Glenn Torrens crunches the numbers on a gearbox that will allow an outback-bound VW Baja Bug to trek better
Gear ratios have a big influence on a car’s performance. Gearing that is too high (or tall) for the engine will hinder the performance of the car. Gearing that is lower (or shorter) results in increased engine revs. Often that is good for performance, but go too low and the driver might be changing gears too often (which can actually make a car slower!) or result in the engine working outside its ‘sweet spot’, and over‐revving.
Gearing is especially important with the little old air-cooled, rear-engine VW Beetles I play with. I’ve competed in motorsport in Beetles for around 15 years and – also in a VW – I’ve been a participant at Australian Speed Week where the ambition is to simply make the car go as fast as possible across a miles-long dry salt lake course. Especially with a little old VW, gearing wrong by just a few per cent can knock 10-20km/h from its top speed or make a first place a fourth!
Another VW project I’ve been involved with recently is my mate Tony’s VW Baja Bug. The two of us built this car several years ago for charity outback treks. A late-1960s VW Beetle chassis and suspension were strengthened and upgraded to cope with the rigours of rough-road travel and the body was modified with a distinctive fibreglass Baja kit that abbreviates the nose, tail and guards of the car to better survive rough terrain. The finishing touch to Tony’s car was a crazy camo paint job!
| Read next: GT’s VW Beetle Baja build
After we finished building the car in 2018, our first Royal Flying Doctor Service Outback Trek was from Tamworth NSW through outback Queensland to Airlie Beach. It was six days of awesome fun! Since then Tony has driven his little Baja to Darwin via Gulf country and later this year it will be again heading for Darwin. This year I plan to again be along for the ride!
These VW Beetle cog-boxes are now at least 45 years old, but they are very durable and thanks to enduring international popularity, parts remain available
Based on my experience ‘cogging’ my motorsport Bugs, for Tony’s Baja I spec’d a gearbox to suit the large 4WD-type rear tyres. This, plus the fact the car was heavier than a standard Bug (our camping and safety gear added 250kg!) meant appropriate gearing was important for the car to maintain speed, even with more than double the standard power from its 1916cc twin-carb engine built by VW engine legend Stan Pobjoy.
With safety/warning lights on top, a chopped‐up body and big tyres, this Baja is probably less aerodynamic than a house brick… so the engine is blowing a fair bit of breeze
Later, we changed to a smaller tyre which meant the engine was spinning at around 4000 rpm at cruising speed… too revvy. To fix this, Tony fitted another gearbox with taller ratios… but those ratios were a little too tall. The engine was lugging at just 3000 rpm, which meant poor acceleration and – because of the slower rotation of the air-cooled engine’s fan – higher temps. So: 4000 too much… 3000 too little…What we needed was something in between!
Too few or too many revs, our gearbox choice is like Goldilocks eating the porridge. Hopefully the new gearbox will be ‘just right’!
Thankfully, VW Bug gearboxes have different ratios for different models built in different years and with different engine capacities. For example, VW Bugs can have a 4.3, 4.1 and 3.8:1 differential ratio. With a selection of diff and gearbox ratios, I was able to specify a gearbox for cruising revs of around 3500rpm.
But this ideal box of ‘factory’ ratios would have to be custom-built. As Tony and I have in the past, we asked VW gearbox guru ‘Shimo’ to build my choice of gearing into a Super Beetle gearbox case.
The VW Beetle has a transaxle: the alloy case houses the gearbox and the diff gears, and carries the flanges for the driveshafts to send drive to the rear
With its carefully calculated cogs, this gearbox should be just right to allow this outback-bound Baja to romp through the trees and cruise with ease.
From Unique Cars #466, May/Jun 2022