So I've got a 73 914 and am having the engine checked . So far the news is all good.
My question is will the stock FI work with 96mm pistons, larger throttle body and a little more aggressive cam?

no personal experience with that yet but some research i have found several discussions of members here that used a bus or Vanagon throttle body on what you are talking about, i am also going to do that, checking the Dub Shop or Go Westy for a 53mm TB i believe. someone else will chime in on this. Seems to make sense, get more air it to help, i also have the 2056/P&C and a more aggressive cam, and will be trying a larger throttle body as well soon.

Phil

for my 1974 2lt, now with mild cam & 2056, I kept the existing stock TB and injection. Had to up the fuel pressure to get it starting and running nice and have the C0 level right at idle. I still have to do some Air / fuel ratio checks across the rev range - piece of mind that its not leaning out up the revs - but it runs fine.

Not sure the the larger TB would add / change to this setup significantly

Has anyone calc'd out the flow possible with the stock fuel rails? I would think a little math in the right hands could tell you what size throttle body you should be shooting for. I've looked on ebay at some of the TB's there, looks like some of those could be made to work.

D-Jet - we are talking D-Jet, right? - uses analogue circuitry with neither microprocessor nor digital logic; the ECU has around 25 transistors to perform all of the processing. So it's pretty basic. The lack of processing power and the unavailability of solid-state sensors meant that the vacuum sensor (the "MPS") was a rather expensive precision instrument, rather like a barometer, with a brass bellow inside to measure the manifold pressure.

Basically, that bellows is the sole inferential input to the ECU for airflow. Coupled to TPS it finds a place in its brain to decide injector duty cycle ("speed density" system).

D-Jet's non-existant processing power means that it is not tunable; it cannot accomodate modifications to the engine. You can get around that somewhat by increasing fuel pressure (as noted above) and tinkering with the springs in the MPS to get it to trick the ECU to demand more fuel.

But it will not be tune-able through a range of RPMs and loads; basically, all you can do is take the existing "ECU map" and move it up or down whole. But it's been done.

If you're going that way I wholeheartedly encourage installation of a wideband O2 for adjustments and monitoring. Without that, you're shooting in the dark and could very easily run the engine too lean, causing damage (or too rich, wasting money, power, and drivability).

Optimally, you could go with an aftermarket EFI system. There's several threads of discussion here for those.

Greg

L-jet you can use the vanagon TB.
D-jet a bigger TB has been done by others with no noticeable difference.

On the L-Jet setup for Betty's 914, using the Bus throttle body woke the motor up. The stock one is way too small for any performance driving.

Clay

I installed a larger Vanagon throttle body. I think it was a 52 or 55mm. You have to modify the 1.8L throttle body shaft to fit the larger butterfly. You will still be able to use the throttle sensor, just drill and tap another hole for a screw. I opened up the plenum with a tail pipe expander to 55mm and trimmed the air cleaner horn to 55mm also. You need a 55 mm throttle body gasket.

"Can be made to work" is a better way to say it.

D-jet systems are designed to run on the exact configuration of motor they were sold on, any deviations from the stock motor config and you basically have to hack the d-jet system to get it to function decently

and

"Decently" != "Ideal"

It can be made to work, within limits, but you will always be leaving something on the table vs say a fully programable FI system.

On a modified engine a larger throttle body can be of benefit but if you are sticking with d-jet I probably wouldn't bother as d-jet is going to be more of a limiting factor in a performance build than your throttle body anyways.

My basic rule at this point is that d-jet is for stock motors. If you want to build anything beyond stock its more than worth it for many reasons to ditch d-jet. Even on stock motors these days an argument could be made to ditch it, but that is opening a whole other can of worms.