I think of the crank as a long torsion bar as you suggest but with a
difference, the more the crank twists, end to end, the farther the cam gets
out of position and the farther the valve timing is off. So if the major
part of the torsioning force is closer to the cam drive the timing won't be
as far off.
The port nozzle in 7 is smaller than in 5 (and the rest) due to plennum air
taken away from 7 by 5 and this probably has more to do with it than
anything else. This can be helped by a little longer intake valve opening on
----- Original Message -----
From: "Kirkwood" <email@example.com>
Sent: Friday, June 03, 2011 13:03
Subject: [Land-speed] BBC Firing order
> Well, if you think of the crank as a long torsion bar with a load at one
> end, what have you accomplished? As John points out, the crank does twist.
> So with a 5-7 you apply a twisting force close to the reactive load. With
> the 4-2 you have moved the twisting force farther away from the load. The
> longer the torsion bar, the less stiffness. You are imparting more twist,
> not less twist, in a 4-2 set-up. But maybe that is the purpose. Skip, if
> you have individual port injection; how can there be interference from
> either firing order?
> I have been reading about this controversy ever since marketing created
> idea to sell more cams. Regardless of the magazine article, I have never
> seen an honest, factual engineering study with good data.
> The altered firing order cam would trade 5-7 interference for 4-2
> interference wouldn't it ? Maybe it would reduce crankshaft harmonics by
> eliminating the normal 7,2 sequence crank twisting . -John
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