History085 Transmissions have a reputation of being weak especially in higher powered boosted applications. Not surprising given the 084 is was based on had to handle 895cc of power and a lightweight Mk1. Add a little weight, and lots more power (about 4 or 5 times the amount of the Mk1) and it’s not surprising a 230bhp G40 will strip gears for fun.
So far my 085 failures have been:
• Inner CV Joint
• 2nd Gear
• 3rd Gear
• 5th Gear
• Diff/Final Drive Bolts
Time for a change...
Gearbox OptionsVarious options are available depending on budget, time and ability. My own headline figures to work to are:
• 200kW (268bhp) Crank Power
• 300Nm (221lbft) Crank Torque
• 750kg + 2 Passengers (150kg)
Other consideration needs to be given for:
• Gear ratio options
• LSD options
• Clutch/Flywheel Combination
• Chassis Modifications Required
• Quantity of non-OEM parts
• Cost
That leaves some obvious choices:
085 Gearset http://www.klaus-bernert.de/rennsport.shtmNot a huge amount of information about these, especially at the power/torque levels required. To benefit from the sequential requires an EMS change, or additional control for torque reduction (ignition/fuel reduction) during shift. Cost is high (6500EUR+)
Getrag R65 5C39 From a Mini Cooper S, 6 speed with triple shafts, so is shorter than most conventional FWD boxes (ala 02M). Custom clutch and flywheel required, along with mounting, shifter mechanism, and must be hydraulic clutch. Good spread of ratios with a 540mm tyre diameter (175/60R13 or 20/54-13 slick) gives 150mph in 6th at 7000rpm. One to think about...
Sequential Sadev/Gemini/Ricardo etc£10k+ not being considered!
02a/02J Validated to 250Nm for production in larger (note heavier) applications, so should be capable of handling more torque in a lighter Polo chassis. A good range of gear ratios, gear sets available, wide range of LSDs readily available. Gearbox can be had from £50 upwards, and have been in production recently. A mix of VW parts means cable change for the clutch, and flywheel/clutch combinations should be achievable without too many custom parts.
Worth noting 02M is a 6 speed, but is heavier and larger than the 02J, it has a torque capacity of 350Nm, and obviously has a bolt on transfer box for the 4wd applications (S3 etc) little overkill though.
Gear RatiosI run various wheel/tyre combinations on the Polo, for gear ratio calculations the diameter/circumference of the tyre is important, here’s mine:
175/60 R13 (Stock G40) – 540.2mm
20/54-13 (Slick) – 540mm
18/53-13 (Wet Slick) – 530mm
20.8-14 (Drag Slick) – 508mm
For the comparison I’ve used a 540mm diameter tyre. I rev the Polo to 7,000rpm on track and have used half max rpm to calculate a cruising speed at 3500rpm.
Stock G40 gives:
150bhp 20v 02J gives:
Not really ideal, similar to a GT box in terms of ratio spread.
180bhp 20v 02J gives:
Ratio spread with ATV on a 540mm tyre:
Next instalment will cover adapter plate design and machining to couple the 02J to the PY block, all for now!
Pete