What causes backfiring?

[Excaliber]

Air pump:
Driven by a belt on the engine it is used to inject fresh air into the exhaust port of the engine head. Usually right near the exhaust valve. The flow of the fresh air into exhaust chamber is controlled (via the computer on new cars, vacuum on old ones) by a valve on the pump that lets the air flow during decelleration.

This fresh air mixes with the spent exhaust gases and helps to insure complete burn of any residual fuel etc. that would normally pass through un-burned. Pollution control device.

Combined with an EGR valve (exhaust gas recirculation) these two items were the most significant pieces of the early smog control methods. The EGR introduces spent exhaust gas BACK INTO the combustion chamber BEFORE igniton takes place. Thus diluting the fresh air/fuel being introduced via the intake valve.

Obvioulsy this diluting of the fresh air/fuel charge negatively impacts performance!! So why do they do it? To lower the "peak combustion temperature" of the "big bang" to control the formation of Nitrous Oxides, which form a BIG part of the "smog" picture.

The timing may also be automatically "retarded" to accomplish the same thing, lower "peak combustion" temperatures. Now the engine (suffering from all this "tinkering") has to push the spent exhaust gases through a restrictive catalytic converter!

Man, now wonder the cars in the 70's lost a LOT of horse power in favor of "clean air". The SAME PRINCIPLES apply today. The engineering to accomplish it is FAR better and does not impact performance "that much".

I prefer paper exhaust gaskets and several "tightening" routines while the "paper" settles in.

Ernie

[speed220mph]

Here's my $0.02 worth:

The air pump, aka "smog pump", has been replaced by scavenging systems, averting the hp loss from driving the pump. After-burning is not needed on a Cobra, unless emissions stds. must be met.

With either system, air must be diverted from the exhaust to prevent backfires . . . if it didn't divert, then bang, whenever the throttle is lifted. Fuel would keep going for an instant out the exhaust after the throttle plate closed, but air stopped abruptly, allowing a slug of fuel to enter the exhaust port looking for oxygen to react with while still hot and go boom.

Leaks near the head will do the same thing as well as big vacuum intake-manifold leaks. At the exhaust, not only does exhaust leak out, air will leak in and cause a similar backfire.

As for leak prevention at the header/exhaust manifold-to-head interface, or where the suckers bolt up and seal, it depends on head and manifold materials. For headers, or tubes with header plates, and cast-iron heads, I prefer soft gaskets, 12-pt. bolts drilled and safety wired. Copper gaskets would work ok, too.

For aluminum heads, go with soft gaskets. Harder gaskets such as copper will seal, but will Brinell the softer aluminum head, making it tougher to get a good seal once the headers are R&R'd for any reason.

If you're using the more rigid cast-iron manifolds against cast-iron heads, rtv silicone sealer works fine high temp or not. A Winston Cup engine builder friend uses only rtv sealer at the header flange with success.

[Tony Radford]

Roscoe, I don't see how "snugging-up" the header bolts will work. I replaced those little square head bolts with socket head bolts and I'm pulling them up pretty tight. I don't think I'm in jeapordy of stripping out the heads, but those headers are not exactly precision assemblies. I'm considering pulling the passenger side header back off and add more of that expensive sealer I bought along with the copper gaskets.

I'm sure my backfiring issue is directly related to the integrity of the seal between the header and the head. I never had a backfiring problem until I replaced the gaskets, and now it's significant (unless I crank down on the header bolts).

Randy, Bill suggested I buy the paper gaskets and spray that red sealer heavily on both sides, install and wait 5 or 6 hours before firing her up. I wanted something a bit more solid and I hate the thought of gunking up the heads with something that has to be scraped off when the connection is broken in the future. As usual, I probably should have taken his advice ver batum and saved myself the expense and grief. Milling out those copper gaskets was a real bummer using a Dremmel tool and dull router bit. I guess this is one more reason to go ahead and purchase the stroker.

