A Hot Dusty Wind from 1981

In reply to paranoid_android :

The system will already deliver more fuel if the engine is ingesting more air to the limit of the system.

You can adjust the base fuel pressure slightly to compensate, but unless you're making huge changes in power output (like adding a super/turbo charger for example) you shouldn't need to make too many adjustments.

CIS works just fine, provided it works, but if it is allowed to fall into disrepair woe is to you.  Basically a carburator threw up all over your engine bay, so any/all vacuum leaks and/or fuel leaks or constriction will drive you to drink.  Heavily.  I would never, ever wish that nightmare system onto anyone.  Seriously.  Install Digifant from a donor VW or MegaSquirt or even carbs, but once CIS goes urp ditch it.  Move on.

I did a study of the CIS/Jetronic stuff a couple months ago, but all my notes and links are on my work computer. I can dig it up on Wednesday if there's interest. Here's the page that made me feel like I could really understand it: https://forums.somethingawful.com/showthread.php?threadid=3806439

I followed on with the bit about the Warm Up Regulator (WUR) being used for tuning and that led to a lot more understanding. It's all a bit foggy again, but ...notes, I have notes somewhere.

And here's a 'random hotlink':

In reply to Stefan :

CIS is like Quadrajet.  When it works, it works really really well.  But when someone mucks around with it, it's horrid.

I find it funny that you recommend Digifant.  When I was into transverse VWs, I had a Digifant car and loved it.  The Internet said that Digifart was the worst thing ever and people either avoided Digi cars altogether or they converted them to CIS, and I was nuts for liking it.

The thing that is somewhat backwards about CIS is the way people approach the tuning.  The flapper plate does constrict power by about 10% compared to an AFM/MAF/speed-density EFI setup.  The key is to, IIRC, REDUCE control pressure so that the flapper can move over its entire range, for the amount of power that you can make, and then reduce a different pressure to get the fueling down from overrich.  That minimizes the losses from opening the flapper against control pressure.  But it's been a long time since I cared about mucking with CIS.

CIS is inefficient at best, and if it has ever been allowed to go dry or contained any water at all the entire system is trash. All of it. Frenchy's comments do not apply to CIS, but do apply to "L Jetronic" or the earlier "D Jetronic" systems. Your VW is too new for D Jet. L Jet can be fooled into providing more fuel with increased fuel pressure and making the system think the car is colder than reality. VW Digifant is best adapted to a Megasquirt, and in my experience, so is L Jetronic. All you re-use is the injectors, pump, and sensors. Also, if you are CIS, it really dislikes any alcohol. Unfortunately, the CIS injectors have intake manifolds that do not directly swap over to electronic injectors, milling machine work is neccessary.

In reply to AAZCD :

Thank you for that link, very informative!