TH400 + 3.08 Gears; Motor and Tire size?

[voxnor]

Putting a TH400 and 3.08 gears in my 69' 1/2T 2wd - deciding between a couple motor options still. From my reading trying to edumacate myself, it sounds like the motor, transmission, gears, and tire size all affect each other?

Given the TH400 and 3.08's - will I sill be able to take good advantage of a high-torque motor like this:

http://www.summitracing.com/parts/edl-45421/overview

How does tire size affect all this? Do I want smaller or larger tires?

[HSRACER]

I have that exact trans and gear set. I run a 383 stroker and a 30 in tall tire. Works great. At 70-75 it tacos around 2500-2700 rpm.

[Molberg]

It depends what you're shootin for voxnor. 3.08 gears will keep rpms down at highway speed... but wont launch quite the same as a 3.73 or 4.11. TH400's have a taller first gear as well compared to a th350. (so less "launch", or snap off the line as well) Larger overall tire diameter will keep rpms down at cruisin speed, compared to smaller. I run a th400 with 3.73 final drive ratio. Cant tell you how it runs.. still being built. I've had great success with this combo in the past.

[voxnor]

@Molberg - Fair enough. To elaborate a bit - the "launch" is exactly what I am worried about. This is a DD so I thought the 3.08 was the way to go (for mpg) - but maybe that was a bad choice? I don't need it to snap my neck from a stop sign, but it does need to get moving quickly and be able to chirp the tires for the fun factor.

I'm worried I'll put in a 6k motor with high torque on the low end and simply be wasting it with the rear 3.08. Perhaps a stock 350 Goodwrench engine is the way to go...

[Molberg]

Originally Posted by voxnor

@Molberg - Fair enough. To elaborate a bit - the "launch" is exactly what I am worried about. This is a DD so I thought the 3.08 was the way to go (for mpg) - but maybe that was a bad choice? I don't need it to snap my neck from a stop sign, but it does need to get moving quickly and be able to chirp the tires for the fun factor.

I'm worried I'll put in a 6k motor with high torque on the low end and simply be wasting it with the rear 3.08. Perhaps a stock 350 Goodwrench engine is the way to go...

I built a 383 stroker/AFR heads/stall etc for the one I'm building now.. I'd go 3.73 final drive if it was me. Had a '72 gmc years back with a built 355/3.08's and 15" rims. Lots of launch when it had a 4 speed standard in it.(M21 I think?) Blew up the tranny racing and dropped in a th400. It was like I lost 100 HP... and it was all gear ratio alone that did it.

[Already Gone]

Originally Posted by voxnor

@Molberg - Fair enough. To elaborate a bit - the "launch" is exactly what I am worried about. This is a DD so I thought the 3.08 was the way to go (for mpg) - but maybe that was a bad choice? I don't need it to snap my neck from a stop sign, but it does need to get moving quickly and be able to chirp the tires for the fun factor.

I'm worried I'll put in a 6k motor with high torque on the low end and simply be wasting it with the rear 3.08. Perhaps a stock 350 Goodwrench engine is the way to go...

I have a basically stock 396/350hp ( from a 67 Chevelle ) in my truck with a TH400 and 3:08's and P255/70 R15 tires all around. The TH400 has a mild shift kit in it ( was in when I bought the truck ) and the truck drives very nicely and has ( for me ) more then adequate power for driving and cruising. If I blast it from a light or stop sign in drive I get rubber on the 1-2 shift. The specs on the modern motor you are looking at should be great for your needs as it will be docile or scream at your leisure and probably get decent gas mileage if you drive it normally.

[HSRACER]

My 383 will smoke the 30 inch tall tires with the 3.08 gears!

[jocko]

Since you say you are putting a TH400 in this package - does that mean you already have the trans? And the 3.08s?

If yes, then disregard this post.

If not, then read on.
While a TH400 and 3.08 is a great setup, it lives in the world of tradeoffs compared to more modern, and readily available, parts. In other words, TH400 and 3.08 is PRIMARILY a cruising setup. TH400 gear multiplication with a 3.08 will net you decent highway mileage at the expense of bottom end acceleration. Whereas a 4.10 rear end will snap your neck with a stout motor in 1st, but you'd be stopping for gas much more often and be taching out at high rpm at highway cruising speeds. i.e. you have to make a choice between the two when using a non-O/D trans.

The answer to your dilemma is a slightly deeper gear set and an O/D trans. While most folks tout the benefits of highway mileage for an O/D trans - the less-often-discussed benefit of an O/D trans is the presence of a deeper 1st gear IN ADDITION to the highway mileage improvements. Take a look at the changes in highway cruising rpm TH400 vs 700R4 or 2004R below - but also note the huge difference in 1st gear torque multiplication.

The only reason I recommend a matched set of O/D trans with rear end gearing is that, while it can be done of course, an O/D trans with a leftover rear gear that was originally designed for acceleration (i.e. a 4.10) or cruising (i.e. a 3.08) will cause problems at high or low speeds, respectively, when only an O/D trans is added to the mix. In other words, if you have a 4.10 and add an O/D, now you'll have neck-snapping acceleration, but it will want to shift halfway through the intersection you're crossing. On the other hand, if you add an O/D trans to a 3.08, you might be lugging the motor (i.e. too low rpm) at the lower highway speeds (50-60 mph).

