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Polishing intake manifold

Posted by MGC2912-6317 
Michael Price
Ottawa, Canada   can
I picked up a North American MGC intake manifold that has been opened up for HS8's at the carb interface. The rest of the manifold has casting debris, seams and a finely pebbled surface. Those of you who use this set up, did you polish the whole manifold intake tract? Thanks.

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robert kirk
Davenport, Iowa, Rock Island, Illinois, Clearwater, USA   usa
If you suspect you will actually gain an aspect of efficiency the entire flow surface is typically polished. There is some credence to a slight interference "step" where the carb mates with the mani. This creates a vortex which may or may not assist flow.

The more saliant issue is however, twin carbs each feeding 3 holes where every time a sister valve slams shut, in effect it creates a back draft for the charging cylinder. The reason a balance tube is used is a design to weaken or augment the effect of that back draft.

Long story short, on a street car it probably won't amount to beans but it might make you feel better and Mr John Powers swears that his engines gain 5 HP when ported & polished and that is just sitting at the stop light....hot smiley



Regards,
Robert Kirk
kirkbrit@yahoo.com
563 323 1017
Moss distributor UK importer
Beat or match any retail/delivered quote



Edited 1 time(s). Last edit at 12/22/2011 01:26PM by kirks-auto.
Kirk's Auto Parts for your classic British and Italian car. 30 years in business.
Bob in Vancouver Avatar
Bob Elwin
Vancouver BC, Canada   can
1968 MG MGC
1969 MG MGC GT
Michael, Problem is you can't get to all the interior surfaces with a die grinder - although a flexible attachment for a Dremel might do it. I used flapper wheels on a die grinder and got a fairly smooth surface on the areas I could reach.
It probably is more cosmetic appearance than real-world horsepower you will generate for your efforts but all good fun. Balance pipe is necessary for a reasonably steady vacuum signal to the carburetors. Cheers, Bob.
Michael Price
Ottawa, Canada   can
Thanks, Bob. Maybe I'll content myself with cleaning up the casting debris.

Cheers,

Mike

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William Panos
Colorado Springs, USA   usa
1969 MG MGB
1969 MG MGC GT
I agree. You might just want to clean up the flashing. A little roughness on the intake manifold walls promotes turbulence at the wall surface, which is supposed to help keep the atomized fuel from separating from the airstream and condensing on the walls. So, manifolds handling dry air streams (exhaust, port fuel injection manifolds) should be mirror polished to promote a laminar flow along the manifold walls. However, intake manifolds handling wet air streams benefit from a little wall roughness, that pebble grain left over from the casting process.
FWIW,
Bill P.
Michael Price
Ottawa, Canada   can
Thanks, Bill.

Cheers,

Mike

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Al Ain, United Arab Emirates   are
In reply to # 1935128 by coloradomgc A little roughness on the intake manifold walls promotes turbulence at the wall surface, which is supposed to help keep the atomized fuel from separating from the airstream and condensing on the walls.
Tuning legend Bill Blydenstein always swore by that theory - and he knew a thing or two about it.
B-racer Avatar
Jeff Schlemmer
Minnesota, USA   usa
1958 MG magnette
1971 MG MGB "POS"
1973 MG Midget MkII
2006 Dodge Charger "Daytona"
2008 Ford Super Duty
A coarse interior surface deoes help keep the fuel suspended. That doesn't mean you want ridges and huge lumps. Use a simple carbide to get rid of the big flashing, the rest should stay.

You do want a step tyo function as anti-reversion if you have the material to support it. Essentially the hole exiting the carb should be slightly smaller than the hole in the manifold, the same scenario at the head, so airflow is less likely to revert out toward the air cleaner when the intake valve slams shut (from another cylinder using that half of the manifold.)



jeff@advanceddistributors.com
Advanced Distributors Lucas distributor repair, restoration and parts supplier.
chuckmosher Avatar
Chuck Mosher
Houston, TX, USA   usa
1962 MG MGA MkII "Othello"
1968 MG MGC GT "Tordos"
Jeff,

Not sure I understand the purpose of the "step function". Any place I could learn more about this and intake / exhaust porting in general ? I'm going to take my first try at this kind of work on an 1800 3-main that is going into my MGA, then on to triple intakes for the C.

Thanks,
Chuck

robert kirk
Davenport, Iowa, Rock Island, Illinois, Clearwater, USA   usa
Chuck,
As Jeff and I mentioned it creates a vortex...a mini ram charge or in his words "anti-reversion". There are a lot of theories about air flow dynamics but the "step" is a pretty much accepted fact.
This entire thread is mostly about the upper end extremes of meting every ounce of potential horse power. If you are not running at near your red line none of this is a practical source of HP or torque.
The biggest gains for a street car are getting a proper cam for the power band you seek with closer attention to the head itself rather thank monkeying with manifolds. When you have same side flow as with most all BMC engines, the neighboring valves upset fuel charged air...it maybe worth a little effort but if your engine build and lift duration are not enhanced all the die grinding in the world will be mostly therapeutic and not HP generating. The SU carb itself can be somewhat of a limiting factor as optimum efficiency does not occur until the needle is fully exposed....pedal on metal all out...eye popping smiley



