Great kudos to Randy Taylor for his essay on carburation, a most
interesting and elusive topic.
The problem of efficient manifold design to obtain performance,economy and
good idling from the fewest number of carburetors is indeed a very
difficult one as so many variables enter into the fray: number of cylinders,
firing order, valve timing, port arrangement and area,type of
carburetor,type of hot spot required,exhaust manifold design.The challenge
of manifold design has been diverted and simplified in modern engines with
the help of direct or multiport FI.
Randy is right when he states that there is little advantage in going
beyond two carburetors in a six cylinder engine, but careful tests before
the war by Alvis for their 4.3 l. six cylinder engine showed the
superiority of the 3-SU arrangement over the 2 SU alternative. Alvis
mentioned that with the 3 SU arrangement they had the best high and low
speed performance. Tests done by the SU company and published in 1962 on an
undisclosed six-cylinder engine ( presumably the AH-3000) showed a small
gain in power for the triple SU HD6, about 3 HP at 4500 rpm, over a dual
SU HD6. A single SU HD8 was also used by comparison and had much worse low
speed performance,but strangely enough, could match the best of the triple
arrangements at 5000 rpm, at the price of 20 % higher specific fuel
consumption.
There are many other cases of six cylinder engines where a triple
set up has been found best, and not only in British engines. The famous
"fastest sports car" race before the war at Brookalnds was won by a
Delahaye with a triple vertical Solex arrangement.
Considering how difficult is the problem and how large manufacturers with
proper research facilities can get it occasionally wrong, my
impression is that the chance of getting a truly efficient aftermarket
manifold are very remote indeed. Few speed-equipment companies, with the
exception of the larger ones catering for the very large hot-rod
fraternity, such as Edelbrock, would have the equipment and know-how. In
the case of the popular Holden OHV six used in Australia, many aftermarket
manifolds were downright failures. The worst was an attempt to use a
centrally located 4bbl.Holley in the straight six.This combination gave
less power than the standard single bbl. carburetor and 8 mpg ! The best
arrangement (for racing) for this engine was,surprise, surprise, triple
SU HD6, used with 3 into 2 exhaust headers.
If you are going to put 2 SU's to replace the single ZS, take care not to
connect manifold vacuum to the distributor. What you want is "ported
vacuum",and must be routed from the proper port at the carburetor butterfly
valve location, never from the manifold itself.This port vacuum has a
rising characteristic with rpm, precisely the opposite of the manifold
vacuum that decreases with engine speed. Your ignition settings would be
completely wrong with manifold vacuum.
Sergio
Sergio Montes Department of Civil and Mechanical Engineering
University of Tasmania
Box 252C,Hobart 7000,Tasmania,Australia
Ph. 56-02-202113 (Int) 002-202113 (Australia)
Fax 56-02-234611 e-mail Sergio.Montes@cmech.utas.edu.au
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