GMC TRANSMISSION gear ratios

[donald w. miller]

Rick, a bit more on this subject.

Wonder of wonders, I found this companies phone number in GMCMI newsletter
#12 (summer 85), dialed it and 14 years later they are still at the same
number.

Changed owners, but still have that kit available under the same Part Number
13438. The man I talked to said these components are somewhat stronger than
original, rated 500 hp versus 400 hp for the original parts.

Forgot to ask if they were straight cut gears but I wouldn't have a problem
if they were.

1985 price was $303

1998 price is $415 plus shipping.

Local transmission shop can install, if you don't need a winter project.

A.C.T.
Fountain Valley, Ca. 92708
(714) 962-6655

Cheers,

Don Miller
75 Glenbrook
Shenandoah Valley of Virginia

>Date: Thu, 5 Nov 1998 12:13:28 EST
>From: RickStapls
>Subject: GMC: TRANSMISSION gear ratios
>
> With all the recent discussion of final drive and transfer chain ratios,
I
>wonder if anyone has any information/experience with changing the actual
GEARS
>inside the transmission for different ratios?
> I read in a GMCMI newsletter from way back in '85 (newsletter #12,
summer
>'85) that a company in Fountain Valley, California was selling replacement
>gearsets which reduced the 1st, 2nd, and Reverse gears to lower
(numerically
>higher) ratios. Specifically, 1st gear went fro, 2.48:1 to 2.75:1, second
>went from 1.48:1 to 1.57:1, and reverse from 2.11:1 to 2.46:1. 3rd (high)
>gear, involving no gears within the transmission, remains at 1.00:1.
> I also recall seeing another reference to such a project, but I can't
>remember where. ("The first thing to go....") IIRC, they said the
replacement
>gears were straight-cut rather than helical like the originals, making them
a
>bit stronger abeit noisier.
> This sounds like the best of both worlds for those of us trying to start
>off on steep hills over 10,000 feet altitude. The lower 2nd gear would
give
>more power on long pulls, although one might not be able to hold 2nd gear
up
>to 70+ mph as you can now. The stock 3rd gear ratio would allow us to get
>across Kansas with minimum noise and fuel.
> Does anyone out there have any experience/information about such
>modifications? Does anyone know if "Art Car Performance Transmission" is
>still in business, or any other such sources? Any leads appreciated.
>
>TIA,
>Rick Staples,
>'75 Eleganza, Louisville, CO
>

 

[donald w. miller]

Zachary, thanks for that interesting report. Isn't it nice when you can
nurse them in to do the repair at home.

If I recall correctly, you now pull a double race car trailer with your GMC
which is quite a bit of weight. It appears your final gears are in an
excellent test bed.

Regarding traction, I inquired on this forum a couple of times regarding
lockers or positraction kits. No luck to date, but with 13,000 GMC's there
is bound to be a machinist out there who has a solution for this one.

Cheers,

Don Miller
75 GlenBrook
Shenandoah Valley of Virginia

- -----------------------------
>
>Date: Thu, 05 Nov 1998 17:33:25 -0500
>From: Zachary Zehnacker
>Subject: Re: GMC: TRANSMISSION gear ratios
>
>Don,
>
> Put us on the list for a final drive failure. We had about 100,000 miles
>on our GMC (we put on about 65,000 of those towing a trailer with the rest
>from the original owner) and we began to hear noises. We didn't know what
>it was (our guess at the time was that it sounded like the chain in the
>trans dragging on the cover). We drove it for quite a while with the noise
>(guess around 5,000 miles) while towing the one-race car trailer. On one
>trip about 3 miles from home we began to smell gear oil, we made it home
>and even backed up into our driveway with no problems. The bearings in the
>final drive had failed. The vibrations from this actually caused most of
>the bolts holding the drivers side axle to the final drive to back out.
>IIRC there were only 1 or 2 bolts left holding on the axle when we got
>home! One of the bearings was totally shot, while the others were not in
>good shape (probably because of the metal from the first one). The gears,
>however, looked to still be in very good shape. We got the numbers off the
>bearings and called around to local bearing places. We were able to get
>Timken replacements from a local bearing supplier. We put in the new
>bearings, reinstalled the final drive, and put another 20,000-25,000 miles
>on it before switching to the 3.42 unit. The final drive was still fine
>when we removed it.
>
> We have not really had any total trans failures. We do, however, have our
>trans rebuilt every 50,000 miles or so to avoid any problems. The final
>drive problem listed above apparently caused some damage to the
>transmission, but it was only found on the rebuild and was not noticeable
>in operation. We feel the loads we place on the trans by towing the
>trailer and the distances from home we travel warrant the more frequent
>rebuilds. We also change the fluid and filter occasionally and have
>installed transmission coolers in-line with the radiator cooler and an
>aluminum trans pan.
>
> We have also considered the different trans gear ratios. We do not
>usually have a problem starting out in first. From what I understand, the
>425 uses the same gear sets as the 400. This allows one to use the
>different and more common 400 gears in the 425. Traction is usually our
>limiting factor when starting to go up a hill from a stop. In our case,
>the shifts bring the motor down quite a bit unless we really bring the RPMs
>up before shifting. The RPM difference would be even greater if the ratio
>difference between the gears was increased. We do, however, like the idea
>that this method does not affect the high gear ratio.
>
> Zachary Zehnacker
> Seemann Motorsports
>

 

[zachary zehnacker]

Don,

We had some ideas on a very manual version of getting power to both tires,
but so far we do not think it is worth the possible safety/failure problems
that this method would have induced with the parts we could find. We are
still looking for more appropriate parts, however, and hope to get a chance
to implement it some day.

