>Gary Boone
> I just had a chassis dyno test on my 2L and see a possibility for a few
> more horses. I have 45 mm TWM SK Racing Carburetors, which are very
> similar to the Solexes. The dyno test also included a air/fuel mixture
> curve versus HP and rpm. The curve was slightly too rich (12 ratio) at
> lower rpms and went too lean (17.5 ratio) above 6000 rpm. The curve was
> also wavy through the rpm range. I've been using 36 mm chokes and Dave
> at Rebello racing reviewed my curves and his only recommendation was to
> try 38 mm chokes. I couldn't find a size spec for the Solex chokes in
> the Nissan roadster service manual. What do the Solexes normally come
> with as far as choke size? Anyone have a good feel whether going to 38
> mm chokes will benefit? I thought I would maybe lose some low end but
> Dave said "Oh Contraire". Any other ideas/comments are welcome.
Basic concept of this type of carb is that the main jet tends to effect most
of the RPM range from just above idle, whereas the air jet has most of it's
effect at higher RPM. Based on that, and if the SKs use basically the same
concepts, then I'd say you want to go smaller in both the main jet and the
air correction jet. The smaller main jet would lean out the lower RPM range
and going with a smaller air jet richens the upper end because it introduces
less air.
I'd suggest doing the main jet first to get things in the low and mid range
where they need to be before you mess with the air jet because anything you
do with the main jet will have some effect on the higher revs too. If you're
going to invest in this process I'd suggest doing it on a dyno with an
exhaust analyzer. It would save time and almost certainly be the most
accurate way to do it. Not cheap, but if the shop has a wide selection of
jets and parts to use you don't have to invest hundreds in jets, tubes,
chokes, etc.
Depending on where you want the power you might want to go with the 38mm
chokes, but be prepared to loose at least some low and possibly mid range. I
just did some quick calcs and it looks like each 2mm increase in choke size
yields about 10% greater area in the choke. I'd guess you'd need a flow
bench to accurately determine the difference in both max flow and mixture
velocity versus RPM, but a 10% increase in area's bound to yield a drop in
velocity below a certain RPM.
If you do want to go with the larger chokes, install them before you play
with the jetting because changing the choke size will probably alter the
air/fuel ratio at least a bit over most of the RPM range.
I already plan at some point to do a back to back dyno comparison of 38mm
SUs versus a pair of 40 DCOE Webers on an L-18. If time, and money, permit I
may also try swapping in different choke sizes to how and where there this
changes the H/P and torque numbers and the mixture numbers, too. Don't hold
you breath, though, I'm a good ways away from getting the car done and it's
WAY too hot for this fat old man outside right now.
FWIW,
Ron
Ronnie Day
ronday@home.com
Dallas/Ft. Worth
'71 510 2-dr (Prepared Class Autocrosser)
'73 510 2-dr (Street Toy)
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