(Anyone out there an MS Windows or elsewise programmer? see the end, at
least)
> From: cusack@gumby.msd.ray.com
> 1976 MGB roadster.
> I have a retro-fitted the engine with a pair of SU HS6 carbs,
> installed a Piper 1/4 race cam, and a free flow exhaust. It growls and purrs
> like an Aston Martin. But leaves me wondering:
>
> 1. How sensitive are SU's to a change in air flow requirements?
> Should/can I change the needle and spring combo? Can I play games with the
> weight of the damping fluid to impact transient response?
Yes, yes, yes. read on.
> 2. The carbs are from a RHD '68 Volvo, and when mated to an MGB intake
> manifold, seem to tilt in towards the engine at about 15 degrees from
> horizontal. Does that change how the float sets the level of the fuel in
> the jet? Is this a second order concern?
yes, that may be a problem. It will make it hard to set the float level,
and float operation may be irregular. You should try to get the float bowls
level somehow.
> 3. What differnce in ride will be incurred by switching to MGBV8
> bushings
Minor
>, or nylon?
Stiffer, but with crisper steering.
> Where have you guys been all my life?
Under a British car?
Any SU can be tuned to work at a single selected RPM by turning the jet
adjusting nut. You *usually* tune at idle, and the metering needle
adjusts the mix as it rises when the throttle is opened. The metering
needle should be matched to the characteristics of the Engine *system*. The
needle taper is selected to give the *desired* (*1*) petrol mix through out
the RPM band. Any change in the system (Cam, carbs, filters, manifolds,
head, etc.), will give less than optimal performance at other than the
adjusted-for RPM setting, unless the needle profile is corrected. Creating
a custom profile is possible, and while easiest to do on an engine Dyno (or
let Charlie Rockwell do it if you're in the San Fran area), it can be done
by trial and error, by making changes and checking performance while
driving....
(*1) -The desired mix can be adjusted leaner or richer at any point, to
tune for economy, performance, etc. You can even create a profile tuned for
best emissions at idle, economy at part throttle, and performance at Wide-
open-throttle.
One real nice way to tune without a dyno is to use a colour-tune, a
special spark plug with a clear view-port which lets you see the colour of
the burning charge inside the engine, to know if it's rich, lean, or just
right. Of course you need to tune at all RPM's under load, so you might
try this:
-remove the bonnet, and have an assistant straddle the engine so as to view
the colour-tune.
-drive down the road at each RPM range, while your assistant notes the
flame colour at each.
-Make needle changes, and repeat.
I know a few people who would actually try this... I wonder how well you
could see the colour-tune with a cam-corder mounted in the engine bay,
rather than a live assistant?
To avoid starting from scratch, when selecting a new needle, it helps to
look at/try the needles from similar carb/engine setups. There are printed
needle profile charts available in several SU manuals. You can also start
with a slightly fat (lean) needle, and carefully file it thinner in the
right places to richen each part of the RPM band.
To help make it easier, (former?) SOL member Dean Deeds wrote a set of
programs which make it easy to compare & select known and custom needle
profiles for SU's. It's a PC program called HAYSTACK, and is available on
the SOL file servers as self-extracting zip file called SUNEEDLE.EXE. This
includes the progams, and needle data files. I also have needle charts for
Datsun Roadsters that could be converted, and it wouldn't take much for
someone to make a file for Z-S carbs as well.
[Programmers, read thes part]
Another (former?) SOLer, Bruce Carter, was going to combine the seperate
parts in one program, and port it to UNIX, and maybe Macintosh. (It may be
a C program) But, if there are any Windows hackers amoungst us, it probably
wouldn't take much to make a Windows shell to make it easier to run the
DOS programs, and maybe even view the profiles as a graph. *That* would be
spiffy. (You can do that now, with a spreadsheet and some tweaking, anyhow
Or, it probably wouldn't take much to finish Bruce's work. If anyone is
willing, we can probably track down the source for more complete re-writes.
Any takers?
________
/___ _ \ Roger Garnett (Roger-Garnett@cornell.edu)
/| || \ \ Agricultural Economics | "The South Lansing Centre
| |___|| _ | 3 Warren Hall | For Wayward Sports Cars"
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