The trailer we tow is about 10,000 lbs when loaded with the two race cars.
It is triple axle and has brakes on all three axles. The toll booth
people LOVE us when we pull up. With the 3 GMC axles (I know two of them
aren't really axles but try and explain that to a toll both collector )
and the 3 trailer axles we have one more axle than a SEMI. Most of the
collectors on the Garden State Parkway have no idea what to charge us which
can really add some time to our trips.

A LONG typical example is listed below.

Our toll booth guy: (Laughs and looks uncomfortably at our rig.)

Our toll booth guy: Hey Joe, do you know what to charge for this thing?

Joe, the other toll booth guy: No, ask them what they normally pay.

Our toll booth guy: What do they normally charge you?

Us: 90 cents

Cars behind us: HONK! HONK! HONK! HONK!

Our toll booth guy: 90 cents, it can't be 90 cents

Our toll booth guy: Joe, they say they normally get charged 90 cents.

Joe, the other toll booth guy: Can't be.

Our toll booth guy: Well what do you think it is Joe?

Joe: 85 cents

Our toll booth guy: I think its 95 cents but I'm not sure.

Us: We really don't care. We will give you a $1 if it will make you happy
and we can get going.

Cars behind us: HONK! HONK! HONK! HONK!

Our toll booth guy: No, I'll call up HQ and find out what it should be.

Our toll booth guy on phone: Hey, I got this thing that looks like a van
but it has three axles, and it is pulling a trailer that has three
axles. What should I charge for it?

Person on phone: What?

Our toll booth guy on phone: I said I got this thing that looks like a van
but it has three axles, and it is pulling a trailer that has three
axles. What should I charge for it?

Person on phone: You say you got something that has three axles that looks
like a van and is towing a three axle trailer?
(Looks around for hidden camera that must be catching the practical
joke his coworkers are playing on him.)

Yeah right! (attempting to play it cool)

Cars behind us: (trying to get into other toll lanes, nearly missing other
cars)

Our toll booth guy on phone: No really, I got this thing that looks like a
van but it has three axles, and it is pulling a trailer that has
three axles. What should I charge for it?

Person on phone: You're serious?

Our toll booth guy on phone: Yes, what should I charge for it?

Person on phone: Ask them what they normally get charged for it.

Our toll booth guy: What do they normally charge you?

Us: 90 cents! Just like we told you before. (starting to get pretty upset now)

Cars behind us: (No cars left behind us)

Our toll booth guy on phone: They said 90 cents.

Person on phone: 90 cents?

Our toll booth guy: Yup, that is what they said

Person on the phone: Let me look it up. Hold on.

Our toll booth guy to person on phone: Ok.

Our toll booth guy to us: They said they are going to look it up.

Us: Whatever.

Everybody: (Waiting.....Waiting......Waiting......Waiting......)

Us: Hello? We'd like to get out of here some time today.

Our toll booth guy to person on phone: Did you figure it out yet?

Person on phone: No, hold on.

Our toll booth guy to person on phone: Ok.

Everybody: (Waiting.....Waiting......Waiting......Waiting......)

Us: Hello? This is getting ridiculous.

Our toll both guy to person on phone: Did you figure it out yet?

Person on phone: No, just charge them whatever they normally get charged.

Our toll both guy: What do they normally charge you again?

Us: 90 freakin' cents!

Our toll both guy: That'll be 90 cents please.

Us: (gives him 90 cents)

Our toll both guy: Thanks! Have a nice day!

Us: We were.

Zak


>Zachary, thanks for that interesting report. Isn't it nice when you can
>nurse them in to do the repair at home.
>
>If I recall correctly, you now pull a double race car trailer with your GMC
>which is quite a bit of weight. It appears your final gears are in an
>excellent test bed.
>
>Regarding traction, I inquired on this forum a couple of times regarding
>lockers or positraction kits. No luck to date, but with 13,000 GMC's there
>is bound to be a machinist out there who has a solution for this one.
>
>Cheers,
>
>Don Miller
>75 GlenBrook
>Shenandoah Valley of Virginia

 

[donald w. miller]

Zachary,

I got a fine giggle out of your toll booth problem. Maybe a system to hoist
up your rear axles at the toll booth so they no longer touch the ground
would simplify this matter for you. You could tell them you like to carry
plenty of spare tires.

Regarding the transmission gear ratio change; the guys at my local speed
shop tell me Art Car, whose shop supplies the lower ratio transmission
gears, is a well known and respected figure in performance circles.

Instead of lockers for the final drive have you considered an ABS brake
system approach.

I may be mistaken, but don't some cars have a feature where they use the ABS
system to apply some brake to the faster (spinning) wheel to increase
traction when accelerating?

If true, then ABS might provide a double benefit for the GMC's.

Cheers,

Don Miller
Shenandoah Valley of Virginia

- -
>Date: Fri, 06 Nov 1998 16:00:51 -0500
>From: Zachary Zehnacker
>Subject: Re: GMC: GMC TRANSMISSION gear ratios
>
>Don,
>
> We had some ideas on a very manual version of getting power to both tires,
>but so far we do not think it is worth the possible safety/failure problems
>that this method would have induced with the parts we could find. We are
>still looking for more appropriate parts, however, and hope to get a chance
>to implement it some day.
>
> The trailer we tow is about 10,000 lbs when loaded with the two race cars.
> It is triple axle and has brakes on all three axles. The toll booth
>people LOVE us when we pull up. With the 3 GMC axles (I know two of them
>aren't really axles but try and explain that to a toll both collector )
>and the 3 trailer axles we have one more axle than a SEMI. Most of the
>collectors on the Garden State Parkway have no idea what to charge us which
>can really add some time to our trips.
>