This guide is intended to cover most of the pain points when fitting a 1.8 20vT into a VW T4, this is as comprehensive as possible, but I’m sure there are pain points that I’ve forgotten about. Credit to Noo Noo, Erinyes, Paul for all their help when I was doing my conversion, Bud the Spud for talking me through his AUQ conversion on the phone and thanks to Tonic and matt-lmr for inspiration, and all the above for all the photos that I’ve stolen and measured and poured over for months. I’ve stolen some of those photos for this thread, if you want them removed then PM me.
Before you start wrenching, theres a few decision’s you need to make on what you want and how you’re going to fit the 1.8T, these impact which engine and what parts you need to buy:
- Throttle type: Cable or drive-by-wire.
- Management: OEM or aftermarket
- Transmission: T4 or 02J/02M.
- Tuned or stock, rebuilt or not
- Which turbo
- Which Engine
- A cable throttle gives better throttle response, when you stomp on the pedal the throttle butterfly opens, with drive-by-wire then there is a small delay while the ECU decides to open the throttle, and on poorly mapped ECUs there is a noticeable delay after bringing the clutch up before the throttle opens. A drive-by-wire throttle allows you to fit cruise control for free, which is pretty nice in a camper van for those long motorway runs, and integrates nicely with the standard pedal box, and means you can remap the standard ECU using free software, defeat the immobiliser yourself, mate it up with stock VAG instruments, etc.
- Standard management gives you VCDS diagnostics, perfect starting in all weather, decent driveability, cheap remaps (including DIY remapping if you want) and the option to retain the standard immobiliser system, but there is a LOT of wiring in a stock loom and its not plug and play into a T4. Aftermarket management means you need a lot less wiring and tweaking the map might be easier (Depending on which ECU you go for), but you might need to splash out on costly interfaces for tuning (looking at you QPEng), you have to use a cable throttle, cold starting and idle is totally dependant on the quality of the map –often not great, and generally aftermarket management seems to need more fettling to get it working happily.
- T4 gearboxes are 5 speed and generally short ratio, but let you leave the gearbox mounting, linkages and driveshafts all standard but requires you to modify the sump to clear the gearbox trumpet. Using the Golf/A3/TT/Octavia/whatever 02M gearbox means you have the option of much better ratios or 6 speed, but much more fabrication is required including custom driveshafts and they will sit at the wrong angle which may reduce CV joint lifetime.
- 1.8T engines will tune to 450bhp+ with enough money, but are also very driveable out of the box, or with a mild tune. Its entirely up to you how far to go with tuning, but remember that big power usually comes with reduced longevity, and traction will be an issue unless you have 4wd or a mechanical LSD. There is also an opinion that 1.8T engines require stronger con-rods when fitted to a T4 (in a golf they are generally regarded as necessary for 350bhp+) due to the increased load/revs/drag of the heavier and less aerodynamic van with a shorter gearbox – two of the first 1.8T T4s threw rods, although there may be other factors that caused or contributed. Conversely, there are currently 4 or 5 1.8T T4s that are running standard rods with no ill effects to date.
- The turbo choice is mostly a factor of how much power you want, although apparently the K03s can be tuned for more torque than a K04 – but when fitting in a T4, there are space implications too – the K04 has nicer boost pipework routing and clears the bulkhead better than the standard K03/K03s. Alternatively the K03 turbo fitted to the Sharan/Alhambra 1.8T is a different design and uses a different manifold to tuck it in lower down on the block, which may clear a T4 bulkhead better, but is probably less good for power.
- The choice of engine code is mostly influenced by choices above – if you are going for a full house build on custom management the AGU is the sensible choice (cable throttle 150bhp from early golf/A3) as it has bigger valves, higher compression, less emissions junk and various internal differences compared to the later engines, and is generally very cheap too. If you want to put in a standard engine on stock management with a drive-by-wire throttle, then an AUM (150bhp) or AUQ(180bhp) is a good choice as they will tune to 240bhp before you need to change the turbo or any internals. The BAM (225bhp) is the most powerful engine out of the box, and has nicer boost pipework routing and a reversed inlet manifold, but fetch a price premium over the other engines.
Im fitting the 1.8T to our 1998 2.5 Petrol LWB T4, which is a bit of a budget build, surf/windsurf/mtb and general workhorse van, my priorities are good driveability and reliability. We spent ages working through the stuff above, and had decided on the following:
- Stock engine – strengthened rods and the associated rebuild takes the cost of the engine from £500 to £1500. I can get bare 20vs for £100, when I blow up the second standard engine, I’ll worry about buying some stronger internals and rebuilding one of my spare engines.
- Standard management with fly-by-wire – Im a big fan of standard management, I’ve played with Emerald/DTA/KMS/QPEng in the past on various cars, and nothing has really worked as well for daily usage as decent standard management, plus I really want cruise control, diagnostics, and toys like Torque. This pretty much limits me to AUM, AUQ and BAM. AUM and AUQ are the same excluding the map, and BAMs cost actual money.
- T4 gearbox – when I started my build no-one had fitted an O2M into a T4, and I wasn’t super confident with fabrication so wanted to make as few mounts as possible. Doing it again, I might consider using an 02M so I have a cheap 6 speed.
- Midly tuned, stock turbo – Im not looking for big power here, just safe overtaking and reasonable performance, so Im going to cap this at around 240bhp with a stage2 tune on a AUQ, with standard K03s. If I do a syncro conversion in the future I might go further with the power, but if Im going to throw money at the van I’d rather fit LPG or go 6 speed. (With hindsight I would consider a K04 just to make the boost pipework nicer and improve the bulkhead clearance, or fit a K03 from a sharan/alhambra and run that maxed at about 200bhp)
Parts & Costs
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