Changing engines and gearing

[cj!]

As I have had conversations with a few people lately about various engine swaps and gearing requirements I thought I might put together a post about my approach.

First thing I do is hunt down the relevant specs so for example I was going to consider swapping in a Jimny G13BB 1.3 engine and gearbox into a narrowtrack Sierra or possibly even a Vit G16B with Vit gearbox I would find out the following info on a Jimny and a Vit.

Jimny G13BB
Power 59kW@6000rpm
Torque 104Nm@4500rpm
Curb weight 1040kg
1st gear 3.652
5th gear 0.864
Diff 3.909
High 1.320
Low 2.145
Stock tyre size 26"

Vitara G16B
Power 70kW@5500rpm
Torque 133Nm@4000rpm
Curb weight 1020 - 1250kg
1st gear 3.652
5th gear 0.795
Diff 5.125
High 1.000
Low 1.816
Stock tyre size 27"

Then I go to a rpm calculator such as the one on izook http://www.izook.com/gearcalc.htm and enter in the relevant data to find the rpm at say 100kmh (63mph) so I know what the Factory had in mind at freeway speed as want to be able to cruise at that.

In the Jimny's case this would give pretty close to 3600rpm and for the Vitara 3200rpm.

Now I can enter the changes to take into account the tyre size I would like to run, in this case 34" Swampers along with Trail Gear 6.5 t-case gears. These gears have a high of 1.7 and a low of 6.5 and the existing stock narrowtrack Sierra diff ratio of 3.9

The end result shows that the Jimny engine will be doing almost 3600rpm and the Vitara engine with Vitara gearbox would be doing almost 3300rpm. What this tells you is that the existing 3.9 diff ratios of the narrowtrack would have the engines turing at almost the same speed as they would be in their stock application. Looking at the curb weights also tells you that the engines won't be struggling with the weight of a Sierra which has a curb weight around 900kg although by the time you add winch bars etc. it will have climbed a bit.

Obviously different engine/gearbox/t-case/diff ratio/tyre size combinations will have different outcomes and sometimes it doesn't hurt to have the engine doing a few more rpm to overcome the increased effort required to turn the larger tyre and rim combinations that you may run so I have a play substituting the various combos in case the original outcome wasn't that desirable or some options are not readily available.

I also check the crawl ratio using the info for 1st gear, diff ratio & low range to make sure that will also be ok for my application. So with a 3.652 1st, 3.9 diff & a 6.5 low you would end up in this case with a crawl ratio of almost 93:1

Also by knowing where the power and torque are developed and the weight of the vehicle it came out of in comparison to the one it is going into, you can get a sense of how well the engine will cope if you can't get the gearing spot on.

Hope this is of some help to others in the furture.