Bosche Mechanical FI on 914?

[ChrisG]

Okay - I am back being a seriously weird person here.
I am about to send that old 1.7 of mine off for a rebuild (top end only) and I have been putting a great deal of thought into how I should set up the fuel system.

1/ is to leave the old bosche FI on it - have it rebuilt and pray the wiring does not give me anymore problems (fat chance of that happening)
2/ Fit twin Dells to the car and watch the performance die, although the car will at least run fairly reliable.
or perhaps something which I have seen done on a few VW's in the past.
3/ Fitting a Bosche Mechanical Fuel Injection System (Golf/Rabbit GTI)
According to many of my sources this system should potentially increase the performance, better the fuel economy, and should in theory only take some minor modifications to fit.
Plus with it being an FI, you dont have the cam problems.
Then all I need is a Bosche 050, and electronic ignition and I have a completely reliable car.

The question is - anyone done this and are their any pitfalls?

Chris G

[Dave at Pelican Parts]

Oooohhhh, you mean Bosch K-jetronic FI--also known as CIS (Continuous Injection System). Rats, I thought you meant *real* MFI, as used in 69-73 911s and BMW 2002 tii's. I've seen that on one 914-4, but it was a REAL WILD race car.

Anyway, the CIS system is a pretty reasonable system. It won't tolerate a lumpy cam any better than the D-jet will (that's what the 911 guys say, the watercooled VW guys say otherwise), but the stock cam won't be a problem. It *should* be pretty reliable once you get it set up correctly.

Someone has done this. That person might even sell a kit for it. I am very sorry to say that I don't remember who it was. I read about it on the Porschelist (now Rennlist) 914 list, so you can try searching their archives for "CIS". That will get you pointed to a website for someone who put CIS on a Type IV in his dune-buggy. Dyno sheets and all--he seemed to like it a LOT more than carbs.

Oh, one thing. Remember that in some areas, the CIS will fail smog because it isn't what originally came on the vehicle. Even if it runs cleaner. (Yet more proof that smog laws aren't about pollution, just about money and power...) Happily, if you're in CA, cars from 73 and before don't have to pass the inspection--and that includes all the original 1.7s.

Good luck on your search!

--DD

[jamesm]

CIS is a pretty good system, but like Dave says, it won't tolerate cams that are much wilder than "very mild". On the 911 engine at least, more than about 70d of valve overlap causes the car to idle very poorly, and surge throughout at least the bottom of the rev range. Duration of more than 230d on the intake may also cause problems. These cam timings cause lots of pulsing in the intake tract from pressure waves, and the CIS airflow sensor bobs around like a cork in the ocean, drastically altering the mixture at a set throttle opening.

I've also heard the water-pumper VW types claiming they use wilder cams with CIS, but I don't know how much "wilder" they are. Probst's book on Bosch FI claims wilder than stock can be used, but there's an unspecified upper limit before problems occur. Porsche never used anything other than very, very mild cams with their CIS-equipped 911 engines, even on Euro models, so I'd be surprised if you could go much hotter than the above numbers.

The nice thing about CIS is that it's very simple, and automatically adapts to changes. Other than the cold-start system, it's all mechanical. The cold-start stuff does tend to be the weak point.

Another alternative is to get a new wiring harness for your D-Jet. There's a place called R.E.S. that sells complete aftermarket harnesses for the injection system that appear to be high quality.

[michel richard]

I've thinking of this too. Went so far as to strip a K-jet off of a rabbit at the boneyard. It's still on one of my shelves, tho.

Other possibility: CIS is mechanical and Bosch still managed to weave a feedback loop into the system, using an O2 sensor, etc . . .

Maybe we might be able to adapt that into the D-jet and make that system a closed loop.

Let us know how you make out !

[7391417]

Have you given any thought to the cb performance fuel injection system like the one on DocNugents 914 lite web site (below).
http://home1.gte.net/nugentmd/914lite.htm
It looks like a real nice setup that is ajustable and reliable, I have seen it on a couple of vws and it appears to work great with performance increase. Plus it looks good with those dual air filters in there.

[ChrisG]

I have thought of going the CB performance way. However it would take up too much of my budget.
With regards to cams - there are many way to get around the uneven idle. Most people forget that on the stock unit there is a rather largeish chamber in the inlet manifolds. This is called the plenuum chamber. Its function is to even out pressure from the airflow.
On a stock 1.7 like mine the cam is not so hot hence you can get away with no plenuum chamber using the Inlet manifold as it has more than enough volume to handle pressure increases.

However on a Type 4 2.0 race engine I used to run I developed a kick ass FI system - it never got finished as the car itself (a chop top drag beetle) was sold on. The cam was a G-Grind on that engine hence I had to use plenuum chambers. It was set up on a twin inlet manifold with one litre plenumms sat on top of each with an output on the top leading to the central body of the FI.
The general rule is match the size of the engine to the size of the plenuums. That was the pressure variance is equal. You also need to connect the pleuums up to balance the pressure waves.
It sounds to me like our 911 friends are trying to run a hot cam on an engine which has low cam plenuum tolerances.
They would most likely get better performance out of their cars by increasing the plenuum size

I will let you all know how it goes

Chris G