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Thread: Gas engine swaps from other VW/Audi models

  1. #31
    Join Date
    Dec 2000
    Canvey Island, Essex, Great Britain


    Transplanting a radically different engine from another vehicle marque, is certainly not a recommended project for the technologically ignorant or faint hearted. I have heard of a few instances where people have stripped gears, as a consequence of mating a high-torque engine (maximum torque is the relevant parameter, rather than maximum power!) to a transaxle of inadaquate strength & durability, so this and other issues, would need to be carefully considered.

    An increasingly large proportion of modern engines, are being electronically integrated with the vehicle's other systems, via some form of databus network, such as CANbus.

    Although various VW-Audi diesel and turbo-diesel engines are available from various donor cars and vans, including the VW Eurovan T4 & T5, plus the VW LT and more recent VW Crafter vans, their relatively large weight & high centre of gravity, would make them less well suited to the rear-engined VWs, than the Subaru, all-aluminium-alloy, quad-cam, 16-valve, flat-four, turbo-diesel engine.

    According to my technical data sheet (LCV/SPEC/TRANSP-40m-12/87), for the British specification, 1987 VW Transporter & Microbus (i.e. VW Vanagon, Type 25 or T3), from Volkswagen Audi Gevaert (United Kingdom) Limited, the engine speed at 70 mph, corresponding to 185 R14 tyres, in conjunction with the 5-speed transaxles for the VW 1700 diesel, VW 1600 turbo-diesel, and VW 2100 EFI petrol & VW 1900 petrol Vanagons, would be circa 3870 rpm, 3620 rpm, 3440 rpm & 3400 rpm respectively.

    However, if one intends to cruise on motorways (i.e. freeways, in North American parlance) at about 70 mph in 5th gear, then it would probably be preferable, to operate a Subaru turbo-diesel engine, close to its peak-torque RPM (i.e. 1800 rpm), which would confer much improved fuel economy, compared with operating close to peak-power RPM (i.e. 3600 rpm).

    Hence, if one intended to use tyres, such as 185R14, 195/70R15, 215/65R15, 195/65R16 or 205/60R16, of external diameter to within 2% of original-equipment specification, then it would be desirable to virtually halve the overall gear ratio in 5th gear (i.e. final-drive ratio x 5th-gear ratio), if one did transplant a modern Subaru 20 litre, flat-four, turbo-diesel engine. Even with this ultra-low gearing, the 350 Nm engine torque, would be more than adequate, to accelerate to higher speed or climb gentle hills. Of course, one would still have the option of changing down to 4th gear, or even 3rd gear, if greater torque at the wheels were needed.

    A final-drive ratio of 457, appears to be the lowest factory-stock option for the 5-speed & 4-speed manual transmissions, whilst final-drive ratios of 409 & 373, were used for the 3-speed automatic transmissions, of the VW 1900 petrol & VW 2100 EFI petrol Vanagons respectively. Whether an after-market ring & pinion set, is available for VW transaxles, having a final-drive ratio of circa 23 to 25, I have no notion at present! In practice, one would also need to consider the required overall gear ratio in 1st gear (i.e. final-drive ratio x 1st-gear ratio), for starting off from rest and slowly climbing steep hills, together with the gear ratio progression from 1st to 5th; recalling that greatest maximum road-speed is achieved, when maximum road-speed corresponds to peak-power RPM for the engine.

    Of course, a CVT (i.e. continuously variable transmission) such as those developed by the British company Torotrak, would be the ideal solution, but assuming a suitable substitute CVT transaxle were available, it would probably be prohibitively expensive!


    Last edited by Capt. Mike; 02-10-2009 at 06:13 PM. Reason: Removed repetative quotes; removed Subaru content & site referneces.

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