Simplifing CIS for ‘78 SC ?

[Vintagemotion]

Is the auxiliary air valve AAV and or Auxiliary Air Device AAD (vacuum control valve) performance and / or emissions relevant? I’m not talking about auxiliary air regulator!!

See picture. What is the consequence of removing these components and plugging connections?

[snbush67]

Yes relevant,

Snipet(s) below from Jim Williams CIS Primer;

http://members.rennlist.com/jimwms/CIS/CIShome.html

Auxiliary Air Valve:
Provides bypass air around the throttle when the engine is first started. When the intake manifold vacuum reaches 5 - 6 inches, the valve closes. The closing is independent of engine heat. Part number 0 280 160 400 pulls in at ~8" vacuum. This regulator supplies air to the intake manifold during warm and hot starts when the auxiliary air regulator is closed. This was added in '76.

Failure mode: When not closing properly, can cause high idle speed , or idle speed to oscillate.

Deceleration valve:
(Also called Auxiliary Air device, or vacuum control.) When decelerating from high RPM with a closed throttle, this valve prevents high vacuum in the intake manifold by providing a path for air bypassing the closed throttle. This provides sufficient air in the air distribution box for a combustible mixture. It is one of three devices that bypasses air around the throttle plate. It opens with about 15 to 18 inches of intake manifold vacuum. Deceleration valve 0 280 160 111 pulls in at ~14" Vacuum. This component was supplied in two configurations on the CIS cars. The first type was on the early cars through the 79 models. It was mounted on the throttle body on the "firewall" side, pretty much out of sight. The second appeared on the '80 through '83 and can be seen on the right hand side of the engine, behind the fuel distributor.

The function of this device is the same for both configurations. For conditions of high intake manifold vacuum, such as when the throttle is abruptly closed, the valve opens, bypassing air around the throttle plate. This adds extra air to the mixture preventing a temporarily over-rich mixture. It opens at about 17 inches of vacuum, and closes again as the vacuum falls. In normal driving, (acceleration, and cruising) the valve remains closed.

In working an cars with the later model valve, I have frequently found the actuation line (small line) intentionally plugged by the owner or mechanic. The reasons for this escape me. I suppose that some drivers don't care for the slow fall in RPM caused by the operation of the valve when the accelerator is lifted at high RPM.

Failure mode: The diaphragm can leak, causing a too-high idle.

Summary...;

If you are having problems with your CIS it is most unlikley that either of the two components you circled in the picture are the cause, more suspect would be the hoses, vacuum lines, cold start valve seal and the airbox itself.

Shane

[Hladun]

Both of these devices act only momentarily. The AAV gives a little bit of extra air ONLY at start up and the VC (deceleration valve) extra air immediately after throttle close. If working correctly, neither affect performance (engine under load, throttle partly open), starting is not affected, but you can get more lean backfires. I've taken the AAV out, but have left the VC in. Why? The AAV adds EXTRA air at start up so it increases the chance of lean back fire (bad) but the VC prevents the engine ocassionally stalling on rapid throttle release on a shift (good).

[Gunter]

I like the Decel. Valve (Vac control) on the '78-'79 SC's because it is adjustable meaning: You can set the "hang time" when shifting.
You can have a slow, or rapid, drop in RPM adjusted to your liking.

I kept the AAV and Decel.Valve but removed (By-passed) the Thermo Valve #4.

[don gilbert]

get a hand throttle from a 75 or earlier car and dump it all. it just a bunch of vacuume leaks waiting to happen.

[Vintagemotion]

Originally Posted by don gilbert

get a hand throttle from a 75 or earlier car and dump it all. it just a bunch of vacuum leaks waiting to happen.

your signature says: working lambda and highly modified CIS....please elaborate I'm curious on what and why. Thanks RJ

[don gilbert]

will pm you rj

[T77911S]

i think the 78 cis is the simplest one out there and probably the best, no freq valve, large runners, decel valve as gunter said(although mine is removed since i put the throttle body from an 80 CIS on and i dont like the decel on the 80), and vacuum controlled WUR.
i would keep the AAV, if it leaks, your rpm's go up and you know it is bad, no performance loss and it helps start the car. some people like to remove the AAR too, i like the hi idle when cold, i just wish it would stay hi longer.

[Paulporsche]

T77911S,

Have you read poster Mystery Train's thread on AAR modification?

[T77911S]

no i dont think so

[Paulporsche]

T77,

It's very good. He took it apart, rewound the internal resistor wiring, and was able to get it to stay open longer.

Don Gilbert,

Could you share your CIS mods w/ the rest of us?

[don gilbert]

paul, not so much on mods except a few things. (adjustable wur, air box mods, on cis, its all in the tuning as u know. just like our 5 second drag bikes, (1/8 mile) the air cooled porsche (with k-jet) is very sensitive to barametric changes, temp changes, humidity, thats why every once in a while your car will feel like ," wow, it running really strong today, why wont it run like this everyday?" well, its because every once in a while the atmospheric condition and your paticular wur are in sync. As most know, once your at operating temp, you are at the mercey of your warm control pressure (also frequency valve) for proper afr. at wot, most wur's that i have put gauges on are around 50-55 lbs. when warm, if you have an 02, I can go as low as 47lbs and still come back to stoiche at cruise and part throttle. once I got more fuel being delivered at wot, i played with my timing and can run 35 degrees advance safely with no spark knock, (i like just a little knock when u first stabb the throttle) Getting more fuel to the engine is not problem with adjustable wur's, its air. so what to do? i am not a fan of turbo's, being an old school hot rodder, and all my old machs and shelby had a ram air system ...so i started experimenting on how to get more air. i have so far come to the pic u see below, it might look a little goofy, but it is very effective. Paul, being a part time drag racer for 25 years, and a 16 year certified factory trained bmw tech, i love to fiddle, i do the most mundane things like indexing plugs, contantly checking afr's and what nots, an adustable reg is not for everyone, as there is A LOT of fiddling to do, but if you want to get the most performance out of these cis cars its essentail i think because the usa specs for our car are so out of whack for performance because of emmision reasons, (the 74-83 cars are set up to run as lean and hot as safely possible.) please dont think im a know it all, as ive only had my pcar on the road for a year now after a 3 year build, and ive read all over this board that nothing can be done to make cis car run any better than factory specs, well, thats not true, as my car was the slowest in the bunch i run with to the fastest by far. (now i'm working to make all my buds cars run like mine. will never stop experimenting, so maybe more to come. see ya

[don gilbert]

this is my little bro's car that we do all our experimenting on. BY the way, if going to an adjustable wur, it makes things much easier to put a perm. fuel gauge between the wur and fuel dist