VR6 throttle body?

[Pat D]

Is it worthwhile fitting a VR6 throttle body?

034 have adapter plates available to fit them on AAN/ABY engines, so I'm assuming it's a bigger or better throttle body?

[Wheeljack]

To answer your question quickly, probably not hugely worthwhile, but there is a potential flow increase of roughly 5% (based solely on the increased cross-sectional flow area and discounted due to the reduction near the plate). But you could also just bore out (with a matching larger throttle plate as well) the AAN/ABY TB to get that increase. And that is assuming everything else around it is not the limiting factor so you can actually see the difference here.

But there are notable pros/cons and differences. First, there are several iterations of the VR6 throttle body, with the later OBD2 drive-by-cable versions being the ideal ones to use for our motors (larger than OBD1 version, but still DBC, unlike later OBD2 VR6 TB, which is DBW). 034 sells the adapter to utilize them because they can be had pretty cheaply here in the US due to the sheer numbers of them around versus AAN/3B (only on '92-'95 urs4/6 and '91 200TQ), as we did not have the ABY/ADU motors here.

The diameter of the VR6 body is roughly 2.5mm larger than the AAN/3B, so it will flow marginally better in stock form. Something around 2.65" versus 2.75" ID.

Perhaps the biggest difference has to do with the cross-sectional shape between the two. The AAN/3B has a mostly constant CS (aside from a step halfway through) and off-idle response is managed by a wedge on the throttle plate. The VR6 utilizes a profiled CS in the vicinity of the throttle plate (and no wedge) to achieve the desired response. This is a later and more developed design than the old wedge approach from the AAN/3B TB. Both the wedge and the shaped CS are to minimize the change in flow as the plate is moved from the idle position.

I believe the updated design in the VR6 TB may also provide better off idle response, so that may be a factor in your build.

I'm currently in the midst of doing a CFD analysis and flow bench confirmation which compares them in two primary respects, as the differences are much more subtle than one flows more than the other at 100% throttle. The other important piece is off idle modulation (and overall modulation), but namely dQ/dtheta at theta near 0, which I am also investigating. Mostly just for curiosity's sake...

For reference, here is the AAN:


and the OBD2 DBC VR6:

[macspring]

Thanks for sharing your experience, was thinking about this after recently seeing a Mk3 Golf VR6 come into in the local breakers. A very rear occurrence where i live.

Kind regards.

[prj]

And I would personally say that under low outputs (under 500hp) the throttle body won't be a major restriction at all.
In fact, even on higher outputs, getting an IC with a good core will do a lot more for your intake restriction than the throttle body ever will.

[Pat D]

Cheers for all the info. I'm doing a full rebuild from scratch and think I'll go with the VR6 throttle. The inbuilt idle control might save some room and clutter around the back of the IM as well.