GREGGPENN

After taking my 89 ride to Superior, their diagnosis was nothing wrong.

Background: I went to a 2.5" front "Y" (no cats) and hiflo main cat. The "catback" was a 2.75" side effects side pipe system. After making this change, my complaint was SEVERE exhaust popping during warm-up. After warm-up it smoothed out. I also reported overly rich smelling exhaust.

My first step was to put a scanner on it. I got no codes.

The I started looking for a ECM tuner in K.C. Didn't have much luck finding one, and a tuner on the forum said the fuel tables were fine so something else had to be wrong.

Officially, Superior said to put the front cats back on, muffler back on, and run the exhaust to the rear. This would cure the popping, noise, and smell. Big help eh? The did say that all sensors were working normally and that compression was fine.

After getting it back, I found what looked like a disconnected vacuum line. Can't figure out where it goes so I plugged it. (There's 2 canisters in the front left corner. One is spherical. The other is shaped like a coffe can. The loose line is in a T connected to the coffee can. Where does this go?)

Anyway, plugging it improved the idle somewhat and I think it helped the smell. However, the popping persisted.

Saturday, I had front hiflo Magnaflow cats put back in the front "Y". The exhaust was a triffle loud anyway cause it sounded like headers with glass packs. (Really sweet at idle and partial throttle, but race car loud at WOT).

Now, it still sounds throaty, but the roar is down to a vroom. More importantly, the low resonance is gone as is the popping.

The MOST interesting thing is the car didn't want to run immediately after installing new front cats. It took 3 tries to keep it running. Then everything was fine. My hunch is the computer had to adjust for the change in exhaust. Why you ask?

Cause now it doesn't pop like before. I do hear an occasional pop and it's certainly still loud enough to hear ANY that occur. So, something changed. Either the O2 sensor reads differently now, the backpressure changed the flow, or both.

You might suggest the front cats "eat" the remainer of the unburnt fumes but during open loop air (from the air pump) is shooting directed into the manifolds where the popping was more likely occurring. So I'd disagree.

So, I'd have to conclude the stock computer system (configuration) is unable to adjust for a wide open exhaust system. If you experience this problem after dumping cats and opening up your exhaust, I recommend going directly to a tuner. If anyone can cure this problem, this would be your only hope.

Had I not already decided to knock the sound down a notch and get it back to "legal" by the book, that's what I would have tried next. Then, as a last resort, I would have installed a MSD Ignition.

But I don't have to do that now after putting front cats back on. Can't say I feel a change with/without them either so I don't think performance is an issue. I will also say that given the option of dumping the midcat or front cats, I'd probably go with just front cats next time. That's because I feel a large, unobstructed front pipe section causes the most cabin resonance on a vette. That's spoken as someone who's on their 3rd configuration. It's also an observation of WHERE I thought the resonance eminated from. This is to say that a majority of the resonance came from the front "Y" area vs. the pipe openings next to me.

The final lesson to be learned concerns sound levels and what you might be shooting for as an exhaust modifier.

A) Open pipes w/o cats would be loud at all levels.
B) Adding a cat and a MILD muffler (glass pack) is mellow at partial throttle, but LOUD at W.O.T.
C) If you want to eliminate cats (L98), leave the fronts. Or buy a 2.5" pipe with cats.
D) Front cats with a moderate sport muffler sounds similar to a sport Mustang.
E) Front cats with two mild mufflers (e.g., glass packs) sounds like D.
F) No cats with stock mufflers sounds is quieter unless hard throttle. Still doesn't get very loud.
G) Cats, glass packs, and milder muffers quiet about the same.
H) Targa mufflers are about the same as glass packs.

Hope this helps anyone setting out to modify their exhaust system.

gp

P.S. If you're old system's not trash, save it. You might find you want to reinstall all or part of it. Even if temporary, being able to see what different combinations can do is very helpful is designing the perfect SOUNDING exhaust.

bogus

this is most interesting.

I have NEVER heard of the L98 popping because the pre-cats were removed. Infact, 1985 L98s didn't have pre-cats.

I would suspect an exhaust leak existed that the repairs found... some fresh air entering at the joint for the front y to the main cat would really amplify that...

AORoads

Very interesting and thorough write up. Thanks! Glad you got it the way you want. Except for that pesky little open vacuum line. Still wondering where it goes. Oh well, it all works, that's what counts.

bogus

Greg is VERY thorough... I am sure he will find the home for that little hose!

Midnite 88

Glad you got your problem fixed. I get a nice gurgle on downshifting out of my Targa's but, no popping like a backfire.

E) Front cats with two mild mufflers (e.g., glass packs) sounds like D.
G) Cats, glass packs, and milder muffers quiet about the same.
H) Targa mufflers are about the same as glass packs.

BTW, what is D.?

bogus

D is a sport Mustang (I assume that's a 302 with pipes).

CentralCoaster

Mine pops. Single high flow cat, then bare pipes. It sounds pretty hairy! I've since learned how to control the popping/gurgling by how I decel and clutch it. I can really make some kablooie noises if I try, or keep it quiet.

