Zenith Stromberg v Lucas Injection

[C M Smith <cmsmith@domain.elided>]

Ah yes, the old carb v FI debate.

 The Zenith Stromberg was successfully modified to allow it to meet very 
stringent emissions regs. The problem with the original design was not 
power related but maintenance related. In fact, both the old SU and the 
Stromberg constant vacuum carbs were excellent designs, much simpler than 
Webers to build and repair. My Triumph repair kit consisted of a BFH, a 
pair of slip joint pliers, two screwdrivers (yup, big and small), and a set 
of six box and open spanners (combination wrenches). The plug wrench came 
with the car and I had three lengths of iron gas pipe for a torque wrench(3 
ft, 2 ft, and 1 ft with the last being 3/4 inch to act as an 
connection/extension for the other two, total length up to five feet for 
torquing the head nuts). For really technical stuff like the spider joints 
you needed circlip pliers which no one I knew had, so the old needle nose 
pliers and a screwdriver were used, as well as a flashlight (torch) to 
locate the circlips after removal!)

 The mixture in a Stromberg (or SU for that matter) was metered by a 
precisely machined needle riding up and down in a precisely machined 
orifice connected to the float bowl. The taper of the needle was such that 
it bore  a precise ratio to the airflow through the gap between the piston 
face holding the top of the needle and the orifice seat. More throttle 
caused the piston to rise ( diaphragm operation for the Stromberg, straight 
piston in bore for the SU) in direct proportion to the airflow. Fuel 
metering was very precise. Snap throttle enrichment was provided by a 
damper which restrained the rate of rise of the piston by the viscosity of 
the oil in a damping piston inside the air metering piston. Refilling the 
oil ( auto tranny fluid?) was a routine maintenance item for these carbs as 
was the delightful task of idle adjustment, which adjusted the mixture 
perfectly across the entire rev range. When the throttle response got 
fluffy you first "checked the oil" using the piston damper as a dipstick, 
and when satisfied that all was well there used a small coin of the correct 
denomination for the country you were in: sixpence for the UK, one thin 
dime for Canada or the US, to screw the idle setting (really just raising 
or lowering the orifice by screwing it up or down from below the carb). 
until the idle reached a maximum, re setting the idle speed as you went. 
Balancing a twin carb set up was really fun and could take a while unless 
you resorted to the old colour tune method or the venturi gauge method. I 
used the old human ear method for matching the hiss and eventually a 
tach/dwell meter for rpm (remember those?!?).

To make the Stromberg carb meet emissions was easy as it turned out. The 
only real error in mixture was due to the tiny fluctuations of fuel flow 
due to the metering needle not being precisely centered in the orifice all 
of the time (another routine maintenance item). So it was redesigned to 
make the needle ride on the side of the orifice, and re machined 
accordingly to match the new flow pattern. This worked great except regular 
maintenance was required (remember that weird concept?) to ensure the carb 
kept performing as designed. SU couldn't meet the standards because the air 
leaked past the piston/bore in very slightly unpredictable amounts.

The big advantage to FI is it is cheap. Once you tool up to build the 
computers, and have figured out the amortization on the software, you build 
it, sling it in, and it never needs maintenance except of the search and 
destroy variety now almost universally practiced (you either can't or won't 
attempt to repair anything you just hunt down the defective bit and chuck 
it, even if it's the entire computer!)

So, carbs weren't dropped because of performance issues, rather cost and 
maintenance militated against the carb continuing. Webers have always been 
the top of the brands, Zenith Stromberg kept up, and Honda did a great job 
of making carbs which met emissions. But FI is so much easier to keep in 
compliance, as well as much easier to make tamper proof!!!

Also, when you get to one throat per cylinder FI is just easier to deal 
with. You can retune an electronically fuel injected engine completely with 
a laptop in a few minutes. With four double throat Webers it can take you a 
week to do the same thing. That's for a V8, for a V12 .....

The SPICA mechanical system made an inherently dirty engine very clean (one 
of the worst modern designs from an emissions standpoint is a hemi head 
engine with big valves. Big bore hemi engines are even worse Lots of power 
but lots of NO also). As for the Lucas Prince of Darkness mechanical 
injection on the Triumph, just thank your lucky stars NA spec TR's had 
Strombergs!!


Cheers


C M Smith

Calgary, Alberta
(403) 289-1076
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