[tommyboy]

Ok , my question has to do with actual side pipe temperatures. I just took my maiden voyage with my SPF 1593 car. Engine is a Dart 351W block bored/stroked to 427cu. in., 48IDA Weber carb induction, complete roller valvetrain and AFR 205 heads. Total timing has been set to 38 degrees advanced, all in by about 2700 rpm. I was extremely careful at initial startup. I set up all the carbs air flows and idle mixtures. Double checked the timing and also pointed fans at each side pipe so as to make sure there was no extra heat to bubble the paint. Anyway the car started up real easy and idled perfect. After 20-30 minutes of run time and no leaks, I shut it down and changed the oil. I then started it up checked everything agian and went for a 12 mile test (shakedown) ride. All seemed well until I got upto around 45-50 MPH and about 2500-3000rpm and started hearing some pretty noticable backfires through the exhaust. So I headed home shut it down and was going to check my header to sidepipe bolts and recheck my timing. The one thing I did notice was that my side pipes seemed very hot where they exit the fenders. Hot enough where I did get a bubble in the paint down low, somewhat out of sight. Needless to say the ride then turned bittersweet. I put my fans back on the sidepipes and that dissipated the heat pretty fast. The overall engine coolant and oil temps were very good. The carbs are setup more on the rich side. Timing and header bolts are ok. So how does one keep those pipes from bubbling the paint? The clearance around the pipes, where they exit the fenders, is good.

[Steve R]

"Back fire" refers to fire coming back into the induction system, which may indicate a lean condition.
Tommy,
My friend's 347 has a temp of around 650deg at idle at the collectors, 900 or so under normal load.
Gents,
Take a look at Earl's Performance Pluming's "Pressuremaster" header gaskets. Summit p/n EAR-29D03A. They have worked well on his Cobra.
Steven

[Tony Radford]

Tommy, that stroker set up sounds awesome. I went through a brief fantasy of putting the quad IDA set up on a 427 stroker that I'm currently waiting on. I was unable to swing the added price of the Weber set up, so that will be a future phase of the Cobra ownership. Please keep us posted as to how it works out after you've solved the header heat issue. My backfiring issue was the result of a small piece of the original paper exhaust manifold gasket remaining on the head. Removing it solved the problem. I've seen previous posts from folks who had the same problem you are having with regards to bubbling the paint at the body cut out area. Good luck.

[Mr.Fixit]

Tommyboy

First fire up for an engine, they very often run much hotter than they ever will again. I have seen ceramic coated headers glowing, only halfway through the cam break-in process. (I think it is excessive heat being created by the piston-ring friction with the freshly cross hatched bore) Anyway, I would always break in cobra motors with fans at either sidepipe exit, and when the headers got too hot, shut it down and resume again after it had cooled a bit. The oil and water wouldn't get too hot during this phase however. If you can shove your hand most of the way between the body and the sidepipe primaries, you are OK for clearance, my knuckles don't fit though.

[Gerry A.]

I'm waiting for my baby to get back from the engine builder and mfg. as I was having the same basic problems you are, the backfiring through the exhaust. Never noticed any excessive heat though and everything on the header/sidepipe side ran perfect with no bubbles. The reason it's at the builder is the engine locked up spinning every single bearing in it.. all the rods and mains, ruining the crank and all 8 rods. In my case and I'm not sure it's yours but it's something to watch out for. I have a roush built 351W, the cam it has is, while not the most radical, still fairly tall. .544 lift and .296 dur. this caused the vaccuum to litterally drop to near zero from the first second the throttle was pushed. While in itself this is great (for a racing engine) it is not great for a stock demon carb with a power valve in it set to open at 6.5". What was happening was the second I pushed the throttle the power valve came open and stayed there about 99 percent of the time the engine was running. Of course you can imaging how much fuel was being dumped into the engine.. not only did it cause the plugs to turn sooty black, foul, but it also turned the oil into water, washing all the bearings and cylinder walls as well. Right from day one I kept saying there was a problem with the way the engine was running cause it was backfiring through the exhaust every time I stepped on the gas. timing was good. everything else checked was good but they kept putting off changing the carb as I requested. Well a simple 20 dollar fix is not costing them several thousand dollars to fix. Anyway what this boils down to is you said that it's a bit rich and for breakin I suppose that's ok but be aware that too much fuel can lead to really nasty results.. Mine had about 2000 miles on it. and yes I just picked up the car in April.. I LOVE!!!! driving that car, LOL...and the looks and comments I get around town are worth every mile I put on it.. besides I didn't buy it to just have it sit in a garage and look at it.. I bought it to ENJOY it. and I intend to.. ooops sorry I got off on a tangent.. anyway. In my humble experience with exhaust "backfiring" I would have to say that the most likely cause is the rich condition on the fuel side.l Just keep an eye on things and have fun.