>> I just finished installing a Monza exhaust system and header on my '76
>> Spit. It is definitely louder than the original equipment!! However, I
>> have some questions.
>>
>> 1. When backing off the throttle during a shift I get backfiring. Is
>> this indicative of timing or carburetor tuning?
>> 2. When at idle, or even at cruise, a lot of noise seems to be coming
>> from the engine compartment, as if the header had a leak. I have
>> inspected for same and been unable to find any leaks. Is this normal due
>> to the seemingly thinner construction of the header? The header is
>> welded to the exhaust pipe, so it is not a gasket problem there.
There are several things that come into play during deceleration with an
exhaust header, especially monza systems.
Firstly, since the entire system optimizes for free flow, the exhaust will
flow freely in both directions. With a standard system the exhaust will be
restricted further in reverse flow, lessening the sound pressure waves
during decel - not happening in the monza.
Second, since the monza does not have a pressure break collector, the sound
pressure wave is not broken at the end of the header (which is why they
sound so throaty and more noise under the hood (bonnet). A standard and many
other headers do have this larger collecter which will reverse the sound
wave within the header. At decel, the sound wave is louder because of the
lean (slow burning) mixture burning in the header. Since the standard
manifold is cast iron, the burn will happen quickly and quietly within the
manifold because of the retained heat.
To reduce the backfire noise, you will have to use a deceleration valve
which will allow the throttle to remain partly open during deceleration.
(The ZS150 standard has this valve, and it works well, even on monza
systems - Webers don't)
In order to reduce the engine noise, you will have to experiment. I used a
3" collector attached to the bottom of the header to remove the sound wave
from my system. (Monza 4-2-1 w/2" output pipe) It also reduced the underhood
noise significantly. It also allowed me to eliminate the resonator in the
centre of the system leaving only the tail tips. I also tried unsuccessfully
using the header wraps, but all they did was make the exhaust pipes hotter.
Good luck with your car!
Wayne Burton
1963 Triumph Spitfire4
1966 Triumph 2000
1954 Austin A40 Somerset
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