When I was growing up car backfires were common. As a kid I had friends who could make it happen at will.
Now that the carburetor is long gone, most younger folk have never heard one. Fuel injection eliminated this, I think; could be wrong.
I never understood exactly what happens. Usually a car would backfire when under heavy load, and the gas suddenly let up. So my guess is the carburetor had a full charge with nowhere to go. But what ignited it?
Or, possibly the lack of air made an overly rich mixture escape the exhaust valves, and the explosion was in the manifold?
That article was pretty pedantic. What they refer to as backfire is more popularly called 'carb farts' in the Harley world. Between a primitive induction system, a wasted spark ignition, and EPA mandated leanness, pre-FI bikes would tend to blow back through the carburetor. That could be interesting if you were running without an air filter.
What the OP and most people refer to as a backfire is what the article calls an afterfire. There were a couple of hills going into town. I'd cut the ignition on my '51 Chevy and pull the choke as I rolled down in gear to make sure people knew I was coming.
You have 2 kinds of things that might be called backfire. It can pop back through the carburetor and that might actually indicate a bad intake valve or you can have a muffler explosion and that is usually a cessation or failure of ignition (how your buddy made it happen, flipping the key off and on) Raw gas builds up in the exhaust and the next time a valve opens on a burning cylinder, kaboom. It can split a muffler open.
I suppose you might still be able to make it happen but it is probably tougher on an EFI with all of the emission controls.
I did that once when I was young. It just blew the muffler wide open. Needless to say, that was the LAST time I did it!
A fairly common practice among "gear heads" back in the 50's and 60's was to connect a spark plug right near the tip of the tailpipe. Somehow they would use a part often called a "vibrator", which was used in old vacuum tube car radios to convert DC to AC. That was fed to a capacitor and a spare ignition coil, which then provided a spark in the spark plug at the tail pipe outlet. The cars back then, had no catalytic converters and allowed raw heated gas to exit the exhaust system. That spark plug ignited those raw gasses and would blow a fairly large flame out of the tailpipe. It looked "COOL". It was pretty harmless, unless someone got too close and set their pants or dress on fire!
Well, as a mechanic I can say definitely it could NOT damage a piston
- as the "explosion" was not in the engine. Bad ignition was responsible for more exhaust bachfires than bad carburetion. My Dad's '69 GMC pickup had a bad habit of slitting the muffler every 6-8000 miles unless you changed the points first. Ends up the factory had not installed the ground wire between the points plate and the distributor housing, so the points burned badly - and intermittent spark would allow the muffler to load up with gas when the spark was off, and then light when it came back on.
An exhaust leak could also cause exhaustr backfire on decel or over-run as the exhaust was rich with gas from the high vacuum, and added air into a hot pipe would light it off, giving bang-bang-bang-BANG!!! coming down a hill.
One day with 22 tons of farm equipment on the float behind the 292 GMC, I came down a long gentle hill to a narrow bridge across a creek where guys were working on the road.. They were in no hurry to get out of the way as I came towards them, so I shut off the ignition for a second or two, then turned it back on. They jumped out of the way real quick!!!. (had a drawn steel "cherry bomb" muffler on it that withstood backfires quite well - after blowing a few Thrush cans off of it.
We had a Mazda RX3 , if you let off the throttle coming down a hill , it would come out with a VERY loud backfire . Something to do with one of the two sets of points being off just a hair IIRC . Anyway , we were coming down a not-too-steep hill into Brigham City Utah , and the traffic light at the bottom of the hill was out ... and there was a Highway Patrol trooper directing traffic ... and I let off on the throttle just right ... and that damn car blew just as we passed the trooper . When I looked in my mirror , he was on the ground with his gun drawn and looking for who shot at him . I still chuckle when I think about that . That particular trooper was a real asshole or I wouldn't have done it .
What wasn't said was that there was also a way to deliver a mist of fuel just ahead of that spark plug . Made a most awesome flamethrower . Didn't do it , but I did see it .
Well, whether it was technically pulsed DC or AC coming out of the vibrator, what came out of the transformer was AC, as it required rectification to turn it back to DC. AC or pulsed DC from the vibrator is just semantics.
An automotive coil is just a transformer that raises 12v to a very high voltage. But all transformers require AC to make them work. But you are correct, it's pulsed DC, but to the transformer, it's the same as AC. The points in older engines simply pulsed the DC. The newer electronic ignitions do a similar thing, but they may be more like actual AC (I'm not sure).
Now a days, one could use an electronic ignition module, like one from a small mower engine, or one made for cars (which would be overkill). For this use, it really dont much matter what makes the pulse as long as there *IS* a pulse to trigger the coil. A set of points could be used to, but there would need to be soem sort of cam external to the engine. So, an electronic module or a one of those old radio vibrators is/was a much easier method to excite the coil.
Someone mentioned in this thread about injecting a spray of gas into the pipe to make a bigger flame. Yes, I heard of that too, but I did not mention it because I have no clue how that was accomplished. In fact I'd like to find out how. Not that I want to do it, but I like to know stuff like that. However, to take a wild guess, I'm thinking in terms of the nozzle inside an oil furnace. Something like that welded into the tailpipe, would send a spray of fuel, bu there would need to be a pump of some sort. But attaching a pump of some sort, to the engine's fan belt, or a pump running off a 12V motor (such as a heater blower motor), would probably work fine. (Just a guess).
To make a "flamethrower" from the exhaust pipe, would probably require injecting fuel into the pipe now-a-days, because newer car engines with catalytic converters are intended to NOT release any raw fuel. But I bet my old late 1940's Farmall tractor would work just fine. I can smell the unburned gas when it's running. But I recall smelling that same odor from the tailpipe on the old cars that I drove in the late
60's and early 70's. (which were 50's and 60's cars with carburetors and no air pollution devices).
Now-a-days, a person would probably be arrested and ticketed for doing something like that on a vehicle driven on public roads, but I could see doing it at a truck or tractor pull, or just doing it for fun on an old farm vehicle or anything that is not used on public roads.
I could not find a web page that described the old type vibrators (not the kind that give women momentary high blood pressure and screaming and moaning). But what I remember is they caused a rapid interruption in the DC, by open and close a switch / contacts. As such they put out pulasting DC, which ended up being pulsating DC on the other side of the transformer.
OTOH, this was a couple decades ago I learned of these.
AC goes through the transformer just fine, but DC does not. Which is why the vibrator is needed. But, then, Clare knows that.
On the output side of the transformere it was AC. That is why a rectifier tube or diode was used. The 0Z4 was a common tube used in the old car radios. There were some syncronous vibrators that had a set of points on the secondary and another on the primary. This eliminated the need for the rectifier and only the capacitors were needed to smooth out the pulsing DC.
Using a viberator is differant than the opening and closing of the points of an engine. By the way that diagram is not correct. "The condenser goes across the points. It is there to prevent arcing. It does this by slowing down the colapsing of the magnetic field which produces the other half of the AC waveform.
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