[Zmagnette] Stub stacks

Fletcher Millmore gofanu at cust.usachoice.net
Sat Nov 16 02:19:07 MST 2013

For the record:
I consulted extensively with Steve Gyles re the design and manufacture 
of the stub stacks for the MGA filter cans, since duplicated by Steve 
Ash and others; it is all documented in the UK board archives, though 
there were a number of private communications. What he made finally was 
almost exactly the design I suggested, less a few subtle details.

I worked from my developed understanding from years of reading and 
observation. The specific critical detail that a 1/4" entry radius would 
give near optimum results came from Vizard's flow bench tests. It should 
be noted that Vizard was testing flow entry from free air, without a 
nearby cover plate, and also, without a backplate. One startling thing 
he found was that air is drawn from behind, or below, the actual pipe 
entry, so he used a 270 degree rollover on his free air stacks. Since 
the carb configuration and space limitations on MGA establish the need 
for a backplate, the external taper was chosen to reduce the effect of a 
backplate as much as possible, giving a 90 degree plus the taper 
rollover radius.

It is my understanding that a cover plate should be more than 1.5x the 
bore diameter from the entry to avoid flow restriction. Systems with 
less clearance should compensate as much as possible; the Bristol 
engine, for instance, has very thin filter cans as a result of space 
limitations - and Bristol were aero experts without compare, so I pay 
attention. They used a "flow turner" stamped into the cover plate. Since 
Steve's goal was to make a drop in and look stock part, modifying the 
outside of the can was not on, so we did the best possible withing the 
space available. This meant reducing the thickness of the stub stack to 
the minimum possible to achieve the 1/4 radius without leaving a sharp 
or indeterminate edge at the carb bore entry, and in order to keep the 
entry as un-obscured by the cover plate as possible. Note that some of 
the comments in this thread, regarding stub stack depth versus can 
depth,  are totally out of line with this reasoning.

I did not know that Barney had posted all that info Robert linked: 
Note that there are several more pages following this one, with dyno 
tests and driving impressions from numerous folk. A most valuable 
resource, thanks guys.

Vizard indicates that about 5% flow improvement can be obtained with a 
1/4" radius in his free air stacks, over a plain cut pipe. Since power 
developed is a function of air ingested, assuming reasonable fueling, it 
can be taken that power increase should be about the same as airflow 
increase. Given the various limitations (including the obstruction of 
the posts inside the MGA cans), I expected and hoped for maybe 4% 
improvement at best. Consequently, it was gratifying to find the 
measured improvements of power very near 5%, which leads to the question 
- why? The question gets bigger when we realize that the tested engine 
was not optimized for fueling, or even in very good tune.

I think the key here is in the reduction of turbulence around the jet, 
and possibly its effect on the vacuum which lifts the air piston. Note 
that dyno tests are done at full throttle, so the air piston is almost 
fully up at any reasonable test point. The unvarying reports of improved 
low speed running and throttle response indicate that improvements are 
happening well below the maximum flow range where the expected 5% 
improvement lies. If you think about it, at low throttle openings, the 
entire active region of the carb throat is within 1/4" of the bottom 
edge of the bore, the edges of this area are even closer, and in any 
event, the jet is always in this area - this is just where we might 
expect turbulence to have the greatest ans least predictable effects. 
The reported noise reductions are also an indicator of reduced 
turbulence - clean flow does not make noise, turbulent flow does.

The Bernoulli effect here is for pressure on a surface which has a fluid 
flowing across it, that would be the surface with the jet in it, and 
that pressure reduction is what pulls fuel into the airstream. If there 
is turbulent flow across the surface, that pressure drop will vary 
randomly, and so will fuel delivery: mixture will never be predictable. 
I expect there is room for some additional optimization of fueling here, 
which might well leads to further improvement in both low and high 
output situations.

At the time of the first test of Steve's parts, I suggested, or begged, 
for somebody to go down to Peter Burgess and run some part throttle 
acceleration tests. Peter's dyno has trhe capability of measuring 
transient effects very quickly, and some tests with blocked throttle 
stops might have yielded some good info here. Unfortunately, I'm not 
there, and nobody seemed interested.

I think these user reports put Kelvin's guess - "I doubt it's going to 
be really noticeable.  " out. And, for Allen and others interested in 
less than full throttle use, I'd say some attention to these details 
might be rewarded well.

While it is not possible on the T/Z plenum to get the full max throttle 
effect, some careful radius work on the inside of the plenum at the carb 
face might well give the light throttle benefits the stub stacks do. 
What you want is as close as possible to a 1/4" radius on the inside 
face of the hole, especially on the air approach side and bottom, but 
NOT introducing a step to the carb bore. I might say that there is 
nothing preventing a bit of judicious radius work on the carb bore 
itself, if you also open up the plenum/filter face to avoid the step. In 
fact, one thing I was a bit concerned about with the folk filing out 
holes was that they might introduce such a step by making fits too 
sloppy - the Gyles dimensions are the max possible for the bore in the 
stub stack, without enlarging the carb bore.


On 11/16/2013 12:21 AM, Allen Bachelder wrote:
> Thanks Kelvin,
> And I'lll look forward to the cold air intake you have in the works.
> Then can we have a discussion about why and how cold air intakes work?
> Cheers,
> Allen
> */****************************************************************/*
> */Allen & Florrie Bachelder   =iii=<
> Spring Creek Home for Wayward MGs/*
> */'57 ZB, '65 B, '69 C/GT, '73 B/GT/*
> */North Street, MI 48049, USA/*
> */http://www.mgexperience.net/member/bachldrs/*
> */****************************************************************/*
> On Nov 15, 2013, at 6:28 PM, Kelvin Dodd wrote:
>> Allen.
>> 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
>> Second.
>> 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>[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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