i'm getting familiar with the d-jet on my sons 73 1.7. i had a 76 westphelia 2.0 with L-jet (not d-jet like 2.0's on 914's). it seems to me both are good but somewhat primitive in operation compared to todays standards. i would think the L-jet would be the best choice but it seems the d-jet is the popular choice, is that becouse it's more common or understood?

L-Jet swaps to carbs best.

L-Jet is newer. Some may argue it's better.

1.7 and 2.0 liter 914 engines came with D-Jet. The 1.8 was L-Jet.

On a stock motor, I figure you should make the best of the system that goes with the motor.

Do you want to upgrade the FI to something modern, or build a high-performance engine? If so, we have options! I figure you should choose between SDS and MegaSquirt. The former costs a bit more, the latter takes a little more work/time.

I think parts for L-jet are easier to find, and more people can fix it because it was used on more cars.

I guess my question is, do you want to keep things stockish or do you want upgrades?

If staying stock then L vs. D is not an issue. Stick with what you have. If you need help dialing that in, you're in the right place.

If you want performance or modernizing upgrades, then let us know that.

I think its a matter of what setup you have. Since its a 1.7L Djet car leave it and get it running the best you can.
I found brand new 1.7 injectors for $40 ea that really cleaned up Blairs black car high rev miss. Lots of 1.7 parts around including lots of used MPS's.

I also like the Ljet as a FI as it simple, easy to tune and tons of parts (bus guys rip it out in favor of the single carb). Vacuum leaks kill Ljet so new hoses and gaskets are a must.

I have both, on running cars.
I like the Djet for start up and idle...its just better in that respect.

LJet can take a bigger motor, the AFM or Air Fuel thingy is easier to get than the MPS...but both can be had used.

The Djet, when properly setup, it rock solid reliable.
The LJet, as Geoff stated is even more sensitive to vacuum leaks and those leaks show up at idle...

Both are good, both run great when all fixed up.
Both require detailed attention paid to the vacuum leaks.

Rich

What? ljet came on 1.8s for 2 years. Djet came on 1.7s for 3 years and 2.0s for 6 years. how can there be more Ljet cars?

There are cars on the road other than 914s.

There are not! Everyone knows that there are only 914s.

Isn't the L-jet barn door air meter closer to modern Motronic pulsed fuel injection?

Yes, Motronic is just an evolution of L-Jet.

The ignition side of the L-Jet system is separate from the fuel metering side. L-Jet ignition is the standard coil and distributor which functions essentially separately from the fuel metering side.

Motronic takes the L-Jet fuel metering system (barn door), and integrates a digital ignition system. But the metering measurment system (ie. the barn door system) is the same on LJet and Motronic.

Ljet is basically half a Motronic system.

I think (not sure) that the l-jet evolved into LHJet with the addition of a lambda sensor, and is still used today on alot of cars.

Rich

the bulk of the 74-75 cars were 1.8L. vs 2.0L.

The bus ljet (75-79)can easily be used IMHO other than the aircleaner (use a cone filter)
Even the later Ljet bus stuff (79 on) can be used but I still have to look at it.

I think modern cars (cars being produced today) no longer have any real connection to either D or L jet.

Certainly every Porsche, BMW, MB, and I'm sure Toyota, Honda, etc. use an air mass sensor, rather than any kind of an air flow sensor.

D-Jet used a manifold pressure sensor, L-Jet used air flow sensor, modern uses air mass sensor. They are all very different in operation.

Air Flow and Air mass are somewhat similar. In D-Motronic (and later versions of L-Jetronic) to determine the 'mass' of the air flowing through the vane, there was an air temperature sensor inside. Similar to MAF sensors which use a heated filament and air flow over the filament to determine the mass of the air. Not like a MAP sensor, though.

No, there is no "vane" in a Djet system and Djet does NOT use a temp sensor to trigger fuel flow. Djet uses manifold pressure, that's why it's called Manifold Pressure system.

