JBurer

Sorry guys - I guess I just can't let this die.
Spoke with a head technician at one of the most well-respected Porsche racing shops in the Southeast. He heard the pops coming from this particular car and confirmed it was not normal (as did the builder of the car - a different, also well-respected shop).

Seeing turboholic's reply, I went a step further, asking the person referred to above about CIS cars and whether repeated popping on deceleration is normal for that fuel delivery system. He's worked with them for 30 years - he should know, right?
He responded, stating that an occasional pop with an aftermarket exhaust is normal and typically related to an improper seal (air entering into the exhaust)... but that repeated popping is not normal for any system.

Short of pulling technical notes on how the CIS system functions, I can't prove that the injection system is shut off on deceleration as I suspect. That said, every other car I've ever had experience with has a mechanism to cut fuel on deceleration - including the old mechanical injection systems on the '70's Alfa Romeos.

I've done my best to bring forward factual information from reliable sources... up to you to make the call on whether you believe it or not. Looking back over the older messages, it seems like Staffan fixed his problem by replacing the CBV.

al 965

Mr. Burer you state these facts with the pretense of "modern fuel systems". You need to do more reading because the CIS fuel systems in our cars has nothing to do with "modern fuel systems" in it's stock conf. Good luck.

cobalt

The CIS runs one liter of fuel through the system every 30 or 60 seconds Can't recall which it is but I believe it is 30 seconds. I can confirm that later. Their is a switch that senses when air flows past the plate in the CIS. As you crank the engine it draws air in and lifts the air flow plate this activates the fuel pump. The fuel pump activates continuously once the air begins to flow. The pump does not shut down until the ignition key is shut off or the engine is starved of air allowing the plate to close. Hens the name CIS or Continuous injection system aka K jetronic.

It has been a while since I had to recall this info so this is a bit rudimentary explanation. I have been around Porsche turbos and some top mechanics in the field for 30+ years. This is the first I have heard anyone say it shouldn't pop on deceleration. In fact on the contrary have had some comment about it being one of the things they love about the earlier cars and listen for it to hear if it is tuned properly. Of the dozens of 930's and 964 turbos I have driven or heard they all pop some. In fact I have always been told if it doesn't pop a little the car is running too lean. If it pops too much it is running too rich. With stock exhaust the popping is muffled mostly and can barely be heard from inside the car if you eliminate the cat or the muffler it becomes more pronounced.

I think If Stephen, Rick Deman, viper bob, Dan Jacobs or any others chime in that some popping on deceleration is expected from a well running engine. This has nothing to do with seals or anything else.

You might find that the particular popping you are talking about might be more extreme than the average car and therefore his response, if he is truly adamant about none of them should pop than I would seriously question his knowledge of the Porsche CIS turbo cars. I have met some mechanics that have been around these cars for 30 years and can be quite clueless.

I can assure you the info you have might be correct for other cars but is not correct regarding the mechanically injected Porsche turbos.

tjr

I know of many 964 turbos that pop on the overrun, usually once the various restrictions in the exhaust system have been removed (primary and secondary muffler/cat). The more straight-through the system the more pops you get. Your well-respected tech may be referring to exhaust system that don't do this - the standard muffler and the B&B to name two.

There are lots of so-called experts out there, with lots of experience, but that doesn't make them right all the time...

turboholic

I fiddled around with my CO settings in the past and one thing I have noticed is that the richer the mixture setting is the less popping I get. If I set the mixture to 3% CO or leaner I have lots of nice pops and bangs on overrun. I run decat with stock muffler.

The 930 and 965 pop by nature, if the car doesnt pop surely something is wrong. I saw plenty of those cars around here and all of them did that popping sound, you just simply gotta love it.

cobalt

I might have it reversed but IIRC when I leaned mine out a little to keep it safe the popping increased but that was 3 years ago, age has a way of affecting your memory.

911addict

Yes, when a garage serviced my straight through 965, the pops were less and the throttle response sluggish. He told me it was set at correct Porsche spec. I told him to lean it out, like I knew it was before. Guess what? Quicker throttle reponse and lots of popping!

lobofraggin

So can I postulate:

Car popping on overrun like a machine gun = engine tuned properly?

brilliant cars, aren´t they?

K.

Mine's making lots of and very noticeable pops because of a swanneck instead of a muffler, it's a glorious sound I don't want to miss (especially in tunnels)!

DonE

On a CIS car, where you don't have the ability to adjust fueling through the RPM range, popping might be cool to hear, but the lean condition it indicates tells the engine isn't tuned correctly. It's another reason I went for EFI. Unless you know how to adjust your WUR, having the car pop on decel means its lean through-out its operating range. I tune my car to be about 14:1 on decel to keep just enough gas in the cylinder to cool the cylinder but not enough to the point where the heat remains due to a lean condition (or, popping). On the other hand, I've seen where CIS cars were made over-rich to compensate for the poor fueling capabilities of CIS during WOT, but at the expense of being too rich on decel too. This puts too much fuel in the cylinders on decel which washes the cylinders (possible premature wear on cylinders and rings) and eventually dilutes engine oil (VERY bad for bearings - as in spun - and other such things).

In a previous post, I read where someone commented that they enjoyed improved throttle response when the car was leaned out. You can also get that feeling with more timing, but it does not mean its better ultimately or the correct way to tune.

I don't agree popping is normal - it just shows less than optimal tuning.

Flame away gents...

cobalt

There is no doubt CIS is old school and has its short comings. Although popping on decel is normal. There is no arguing otherwise. It might not be optimum but it is the nature of a CIS turboed cars that don't have a factory exhaust. My car is tuned to its optimum level for CIS. I monitor my AFR's using a very nice gauge I have installed from In Your Face Gauges. It does not pop excessively but it does POP. Yes if i were to convert my car to EFI the sound would no longer be there and it might be a better system but the CIS has proven to be adequate for many years. These cars are fast enough and most of us are not racing them just street driving so there s no harm done.

BTW my car runs around 12:1 on WOT 14:1 on decel and i still have popping on decel.

turboholic

100% agree with Cobalt

911addict

To add, lots of 965's pop more than you know, you just don't hear them all the time due to silencers and cats. Remove these and most will pop.
PS: My car had over 90K miles when I sold it, and no sign of an engine rebuild, so can't have been that badly tuned :-)

Vince964T

125 000 kms, tons of track time, air fuel ratio is good up to max rpm, leak down test done this summer was perfect. Pops like there is no tomorrow on throttle lift and has been doing so since I removed the muffler and put a race cat in.