In a message dated 96-10-09 09:22:21 EDT, you write:
<< What I found on his car was a little bit surprising for me:
He used the SU's on the (unflowed) pre TR4A manifold without Rampipes or
filter
and he switched to this configuration, 'cos with the flowed later manifold
he
found the (as I remember) plugs on 1 and 4 running lean. My first thought
was the
bridge between 2 and 3 manifold pipes, but this was closed by welding. Next
guess
was a not centered wall inside the manifold, where the gas is splitted and
on a
close look, this might be the point.
Additionly I was told, that the engine produces more power on the new setup
with the older manifold....
I took another close look into the SU's and found two things where I'm not
shure
about, wheather it's good or not!
-the carb' input was widend up, it looked like it was done on a lathe.
-the edge at the bridge where the piston falls onto, was rounded.
I can't say anything about the first, but IMO and as far as I remember
(can't
find anything written) the second might be worse, 'cos the edges, on the
bottom
of the piston and the bridge should be sharp, in order to swirl the air
quite a
bit. Right?
Another question:
What can be done to racetune SU's, instead of changing jet and needle ?? >>
Peter-
Re: switching from the long manifold top the short one...
As I remember (oh oh not the memory whatchamacallit) the short manifold has
larger openings and with alot of internal grinding, it would indeed make
pretty good hp. However, races are not won on the dyno, and the longer
manifold allows the power to be utilized over a wider, and lower, rev range
than the shorter manifold. The problem that the racer was experiencing may be
due to the fact that the long manifold (especially if it has been ported,
making it thinner) is very prone to cracking just outboard of the manifold to
head junction, especially on the end runners (cyl's 1 and 4). This problem is
so prevelant that I have seen it on street cars also. I have 2 manifolds
afflicted with this problem and you can NOT see anything at all, but the air
leak on the maniflod runner is extreme. The problem is created with the
addition of headers since the intake is no longer supported by the exhaust
manifold.
If you think that you may have this problem, put some soapy water on the tops
of the intake runners and apply air pressure to a cylinder with an open
intake valve, then block off the carb openings. If you see bubbles, you got
problems.
Often you will find racers with a large section of aluminum welded to the top
of each runner to keep them from cracking. Unfortunately the SCCA doesn't
allow any additional material to be added to the manifold (I'm sure they were
thinking of the inside, but the rule applies to anywhere) so this was always
a protestable thing to do.
Opening up the mouth of the carb is also an acceptable proceedure. It allows
more air through the carb, but does tend to create a low air speed problem
with regard to piston rise (not the hot lick for a street car). Grinding an
angle, or rounding the front of the piston also is an acceptable mod.
Rounding the bridge is NOT a good idea. The fuel needs a sharp edge from
which to "wick" and vaporize. Take a good look at the bridge at WOT, and you
will see the fuel "dancing" along the bridge as it vaporizes.
All that being said, you can actually cut the bridge completely away inside
the carb, so that it is basically a straight tube inside. To replace the
needed venturi, you must braze an angled piece of brass to the top of the jet
head. This piece can be fairly thin and not quite as wide as the original
bridge. This will allow some additional airflow through the carb, but will
really cause poor running at anything less than about 3/4 throttle, again NOT
for a street setup.
experiencing manifold problems...
Nick in Nor Cal
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