Alec, in my experience (also a TR-4), you can make just as much power
with a well tuned set of SU's as with Webers. My dad (Jack) ran SU's, I
have Weber 42's (came with the car). We were virtually the same speed /
power. Took us both a while to get there and required running on the
Dyno to dial them in.
From what I understand, the "Cannon" intake manifolds are considered
the best. I discovered that the 42's (which are rare as hen's teeth)
take the same internal bits as the 40's, so I think the advantage is
more psychological than anything over the 40's. I ended up with the
biggest chokes (venturi) that would fit - I think 36 mm was as big as I
could go. I do have a set of 45's that I keep threatening to spiff up
and try. I suspect that going to a 38 mm choke would be the hot ticket
for my engine. Depending on the build, 40's work pretty well. Big
bore, billet crank, hairy cam, high RPM would probably benefit from 45's.
What I found on the Dyno is that there's a ton of power in just getting
the mixture right across the rev range. I ended up with pretty small
air correction jets to keep it from going lean at high RPM's. I
probably gained around 20 HP just by tuning the carbs.
From what I understand, comparing jetting from one engine build to
another is pretty haphazard, it's best to tune it to the specific engine
rather than just copy someone else's jetting. Also, I believe the
emulsion tube can make quite a bit of difference, so just comparing main
/ air jet sizes if you're not running the same emulsion tube kinda
invalidates the comparison. In my case I just used the emulsion tubes
that came in the carb's and didn't try alternates. I believe the big
differences in those is more for part-throttle driveability which I
generally ignore. If I can drive it to the grid it's fine, otherwise
it's pretty much just wide open or closed, not a lot of part throttle.Â
But other drivers may use more part throttle and so the emulsion tubes
would be of greater importance.
I love the looks of the webers, but Jack was able to get decent mixture
control with SU's and some slighly custom needles - I think he took RB's
and skinnied down the tips to be more of the RA needle profile there.Â
He also ran 0.104" jets to get enough fuel out (stock is 0.100"). We
would draft each other at Road America, so were pretty close in overall
power.
I'm not quite sure what kind of advice you're looking for - should I do
it or not? What specific setup should I get? What carb settings should
I start with? None of these have cut and dried answers in my opinion.
Should I do it? Maybe, but you can also get similar results with good
well tuned SU's. The webers look awesome though.
Which setup to get? Partially depends on the engine build. I suspect
you'd be happier with 40's on a decent manifold with a decent linkage.Â
If you have a monster motor you'll want 45's.
Starting setup? If 40's, 36 mm chokes, possibly 150 mains with about
130 airs is roughly where I ended up (need to dig through the expensive
box of brass bits to verify). No idea what emulsion tube or accelerator
pumps I'm running.
One advantage of the webers is that they DO have accel pumps, so getting
the engine started on a cold day without the choke is easier than with
the SU's.
I found the tuning of SU's to be more obvious than the Webers. I used a
couple of books and Teri Ann's website to get a good understanding of
what I was doing with the Webers. The big aha for me was understanding
that the main jet affects the mixture across the entire rev range while
the air correction affects high RPM mixture more than low RPM mixture.Â
Changing the air correction does also affect lower RPM mixture so they
need to be tweaked in combination with each other. Getting the air
correction small enough that it didn't lean out at high RPM but big
enough so it didn't richen at high RPM was my key, then I tweaked the
main jets for max power and maybe went a step richer to keep the pistons
un-molten.
Regards, Tony Drews
On 1/18/2021 2:54 PM, Alec Buchan via Fot wrote:
> It looks like I am moving up a group and in order to not maintain my
> 'blue caboose' moniker, I am looking at converting to Webers. Moss and
> Pierce sell a 2 x 40 DCOE kit but I've also located a 2 x 45 DCOE kit
> (not from Moss or Pierce). looking for recommendations, if you might
> be so kind.
>
> On a side note, the few of us here in the PNW are looking forward to
> seeing all you 'front of the packers' in Portland this summer.
>
> Kindly,
>
> Alec Buchan
> Bellingham, WA
> '62 TR4 #5 (aka the Blue Caboose)
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