[Fot] Race headers.

[John Davies]

Because the six has three pairs of pistons, moving in synch, but 360 degrees
out for ignition timing (EG No.1 on firing stroke, when No.6 is drawing
charge into the cylinder) a 6-2-1 design receives impulses into the
secondaries at irregular intervals.    A 6-3-1 gets an impulse regularly.
This impulse will be reflected back to the other of the pair, and if the
primary is the correct length, that impulse will be in its negative phase
when the exhaust valve opens.      This will improve flow out of the
cylinder, so that this design is called a "scavenger" or "extractor".    

The length of primary is critical.    The enclosed diagram refers to inlet
tuning but the same applies to exhaust manifolds.    As can be seen, the
usual length of 17-19" will boost extraction and power in the mid range,
where it will be most useful for a road car.    An out and out race engine
builder might consider longer, to gain at the top end.

For such pure race engines, 6-1 manifolds may be considered.     Look at any
F1 engine, or at the extraordinary manifold built by Gareth Thomas - see
enclosed.

John





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