What causes backfiring?

Tony Radford · #1 (**permalink**) 05-03-2003, 05:58 PM

All of a sudden, my car has started backfiring almost everytime I change gears (when I'm winding her up). I just finished changing out my header gaskets to copper as I had a leak. I also lost one of the threaded plugs that go in the heads, which I replaced this morning. It seems a bit coincidental that the backfiring. Performance is fine (she idles and winds up smoothly). But it's a bit embarassing when I get on it and I become a "drive by shooting" suspect. Any ideas?

Excaliber · #2 (**permalink**) 05-03-2003, 06:15 PM

I assume your back fire is when you let off the gas to change gears? On deceleration of the engine? Many things can cause back fire but a couple of the more common ones are:

Exhaust leak and/or to rich on fuel. Some will say "to lean" can cause backfire but this is generally due to exhaust leak introducing fresh air (additional oxygen) in the flow of spent gases causing additional combustion. Combine this with a "rich" exhaust mixture and you get a BIG back fire going!

Ernie

Roscoe · #3 (**permalink**) 05-04-2003, 08:15 AM

Here is the Demon carb tech sheet. No matter what carb you have some of it will apply.

http://www.priveye.com/dls/Demon2.jpg

Also check to see if your headers have loosened up.

Roscoe

BANDIT 1 · #4 (**permalink**) 05-04-2003, 08:34 AM

I would say that if the only thing that has changed is the exhaust repair you did, than I would have to think that it is some sort of exhaust issue. When I lost 7 out of the 8 nuts that hold on one of the sidepipes to the header, the car acted exactly how you described.

Lew the taxman · #5 (**permalink**) 05-04-2003, 10:52 AM

Bandit 1

The same thing happen to me. I lost 7 bolts & nuts out of the right side, and 5 out of the left. All in less than 4,000 miles. I had back fire like you wouldn't believe.

Lew

Jahred · #6 (**permalink**) 05-04-2003, 11:54 AM

OK, you guys have me real curious. By "Backfiring", I assume you are talking about the "ignition" BANG! of unburned fuel at the exit area of exhaust pipe???

My 514 has always done that.........when I rev it and let the car back down to about 3000 RPM.....I always understood that was just unburned fuel that reignited when it hit a "fresh supply" of oxygen at the end of the pipe.......

I am curious though, as of late I have noted excessive backfiring on the right exhaust pipe.......but little or none on the left.....which made me a bit nervous.

Not with the machine at present, but think I will definately check for lose bolts? But clarify if you can.......if "post-ignition" is at rear of exhaust pipe, this is not really a loose bolt issue, I do not guess........are we talking about the same thing???

I like the flashes of light in the dark! But I'd like to think they are supposed to be there!!

Has anybody used ignitors like you see on some Hot Rods?? Is there a benefit to those or just for show? Wonder if they would cut down on exhaust fumes?........my contacts get a little owly after a drive in the beast..........

Excaliber · #7 (**permalink**) 05-04-2003, 12:13 PM

Loose bolts on the exhaust can cetainly be a cause of backfire! Now, no matter HOW tight your exhasut system is some cars will STILL have some backfire, which you could consider "normal".

My Excal has a "nice" backfire rumble to it on decelleration. I love to down shift and hear the rumble/pop pop slowing down for a stop sign. My headers are cracked,,,,,,,I'll loose that great sound once I fix them, lol.

Ernie

Ron61 · #8 (**permalink**) 05-04-2003, 01:47 PM

Watch one of the night Winston Cup NASCAR races and you will see hugh flames come out of some of the cars in the corners as the unburned fuel ignites. I think most cars have a little unburned fuel that goes out the exhaust when you suddenly decelerate from a pretty good speed.

Ron

Tony Radford · #9 (**permalink**) 05-04-2003, 07:33 PM

I went back and re-torqued all the header bolts and the backfiring stopped. Later in the day, I took another joy ride and some of the backfiring had returned. After the exhaust system cooled, I went through and was able to pull a tad bit more on two or three of the bolts. Has anyone else installed copper exhaust gaskets? I'm wondering if I will always have this problem or if they will eventually pull up and stay. It is amazing how little it takes to create a backfiring situation.

poorboy · #10 (**permalink**) 05-04-2003, 07:59 PM

Tony,
No, you will always have to keep check on the header bolts as they will work loose.

Poorboy

Ron61 · #11 (**permalink**) 05-04-2003, 08:19 PM

Poorboy is correct. Also if you have an aluminum intake and heads check the torque on those bolts once in a while. I normally go over both of my cars after any hard drives and about once a month if I am just driving them around and I nearly always find at least one ot two bolts that have backed off a tad.

