Input needed - gears or stroke?

[goaztecs]

My brother has a 65 Mustang with a mildly modified 289. He is doing a ground-up resto, and is working on his engine strategy. He was fairly blown-away by the quotes that he got for rebuilds with a 331 or 347 stroker kit.

Knowing that his current build is not too old, I asked him "what's wrong with your motor?" In the heavier Mustang, he wants more low-end torque for off the line performance.

Since he is replacing his toploader with a T-5 with a tall overdrive, I suggested that he just swap-in a super-short set of rear-end gears. Mathematically, he should realize a huge torque gain at the rear wheels, and his overdrive will make up for any high-rpm freeway engine speeds that the short rear-end would cause.

Is there a hole in my logic? Can't he get the low-end torque he's looking for with a new rear end, and leave the motor essentially alone. Perhaps bolt-on some new heads and a cam... Thoughts???

[joedeats]

Considering both the weight of your car and the trans your going to use I would say a set of 3.73s would be great maybe a tad higher but not to much more. You stated it is a mild small block, if you gear it to high you will be in an rpm range where you dont make your power. I suggest to most with typical 5.0(302)set ups to go with 3.55 or 3.73 as they are both great for daily driving but you can really get up and go. If you go with a bigger cam, a set of ported heads with bigger valves and a stroker set up I might reccommend more gear but your set up as it is would unsuitable for big gears.

[goaztecs]

Good advice, by the heart of my question is:

- Can you make up for torque at the rear wheels with lower gearing, without actually building more torque into the engine?

Logically, it would seem to be true, considering that rock-crawler 4x4's make tens of thousands of lb-ft of torque at the wheels through extreme gear reduction.

Is there a hole in my logic, or is this accurate?

[Ron61]

Not exactly sure I understand your question, but your engine's torque is going to stay at whatever RPM it is now. But the lower gears will get you to that range quicker and it will feel a lot stronger from a stop. Tire height will also play into the factor. With 25" diameter tires and 3:27 gears, I am taching 2525 RPM at 60 in 4th but just 1900 RPM at 70 in 5th as it kicks my ratio up to 2:10s. That is in the Cobra which is lighter than your brother's Mustang. I have a .6 split and if you went to a .8 that would lower the 5th gear top ratio. I think Joe is right on about the gears as the Mustang weighs more than the Cobra.

Ron

[goaztecs]

Ron,

You're right. I'm probably not wording this as clearly as I'd like to. Torque is a rotational force that can be measured in a number of places in your car. Two of those places are at the flywheel and at your rear wheels.

A certain mechanical advantage is gained in gear reduction. Think about the tiny gear in your starter turning the large gear that is your flywheel. The fast-spinning small-geared starter gear is able to apply the necessary torque to the flywheel due to the mechanical advantage that is gained by using a flywheel gear that is many times its size. If the flywheel were the size of the starter gear, your starter would not be able to turn your crank against the compression in the cylinders.

In the same way, your actual torque at the rear wheels would be your engine's torque multiplied by the gear ratio in your rear-end, in theory.

Without modifying the engine - wouldn't a change in gear ratio effectively produce more absolute torque at the rear wheels?

[clayfoushee]

Goaztecs,

I have a 65 Mustang (66 GT-350 clone) w/ 289 set up exactly the way your brother is considering, T-5 w/ 3.73. It does perform well. You can burn plenty of rubber in 1st, and cruise in 5th at 1900-2000 at 65mph.

[ItBites]

Yes,

Torque at the rear axle is the product (multiplication) of the torque at the flywheel times the total gear reduction (transmission gear ratio X rear gear ratio). Losses are incurred in the driveline, but the method presented means a lower rear gear produces more torque at the rear axle, given the same torque at the flywheel and the same first gear ratio.

I don't know squat about Brand-X (Ford) trannies, but lets take an example case:

First gear ratio = 3.23 (example)
Old Rear ratio = 3.08 (typical chevy hiway gear)
Engine torque = 250 lb-ft (since you got a 289)

Old Rear axle torque (ignoring driveline losses) = 2487 lb-ft

First gear ratio = 3.23
New Rear ratio = 3.73
Engine torque = 250 lb-ft

New Rear axle torque (ignoring driveline losses) = 3012 lb-ft


Big increase (21%) huh?

Now, torque equates to tractive force (what makes ya go forward) by dividing the torque (lb-ft) by the tire radius (half the height of the tire). Remember to do this in feet, not inches of tire radius. So, if your tire is 26 inch tall, the radius is 1.083 ft. Using the example above you get:

Old tractive force = 2296 lb (force)

New tractive force = 2781 lb (force)

All this assumes the tires do not slip and is of course a snapshot at the point where the engine makes 250 lb-ft of torque.


You can take this one step farther and throw in about 15% drivetrain loss and then use F=mA to determine an approximate acceleration at this snapshot. I again don't know squat about Brand-X, but lets assume your mustang weighs 3220 lb (=100 slugs - trust me on the slugs part).

Old acceleration = .61 G

New acceleration = .73 G



Additionally, if you're looking to lower quarter mile times, the lower gear you go to will always lower times, until the point when you are at peak rpm in high gear before you reach the traps. So, in spite of other opinions in this post, a 4.11 gear might give you the lowest times as long as the engine is not yet at redline when you trap. Drag racers play this game all the time. They put in the gear that gives the max RPM in high gear in the traps. This way they use all of the engine torque (power) curve in every gear to get there.

I can explain this statement as well, but it involves integrating area under the curve and total energy (work) expended to reach the 1/4 mile in the least time (power), but I'll save that for another post.


Put in the lowest gears you can stand with a low torque 289. If you drive hiways this will be a compromise between cruise rpm and acceleration potential.

[goaztecs]

Now that's the kind of science I was looking for! Your sample case shows a 21% gain in absolute rotational force at the rear axle with no modification to the engine. I think that's enough evidence that my idea had merit.

He was going to change out his rear leaf-springs anyway, so might as well take that old 2.73 (yes, 2.73) non-locking rear-end out anyway, and put in a whole new axle with discs and nice, sturdy 3.73 gears....

Thanks for the analysis!