Spitfolks,
A Spitfire with no diverter valve (i.e. '78 model) and
no (or insufficient) catalytic coverter, but WITH an air
pump fitted, will backfire on the over-run. Not the
"desirable" kind of backfire, but more like small arms
fire which causes pedestrians to dive for cover. Running
a car like this will blow out a muffler in short order.
Disconnecting the air pump (as mentioned) will stop the
bang, so will installing a good catalytic converter.
The backfiring is not due to cat-con obstruction, as a
"gutted" unit (innards removed) will backfire loudly too.
This happens because you have:
a) high intake vacuum (thottle closed, high rpm)
b) high exhaust side pressure (no diverter, high rpm)
c) residual unburned fuel on the exhaust side
I believe the unburned fuel gets forced back into the
motor, hits hot exhaust valves (or other hot spots) and
BANG! heads turn. I suspect (but have not verified)that
less-than-perfect valves play a part in all this.
BCingU,
(the other) Bob
--
Bob Sykes
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