[Zmagnette] Stub stacks

Kelvin Dodd doddk at mossmotors.com
Fri Nov 15 16:28:05 MST 2013


First off they are pricey.

There are two types of stub stack available for the 1.5" SU. Cast and machined. I've not been impressed with the design of the cast ones which run about $20 each. The machined ones look like they will do the job, but run about $32 each.
APT offers both types: http://www.aptfast.com/ListItems/SubCategory/SU%20Carburetors/Stub%20Stacks%20and%20Ram%20Pipes.aspx


At lower rpm the carburetor piston offers more flow blockage than poor fluid dynamics at the air filter plate. At WOT, it's a different matter.

There are no doubt all kinds of peaks and valleys in fluid flow during the dynamics of accelerating and cruising up to 3500 rpm. I was told by one of my engineering teachers that anyone who professes to be an expert on fluid flow is a liar. He stated that it is still more of an art than a science and a dark art at that. In fact, after receiving a doctorate in the field he determined that aircraft are kept in the air by smoke and mirrors because the dynamic forces at work are too complex to be understood by the likes of mankind.

If you have to run a pancake style filter, then by all means use a stub stack. Before they became available for the DCOE, I used to cut down the standard long trumpets and braze them up into short stacks. They will make some difference, though I doubt it's going to be really noticeable.

Just work on the basics first.

From: zmagnette-bounces at autox.team.net [mailto:zmagnette-bounces at autox.team.net] On Behalf Of Allen Bachelder
Sent: Friday, November 15, 2013 11:03 AM
To: List for the Z Magnette Group - North America
Subject: Re: [Zmagnette] Stub stacks

Kelvin - your point is well-taken. Are you saying that even if everything else is spot-on, it's still not worthwhile to mess with velocity stacks for those of us who spend most of our lives below 3500 rpm?

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