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Polishing intake manifold

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Polishing intake manifold #1
MGC2912-6317 Michael Price Ottawa, Canada	Posted: December 21, 2011 01:44PM Registered: 2 years ago Posts: 124

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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Re: Polishing intake manifold #2
kirks-auto robert kirk Davenport, Iowa, Rock Island, Illinois, Clearwater, USA	Posted: December 21, 2011 02:13PM Registered: 4 years ago Posts: 13,090

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
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Edited 1 time(s). Last edit at 12/22/2011 01:26PM by kirks-auto.

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Re: Polishing intake manifold #3
Bob in Vancouver Bob Elwin Vancouver BC, Canada 1968 MG MGC 1969 MG MGC GT	Posted: December 21, 2011 07:05PM Registered: 5 years ago Posts: 187

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.

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Re: Polishing intake manifold #4
MGC2912-6317 Michael Price Ottawa, Canada	Posted: December 22, 2011 01:22PM Registered: 2 years ago Posts: 124

Thanks, Bob. Maybe I'll content myself with cleaning up the casting debris.

Cheers,

Mike

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Re: Polishing intake manifold #5
coloradomgc William Panos Colorado Springs, USA 1969 MG MGB 1969 MG MGC GT	Posted: December 22, 2011 02:07PM Registered: 5 years ago Posts: 195

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.

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Re: Polishing intake manifold #6
MGC2912-6317 Michael Price Ottawa, Canada	Posted: December 22, 2011 02:30PM Registered: 2 years ago Posts: 124

Thanks, Bill.

Cheers,

Mike

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Re: Polishing intake manifold #7
Richard Moss Al Ain, United Arab Emirates	Posted: December 29, 2011 06:30PM Registered: 1 year ago Posts: 245

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.

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Re: Polishing intake manifold #8
B-racer Jeff Schlemmer Minnesota, USA 1958 MG magnette 1971 MG MGB "POS" 1973 MG Midget MkII 2006 Dodge Charger "Daytona" 2008 Ford Super Duty	Posted: December 29, 2011 08:47PM Registered: 6 years ago Posts: 12,506

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

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Re: Polishing intake manifold #9
chuckmosher Chuck Mosher Houston, TX, USA 1962 MG MGA MkII "Othello" 1968 MG MGC GT "Tordos"	Posted: December 30, 2011 07:21AM Registered: 3 years ago Posts: 189

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

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Re: Polishing intake manifold #10
kirks-auto robert kirk Davenport, Iowa, Rock Island, Illinois, Clearwater, USA	Posted: December 30, 2011 08:47AM Registered: 4 years ago Posts: 13,090

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
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Re: Polishing intake manifold #11
jharden73 John Harden Birch Run, MI, USA 1973 MG MGB MkIII "The Grine Hawnet" 1973 MG MGB MkIII "The Grine Hawnet Pic 2"	Posted: December 30, 2011 09:10AM Registered: 7 years ago Posts: 499

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.

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Re: Polishing intake manifold #12
Swamperca Swamper CA Nor Cal, 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"	Posted: December 30, 2011 12:04PM Registered: 8 years ago Posts: 7,658

Sean Brown will P&P the head also do intake manifold for maximum flow.

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Re: Polishing intake manifold #13
MGC2912-6317 Michael Price Ottawa, Canada	Posted: December 30, 2011 02:34PM Registered: 2 years ago Posts: 124

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.

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Re: Polishing intake manifold #14
kirks-auto robert kirk Davenport, Iowa, Rock Island, Illinois, Clearwater, USA	Posted: December 30, 2011 02:53PM Registered: 4 years ago Posts: 13,090

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
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Re: Polishing intake manifold #15
MGC2912-6317 Michael Price Ottawa, Canada	Posted: December 30, 2011 03:18PM Registered: 2 years ago Posts: 124

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

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Re: Polishing intake manifold #16
kirks-auto robert kirk Davenport, Iowa, Rock Island, Illinois, Clearwater, USA	Posted: December 30, 2011 03:34PM Registered: 4 years ago Posts: 13,090

Michael,
The idea is you want to flow toward a bigger canyon not into a butte/wall...

Regards,
Robert Kirk
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563 323 1017
Moss distributor UK importer
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Re: Polishing intake manifold #17
Big6Mark Mark Miller Springfield, OR, 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"	Posted: December 30, 2011 05:35PM Registered: 5 years ago Posts: 1,219

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

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Re: Polishing intake manifold #18
chuckmosher Chuck Mosher Houston, TX, USA 1962 MG MGA MkII "Othello" 1968 MG MGC GT "Tordos"	Posted: January 01, 2012 07:29AM Registered: 3 years ago Posts: 189

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

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Re: Polishing intake manifold #19
kirks-auto robert kirk Davenport, Iowa, Rock Island, Illinois, Clearwater, USA	Posted: January 01, 2012 09:39AM Registered: 4 years ago Posts: 13,090

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

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Re: Polishing intake manifold #20
MGC2912-6317 Michael Price Ottawa, Canada	Posted: January 01, 2012 01:38PM Registered: 2 years ago Posts: 124

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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