I'm still thinking that your popping is still there, you just don't hear it with cats/mufflers in place. I've tried pretty hard to seal up my exhaust leaks, but who knows.

GREGGPENN

I really, really, REALLY, REALLY, REALLY don't think the popping is still there. I am POSITIVE I can still hear it. I should also say that not ALL of the popping is even gone -- just 98% of it! Also, what's left is milder popping (not to be confused with gurgle).

The only other thing I can add is the dealership said the O2 sensor read a little "weak" -- though still in spec. Still, I didn't replace it until two days AFTER the install and the popping started immediately. Don't think the O2 sensor was it.

There's a remote possibility it's the original ignition wires/cap. I finally got that pesky 3rd torx out last weekend. (I used Bogus' dremel slotting idea. Thanks!).

That was the day AFTER I added cats back in. I spent the rest of the day finishing the hood fitting and removing the remainer of the weatherstrip. Yesterday, it went to the bodyshop for paint -- so I won't be able to put the new wires/cap/rotor/coil on 'til after it comes home. But, related to this thread, the 20yr tech @ Superior said zero chance that would have cured the popping anyway.

I still think the answer lies in the computer system.

Gregg

BTW: "sounds like D" means it sounds like my option letter "D". Remember, I lettered (vs. numbering) my combinations! And, that is to say D & E both sound SIMILAR to a sport Mustang. (Man, I really should be careful about saying a vette sounds like a stang! I must be

GREGGPENN

An excellent point. I have heard of others complaining of popping. CC's post gives evidence of that. Has to be a difference in fuel tables, computer system, sensors or something computer related. (Wonder if the air pump is different on 84/85?)

I was under the car during the cat install. I did some adjustments to my side pipe mounting. I asked about a leak before we started and when complete. There was no leak and the shop did not tighten front or rear connections. They ONLY cut in and installed the MAGNAFLOW cats.

BTW: Since they pulled a boner and thru my old front pipe away (even though I asked for it to be saved), I got free cats/installation! I couldn't get the old exhaust home with my vette and told them I would return to get it with my Jeep. (I did get my rear "Y" back. And, it has some sweet 3" dual stainless slant tips I'll sell one of these days).

gp

bogus

my LT1 has always popped a bit... I think that GM even had a TSB out for it, but no fix.

And it pops even more with the smoother bore Corsa exhaust.

C4DC

same with my LT4

tequilaboy

My take on exhaust popping:

Car is running rich prior to deceleration, leaving unburnt fuel in exhaust.

During decleration, fuel is cut off (DFCO). The engine is now pumping fresh air into exhaust. When DFCO conditions are no longer met, fuel injection is resumed. Hot exhaust gas then mixes with previous volume of air and unburnt fuel.

Heat, air and fuel create favorable conditions for combustion and popping occurs.

The mixture prior to the deceleration, the effectiveness of the cats, and the DFCO tuning and possibly egr and air tuning are all contibuting factors. Leaner mixture and/or better cats less unburnt fuel, less popping.

Interesting note: After installing my Comp 503 cam, no more exhaust popping. My assumption is that due to the overlap there is not enough fresh air being pumped into the exhaust anymore during DFCO.

Since I don't have any popping now, I can't really perform a test with disabling DFCO tuning to verify. Anybody wanna experiment in this area?

GREGGPENN

Gotta agree with that. But why too rich? Tech says fuel tables are correct? Also, it doesn't pop until opened way up. Seems like it should be too rich and pop even with front cats on it.

First para implies popping doesn't occur until resumption of fuel. My popped when letting off exhaust (between shifts and decel). And, the popping was IMMEDIATE. No time for injection to cease/resume.

Since air is routed directly into manifolds during open loop, popping SHOULD occur there. And, that's WAY before the first set of cats. How could they possibly eat up the extra fumes in the manifold. They couldn't.

I've also heard that monitoring (e.g., via O2 sensor, etc) doesn't commence until closed loop. So, the computer and mixture should be "static" during warm-up. In that regard, it shouldn't be the computer. But, with air pumping into the manifold during open loop, it CAN'T be the front cats, right?

Valve springs and back pressure could be debated, but the difference is night/day. The amount of back pressure from hiflo Magnaflow cats should NOT be night and day.

Kinda brings me back to fuel/ignition. And, that means computer.

If a cam helps the problem, it's probably letting more air into the mixture and leaning it out. Did you increase fuel pressure?

CentralCoaster

FWIW, I have no smog pump. The air tube on the cat is capped.

I get popping during normal driving on shifts. Its worst(best) when decellerating in gear then pushing the clutch in.

I also get 27 mpg on the highway, and the car runs excellent at the track, I doubt its running rich in open or closed loop.

I'll pay more attention to see if its worse during warmup or not. How long does it take to get to closed loop? A few minutes?

My O2 sensor is maybe 2 or 3 years old at most.

Red Tornado

check your base timing at the distributor as well......make sure its not retarded by a few degrees or more.

CentralCoaster

You lost me there man.

GREGGPENN

That is correct. On mine, you can even hear a difference in the exhaust when it changes! It gets smoother and slightly quieter.

On a 70 degree day, mine takes about 3-4 minutes.