Simplifying CIS
 

In an effort to simplify the CIS system to eliminate point of failure issues, I was wondering if it's possible to eliminate the AAV and Decel valve, leaving only the AAR to control idle speed. This is on an '83SC. I know the Decel can be eliminated, and thinking that the AAV only provides throttle bypass air on start, this might be eliminated by stepping on the gas when starting.

What brought me to this point, was being stranded this weekend due to a split intake runner boot. Troubleshooting those valves is difficult, and if the AAV sticks open, this could lead to a similar vacuum leak.

I have the motor out, and would like to simplify the system, but if it doesn't work, that would mean dropping the motor again to re-install everything. Has anyone been down this road before?

http://forums.pelicanparts.com/uploa...1758550676.jpg

https://www.cis911primer.com/components/images/Auxairparts.jpg

Didn't exist before 76 and those CIS systems worked fine.

Seems like an easier and potentially less problematic approach would be to replace all the intake runner boots which if original are likely worn and prone to cracking.

I have maintained numerous cars (not all Porsche) with CIS (and still running mostly stock CIS on my '75 Targa). The CIS setup on the '75 is, my opinion, the simplest you can live with (at the expense of a slightly more complicated WUR).

The AAV is almost never the cause of a vac leak. Bad boots, hoses, injector seals and sleeves, and air boxes are almost always the cause of the vac leak. Intake runner boots are a maintenance item - you just replace them every so often (or you replace them when they fail and hunt for the reason why nothing runs right).

You want the decel valve - CIS with no decel valve is a popping annoyance.

You can eliminate the AAV, but you'd need to add a hand throttle to keep the throttle open a bit until it's warmed up.

You can pull the CIS off with a partial drop. Not easy, but not impossible either...

I'm in the process of replacing all boots and seals that might cause air leaks.

I unhooked the decel valve long ago, and the minor popping that occurs every once in a while is not an issue for me. I believe the AAV is either open or closed. Open when no vacuum present, and then closed with vacuum. It's designed to bypass the throttle when first starting the engine, and then closes when the engine starts and vacuum is present. That's the one I'm concerned about eliminating. Doesn't the AAR control engine speed while it's warming up? I plan to keep the AAR, but was hoping to back-date and eliminate all the tubing and hoses associated with the decel and AAV valves. Less weight and complexity, and less chance of something failing -

That's right.

In CIS terminology, AAR is the automatic hand throttle.

I've recently gotten my '74 back on the road this year after cleaning up the CIS system (among many other things).

Yes, you can remove the DV, AAR, and AAV. The only "extra" bits I have on my CIS unit are the WUR and CSV as these are required.

Cold starts are fine -- sometimes you may need to start it twice when cold for it to catch and run. No throttle needed. Let it warm up and it's fine. The AAV/AAR are mostly related to cold starts anyways, but they don't stop it from starting. DV is personal preference. I saw all of them as potential vacuum leaks down the road.

https://forums.pelicanparts.com/porsche-911-technical-forum/1184059-79-sc-plugged-rubber-pipe-sensor-plate-boot.html

Looks like that's what was done to the CIS setup in this other thread.

The 2/3-way AL fork is also prone to cracks.

Thanks! - that's what I was hoping to hear. I'm going to ditch the decel and AAV, but keep the AAR. If nothing else, it will look less cluttered and eliminate a source of air leaks -

Agree here with the AAV (and the AAR). Both are simple devices — the AAV especially so — and most of the leaks just come from the hoses. That big 3-way hard line part that goes into the AAV and the back of the intake can be a source of small leaks over the year. The hoses fatigue. All of that is easily repairable.

If you're in California or Florida where it's rarely that cold you might be able to do without both, but anywhere else it'll be a bit of a pain (esp. w/o a hand throttle/choke to make up for it).

IMO: the decel valve can go. I'm OK with some overrun and never liked the way it held revs.

I haven't missed not having the AAV op for years.

As biergekuhlt said, you might need an extra crank after the car has been sitting for a while but I'll take that over a massive bolus of air leaning out the mixture at startup when the engine is already starved of fuel.

http://http://forums.pelicanparts.com/911-engine-rebuilding-forum/865104-how-much-power-increase-3-0-a.html

Henry Schmidt's take on simplified CIS.

Interesting read. Henry seems to knows his stuff. He's also generous with his time and has helped me out on the phone - really enjoyed talking to him. I feel better about ditching those parts and simplifying the system.

Im interested in your outcome...

Please keep us posted.

Thank you,
Dave K

I will. It might be a couple of weeks as I'm planning a few upgrades "while I'm in there"