Technical C4 automatic transmission

Hello, could someone please tell me how much horsepower it takes to turn a c4 automatic transmission?
I'm trying to do some calculations. Thanks.

 

Specs are here: https://www.460ford.com/threads/transmission-specifications-including-parasitic-hp-losses.118570/ The AOD is 30-35 HP.

 

That chart says the C4 uses 28 hp.
Who knows at what rpm and load that is.

 

Good point. I have heard everything from a actual hp number to a percentage.

 

I think percentages are more worthwhile. I/e at idle I would think it’s not using up 28 HP, but at 5250 RPM it might be.

 

That seems ballpark. A stock 302 would make about 130 hp and would match up to a 22 to 24% ratio as I have heard. It has to fluctuate throughout the rpm range, I would think.

 

What do you guys mean by percentage??.... if the transmission needs 20 HP AT 2000 rpm it matters not what percentage that is of engine HP...100 or 1000 hp engine, the transmission will still use 20 HP AT 2000 rpm....or 28 HP at what the factory figures max rpm to be....

 

Well, let’s look at your question a bit. A 9 inch rear is supposed to use more HP than a 12 bolt , which uses more HP to run than a Dana 44, etc


So, how much HP is needed to drive a 4000lb vehicle at 5 MPH, how much is needed to drive it at 50 MPH? If you have ever looked at a dyno chart, you’ll notice where they are mapped from. Just to give you an idea

 

@chessterd5 what I mean is I can spin a rearend over from the pinion gear, as can you. But that’s just break over, let’s say I for shows it takes 12 hp to run a 9 inch, I don’t think I am making 12 HP to turn the pinion by hand. Now to turn it at 3000 RPMs, it takes more tq and HP, just my thoughts. Kinda like walking through a lake, versus running. vs doing the same thing on a paved road.

 

I agree. I have thought about some of these things. And you are right, gearing, weight, and parasitic loss are factors.
I guess what I am really trying to figure out is how much minimum torque does it take to make a c4 auto operate under its own power to move a vehicle regardless of speed.

 

A lot less power than it takes to turn a C-6.

 

Is there a reason for this line of questioning? It might help to spill a few beans.

 

Once you get over about 45 mph, most of the HP required to maintain and/or accelerate is due to wind resistance. The amount of power required to overcome wind resistance goes up by the square of road speed. If you are building a traditional hot rod, it probably isn't very aerodynamic. Unless it is a 54 Studebaker, that is.

 

True. The c6 is a great trans. Nothing better for abuse except maybe a cast iron powerglide. I dropped a cast iron powerglide into reverse once doing 70 mph and it never hurt that transmission. It did completely rip apart the front body mounts. Lol

 

That sounds like land speed stuff. Which is cool but I'm dealing on the other end of the spectrum. I'm dealing with a top s speed of 5 to 10 miles per hour.

 

As said previously, it depends on load and speed. There are thrust washers between the gear assemblies. The more torque transferred, the more friction on the washer. A disengaged clutch assembly has plates spinning next to each other. The oil between them has shear drag which increases with rpm. The rotating assembly has mass. The more rapid the acceleration, the more power it takes to change the rpm of that mass.

 

Eliminating or lowering parasitic load is the dreams of under powered racers, limited racers, fuel mileage freaks, and today’s automobile manufacturers.
There are things that help like choosing the best rear end, transmission, oils, wheel bearings etc. I’ve been doing it for years and it’s been fun.

 

A clue!

 

Ok, here's my madness. I am experimenting with building a parade car. The vehicle is HAMB friendly including the transmission and rear end, but the engine is not. So I don't want to mention it. I can supply any numbers pertaining to the drive line, vehicle weight, gear ratios, etc.
Even if it has a top speed of 5 to 10 miles per hour, it would accomplish it's purpose. Thanks

 

Gear ratio would benefit you more than parasitic loss at those speeds. But just my opinion I think at 5-10 MPH the difference is an auto tranny loss would be negligible. Could you use/adapt some like. a auto forklift/tractor/ect transmission? They have pretty steep gears.

 

Yes that is possible. But I would like to see if what I have will work. I know, classic hot rodder excuse.
Plus everything is set up from the bellhousing back.
Here are the numbers
19 ftlbs. Of torque, 13 hp., 1000 lbs gross vehicle weight including driver.
2.78 final drive.

 

Seems like a mismatch for a c4 at that speed you mention. You’re basically idling or a bit off idle to have 10 MPH with 2.78 gears. Now next question, will said vehicle be towed/ trailered to places, or driven?


FWIW, my MF65 will run for 10 hours under heavy work with 4/5 gallons of fuel. Now I surely wouldn’t drive it for 20 miles to do that tho, is it just the auto you need/want?

 

Another important question, how heavy is this vehicle going to be?

 

What is a MF65?
I just have the auto. and don't want to buy something if I don't have to.

 

1000 lbs. Including driver.
Its a very stripped down model T.

 

Guessing you are using an IC engine, around 3600 rpm. Also guessing around a 28" tire. 120 28" tire rpm is 10mph. With a 2.78 rear that's 333 driveshaft rpm. You are going to need ~10:1 reduction between the motor and driveshaft. Guessing first in a C4 is ~3:1, you may be turning the C4 1000 rpm in first to go 10mph (~3:1 between the engine and trans). You would need a pretty low stall to make it go. With the auto trans losses, it would be on the edge. I think a hydrostat out of a lawn tractor or torque converter drive from a utv would be better. Have fun, Mike

 

Golf cart.

 

Thanks for the math! I think I understand. What you are suggesting in laymen terms is that it could be possible but with a stock converter the vehicle may have trouble starting to roll from a dead stop? But once moving it may stay moving.
I agree about the 10 to 1 reduction. From what I have seen with other people doing this, it's a 10 to 1 reduction into a manual transmission using a centrifugal clutch. I have seen very large vehicles move using that set up. Read 6.5 hp, 10 to 1 reduction, 5 speed manual, 3.83 to 4.11 gears and a vehicle weight of over 6,000 lbs.
I just wanted to see if I could do it with a automatic.

 

Yeah, I know boring. Lol
This engine probably isn't going to stay. I just wanted to participate in my hometown parade and maybe some other community function.
In my defense, I do have a couple of other real engines I would like to build for it when I get time and money.