Hi Les and other interested participants
OK Les, I accept this thesis, and I must say, this post has made me think a
little deeper on the subject. I certainly agree with your observations
about the piston and choke area in terms of their aerodynamics; but, more
importantly, what I didn’t take account of yesterday was the number of
“throats” feeding the cylinders. This is where I think the Weber setup has
some advantage.
I see it like this. I think all would agree that the most efficient fuel
delivery setup is to have a mixture tuned and fed which meets the individual
requirements of each cylinder in the engine, especially in an older engine
exhibiting diffent compression readings in each cylinder etc… Also
importantly, the shortest distance between two points is a straight line.
So, the most efficient carburretion arrangement (just like with fuel
injection) would be to have a single carburettor feeding each individual
cylinder with gas flow “straight” into the combustion chamber. On my
rethink, this is exactly
what the Weber setup tends to facilitate. Triple Webers in fact, operate
like 6 individual carbs (Sort of like having 6 Amals) with a “throat”
feeding each cylinder.
The SUs, on the other hand have one “throat” feeding two cylinders and we
also start to “turn” the mixture before it enters the head. So the
adjustment of the carburettor has to meet an averaging arrangement to suit
the needs of the two cylinders receiving the air/fuel mix, and the pathway
from the jet to the combustion chamber has two turns in it.
And, of course, the two carb E Types with the emission stuff, Strombergs etc
bring this formula down to two “throats” feeding three cylinders each. The
Stromberg arrangements also include an incredibly inefficient gas flow
transfer arrangement before anything even gets near a bank of cylinders.
So, on my humble reanalysis as a result of Les’ post, I can see some logic
behind “just fitting a Weber” to get improved performance.
;o)
Cheers
Noel Annett
68 2+2 E Type
67 Daimler Sovereign (420)
66 Daimler 2.5L V8
84 BMW 528i
Canberra, Australia> ----------
From: Les Halls[SMTP:blakjag@ibm.net]
Sent: Wednesday, September 16, 1998 10:14 AM
To: e-type@jag-lovers.org
Subject: Re: [E-Type] Re: Webers vs SU - power
Noel, everything you mention is fine, except I disagree with the notion
that you must re-engineer the head in order to use Webers.
Re-engineering the head would most likely do the car some good, even
with the SU arrangement. The Webers improve things on the “standard”
engine because they simply allow the engine to breath better, and THAT
excludes the obvious performance advantages obtained by being able to
vary the choke/venturi/jet arrnagements so that performance can be
“tuned” to the specific range you are looking for (low down by using
short air horns or high end by using extended ones etc.). Webers were
always considered to be one of the quickest “bolt on” ways to improve
performance. The REAL problem with using Webers is simply that to get
them right, you have to go through many permutations of jet swap out and
road tests, and it’s diffcult to find suppliers of the jets etc and when
you do, they’re very expensive. You can easily spend hudreds of dollars
just arming yourself with a supply of these things for test purposes.
If I had access to all the necessary jets, I wouldn’t be having any
problems, AND I’d have a “standard” E that runs better and quicker that
a “standard” E with SU’s. Take a look down the throat of the SU
sometime. The piston and choke area is a damned awful aerodynamic
arrangement. Compare that with a Weber setup and quite frankly the
difference is like chalk and cheese. IF you set them up right, they
almost HAVE to improve vehicle performance…IMHO.
Les.