after you rev my car and it is revin back down it pops and cracks, sometimes it sounds decent, and other times it doesnt. I mean its not like a crappy backfire, but well...anyway

my set up is

L98 (1987)
Edelbrock T.E.S Headers
Catco High Flow Cat
Borla 3" Cat Back
Jet Stage II Chip
Ported & Siamesed Plenum
SLP Siamesed Runners
Suncoast Ram air airbox.

oh yeah, this hasnt "developed" it has done it since I put the engine/exhaust in.

 

well that was a heck of a statement.. ..heres my question, what is causing it, what can I do to fix it, is it the high flow cat?

 

it's from the lack of back pressure and yes it is from the high flow cat. the only way that i know to not have them do that is to get resignated tips.

 

What you're hearing is backfiring. Very little ones.

It's mainly caused from running too lean. This causes hot spots in the exhaust. When your RPMs drop, then the unburnt fuel is finding hot spots in the exhaust to ignite itself with. This can also be caused by too little scavenging, not backpressure.

I'm sure you've heard a Harley do that, right? They run the carb lean to get it to do that. But they don't have to worry about a cat burning out.

AJ

 

You don't have any exhaust leaks do you? I wouldn't think a hi-flo cat would cause backfires, just the same way not having a cat doesn't cause it.

 

my rx-7 would do it when running rich


like when I was just showin off I would take off my TPS( car thinks it is at WOT all the time now so gives mroe fuel) and I have always had the dashpot removed.
the car runs a little rich as it is and it would always crack and pop when letting off the throttle
though the exhaust temps on my car are near 1800* or so
just a little hotter then what you guys run

also my maverick does the same thing and it is carbed and I know that it is running rich

just that with a rich mixture you have the unburnt fuel to make the noise

 

I don't think it's from the cat. My friends all stock TPI Camaro except Mac Cat-back does it. He has the stock cat. I always thought it was from running rich and the unbernt fuel hits the "hot spots." He also has the "resonated" tips that came with the cat-back. What's the real truth?

-Matt

 

I LOVE THAT SOUND!!!!!

 

How would i make it run Leaner, I mean with the Ram Air, Plenum, Runner Work, Headers, Cat, and Cat-back, I have plenty of resources of getting air into and out of the engine.

 

I have the same problem, but I don't mind it. its from lack of backpressure on mine.

 

My 92 Z28 convt 5spd does that when cold after I gutted the cat. She also has a Dynomax cat back. I'd imagine it's not enough back pressure on the valves when cold, but who knows?

 

can you explain that one to me?

 

And me.

 

my 91z popped when the revs went down (after i put a full exhaust on). it went away after i put a chip in. the car runs much better now

 

All cars do that, it just you can't hear it with a stock muffler. The other word for it is Backpressure. If you car is doing that that means your car is runing properly. Thats why our cars come with AIR pumps, what the AIR pump does is pump air into the cat and the exhaust to equlize the pressure in the in the tubes so it will decrease poping or stop it completely. You have nothing to worry about thats what a performance systeam is all about. To have some fun, the next time your around people, gun it in first gear and then completely let off the gas, it will **** people off so much and it sound like gun shots. i'll have to take a sound clip of my car and show you how loud it get when i tell of the gas.

La La's :lala: :lala: :lala:
Robert

 

Gee, did I stutter?
After I gutted the cat, the car pops thru the exhaust when the car is cold, sometimes between shifts, and usually if I decelerate using the engine vice the brakes. It does not pop thru the exhaust once it warms up.
Notice I did not say "I'm sure it's because of backpressure on the valves". I think I said, "but who knows".

 

in other words when the car slow down or "creates" back pressure in a high gear. what happening is the motor is trying to slow itself down by pump air into the cylinders by sucking it through the exhaust pipes at the sametime it's trying to push the gasses out creating "popping" or small backfires.

 

"Thats why our cars come with AIR pumps, what the AIR pump does is pump air into the cat and the exhaust to equlize the pressure in the in the tubes so it will decrease poping or stop it completely. "

Not to argue man but I thought it was because of cat technology at the time they had to pump air into the catalyst to keep it from clogging. You could be right I've just never heard that before thats all.

I always thought that popping was from unburnt fuel being pulled through by the extra air still in the pipes and the fuel hitting hot spots. But I don't claim to know everything.

 

Quote by HrdRockA4305

You are right to, that the other prupose of the air pump. when air is pumped into the cat to keep it from cloging, it also put air into the exhaust pipes.

 

To get a backfire three things are necessary fuel, oxygen and ignitionsource. These occur several ways:

1. running lean (or non-steady flow of fuel)
2. an exhaust leak (allowing oxygen to enter the exhaust)
3. ignition misfire (allowing oxygen and fuel into the exhaust)
4. and on vehicles with an air pump the malfunctioning a diverter valve (gulp or anti-backfire valve)
You should check the gulp valve, if this is OK check for exhaust leaks and the ignition system. The check valve is the most likely cause. The valve is suppose to stop the outlet of the air pump from going to theexhaust when decelerating.
Sometimes advancing your timing will help also.

