SSI carburetors and velocity stacks

[Rick]

I'm committed to getting an SSI carburetor to work on my single projects(s), and am trying to resolve some of the problems they have- low speed/idling seems to be the reason most people give up on them.
I'll attach a picture of the data David Vizard collected while he was flow testing 'ram pipes', what I call a velocity stack, but not sure if either is a good term. If you're not familiar with David Vizard, he's one of the well known US engine builders- sort of a Smokey Yunick, C.R. Axtell, Jerry Branch, etc, guy. There was a time in the US when these backyard engineers did amazing work, but that's another story.
It may be hard to see, but he uses a bare carburetor as a baseline and tested different ram pipe configurations to see how they affected the flow. The standard SSI velocity stack is close to his test #4, which reduced the flow by 5.5% over just the bare carburetor. Not good.
You can see what he found to work- I expect he tested dozens if not hundreds of intakes to distill down to these examples.
Here's his data- I got if from a web site, maybe not entirely ethical, but I'm not trying to make any money from his work, so I hope he won't be upset.

velocity stacks.jpg

Here's the SSI stack:

SSI velocity stack.jpg

So, a little work on the front end of an SSI should improve the flow over the standard stack- I'm hoping it will help the low speed running- since there's so little air flow at idle every little should help.
And, here's a picture of a Mallosi modified Dellorto that I always thought had a bizarre intake, but it turns out that the folks at Malossi knew what they were doing. I traded a bunch of F1 bikes and parts to get my single parts, so it's too late to turn back now.

F1 Malossi.jpg

Rick

[wcorey]

Flow is only one part of the equation. I'm sure you know that velocity stacks don't only facilitate flow but also regulate the length of the intake tract, which affects/tunes the resonance/pulse of what goes in and out of the combustion chamber. A large number of things come into play there such as exhaust length/volume and at what rpm range you'd like to have peak torque and/or hp. Stacks come in a range of lengths for good reason, every application is different and there's no one-size-fits-all...
I've done a lot of performance tuning on the newer belty stuff and something that comes immediately to mind is the Mallosi short manifold conversions on the 900 2-valve motors. The longer stock manifolds gave increased torque in the midrange and the short ones higher hp way up top. You couldn't have both so generally only the track bikes used the Mallosi's. On the 4-valve stuff it got really crazy where there were 'dips' on the power curve that were significantly affected by thigs like airbox volume and how for away the stacks were from the airbox floor or air cleaners. And then when you change one thing, it changes everything else right down the line...


Bill

[Bevel bob]

My 250 (which had been rebuilt from a racer) came with an Amal mono which has a huge reverse rolled lip at the bell mouth , It is now clear that this is the most efficient shape , it looked very ugly!! But from the information posted it should give 10% better flow than the SSi trumpet. I may have to give it a try.