To External users:
BRITISH-CARS@AUTOX.TEAM.NET at Internet
I have two minis myself - although neither one is much of a performance
car...a '61 Woody Wagon and a '79 saloon. Enough about that. Anyway,
a friend of mine was over a couple of days ago, and he is building an
engine that sounds much like the one being discussed here. He plans
to use a VP3 cam (for road use).
We started to chat about scatter cams, and this is (to the best of my
sometimes fuzzy recollection!) how/why they work better than normal
cams:
#1. A series fires like this: 1-3-4-2.
#2. Think of the firing order like this: 3-4-2-1.
#3. As we all know, the Mini head is siamese ported.
#4. Chain of events with normal cam: Cyl #1 doing intake stroke. Cyl
#2 Will be doing its exhaust stroke. Just as Cyl #1 is finishing
its intake stroke, (the intake valve won't have had time to fully
close), Cyl #2 goes suuuuuck, and starts it intake stroke, and
because they are sharing the same intake port, it will rob Cyl# 1
of part of its charge, resulting in less power. Cyl #4 does the
same thing to cyl number 3.
The idea behind a scatter cam is to spread the timing of these pairs
apart to stop the robbing effect. Apparently lower horsepower engines
won't notice much of a difference, but the performers should...
If you want to read about it from "the" authority, get your hands on the
_NEW_ edition of "Tuning BL's A-series Engine" by David Vizard. You can
tell the new edition from the old one by the cover. The new cover is
yellow, the old one is blue.
All I can say now is I hope that I explained that right! (You should
see the mess of diagrams that cover my desk as I type this in...)
Ian.
----------------------------------------------------------------
Ian Hobday Phone: (604) 389-3822
SNA Production Group, OV ID: IGHOBDAY
BC Systems Corp. Internet: IGHOBDAY@BCSC02.BITNET
|