Deceleration Popping

[Superman]

Many of you knwo about Superman's inane questions in the gray area between philisophy and motoring, and some will recall our discussion about popping in the intake manifold when the mixture is too lean. I still don't understand this phenomenon, but have a new question.

What causes the popping in an exhaust system due to an exhaust leak. it is widely known that exhaust popping during deceleration is caused by an exhaust leak. What is burning in there, how does it catch of fire, and what does the exhaust leak do to cause this?

Also, is is possible that my broken head stud is causing this popping during deceleration?

------------------
'83 SC

[Superman]

Are you guys a bunch of dummies?

------------------
'83 SC

[twin plugged targa]

wish my car made any kind of sound at the moment...... booo hoooo!

[Pillow]

Beats me? I would like to know too.

Maybe uneven back pressure? Then again race cars have almost no back pressure and do not backfire.

I have no idea why fire would blow out the back though. Always has perplexed me.

Someone out there must have an idea?



------------------
Adrian Pillow
1979 911 SC
1966 VW Microbus
PCA - Peachstate Region

[Superman]

Guys, Adrian wants to know about this, so whoever knows.....

Added question: Will badly worn exhaust valve guides have this same (popping) effect?

------------------
'83 SC

[Superman]

I'll swallow the bait, Noah (I don't care if people find out that I'm an idiot). Perhaps longer strokes do wear rings quicker, but maybe not by much. You see, I find it almost unbelievable that engines survive trips of 100 miles. In that time, those rings will scrape long the cylinder walls more than a quarter-million times.

You see, I subscribe to the theory that engines wear primarily at startup. Once the lubrication suystem comes up to pressure and an oils coating is on all wear surfaces, I believe metal parts do not touch each other. If they did, we'd have to rebuild engines weekly.

Now then, what is exploding in my exhaust pipes (unburned fuel, I suspect), how is it getting in there, and how is it getting ignited? Dazzle us, someone.

I'll bet Roland knows this one.

------------------
'83 SC

[island911]

I'll take a stab.
I'm thinking a sudden timing retard along with lots of fuel flow (coming off the high rpm) not enough air to match the fuel, and you have really hot, *partially* burned fuel seeing some more air to party with in the exhaust. . .due to the leak.
All in one run on sentence.

------------------
'81 Platinum Metalic SC Coupe

[Bob W.]

I believe the biggest wear factor with longer stroke engines is not the farther distance that the piston travels but the accelerated piston speed that occures with a long stroke engine. I did mention this on a past post.
Bob W.

[Superman]

Thanks, Island. I hadn't thought about the extreme air vacuum, and the ignition retard. Sounds reasonable.

You other guys: Island is smarter than you, I guess.

------------------
'83 SC

[Early_S_Man]

Super,

If you limit the discussion to exhaust leaks near the cylinder head, or that vicinity, say within 18" to 24" or so, since other leaks, beyond that range (or beyond the 'collector' anyway) probably do not cause popping ...

Then, typical broken studs or welds, cracks in headers, etc. near the head can allow intake of outside air do to the 'reversion effect' which normally affects intake tuning, extraction, etc. ... i.e., a 'suction effect' due to exhaust pulses from adjacent cylinders causing a negative pressure that can ingest outside air in through the opening of the 'leak' ...

If you wonder what 'intake tunining' has to do with an exhaust leak, well, the reversion effect from the exhaust system pulses in engines with extremely radical racing cams (by Detroit standards) or of the 911S, 906, and RSR variety ... extends all the way backwards to a 'reversion fog' of fuel-air mix above the carburetor horns, and is a factor in the design or usage of extended booster venturis in 46 IDA carbs, and tall manifolds on 906 engines! In fact, reversion pulses are so troublesome to the metering plates of CIS fuel systems, that cam overlap is drastically reduced to minimize flutter of the plate/disk!

But, even in CIS engines the reversion effect occurs within the exhaust system, so a leak at the exhaust port/heat exchanger junction, or in one of the exhaust runners prior to the collector, or even at the cylinder/head junction 'CE' gasket has the potential of being affected by reversion pulses from adjacent cylinders 'pulling in' air.

Noah,

Your ring wear question has too many variables, such as the metallurgy of the rings, cylinder wall composition, oil jet squirters, etc. Many people ask why early 911S don't wear out faster than 'T' engines, but different ring materials are used, so conditions are not the same, and comparisons in piston speed are not the only factor at play.

As an interesting corollary to your 'stroke' question ... why did most, if not all of the new 4-valve engine designs of the late '80s choose undersquare bore/stroke ratios ... VW, Mercedes, Honda all are undersquare! Obviously, improved metallurgy played a factor, offsetting any wear concerns with high rpm operation of long-stroke engines!

------------------
Warren Hall
1973 911S Targa

[Doug Zielke]

Super,
Idle mixture seems to have the most effect in my car as to decel popping in the exhaust.
Lean idle mixture = more popping
Rich idle mixture = less (or none)
This effect shows up on my older (with carbs) m/c's as well.

Until I took out the cat and installed the new free-flowing muffler, I never heard as much popping. Now, if the idle mixture is on the lean side, I can readily hear it.

------------------
'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
The Blue Bomber's Website

[Jim T]

I agree with Doug, at least for CIS 911s, most of that popping on decel seems to be mixture related, from what I've seen, including on SCs that I've owned.

[chuckr]

My 83SC had the exhaust popping thing.It turned out to be two bad intake boots.

[Superman]

Whoa, Warren. I guess I asked for it! I think you're pointing out that a leaking cylinder/head junction could suck air and cause the popping, since the pulses can go all the way to carb horns. I have seen the spooky pulsing fog in race carburetors, so I think I know what you mean. I did not leave my face over that carburetor very long though, as I've learned my lesson about holding your face over carb horns.

Chuckr: Are you sure your popping was in the exhaust? Conventional wisdom says that intake leaks (lean mixture) causes popping in the intake, not the exhaust. Exhaust popping, according to the ancient grease monkeys, is caused by exhaust leaks.

My intake boots were changed a year ago. My CAT lots it's guts a couple of weeks ago, which makes the car sound louder, better, but the popping is much more audible. Richening the mixture does not eliminate this, or even control it well. My mixture is richer than spec, and the popping is plentiful.



------------------
'83 SC