NissanDiesel Forums

If you add a GearVendors OD unit . . . you triple the value of your truck!

I've known about Gear Vendors for years, but have never even seen one in person. Are they any good? Been around a long time under the same name if they are not.

I've seen two close enough to touch They come in "early" (Type J) and "late" versions. They have their own oil supply (ATF) and sump and filter, and they tend to not get serviced after a while. I serviced a Type J about a year ago by removing the sump and filter and spraying it all out, and though the oil looked pretty bad, the unit itself is well made, and was still working fine.

Yes, they have earned a sterling reputation -- except for the cost! They are quite expensive, compounded by the fact that there is NO parts supply, so you can't refurb your own when they finally wear out. You can't buy clutches, bearings, nothing. The only recourse is to use your worn-out or broken unit as a core for a GV rebuild. Which POs me.

If someone knows of a GV parts source, I'm willing to be proven wrong. Hell, I encourage it! I've passed up two broken GVs in the past two years, because what can you do with one? I've heard that the early Type J can use Volvo OD parts for servicing.

The big advantage of the GV is that it can be shifted power-on, so it's fine for use with ATs. The GV is basically a planetary set from your typical AT. The leading competitor, US Gear Overdrive (used to be Doug Nash OD) does a non-synchronized gear shift, so it's tricky to shift on-the-fly. It's designed to be used as a high-range/low-range device, shifted at a standstill. The GV splits your gears, so you get more gears, which is a lot more flexible.

There are a couple of disadvantages to the GV though. Though electrically controlled, it uses hydraulic pressure to apply the clutches, and the pressure pump is driveshaft driven. That means on a MT that you can't split or overdrive 1st gear, because the driveshaft isn't moving fast enough to bring up the hydraulic pressure.

And in 4WD applications, the GV only affects the rear diff, so that's a factor if you want to be able to split gears while in 4WD mode.

I want one. Or two.

When I was still turning wrenches for a living, I starting driving part time a straight truck (22' box) Ford C700 tilt cab. Don't ask how long ago that was, ok!

It had a two speed split rear end and as such you split each main box shift. The differential had a removeable 3rd member that contained the extra gear set (no clutches). That's the sort of OD I would like to have on the little Datsun due to space.

Otherwise, a taller ring/pinion set could be acceptable since 1st gear is so low. Maybe 3.25?

Two-speed rear axles are nice, but they do not shift themselves, and I've never seen a light-duty one. I've driven several, and I'd just as soon have a Brownie, but they're nice just the same.

The GV can be put into an automatic mode where it shifts itself in and out. There are people who can use this who would never be comfortable with dealing with the two-speed diff.

I looked into getting an under/overdrive unit a while back for my Dodge, US Gear made one that would go between the tranny and transfer case for most of the popular 3/4 and 1 ton trucks on the market. I know it doesn't help much on the Nissan trucks but the units are made. Suckers are pricey, about $1800 IIRC.

Look was ALL I did, it was cheaper to add power and upgrade the clutch to hold it

Billysgoat wrote:SNIP- Look was ALL I did, it was cheaper to add power and upgrade the clutch to hold it.

Neither engine hop up nor a beefier clutch changes gearing.

Here are the gearing numbers for my truck with OEM wheels.

Tire: 185/75x14 passenger car tire. 23.75" diameter
OD gear ratio: 0.81
Final reduction ratio: 3.88

55 mph = 2450 rpm

Engine peak torque band: 1900-2400 rpm
Power Chart

http://www.izook.com/gearcalc.htm

If Billy needed more pulling power in his Dodge, using the US Gear as a gear splitter would be an alternative to having more power.

asavage wrote:If Billy needed more pulling power in his Dodge, using the US Gear as a gear splitter would be an alternative to having more power.

But the torque multiplication would make increased torque at the wheels occur at a lower vehicle speed. More gears does not change the engine's torque curve nor redline.

philip wrote:But the torque multiplication would make increased torque at the wheels occur at a lower vehicle speed.

Not necessarily. If you can't pull a grade in 4th, but can go over the torque peak in 3rd, you can get up the hill faster in 3.5rd.

asavage wrote:

philip wrote:But the torque multiplication would make increased torque at the wheels occur at a lower vehicle speed.

Not necessarily. If you can't pull a grade in 4th, but can go over the torque peak in 3rd, you can get up the hill faster in 3.5rd.

Gearing does not change the engine's torque or hp curve. That's not argueable.

What I hear you saying is that in 4th gear, the engine would not hold vehicle speed at maximum torque RPM but ... with lower gearing the engine would spin past maximum torque and crest the hill at a higher vehicle speed.

RPM past max torque results in less engine torque but ... the engine torque loss is offset to a degree by the higher torque multiplication from lower gearing. This over rev strategy works well with engines having peak torque and peak horsepower fairly close together and particularly engines designed to produce much more horsepower than torque (ie, four cylinder motorcycle engines). But such is NOT the case with medium/high speed automotive diesels (especially with only two valves per cylinder and natural aspiration) where maximum torque occurs at an RPM around HALF that of maximum horsepower RPM.

When you mate this kind of power band with a wide ratio gearbox, the driver pulling a hill will find it necessary to upshift at an RPM high enough that engaging the next taller gear puts the engine back down to no less than peak torque RPM.

philip wrote:What I hear you saying is that in 4th gear, the engine would not hold vehicle speed at maximum torque RPM but ... with lower gearing the engine would spin past maximum torque and crest the hill at a higher vehicle speed.

3rd gear adequate to get up the hill. Engine will pull, say, another 1000 RPM beyond torque peak, but will not pull 4th, which might put the engine into its torque peak again (if sufficient headway was gained in 3rd), but still not enough torque for that particular road speed.

Splitting the gear would put the engine into peak torque RPM and at a road speed it can pull.

asavage wrote:SNIP- Splitting the gear would put the engine into peak torque RPM and at a road speed it can pull.

The message here is to keep the diesel engine operating at peak torque for the load (grade) and speed at the moment. Ok ... where's that Continually Variable Transmission for the Datsun?

(Note: In order to get this "calculator" to use the actual 24.25" tire diameter I have installed, the tire aspect ratio had to be adjusted down to 165 from 185/75x14)

phillip, where did you get that program? Thats so cool!

It's just one of many specialty calculators on The Net.