(Yes, there is a minor temp sensor in the Djet system, but that's just to make minor adjustments based on temperature, it does not define the system. all fuel systems for decades have some kind of temp compensating system, carbs have automatic chokes, etc.)

Re-read my reply. I'm talking about D-Motronic and L-Jetronic. Not D-Jet (manifold pressure).

There are 3 basic types of fuel injection....

1. Mass Air flow systems.
Think L-Jet.. Load is determined by amount of air coming into the engine.
2. Speed Density Systems
Think D-Jet..Load is determined by throttle position and and manifold vacuum.
3. Alpha-N systems.
This is MFI. Load is determined by engine speed and throttle position.


The L-Jet system was the first Mass Airflow system Fuel injection system. With the addition of the Lambda (O2) sensor, it had feedback. When they changed from the Vane Air Meter to the Hot Wire Mass Airflow sensor, it became LH Jetronic. With the addition of Ignition control, it became Motronic.

The Change from a Vane Air Meter to a Hot Wire Mass Airflow Sensor removed the limitation on camshaft overlap. The new computer could be programmed to compensate for the backward pulse of air caused by the overlap, and the Hot Wire Mass airflow sensor didn't have a mechanical flap to flutter from the back pulse.


The Speed Density systems are still being produced. GM used it for a long time on a bunch of their cars. It works well too. But the best and most efficent injection systems measure the air coming into the motor. It gives the most accurate determination of the amount of fuel to inject

Oh, ok.

D-Motronic is not a term I'm familiar with. What is it?

Digital Motronic. It's a digital (not analog) system that controls fuel and ignition. Think of... on a mid 80's 911 carerra, or a 944/951/968. Instead of using the wiper board inside the air meter (vane) it has a pre-programmed set of fuel maps, and uses the vane as reference. The same system also came with knock-sensing that could automatically retard timing or advance timing. No more points! It also used a magnetic pickup with/on the flywheel to determine the engine's position and give precise ignition timing (think crankfire).

I wonder if that could be adapted to a 914 for simple FI under boost

PEFI is probably easier tho

Jim, I've contemplated that but i think there would be too much fab work.

You would have to make sure your flywheel had the right number of teeth, as well as have a magnetic pole installed at the proper location for the Reference sensor. And then fab a bracket after cutting a hole through your transmission bellhousing to mount it up. The parts could be adapted if you had the time and developmental skills to do such a swap. Then, if that wasn't enough, you would have to re-program the EPROM in the DME to have the proper fuel maps for your displacement.

I've just never heard the term "D-Motronic," all Motronic uses a digital computer so the D seems redundant.

An 80s Carrera has a Motronic airflow meter that *has* a wiper board inside of it. I've taken them apart and seen 'em. So does BMW Motronic, I think all non-hotwire Motronic injection systems use the wiper board inside the air meter, they used that up until they went to the hot wire air mass sensor, I think.

Points were gone a long time ago, well before Motronic. For ex., the '78 911 came with CIS and no points. I don't think any Porsche outside of the 70s had points anymore.

Motronic didn't have knock sensing added in for a while, I know up to the 89 Carrera didn't have a knock sensor.

I've always found the evolution of injection interesting, from DJet through LJet, CIS, Motronic, etc. Through all of it, I'm still most amazed by DJet, which is so primitive, done well before the pre-digital computer age (developed in the 60s), is fairly simple/elegant by injection standards, and can still deliver good driveability, reliability, power and economy even 45 years later.

I've been impressed with Djet since my first 1.7 and now my 73 2.0. I just re-furbed the FI and I can't believe how good the throttle response is.

My buddy manages a local VW shop. I was there yesterday to put my car on the CO analyzer before a trip to emissions. While I was there, one of the shop kids said he would rip that FI out and put carbs in. I just smiled quietly. After that, the car flew through emissions.

BTW, I've been reading the "Bosch FI and Engine Management" book as of late. Just for shits and grins you know. Its been sitting next to my favorite seat in the house.

Also, my 944S2 has Motronic and I <believe> it has a knock sensor.

yep, shop kids and Joe R love to rip out that FI in favor of carbs.