That is why the factory combos make sense - the milder, "in the middle" rear gears take advantage of BOTH ends of the range in a vehicle.

This is why the 700R4 (or 2004R) + 3.73 gears are SO popular. Best drivability out there. If you lean a little more toward highway cruising than chirping, then a 3.42 might be more appropriate. With your motor, burnouts won't be a problem either way.

Tire size is only a small factor unless you extremely change from stock. Bigger tires equate to reducing the numerical gear ratio of the rear end. So, if the car was produced with a 4.10 gear and 29" tire - and you swapped on 31" tires, you would be effectively be changing you rear gearing to maybe a 4.08, or 4.02, or whatever... Bottom line, it becomes more of a "highway gear" with taller tires. The opposite is true of a smaller diameter tire. Do nothing but switch to a smaller diameter tire and you effectively increase the rpm required to drive the same speed on the highway that you were at before (and keep in mind, you could only really confirm this with a gps - your speedometer will be incorrect!)

Anyway - sorry to waste space if you already have the TH400 and 3.08, but felt it worth discussing since you were kind of asking about the how the whole shebang works together.

All the stuff below is based on a 29" dia tire.

[Already Gone]

Jocko is right on with his info. Personally if I was going to a 700R I would go with 3:42's in the rear.

[voxnor]

Thanks guys. Really appreciate the write up Jocko.

I do already have the TH400 and the 3.08. I'm going to view this in a positive manner in that it has helped force my hand to stick with my original plans for the project - which was just a reliable, comfortable driver with mostly stock looks.

It sounds like if I just add a Goodwrench 350/260 or the 350/290 to the TH400 and 3.08's, running on 28-29 inch tires i'll have a real nice cruising setup. Might help keep my foot off the pedal too....Should make the wife and my wallet happier.

[jocko]

you're welcome voxnor - yes, your combo is great as is - it was how the general made some of em in the first place. Enjoy! Just wasn't sure if you already had all the ingredients - sounds like you do, so run with it! Looking fwd to the progress pics.

[ole dollor]

Originally Posted by jocko

Since you say you are putting a TH400 in this package - does that mean you already have the trans? And the 3.08s?

If yes, then disregard this post.

If not, then read on.
While a TH400 and 3.08 is a great setup, it lives in the world of tradeoffs compared to more modern, and readily available, parts. In other words, TH400 and 3.08 is PRIMARILY a cruising setup. TH400 1st gear multiplication with a 3.08 net you decent highway mileage at the expense of bottom end acceleration. Whereas a 4.10 rear end will snap your neck with a stout motor but you'd be stopping for gas much more often and be taching out at high rpm at highway cruising speeds. i.e. you have to make a choice between the two when using a non-O/D trans.

The answer to your dilemma is a slightly deeper gear set and an O/D trans. While most folks tout the benefits of highway mileage for an O/D trans - the less-often-discussed benefit of an O/D trans is the presence of a deeper 1st gear IN ADDITION to the highway mileage improvements. Take a look at the changes in highway cruising rpm TH400 vs 2004R below - but also note the huge difference in 1st gear torque multiplication.

The only reason I recommend a matched set of O/D trans with rear end gearing is that, while it can be done of course, an O/D trans with a leftover rear gear that was originally designed for acceleration (i.e. a 4.10) or cruising (i.e. a 3.08) will cause problems at the extremes when an only an O/D trans is added to the mix. In other words, if you have a 4.10 and add an O/D, now you'll have neck-snapping acceleration, but it will want to shift halfway through the intersection you're crossing. On the other hand, if you add an O/D trans to a 3.08, you might be lugging the motor (i.e. too low rpm) at the lower highway speeds (50-60 mph).

That is why the factory combos make sense - the milder, "in the middle" rear gears take advantage of BOTH ends of the range in a vehicle.

This is why the 700R4 (or 2004R) + 3.73 gears are SO popular. Best drivability out there. If you lean a little more toward highway cruising than chirping, then a 3.42 might be more appropriate. With your motor, burnouts won't be a problem either way.

Tire size is only a small factor unless you extremely change from stock. Bigger tires equate to reducing the numerical gear ratio of the rear end. So, if the car was produced with a 4.10 gear and 29" tire - and you swapped on 31" tires, you would be effectively be changing you rear gearing to maybe a 4.08, or 4.02, or whatever... Bottom line, it becomes more of a "highway gear" with taller tires. The opposite is true of a smaller diameter tire. Do nothing but switch to a smaller diameter tire and you effectively increase the rpm required to drive the same speed on the highway that you were at before (and keep in mind, you could only really confirm this with a gps - your speedometer will be incorrect!)

Anyway - sorry to waste space if you already have the TH400 and 3.08, but felt it worth discussing since you were kind of asking about the how the whole shebang works together.

All the stuff below is based on a 29" dia tire.

good read. I thank you also.