924Board.org

[emoore924]

Here's the situation: I have an 80 931, head mods, ic, microfueler, no cat.

Upon deceleration, the car sounds like a 2-cycle, heavily popping through the exhaust. It sounds a lot like it is probably running too rich and is burning off fuel in the exhaust. Slight opening of the throttle mitigates the problem almost completely. Think 125cc Yamaha dirt bike.

But here's the worst part.

More often than not, like 4 out of 5 times, when I shut off the motor, 2-3 seconds to 1 minute after shutdown, ka-BOOM!! I get a major backfire through the exhaust system. I'm afraid this ka-BOOMing is going to shorten the life of my turbo considerably or rip the exhaust off the car. Scary to think which way the turbo is spinning and which way the shock wave goes through the exhaust.

I have two major suspicions -- fuel mixture and something going on with the microfueler. It is running rich as evidenced by black on the underside of the bumper by the exhaust. FWIW I idle for at least 4-5 minutes after coming off the road to cool the turbo so it is not like I'm shutting off red hot.

Suggestions? Advice?

[Lizard]

definetly a fuel related issue, however verify that you have no unmetered air (vacuum leaks) aswell to check the timing and verify that you have appropriate timing
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3 928s,

[kaffine]

what is a microfueler?
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80 924
80 931

The best desciption of an atom boils down to something unknown is doing we don't know what.
Sir Arthur Eddington

[emoore924]

A Calloway Microfueler is a gizmo from the late 80's that detects when you are on boost and turns on fuel to a 5th injector that shoots gas straight into the intake manifold.

Supposed the richen the mixture to reduce the risk of detonation and an over-lean condition under boost.

[Paul]

I'd start by disconnecting the micro fueler.

If that isn't it, make sure the 5th injector is not leaking.

[numbbers]

What are you running for exhaust. Every since I put a Borla low restriction muffler in my '80, I will get an afterfire in the exhaust on shutdown. Particulary, if I have the idle a little fast. I just assume that the Borla is letting a little more fresh air reach the unburned gas that the CIS continues to feed to the engine until it stops turning over. I haven't found a solution to it yet, so keep me posted on what you find out.
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1980 924 Turbo

[emoore924]

The exhaust is log manifold, and after the turbo it is stainless hand-fabricated to the cat case. There is an empty cat case. Then stock porsche 944 turbo from there to the back.

Your borla experience is a bit disconcerting as it sounds very similar to my situation.

If I *knew* the ka-BOOM was not hurting the turbo, I could probably live with it. (If that's the case, what's the worst thing that could happen -- blow out the muffler, right?). But I'm concerned because it is a pretty big bang and that ain't good no matter which way its going.

[Smoothie]

Yeah my old Honda Accord CVCC used to do that. I'd park and quickly exit the area and watch for the reactions of strangers in the parking lot when it went off. It made a crazy loud bang like a shotgun going off.
I'd suspect the microfueler and cold start valve too, but I wonder if there's another contributing factor like a hot spot. Maybe try cooler plugs?

That old Accord CVCC (Combined Vector Combustion Chamber) had additional small combustion chambers with their own small intake valves built into the head (one for each cylinder). A richer mix was sent to these small combustion chambers to start combustion which spread out to the larger combustion chamber in the cylinder (the spark plugs screwed into the head positioning them within the small combustion chambers). A leaner mix was sent to the larger combustion chamber. The combined end result was a leaner running engine. That meant a hotter running engine as well. Long story short - the lean and hot running of the CVCC engine probably generated some hot spots that in conjunction with some additional fuel delivery after shutoff, caused the loud afterfires.
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"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

[Paul]

Got ya!

It's Compound Vortex Combustion Chamber!

I used to work on them, I remember one that belonged to Rhonda, a PSU coed.....

[Smoothie]

No prob - you can have that one.
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"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

[Smoothie]

..or maybe it's Compound Vortex Controlled Combustion? - http://hubcap.clemson.edu/~wstef/engine.html
http://www.luk-korbmacher.de/Autos/randt.htm
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"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

[Paul]

CVCC
CVCC is a trademark by the Honda Motor Company for a device used to reduce automotive emissions called a Compound Vortex Combustion Chamber. This technology allowed Honda's cars to meet the 1970s US Emission requirements without a catalytic converter, and first appeared on the 1975 ED1 engine. It is a form of stratified charge engine.

Honda CVCC engines have normal inlet and exhaust valves, plus a small auxilary inlet valve which provides a relatively rich air / fuel mixture to a volume near the spark plug. The remaining air / fuel charge, drawn into the cylinder through the main inlet valve is leaner than normal. The volume near the spark plug is contained by a small perforated metal plate. Upon ignition flame fronts emerge from the perforations and ignite the remainder of the air / fuel charge. The remaining engine cycle is as per a standard four stroke engine.

This mix of rich mixture near the spark plug, and lean mixture in the cylinder allowed stable running yet complete combustion of fuel, thus reducing CO (carbon monoxide) and hydrocarbon emissions.

The initialism, "C V C C", is probably derived from the name of the Honda Civic car in which these names were used.


Advantages over previous stratified charge engines
Honda's big advancement with CVCC was that they were able to use carburetors and they did not rely on intake swirl. Previous versions of stratified charge engines needed costly fuel injection systems. Additionally, previous engines tried to increase the velocity and swirl of the intake charge in keeping the rich and lean mixtures separated. Honda was able to keep the charges adequately separated by combustion chamber shape.


References
Setright, L. J. K. (1975). Some Unusual Engines. London: Mechanical Engineering Publications Limited.

[Paul]

http://www.edmunds.com/insideline/do/Features/articleId=68272

For 1974, the Civic's engine size grew slightly, to 1,237 cc and power went up to 52 horsepower. In order to meet the new 5-mph bumper impact standard, the Civic's bumpers grew, as did its overall length, which was now 146.9 inches.

The CVCC (or Controlled Vortex Combustion Chamber) engine debuted in 1975. Offered alongside the standard Civic engine, the 53-horsepower CVCC engine displaced 1,488 cc and had a head design that promoted cleaner, more efficient combustion. The CVCC design eliminated a need for a catalytic converter or unleaded fuel to meet emissions standards. (Nearly every other U.S. market car for this year underwent the change to exhaust catalysts and the requirement to use only unleaded fuel.) Due to California's stricter emissions standards, only the Civic CVCC was available in that state. A five-speed manual gearbox became available this year, as did a Civic station wagon (only with the CVCC engine), which had a wheelbase of 89.9 inches and an overall length of 160 inches. Civic sales topped 100,000 units for this year.

[Smoothie]

Interesting - yet another iteration of it claims it's Controlled Vortex Combustion Chamber - http://www.edmunds.com/insideline/do/Features/articleId=68272
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"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

[Paul]

D*mn I can't get that song out of my head, every time the CVCC showed up, we all sang "help me Rhonda".