The Stovebolt Site Forums The Parking Lot Big Bolts brownie box with an Auto tranny?

Can I use an auxiliary transmission with an auto tranny? I am gear bound when I come out of overdrive on hills in my 57 LCF. I am running a 350 with a 4l80 tranny and 5.38 gears 9r22.5 tires. I am comming up over hills pulling my camper at around 50 mph and the 350 is tacking pretty high. As soon as I get back on the flats it shifts into overdrive and I can cruise right along. I do not want to swap out the rear because I think I will not be able to get going from a stop. Camper is around 9k pounds.

sounds like a torque problem more than a gear problem .... but maybe you just need to relax on the hills, what's the rush?

Bill

Never heard of using a traditional aux tranny, but you might consider a "gear vendors" o.d. (or some other ratio) splitter (just a two speed add on box)

No hurry Bill, just seems like I am in everyones way. I am not pushing the engine either. Run it about 2/3 throttle on the hills. Could probably get more speed but no sense making the motor work that hard. Seems like if I could get the rpms down some in drive it would motivate along a little better.

If you're already cruising level ground at comfortable road and engine speeds you can't really do a lot better.

Perhaps you want an in between gear from OD and direct that you have now? I doubt it's worth it to install or fun to shift an auxiliary with an automatic in front of it.

Not that you can;t install an auxiliary with an automatic, it's been done before. but not something you'd want to shift on the fly much. The slip of the converter and lack of a clutch to disengage might make it terribly difficult to match speeds and get it back in gear while moving.
Also note that an auxiliary shouldn't be left in OD whine the main transmission is in lower gear gears. The OD should only be used in the higher gears (less torque/stress and heat).

Grigg

you want more speed up the hills, but also want lower RPM .... gear changes will not get you there, a lot more torque and HP is what you're wishing for, running the 350 at lower RPMs will slow you down under load, which is why the trans kicks out of OD on those hills .... "upgrade" to a big block or ignore those impatient folks behind you

Bill

Are you looking at a tach, or just listening to the engine? If the 350 is under 4,000 RPM, you're not likely to be doing any damage. The 4L80 is a very good transmission. It's rugged, and the gear ratio spread is about right for most engines. I run a 454/4L80 setup in a 35 foot class A motorhome, and I have no problems pulling most hills. It does wind up a little on a long uphill pull in direct drive, which sounds like the same situation you've got. The big block definitely has a lot more torque than a 350.
Jerry

At 15,000lb you are probably pushing the limits of what your 350 is capable of. The problem you are looking at is that even if you do add an auxiliary trans you may not get the right split and still not be able to pull the hill in the new split ratio. Your 4L80 is 1:1 in 3rd and .75:1 in 4th. You would probably need to accomplish a .85 or higher to have a chance of being able to pull in that ratio, otherwise as others have said you will simply not have enough torque at the lower engine speed.

Tad

thanks for the replies. I know I am probably at the limits of the 350. It was just a thought and was wondering if anyone had done it. sounds like more work than it is worth. I am not in any big hurry so i will just enjoy the scenery a little more.

Another option for you to think about is a two-speed rear. I now this will work as I've seen it several times as a factory GM option behind some junior sized Allison's.
(even in school bus chassis if you can believe it)

We went camping this weekend and maybe it is not as bad as I thought. Pulled some hills at 65 mph. I think it may just be how loud it is when I am working the engine that hard or that it was on the road 2 days before we used it the first time. The pipes really crack out some sound when running it hard. I think I will just run her at 50 on the hills. It will make me feel better.

Shane,

The TF trucks have very loud cabs. Did you sound deaden the inside of the cab and doors? Did you turn the exhaust out behind the rear?

Mike B

Exhaust is out the rear of the truck and I have dynomat type insulation throughout the cab. I think the flowmasters may have been a bit much but its not bad just loud when you mash down trhe pedal. Wife does not complain so I guess it is ok. Of course she has flow masters on her Suburban so she likes to hear it.