Don Terrill’s Speed-Talk

What are the worst damages that can be caused by backfires when cranking a naturally aspirated engine.

bent conrod, intake valve blown open causing hydro lifter to pump up and in some engines cause a bent valve, bent push rods, broken rocker, starter motor damage(not really actual engine damage)

The only damage I've ever had was blown power valves, even in the carbs with power valve protection. But you can bend the throttle blades or distroy the whole carb in a really bad one. With a supercharger, you can blow the whole supercharger off. I'm sure you've seen that on TV with Top Fuel Dragsters. But I've never heard of anyone bending valves or pushrods. I'm not sure if a backfire can be powerful enough to open a valve. Guess it could.

I had a misfire when I brilliantly installed the dist. It seemed like it fired on the comp. Stroke while cranking, Both valves shut. Didn't come out the carb or the exhaust. Sounded like a sledgehammer hitting the top of the piston. I hope that the hi torque starter was weaker than the h beam 800 hp rated rods. Motor has no tight spots when turned over, and started up fine, no odd noises. In the words of Winnie the pooh.... Oh bother.

While waiting for my msd dist to come back form repairs I had a backfire (you can not repair the E curve dists yourself-and it look like a brand new dist msd sent me back)

The rotor slip on some 30 year old dual point I was using but right by my house, yeah it was loud and the motor then shut off. That was last summer and all is still fine. I'm sure all my neighbors heard it, lol

Watch some of the old 60's Munsters reruns, that old hot rod car they used loved to backfire.

Now I would not be surprise if backfiring can cause major damage.....I think there is safe backfiring and unsafe I know a crack header pipe will cause loud backfiring noises on deceleration, give it a bit of gas it stop, let up and bang bang if its in a low gear or running steep gears in back...I ran it for a month or two like that until the weather warmed up and too many people outside would jump and look--think it sounded a bit like gun fire--but the sound doesn't carry--suburban folks don't know the difference

I've lost mufflers to a good backfire before..

the only other backfire damage I've seen is valve cover and intake tray on BB Mopars get blown out. but it only can happen if engine cranked and gas fumes build in crankcase. don't see how rod can get bent either.

we had a limited sprint car running a 347 ci sbf. the distributor gear pin sheared on the backstretch at around 7000. The "BANG" made everyone gasp, and we started thinking about what we had for a backup engine.... We pulled it back to the pits, cranked it by hand and it felt fine, (thats when we checked the dist and found out what happened). Put in our backup magneto and pushed it off and it ran fine, no issues. Finished the season.
......Now that I think about it, when we pulled that engine apart one of the main webs came out with the crank. We felt that the only thing holding it together was the main girdle. Maybe the block was damaged when it backfired?

When I mentioned bent conrods Im talking of a backfire or crossfire which fires on the comp stroke or when the intake valve is just open, NOT a backfire thru the exhaust.
On the Ford Australian OHC 4.0L 6cyl engines we have a factory option for LPG. On the early 2000 models there was an ECU glitch which on occasion caused the wrong cyl to fire on a coil over plug setup. On the LPG models, if this happened at the right time, they can and did bend a conrod. After this Ford fitted all factory LPG engines with the conrods from the Turbocharged model. The stronger rods would not bend under this condition. Then they fixed the ignition glitch.

If the crossfire occurs when the intake valve is open, and the intake manifold is full of air and fuel, this ignites and can push another intake valve open and the hydro lifter will then ride the valve open till it settles. This at times can bend an intake valve and or the pushrod.

that I can see happening

don't forget the teeth on the flywheel taking a dump, pita.

crazyman wrote:I've lost mufflers to a good backfire before..

Had an old Ford pickup with a healthy 390, when towing the first thing I'd do is pump the accelerator a couple times then hammer it once, that pesky muffler would blow off with a back fire and she'd run right, a little louder, just harley loud...

I've had 3 items damaged, I run alcohol and when cold, it’s backfired and once blew an intake manifold gasket. I recently broke a starter pinion gear and took off a couple teeth on the flywheel.