Ron

Janusz · #12 (**permalink**) 05-04-2003, 08:24 PM

Great information. I have the same type of back fire that occurs when I come off of the throttle after hard acceleration. Always assumed this was due to a rich fuel mixture, but I do have some minor exhaust leakage where my side pipes slip over the headers (not a flanged connection), which may be where the detonation is occurring. Any suggestions for sealing this area (sealant, heat resistant tape)?

BANDIT 1 · #13 (**permalink**) 05-04-2003, 09:45 PM

I would recommend contacting a Cobra engine builder that has alot of experience with Cobra exhaust systems and what setup works. There should not be "backfiring" in any situation, if there is there is a reason. One example is I use to have a 1985 5.0 Mercury Capri (Mercury Mustang GT), for some reason there were no cat. converters on the exhaust system. The car would backfire because the person that had removed the cat. converters didn't remove the air pump. I fixed the problem by routing the air pump to the inlet on the chrome aircleaner and then I had a poor man's turbo.

Tony Radford · #14 (**permalink**) 05-05-2003, 03:38 AM

I never had this problem with the paper gaskets. My problem began when a small piece of the corner of the gasket blew off while screaming down the interstate. I figured the copper gaskets would prevent this from happening in the future. Now I can't seem to get them to seal and stay. Rose Ann Rosannadanna was right, "it's always something".

Roscoe · #15 (**permalink**) 05-05-2003, 06:50 AM

I use copper gaskets and stainless bolts drilled for safety wire.

But there is more than a gas leak that can cause backfiring.

Roscoe

GeorgiaSnake · #16 (**permalink**) 05-05-2003, 07:02 AM

I'm getting ready to install my headers and I'm considering no gasket - just high temp sealant. Any comments from those with experience ? Also what is the proper torque for the header bolts on aluminum heads.

Randy

Roscoe · #17 (**permalink**) 05-05-2003, 07:06 AM

I would not recommend no gasket. Use the solid copper with no sealant. AFR states that you should not torque the intake or exhaust manifold bolts...just snug them up tight.

Since going with the stainless bolts drilled for safety wire I've not had one bolt loosen up.

Roscoe

scottj · #18 (**permalink**) 05-05-2003, 07:44 AM

I use clear silicone and no gaskets. I also use the silicone on the threads and a calibrated "torque-wrist" to keep the bolts tight. For the header to sidepipe flange I use the high-temp sealant since regular silicone will burn out there, again no gasket. No leaks.

Scott

Jahred · #19 (**permalink**) 05-05-2003, 09:11 AM

Bandit, I hate to ask, but what is an "air pump" as you reference it???

I think this converstation about backfiring is very interesting. I am not sure we are getting anywhere, but I think there are some useful answers out there if we work at it.

My Exhaust system consists of nothing more than some custom made headers, and some pretty lightweight steel sidepipes........no mufflers, no cats anywhere here.

So it makes sense to me that there would be backfiring - which I think is a misnormer - at the exit end of the tailpipe.

When I ran with the Porsche club (not in cobra), a few of the really high end cars (prob also in the 600 HP range) would exhibit exhaust flames near the end of the first straight, which is nice and long........nearly a mile.

If the pipes are hot, and all the gas is not burned (I run a Holley 1000 HP)....and those hot fumes hit fresh cool air on the exit end of the pipe, BAMMMMMM!.....

I still think that's natural....but it is not a traditional backfire.....though I don't know what else to call it......

All that said, I would certainly believe that everything else being equal, no backfire would mean better fuel burn, better mileage, more power etc. And as I see my car now backfiring more prevalenting on the passenger side..........I do have to wonder...........Wasssuppp?? Something is slightly amiss, at best.

Question: What is a backfire? Is it a backfire if it is at rear of exhaust??

Question: Is anybody out there running a 514 with 1000 or greater carburation and straight pipes without getting some crackling and bangin when they back the car down from higher revs???????

I kinda like it, but it is hard on the ears and wanna make sure she is runnin right...........

Al Bockman · #20 (**permalink**) 05-05-2003, 09:34 AM

When I put my SPF together I only had 5 days from the time I got the engine until VIN inspection. I used paper gaskets because I couldn't get copper in time to get everything put together. I used ARP stainless 12-point bolts. Sure looked good. The paper gaskets went out after about 200 miles. We were going to Aspen CO with a group of Cobras when both gaskets burned out near the top of Independence Pass. I had left 3 ARP bolts on the road somewhere. Coming back through Eisenhower Tunnel (a mile and a half downhill) the guys with us said I was shooting flames out of the pipes when I left off the gas. A lot of backfiring going on, too.

I got the copper gaskets and used Stage 8 locking header bolts and haven't had a problem since. The Stage 8 bolts have cam-like devices that slip over the bolt head and rest against the header tubes. These are held on with C-clips. No way these bolts will back out. Check them out at www.stage8.com .

Al Bockman