[Zmagnette] Stub stacks

Michael Anderson mjamgb at sbcglobal.net
Fri Nov 15 07:58:56 MST 2013

I respectfully disagree that more air = more noise by default. "Noise" to a mechanical engineer (or most any engineer) is an undesireable by-product of inefficiency, i.e. whatever is causing the noise is sapping energy from whatever the machine is supposed to do (unless it's job is to make noise). Having said that, stub stacks should reduce noise -all else being the same - because the efficiency of the system is increased.

To that end, Allen opens up the issue of Bernoulli. In our carbs Bernoulli is working hard for us at the piston/bridge, not the inlet throat. Without some kind of "stack" Bernoulli is screwing with us at the carb inlet as the "apparent" inlet diameter gets smaller due to the air having to make a hard 90-degreee turn... turbulence is also introduced before the piston/bridge which is not a happy thing for nice even fuel induction.

Those two reasons are what makes the mixture "change" when you put a stack on. Not more air, per se, but quality of air flow. 


Michael Anderson
One of a Kind Stuff/AEU
775 772-2056

 From: Wayne & Isabel Hardy <gwhardy at suddenlink.net>
To: List for the Z Magnette Group - North America <zmagnette at autox.team.net> 
Sent: Friday, November 15, 2013 1:19 AM
Subject: Re: [Zmagnette] Stub stacks

If the carbs are tuned again to compensate for more air to the 
inlet, then more power should always be the result. If more power is “Better”, 
then the answer is yes. But you must adjust for more airflow to get a more 
better running motor..ie more air will need more fuel too if a result is to be 
more power. Maybe a new, slightly richer needle is the ticket to heaven. I know 
the K&N filter people caution that their filters flow so much more air than 
the usual paper ones, that a richer needle, or jet, or slight enrichment 
adjustment may be desirable for best running.
Of course all this extra air flow is also likely to create 
more inlet roar under acceleration. Is that “Better” or not???
I personally run the MGA cleaner assemblies rather than the 
Magnette one for better access to the carb adjustment screws for keeping up with 
summer 100 degree days now swinging to winter 50 - 60 degree days and the 
resultant changes in idle speeds due to cooler, denser air, and the need to 
richen the mix by about 1/8th of a turn.
MG put on the big old ugly oil bath filter for better engine 
life in a variety of dirty dusty operating situations around the world, and for 
more civilized operation in a quieter manner. Their true race motors ran with 
velocity stacks and no filters usually.
From: Allen Bachelder 
Sent: Thursday, November 14, 2013 10:21 PM
To: List for the Z Magnette Group - North 
Subject: Re: [Zmagnette] Stub stacks
cautiously venturing beyond my paygrade here, but are we not talking about the 
Bernoulli Principle: stating that as air enters a conical tube the air pressure 
is maximized, but as the tube diameter conically diminishes, air pressure 
decreases as air speed increases inside the carb.  Without a conical stub 
stack, there is no acceleration in the air entering the carb inlet.  How 
much lift would we get from a cross-sectionally flat airplane wing-top with a 
squared-off leading edge? 
That being said, how can the original Magnette intake plenum perform any of 
this function? Same question for the intake plenums on T-series cars. I would 
think they would be Bernoullian disasters.  Would a Magnette engine always 
run better with stub stacks rather than the original plenum?
Curious in Michigan.
& Florrie Bachelder       =iii=<
Creek Home for Wayward MGs
ZB, '65 B, '69 C/GT, '73 B/GT
Street, MI 48049, 
On Nov 13, 2013, at 7:33 PM, John Park wrote:

Depending  on length it can produce a much smoother flow, some say a bit of a ram effect  which increases power and torque . Longer stacks as in longer runners means  more torque while shorter ones mean more power.  Not lots, but it all  contributes to a freer flowing intake which helps your freer flowing exhaust  to make a bit more power/torque and slightly better fuel economy.  You  note vintage racers usually have anywhere from2.5 to 6 inch stacks. Some with  a stocking for a coarse filter, most just hoping nothing of consequence enters  the throat… I have a set of 2.25 inch stacks in my 3.25 inch K&N’s on my  MGB and they seem to help. Just the ho hum stock setup on my  Magnette.
>From: zmagnette-bounces at autox.team.net [mailto:zmagnette-bounces at autox.team.net] On Behalf Of Richard  Greenberg
>Sent: Wednesday, November 13, 2013 12:34  PM
>To: List for the Z  Magnette Group - North America
>Subject: Re: [Zmagnette] Stub  stacks
>What  is a stub stack? Would it allow for a filter that lies closer to the engine  than a regular pancake filter?
>On  Nov 13, 2013, at 9:53 AM, Allen Bachelder wrote:
>I've  always just used the item #54 from the Moss catalog: http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=28996#top  Are these from APT or Rimmer  better?
>>Always  anxious to learn...
>>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
>>On  Nov 13, 2013, at 4:10 AM, monster at caverock.net.nz wrote:
>>Hi  everyone,
>>>I'd like to get a pair of 1.5" H4 stub stacks and I've 
      tried emailing Steve Ash several times to see if he would make some more, 
      but I haven't got an answer from him.
>>>So... from a light scour of 
      the internet there appears to be two types available at a reasonable 
>>>$32.50 ea -http://www.aptfast.com/ShowItem/101221%20Billet%20Alloy%20Stub%20Stack%20SU%20HS4.aspx
      pair - http://www.rimmerbros.co.uk/Item--i-RL1396ALT
      excluded the K&N ones as they are very rough, by all accounts.
      there any others that people would recommend?
>>>They're going to fit 
      inside K&N filters that are 2 1/8" thick.
>>>Robert (in 
      mailing list
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