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Question On Airflow From Filter To Inlet Ports

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#1 DamoMini


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Posted 27 June 2020 - 02:08 AM

1978 Mini 1000. Head is off at the moment - burned an exhaust valve :goaway: 


While I wait for tools / parts I examined my carb / inlet setup.


Carb is an HS4,  K&N conical filter with a small stub stack installed.  There is a spacer between the carb and the inlet manifold which is a Minisport alloy unit to which either HS4 or HS6 carbs can be installed (1.75 inch opening).


In looking carefully at the parts I came across a couple of things that I would like to understand better,  I am not looking to replace any of the parts at this time just figuring out if I need to modify or remove them


(1) The stub stack inside the K&N filter has a slightly narrower ID (~ 1/16th) than the inlet for the HS4 creating a small step for air entering the carb. Should I remove some material from the stub stack, leave the step as is, or just remove the stub stack.


(2) The throat ID leaving the HS4 is 1.5 inches but a chamfer has been added (not sure if this is stock or not) adding another 1/16th to the ID.  The spacer is 1.5 ID so again, there is a step.  Should I remove some material from the spacer to match or leave it alone.


(3) Finally, the inlet ID to the manifold is 1.75 inches which I understand means you can mount either an HS4 or an HS6 to this manifold.  But this makes a pretty big step for the air leaving the HS4 carb and entering the manifold - is this okay or should I consider tapering the ID of the spacer to "blend" the air flow into the manifold. 


Thoughts / Comments appreciated.

#2 Ethel


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Posted 27 June 2020 - 06:21 AM

A step from larger to smaller is obviously an obstruction, but it'll be exacerbated by sharp edges that can create turbulent flow. Going from small to large is much less of a problem. If there is turbulence it tends to be trapped in eddies  behind the step with laminar flow passing over them unhindered. In your induction system this can even be an advantage as it presents an obstacle to the flow reversion you get when an inlet valve closes and the induction charge "rebounds" like a cut rubber band.


If I'd noticed that with stub stacks I must've forgotten, but it's likely deliberate because of the above. Especially with the tendency of gaskets to not be that accurately made and to squash out when compressed. I have attempted to flow carb spacers, but by introducing a slight transition that won't cause turbulence, maximum of 15 degrees slope, which will leave some step down to the manifold. You could do similar and widen the spacer out  so there's no step up from the radiused carb edge.

#3 Gilles1000


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Posted 27 June 2020 - 06:21 AM

First, if you can, get a copy of David Vizard's book "Tuning the A-series engine".

Very helpful to get loads of info about tuning the engine, even for economy.


To answer your questions, from what I remember in the book.

Going from a smaller to bigger port is not a big issue. as you do not reduce the diameter and therefore create turbulences which reduce even more the working ID.

But I do not know how big the difference can be. Same for the exhaust side.

You will have some turbulences in the corner at the step, but they do not affect a lot the flow. It's a huge work to adjust them, and as per Vizard does not improve a lot.


For 3), you could tap the spacer on it's whole thickness to smooth the step to the manifold.


Hope this helps

#4 sonscar


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Posted 27 June 2020 - 06:24 AM

I would try to match the parts as much as possible,though bigger is apparently not always better.Your call,will you notice the difference?maybe yes maybe no but you will feel as if you have done something.Steve..

#5 Dusky


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Posted 27 June 2020 - 06:51 AM

I bolt everything up and add small pins on both the carb mounting face and the intake manifold. Then take the dremel out and smooth everything out.

#6 DamoMini


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Posted 27 June 2020 - 05:25 PM

Thanks all. I’ll do a little work on the spacer as it’s the only large-to-small transition in the flow.

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