Regards,
Robert Kirk
kirkbrit@yahoo.com
563 323 1017
Moss distributor UK importer
Beat or match any retail/delivered quote
Kirk's Auto Parts for your classic British and Italian car. 30 years in business.
jharden73 Avatar
John Harden
Birch Run, MI, USA   usa
1973 MG MGB MkIII "The Grine Hawnet"
1973 MG MGB MkIII "The Grine Hawnet Pic 2"
Michael,
Rob Burgess has a nicely written section on porting and polishing in his book "How to Power Tune MGB Four Cylinder Engines."
A must-have for any speed and power interest.
Swamperca Avatar
Swamper CA
Nor Cal, USA   usa
1969 MG MGB GT "Rat"
1969 MG MGC GT "A 69 MGC"
1970 MG MGB GT "Widow"
1971 MG MGB "Ruby"
1974 MG MGB "Groovy B"
Sean Brown will P&P the head also do intake manifold for maximum flow.

Michael Price
Ottawa, Canada   can
Thanks, John et al. By my measurements the difference between the output end of a 2 inch SU and the input port of the stock NA MGC manifold is about 1/16th of an inch at the circumference. Giiven this discussion on "steps" perhaps it is not necessary to open up the manifold. Maybe a bit of tapering in the distance piece would provide the transitional step.
robert kirk
Davenport, Iowa, Rock Island, Illinois, Clearwater, USA   usa
I am not following you Michael but the idea is to have a slightly larger OD on the manifold at the point the SU delivers its fuel laden charge. 0.0625 all round or would seem to be close to ideal. The idea of porting and polishing beyond that is to make sure the "chute" is at least the same size to the head. As mentioned a slight roughness in the manifold can be beneficial but there can be a point of diminishing return as the rougher it is the more surface area the fuel mixture must flow across. The most worthwhile effort to porting and polishing is spent on the head itself and even more benefit if, in this case, you have a triple intake set up as opposed to a dual intake manifold.



Regards,
Robert Kirk
kirkbrit@yahoo.com
563 323 1017
Moss distributor UK importer
Beat or match any retail/delivered quote
Kirk's Auto Parts for your classic British and Italian car. 30 years in business.
Michael Price
Ottawa, Canada   can
Sorry, guys, I got the step theory ass backwards, comme d'habitude. In flow dynamics as in anatomy one definitely does not want a back flow.

Mike

robert kirk
Davenport, Iowa, Rock Island, Illinois, Clearwater, USA   usa
Michael,
The idea is you want to flow toward a bigger canyon not into a butte/wall...



Regards,
Robert Kirk
kirkbrit@yahoo.com
563 323 1017
Moss distributor UK importer
Beat or match any retail/delivered quote
Kirk's Auto Parts for your classic British and Italian car. 30 years in business.
Big6Mark Avatar
Mark Miller
Springfield, OR, USA   usa
1960 Austin-Healey 3000
1968 MG MGC "Red Tail"
1968 MG MGC
1968 MG MGC GT "The Wreck"
1969 MG MGC GT "The Lump"
Michael:

The step or anti-reversion function at the junction of the carb-to-manifold or manifold-to-head is such that the flow will always be easier over a step into a larger port from a smaller one. And much harder in the opposite direction.

Anology like Mr, Kirk's: A speedboat can go over an overflowing dam easier than an empty one.

Some header manufacturers have utilized this for years making a slightly crimped funnel end to the pipes they weld into collectors. The exhaust gasses will be forced to go out the collector and the resistance to revert back to other cylinders is much greater.

The step from the 2" carb throat can be into a spacer that is slightly larger than the 2" to make sure there is no blocking edge in the spacer, but the spacer can be tapered down to a bit smaller by the time it reaches the manifold. After all, the 2" throat is 2" MINUS the cross section area of the throttle shaft. (About 1.789" diameter IIRC) As long as it is smooth and of sufficient diameter for maximum flow it won't matter. And easy to do if you use the original 1/2" thick spacer for the HS6.

The inside of the manifold can be smoothed a bit, but without a WET flowbench you don't know how smooth. And if I recall David Visard even proposed that a spiral pattern in the manifold ports would be helpful as well. Like left by pulling out a flapwheel (Sand-o-flex) while turning?

C Ya,
Mark
chuckmosher Avatar
Chuck Mosher
Houston, TX, USA   usa
1962 MG MGA MkII "Othello"
1968 MG MGC GT "Tordos"
Thanks for the discussion, guys, fits better with my understanding of fluid flow dynamics as described by the Hagen-Poiseuille equation for non-laminar transitional flow in conic sections where the Reynolds number exceeds the laminar to turbulent flow transitional value in the range 2000-4000

robert kirk
Davenport, Iowa, Rock Island, Illinois, Clearwater, USA   usa
Re-elucidation is always a pleasure. eye rolling smiley



Regards,
Robert Kirk
kirkbrit@yahoo.com
563 323 1017
Moss distributor UK importer
Beat or match any retail/delivered quote
Kirk's Auto Parts for your classic British and Italian car. 30 years in business.
Michael Price
Ottawa, Canada   can
Thanks, for the explanation, Mark. Presumably the same principle as the expansion chamber on a two stroke exhaust manifold, to keep the spent charge from mixing with the new charge.

Cheers,

Mike

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