backfire = bad?? help please

rtoms · 05-05-2006, 08:20 AM

so yesterday early morning i installed the fabspeed cat bypass(premuff) and the muffler delete, it sounds great, little loud but its fun. my question is, the heads were redone an gone through recently, and he said that i will be damaging the valves with the backfires??? any help thanks a bunch
tom pearson

rtoms · 05-05-2006, 09:59 AM

bump... anyone?

RoninLB · 05-05-2006, 12:42 PM

Muffler popping, detonation, misfire, etc are all different.

set up a signature w/engine info so readers will know what's going on.

afaik... running an engine bare without any exhaust will burn exhaust valves.

rtoms · 05-05-2006, 01:32 PM

its an 86 911 3.2 carrera with fabspeed muffler and cat bypass? sounds awsome i dont wanna take it off, but if its hurting my engine off it comes

strupgolf · 05-05-2006, 05:04 PM

rtoms, I also get a backfire when cruising about 2000 rpm: 86 3.2 with a flowmaster, and a cat bypass. I just changed back to my original chip(from a High Perf. one) and it does'nt run as well. I'll be interested to hear from others on this.

Steve W · Steve W's Garage

Your part throttle fuel mixture is too lean, as a subsequence of deleting your muffler and cat. You need more fuel.

rtoms · 05-05-2006, 07:43 PM

is this hurting my motor? how much to add more fuel, which chip ect? thanks tom

Steve W · Steve W's Garage

I think the first question should be, are you planning on keeping the muffler delete pipe? Everyone I have known that had installed one thought it was fun for a couple of weeks, then went back to a muffler because they thought the noise was too much.

rtoms · 05-06-2006, 03:06 AM

i actually really like the sound, i just dont like driving the car if the exhuast ( or lack of) is hurting the car...

Rick V · 05-06-2006, 03:07 AM

With the exhause system you now have all you are running is "straight pipe" Your back pressure is nada. I don't care what you do to your fuel or timing you ARE going to backfire, most of all when you lift off the gas. You are also killing your lowend torque. Systems like you have are designed for high "R's".
I did this on my SC for about 30 min. It was fun to see the looks on peoples faces when I went by, but way too loud. As Steve W said Most folks replace one of the units back to stock.
IMO if you want the sound and healthy back pressure, put your factiry cat back and keep the muffler by-pass. You will have what the engine needs, your backfires will be almost non existant (not gone) and it will still be police atracting loud.
You can run the cat bypass and a muffler setup, You will keep the law off your back, and it will only be rude when you hang foot.
Anytime you backfire you are taking a risk of damaging something.

rtoms · 05-06-2006, 05:02 AM

thanks for the heads up, im gonna contact fabspeed and see what they have to say about it? it sounds like most likly i will be returning to put the stock cat back on with the muff bypass, thanks for the help and more help and experience is appreciated

tom

rtoms · 05-06-2006, 08:15 AM

im gonna try to get my buddy to take pics tonight, he was behind me last night, and said there were burst's of blue flames comin out when i was letting off the gas and shifting

thumbs up on flames thumbs down on it being bad for my motor

North Coast Cab · North Coast Cab's Garage

Interesting post. I removed my Fabspeed euro-bypass, Steve W chip and custome dual out muffler(still for sale if interested). IThe car now idles lower, around 650, and I get some backfiring upon deceleration. I'm trying to get some equipment to check the mixture.

John

rnln · 05-06-2006, 08:44 PM

RoninLB,
mine has 88 engine, low miles, all stock except Borla muffler. Once in a while I have back fire when shifting 1 to 2, or 2 to 3 when quick shift and let off the gas VERY quick. Slow shift, won't happen. And too agressive on the gas, won't happen either. If I want to reproduce it, it's hard.
Can you diagnosis this?

RoninLB · 05-06-2006, 10:01 PM

Quote:

Originally posted by rnln

88, low miles, stock Borla.

back fire when shifting 1 to 2, or 2 to 3 when quick shift and let off the gas VERY quick.

Slow shift, won't happen.

If I want to reproduce it, it's hard.

I never became involved in your model's induction or exhaust so I'll defer to those above.

But combustion heat, speed, and ability to ignite is partially controlled by the porportion of air and fuel. A leaner deceleration mix will heat up your muffler more than a richer mix. A leaner faster deceleration mix will ignite fast in a hot muffler.. assuming your exhaust gases are dirty enough to ignite.

What to do about the situation specifically is beyond me.

Steve W said it way above...
"Your part throttle fuel mixture is too lean, as a subsequence of deleting your muffler and cat. You need more fuel."

You need carbs

Steve W · Steve W's Garage

Slow shifts allow the O2 sensor to try and adjust for the lean mixture. Fast shifts and the O2 sensor isn't fast enough to compensate. Stock programming is already on the edge of lean for emissions with the stock muffler and cat. Releasing exhaust backpressure from sport mufflers, cat bypasses, or deleting mufflers in general will make a lean condition worse. Lean mixtures are hard to ignite in the cylinders, so they get spewed out as raw fuel in your exhaust. With enough raw fuel in the exhaust, ignited through one of the open exhaust ports, you get a explosion in your exhaust with flames belching out your tailpipe.

rnln · 12:04 AM

Ronin, Steve,
First Thanks.
Second, more question.... thanks in advance first.
So far.. mine is a little lean? But I it smell so much gas. Whenever I start the car in front of my house, someone come out and close the door (if I forgot and leave it open). Smelling gas at the exhaust pipe is a sight of lean, or rich?

Now question on the fix. If my car is running lean, should I adjust more air in? Or should I adjust less fuel? I am not sure how to adjust these on this car yet, but I want to know for future fix since we are here.

Ronin,
Why carb is better? I used to have a carbs car and it's killing me. Adjust those screws all night long after work, almost everyday during the time owning it. Gas "spray" everywhere I go. People cover their nose and roll up windows all the time. proximately 11-12 MPG... arghh.. I just hate carbs, I know it's because I don't kown how to adjust them, but isn't computerized better?

Thanks.

rtoms · 05-07-2006, 03:44 AM

i think the carb comment was a joke, i assume an air filter with higher flow and a chip might help add the proper fuel ratio that i need, as well as possibly adding the stock cat or muffler

strupgolf · 05-07-2006, 04:28 PM

Its starting to make more sense. My car also smells bad at start up and the house will smell as well. I thought it was rich burning, but now I can see why its lean, the engine isn't burning the fuel, spewing it out the pipe. Whenever I drive it, I always get the sensation of raw fuel odors from it. Now, who do you adjust it to richen it up? Thanks for any help.