Single Throttle Body Setup Question Options

[Craigers17]

This thread is meant to "sort of" piggy back off the current thread on fuel delivery setup options. I see that Clay has this link for a complete modern EFI system:

https://store.allzim.com/914-parts/fuel-del...efi-conversion/

...and I know the Dub Shop has a similar setup. I also see that the Dub Shop has both a single throttle body & a turbo plenum single throttle body setup. Here:

https://thedubshop.com/single-throttle-body-intake-package/

and...here: https://thedubshop.com/single-throttle-body...-turbo-plenums/

Sooooo....since I'm not the sharpest knife in the drawer, can someone educate my dumb ass on why it seems much harder to develop an aftermarket single throttle body setup for a Type4 / 914 engine as opposed to a Type 1? Or maybe I should say as opposed to one with a Type 1 shroud. It seems like the air and fuel travel the same distance. What am I missing? I do realize that 2-3 companies are working on this....just wondered why it seems to be more of a challenge.

[ClayPerrine]

This thread is meant to "sort of" piggy back off the current thread on fuel delivery setup options. I see that Clay has this link for a complete modern EFI system:

https://store.allzim.com/914-parts/fuel-del...efi-conversion/

...and I know the Dub Shop has a similar setup. I also see that the Dub Shop has both a single throttle body & a turbo plenum single throttle body setup. Here:

https://thedubshop.com/single-throttle-body-intake-package/

and...here: https://thedubshop.com/single-throttle-body...-turbo-plenums/

Sooooo....since I'm not the sharpest knife in the drawer, can someone educate my dumb ass on why it seems much harder to develop an aftermarket single throttle body setup for a Type4 / 914 engine as opposed to a Type 1? Or maybe I should say as opposed to one with a Type 1 shroud. It seems like the air and fuel travel the same distance. What am I missing? I do realize that 2-3 companies are working on this....just wondered why it seems to be more of a challenge.

I have talked with Aaron @zims ( @Partsguy22 ) and while it would be easy to setup an aftermarket EFI system that used the stock 2.0 intake plenum, throttle body, and air cleaner, it has two disadvantages. 1. The throttle body is a restriction in the intake, and 2. it would require completely different programming. Most people that put aftermarket EFI on their cars want the extra performance.

Granted, you could put a new plenum on with a larger throttle body, but you are still faced with the same pressure drop issues at low RPMs that are resolved by multiple throttle bodies. Smaller runners increase air velocity at low RPMs, increasing performance. But a single small throttle body restricts air at high RPMs. That's the reason that car manufacturers used 4 barrel carbs instead of just one giant barrel.

Now if someone wants a "Stealth" EFI system that looks like a factory system, it can be done. But it would require some lateral thinking on how to hide all the "modern" components.

[GregAmy]

...it has two disadvantages. 1. The throttle body is a restriction in the intake, and 2. it would require completely different programming. Most people that put aftermarket EFI on their cars want the extra performance.

I'll also disagree with the "most" word.

First, on throttle body airflow. My D-Jet conversion, on a dead stock engine, was 96 horsepower at 2900 and 88 torques at 4400, with the torque flat to just over 5200. This is at the hubs on a Dynapack.

But more importantly, my MAP is usually in the 98 kPA range at WOT and 6,000 RPM in the plenum. If I'm reading the data correctly then that mean I'm roughly 97% or so of atmospheric in the plenum of the D-Jet system. That implies the stock throttle body isn't much of a restriction.

Now, the runners will no doubt offer some additional restriction. And the throats are not pointing right down into the cylinders. So, yes, someone building up an engine with cams, compression, and porting will certainly want more airflow than what D-Jet will provide. But I offer that on a mostly-stock engine the D-Jet induction system is surprisingly good for the application.

I actually bought throttle bodies and all required components from Mario to convert my D-Jet system to ITBs: https://thedubshop.com/dual-idf-throttle-bo...injectors-vwss/

I had everything I needed to install it, short of motivation and some electrical pigtail adapters. I ended up selling all the components to a member here because while I would have liked to have pursued it for the knowledge, on a mostly-stock engine I just wasn't going to get much out of it.

It would have been nice add to my historics race car (high compression, big valves, porting, 86B cam) but that's running fine in dual Dells so I'm not gonna fark with it.

As for "completely different programming"...that concern I don't get. Different from D-Jet? Of course it is. But it would be that way with any other system, even carbs (jet tuning is an analog art all by itself). There is a learning curve for anyone that was customized programming (or carbs); in fact, everyone would have to learn how to do it themselves because every one of their engines would probably be differently built and a supplier cannot account for that (D-Jet was designed for a single build spec).

As I noted in my opening monologue in the blog, the D-Jet system is pretty damned remarkable for something that was designed over half a century ago; I think we are now all beginning to believe that. And if we could continue to get replacements parts at low costs for it, then I see no reason to move away from it on a stock engine.

But with components becoming scarce and spendy, and a desire for moar powah engine components, a modern interpretation is warranted. I'm delighted we have several options now.

- GA