Petza914

Maybe something like this in the appropriate size would work well and look good.

Kraken Automotive - 1/8 NPT to 4AN Adapter Straight Pipe Thread to 4 AN Flare Fitting Black

dr bob

I think that you'll be happy with something cheap like the 1/2" copper pipe cap you mentioned, stuffed in an AAV hose. That way you can start with a small drill and work your way up to the size that gives you a stable cold idle. The idle just needs to be high enough to offset the effects of fuel vapor condensing on the port walls. Your target cold idle need be no higher than maybe 1000 RPM, so long as you don't have a lean stumble or misfire while idling and as you roll into a cold throttle. Once the head gets warm enough to make sure that the fuel doesn't condense, there's really no need for the extra air. Ditto the operation of any warm-up valves that add fuel while cold. "Just enough" is plenty.

In legacy carburetor days, a combination of a choke to limit air into the carb, a cam on the choke shaft that held the idle high while cold, and an exhaust passage in the manifold under the carburetor were used to combat the effects of fuel condensation in the cold intake and heads. Exhaust flow would be restricted on one side of the engine until the exhaust warmed up a bunch, causing hot gas flow to heat the intake manifold. A little housing called a "choke stove" managed heat build up, and coiled bimetallic springs operated choke/throttle plates on the carburetor and in the exhaust manifold. Getting all that stuff "tuned" for a particular car and temperatures bordered on an art form. Fuel injection solves so many of those "off-design" starting and driveability issues. Thank goodness!

j.kenzie@sbcglobal.net

Sounds good, Dr. Bob. Thanks for the ideas.
Dave

Petza914

As a frame of reference, the cold start protocol on my fully DME controlled 997 3.8L when first started idles at 1,200 RPM then over then next 30 seconds slowly declines to around 700 and stays there. 1,000-1,200 RPM would be my target for this project too. Part of that elevated 997 idle is to heat the cats from an over enriched mixture as it runs in closed loop when first started, which may not be as important on these older 928s as many of them are probably cat delete (like mine). On the later 997.2 DFI motors, Porsche handled it differently with an auxiliary air pump to inject extra air into the system and have the dme adjust the mixture richer to compensate for that based on feedback from the O2 sensors..

j.kenzie@sbcglobal.net

Sounds good, Pete.
1200 RPM it is.
Thanks,
Dave

PorKen

Yours being an '84 automatic you should not have a vacuum limiter (factory deleted '83-up autos) but on earlier engines they can cause variable and high idle speeds. (Unfortunately, manuals usually need this for idle recovery.)

Deleted on my '80 auto, here - https://rennlist.com/forums/928-foru...l#post16234783

karl ruiter

I think the AAV makes a relatively small opening when cold. Maybe 1/8" dia or so. If my recollection is correct, you will want to start much smaller then 1/4" for your opening. In fact, probably good to start with it totally blocked to confirm that you are barking up the correct tree.

j.kenzie@sbcglobal.net

Thanks Karl. When I close the AAV electrically by jumpering the fuel pump for 5 minutes, I get normal warm state idle of 800 RPM. I think you are right in saying I should start with a small opening. Maybe I'll try 1/8". I need some opening of the cold start system won't work.
Dave

hacker-pschorr

Before starting to modify anything I'd pull the supercharger pipe, reset the idle without the supercharger in the equation (if needed) and see what happens.

j.kenzie@sbcglobal.net

Hi Eric,
I thought about disconnecting the SC pipe for testing, but in a way, getting it right without the SC is a little counter productive. Since I got good idle speed with the AAV warmed/closed, and too high with it cold/open, I figure somewhere between 0% and 100% will be a sweet spot for AAV flow when cold. I think I can just triy a few restrictor sizes until I get 1200 RPM cold. I know my current AAV closes and I know it opens, I just thought I'd limit the opening with a restrictor. I think it should be easy and reversible.
Thanks,
Dave

hacker-pschorr

You're missing my point. The idea is to see if you can even get it to behave correctly without the supercharger in the picture. If you can, then you've definitely pinpointed the problem (or at least narrowing it down). If it still idles high when cold without the supercharger in the equation, then you know the blower has nothing to do with the issue and you can focus on the real problem.

I'm not a fan of modifications to fix something else that may be broken. Such solutions mask the real problem that could get worse and become a bigger problem.

j.kenzie@sbcglobal.net

OK Eric
It's worth a try to see what the warm-up program is with the current AAV, with the SC defunctionalized. Before doing any mods, I will open a SC pipe to remove any pressure and see how it starts. I will try it and report back. Out of curiosity, if it behaves the same, what would you look at? Remember in 5 minutes, it's fine. It's just the warm-up over-shoot that is bothering me.
Thanks again,
Dave

Petza914

Dave, the other alternative is to pull the supercharger belt but leave the rest of the plumbing connected - whichever is easier on your particular setup.

j.kenzie@sbcglobal.net

Yep. Good idea, Pete. I think I can crack open an elbow in the piping as the easiest, but if not, then it's the belt.
Thanks,
Dave

j.kenzie@sbcglobal.net

BTW, my AAV electrical closing by jumpering the fuel pump relay only effects the AAV heating element and bimetallic strip, and circulation of fuel. The engine itself remains cool, so no change in Temp 2 values, therefore no L-jet enrichment. When closed this way the idle is fine. When open the idle is too high. Maybe the AAV has an internal fault so that it somehow opens too far. This valve is actually a used 928Intl replacement unit that checked out when compared to open and closed pictures, back before install a few years ago. Once again, I think this high idle condition started when I fixed some SC piping leaks and got my boost up to spec. Not sure why this is acting this way, but I think I have properly isolated the problem to air flow through the AAV, independent of engine temp. Makes me think a direct method to reduce this flow would work and would only effect the 3-5 minute warm-up program. It's fun to think about these problems, but in the next few days, I will try a couple of experiments and see what works and to what extent.
Thanks for your patience,
Dave