 

man, I love this message board. I've been getting more info here than I have in the past 5 years. LOL.

 

in a basic thing the pops in the exhaust is caused by air/fuel making it into the exhaust and then going boom

now things that can cause that are as jobryan26 stated, though you can also have running rich in there also I would say. well in my car at least though my exhaust temps are around 1800* or so.

one thing that could cause it as you said rob p on your gutted cat thing is that it will cause a lot of turbulence. and when you let off the gas you will run rich for a moment b/c the TB will slam shut and cut the air off but keep the fuel going. so this unburnt fuel will make it to the cat then could get stuck inside an eddie (sp?). and then you prolly will get some air in there from the airpump so now you have some a/f mixture inside that cat and with exhaust temps being high from the mixture starting to lean out ( if I remember right rich right at first then starts to lean out while you are engine braking) it will start to go off.
it has nothing to do with pressure on the valves.


the only thing I can think of that might cause your thing 86FyrBrd is it might be the high flow cat. those things are made to make the fuel go and burn if it doesn't do so in the cyl.

and as a guess here as another thing that can cause it on some cars, though dont think it would be your case, I think if you have a really good exhaust setup and have a really high velocity inside the exhaust it might atomize the fuel a little better and make it more prone to burn with one of the things that jobryan26 said.

 

Last I heard, the AIR tube going to the Catalytic Converter is there too inject air into the Catalytic Converter to help Increase the temperature in the Cat so the Cat will be more efficient. Catalytic Converters work best when they're up to temperature (2200*) I think. Also, I have that popping on the down rev and it started after I installed my SLP Y-Pipe. It's only noticeable when I really rev it up and let off but, in normal driving I don't notice it.

 

I think RXSpeeds latest explanation is what my problem is, not sure of other peoples though, cause if I rev it and bring it down a lil slower it doesnt do it, but if I rev it and let off the pedal totally it pops and cracks on the way down.

 

Yeah, my 72 LeMans does that too when you let off the gas. Its cool sometimes but othertimes annoying. Just a little lean

 

Mike - dont'cha mean rich? Cause I know your cars not running lean not to mention the carbs a little overkill for the motor

 

whoops, i meant rich, why did i put lean?

 

It is caused by a RICH condition....not lean. Mine did the same thing ...when I added the MSD ...ITS ALL GONE NOW...why??? Because more of the fuel is burnt now....

 

YOU PLAY WITH YOUR WANKEL!?!?!?!?!

 

Air is pumped in at the exhaust manifold to help finish oxidizing unburnt fuel to lower emissions. The air is pumped to the CAT to help prevent clogging.

 

I wish I had a wankel!!! haha LOL! Even better yet... an Allison! 1500 naturaly aspirated hp aint bad with no mods.

 

hell ya I do

you would also if you had one

 

I wish I had a wankel!!! haha LOL! Even better yet... an Allison! 1500 naturaly aspirated hp aint bad with no mods

Man, I feel kinda ******* mentioning this, oh well
Me and my grandpa went to a tractor pull one time (hey anything with mad HP is cool to me, and Grandpa is a retired Caterpillar powertrain engineer so we get a major kick outa stuff like this) and there were 2 rigs that were actually running 16 or so cyl Allison engines, probably from old fighter planes or something. One had 1, the other had 2!! Where they got them and how they rebuilt them But man.... you think 4 V8s with 8-71 Blowers is loud you gotta hear one of those *****! They even had from mount centrifugal (sp?) blowers on them, likely straight off the airplanes which needed them at really high altitudes. I wish I had pictures of that.

 

its called BORLA my friend. My friends LS1 w/ borla even does it every borla i have ever heard is like that.... sounds like an expensive set of straight pips if you ask me. I dont get where people think flowmasters sound tinny when there is borla around. the stuff is made well, i just dont like how it sounds

 

if my car is cold, and i rev it up, when the rpms fall, it makes a crackling sound... but it goes away when the car is atan opt. temp

 

Hey 86FyrBrd, I couldn't help but notice that your car is always parked at the texaco two doors down form my dads dealership. I work at Coopersburg Kenworth and I admire your car every time I drive past it, it looks very nice. I really like the color.

 

yeah thats my car, unfortuentally I work there. Have you even been in or seen it up close. Stop in anytime, If I have time I'll BS with ya.

Rite now my car is at the body shop getting the trans re-built so I wont have it till like late monday or tuesday.

 

Yeah, I think I'll stop by and chat sometime.

 

Yeah my Freinds 97 Bird makes a load @$$ popping sound and now that you mention it that car doent burn enoght gas. When he starts it and the exhaust is pointed in the garage you eyes start to burn. Damn thats bad:nono:


>hey rx7speed thanx for putting that rotary pic there. Now I know how that engine works cool!

 

hope its not too late... this happens cuz the stock hei distributor is good to 4500 rpm... if u rev it up to 4500 and let it decelerate, u will not hear backfiring. then when u go over and let go of the gas, thats when u hear backfirin.