I just hung up with TWM,
and they believe those ports are completely accurate in their TB. He
asked what size TB I had, what engine, and about what HP I thought it was
capable of. After hearing all that, he said my 42 mm bores were
simply too small, and that I'd need at least 45's. When I pointed
out that my runners were only about 42 mm, he basically said "start
over" :-) He did mention the possibility of adding a crossover
tube between the two runners, to allow one to steal from the other, but I
probably wouldn't do that unless I was committed to building another intake if
it didn't work.
Without hard evidence
to the contrary; I think it is more likely that the MAP readings we see on the
TWM TB is valid, or close to it. The MAP port is ½” downstream, directly
in line with the butterfly shaft, and supposed to measure ‘static’
pressure in the manifold. Seems like a pretty good spot to me. Of course
there is all kinds of turbulence going on there, but my judgment says that it
is a lot more likely that the pressure drop is in the relatively long path
upstream than some local condition that could exist over a cm or 2 in the TB
barrel. I don’t think there is any way to sustain 3-4” HG pressure
differential over a very short distance in the area of the port.
Tracy and others are
measuring MAP at the TB, BEFORE any losses in the intake runners. We are
measuring near the intake port downstream from a single 1 ¾” runner per
rotor (the TB barrel itself). So of course we are not going to see ambient
atmospheric pressure with WOT. OTOH; I also find it hard to believe there
could be that much pressure drop upstream.
Atkins recommended to
me not going larger than 44mm barrel dia./rotor for better throttle response. That
may not be important in our application. In discussions with Paul Yaw; he
seemed to agree that 44 mm was a good choice based on getting the higher flow
velocity. So who knows? What I know is that I can’t complain about the
power my engine put out on the dyno. 85 hp/rotor at 6000 rpm, and 96 hp/rotor
at 7000 is not all that bad for NA, 9.0 rotors and “untuned” intake.
And at 7000 rpm WOT I was reading only about 24” MAP at that TWM port.
Since the power is
there, what is the concern? Maybe something about the EC2 response that
expects a MAP reading taken at some other point? Running WOT hp vs rpm curves
on the dyno is a bit different than running on a prop load curve. But I would
think that setting up the mixture correction table on the EC2 with the MAP port
where it is should still give good results.
On the 3003 TB that I
have, the MAP port connects to two of the three runners. I added an additional
backup port in the third runner. The new port is similarly placed, but is 1”
downstream from the butterfly shaft instead of ½”. When I get running again,
I’ll check to see if the readings are any different.
Al