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EFV-8 Club Forum / General Ford Discussion / Backfire in exhaust

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Esteban34

08-17-2013 @ 1:46 PM

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My 34 Tudor just underwent valve job, new cam gear. Backfires on acceleration, not deceleration. What causes that? Timing is correct on cam/crank.

adonde vas, estas alli
Esteban

Esteban34

08-17-2013 @ 1:46 PM

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My 34 Tudor just underwent valve job, new cam gear. Backfires on acceleration, not deceleration. What causes that? Timing is correct on cam/crank.

adonde vas, estas alli
Esteban

Esteban34

08-17-2013 @ 1:46 PM

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My 34 Tudor just underwent valve job, new cam gear. Backfires on acceleration, not deceleration. What causes that? Timing is correct on cam/crank.

adonde vas, estas alli
Esteban

Esteban34

08-17-2013 @ 1:46 PM

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My 34 Tudor just underwent valve job, new cam gear. Backfires on acceleration, not deceleration. What causes that? Timing is correct on cam/crank.

adonde vas, estas alli
Esteban

supereal

07-12-2013 @ 6:53 PM

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The answer is that exhaust flow thru the system creates suction at any breaches in the conduit between the combustion chamber and the taiipipe. This allows unburned fuel to enter and be ignited with the air and be ignited, producing the "back fire".

Dustbowl

07-12-2013 @ 6:43 PM

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OK, I will almost give up and accept above comments. My reservation is re the leak in exhaust system. The gas in the exhaust system is at above atmospheric pressure, or otherwise the gas will not flow out the tailpipe. If so, how does air lead in from lower to higher pressure?

I am, incidentally, an old chemical engineer who worked in oil refineries so I think I understand combustion and fluid flow fairly well.

shogun1940

07-09-2013 @ 6:53 PM

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my 48 backfires because it has leak in the exhaust header,, hot exhaust plus unburned fuel plus plus new oxygen is an explosion

Stroker

07-06-2013 @ 4:51 PM

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TomO/Dustbowl: I agree that a rich mixture will also retard the burning rate. For our old Fords, the "Goldilocks" mixture is probably closer to 16:1 rather than the more environmentally acceptable 14+, wherein the mixtures burns where it is supposed to and finishes burning before the exhaust valve opens. Ignition timing is also a factor, as a late spark obviously delays the onset of combustion. Modern fuels, particularly those which contain ethanol alter this ratio as well, requiring more fuel to air to achieve a burn rate that will complete on time. Contrary to
popular notion, the higher the octane the fuel, the slower the burn rate. Back in the 60's, Honda had some 250cc 4 cylinder motorcycle racing engines that basically would outrun the burn rate of contemporary racing fuels, and had to run quicker-burning fuels that were actually very low octane. I've used acetone mixtures in the past to accelerate the burn rate.

TomO

07-06-2013 @ 8:26 AM

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A rich mixture will also cause backfires in the exhaust. The power valve should be closed during deceleration and if the gasket or the valve is leaking, there will be excess gas in the exhaust system that will explode when it hits the hot muffler parts.

Tom

Stroker

07-05-2013 @ 6:24 PM

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Dustbowl:

The ideal air-fuel mixture of air and gasoline that is designed to burn properly in an internal combustion engine is called "stoichiometric" or a 14.7:1 ratio by mass. A lean mixture burns much slower than a stoichiometrically "correct" mixture, so it is still completing its' burn late in the exhaust cycle, often continuing after the exhaust valve closes, and the combustion byproducts have entered the exhaust plumbing. These byproducts obviously contain yet un-burned fuel, which upon contact with the hot exhaust plumbing, ignite. So...it's simply a matter of timing; eg how long does it take for the mixture to completely burn.

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