust), at 60mph on the highway, throttle response is a bit soggy, but at and past 62mph, the response to small inputs is very responsive, by comparison.

Another side dynamic is how the torque converter "loads" the engine at 700rpm hot base idle. How the intake manifold vacuum changes between "P", "N", and "D", for example. A too tight of a converter will put too much load on the engine and lower the idle intake manifold vacuum to less than 10"Hg, which can trigger initial power valve circuits for additional enrichment. Not good!

So, IF you "size" the converter to match the cam and such, that can be good. BUT with the GV OD unit engaged on the highway, the converter might not be past the lowest rpm level for good operation. In which case going to a 3.91 or 4.10 axle ratio might be advised.

Just some thoughts,
CBODY67
Now thats the first time somebody has written something that makes sense of of this torque converter cloud, at least for me. Thank you.
 
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My $.02
Looking for 450 to 500 horsepower with a 383 is going to require a pretty large cam.
They don't make good torque at the bottom of the RPM range.
The slip of a higher stall converter is also more slip at lower rpm.
A converter has a stator that begins to spin at higher RPM that reduces slip.
If you end up with a cruising RPM that is too low, the motor won't be responsive and the converter will make a lot of heat.
If you are running a 3.55 it will be the same as 2.7 when you are in overdrive.
I believe that with that tall of ratio won't work well until you are approaching 90 to 100MPH
the OD ratio of a gear vendors OD is 0.78:1 (3.55X0.78=2.769)
If you have 4.10 rear end in overdrive, that works out to 3.3 and cruising RPM at 65 ends up about 2600 with 26" tall tires
that would be acceptable in my opinion. (4.1X0.78=3.2) gear ratio in OD and [(65X3.2X336)/26=2688] MPH X gear ratio X 336 all divided by tire diameter.
I hope this isn't too much BS!
bottom line: too tall of gearing with a large cam is disappointing.
my friend put a large cam in a stock '69 350 horsepower 396 with no other change, and with 3.5 gears and no posi, it couldn't even do a burnout.
Scott
 
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