Carb-intake manifold-intake port matching

The question though is how scientific this has been.

Have you dynoed the result, or some sort of performance yardstick to measure by.
Or is this seat-of-the-pants-it-must-be-better type tales....

Ugh yes and no Rohan. Peel has about 3 seasons in prime conditon before new Combat head and new Amal carbs and 2-1-mega fall off knocked off any bragging rights for another season comprehending all that went wrong and how to put back right while dealing with way bigger priorities she was still my daily life as long as nil ice the throttle stuck open and killed her off. You know my tales and sticking to em with more to come. Made plans to reproduce Peels Combat combo again against the grain down to ligher smaller flywheel beside ugly intake path and sewer pipe exhaust. Power power power BFD, if can't hook it all up, hehe haha hoho to the funny farm. Good engines make ya listen up and draw near, great engines will involuaarity recoil you back and cause piercing pains like a Mt. Lion Scream and are more scary than triling to hold trigger on full auto.

hobot said:

Its fairly easy to mimic some of my Ms Peels intake by crafting an intruding rough edged gasket at head, all the way around or more or less of a crescent. Of course if something else in chamber or exhaust does not allow much more flow then may not work as advertised.

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You are right to make this mod at the head if the power increase mechanism is more mass flow from boundary layer boost to help negotiate turn at valve guide. Getting the flow tripper closer to the valve is the right thing to do in this case, and a sharp edged weir-like obstruction is a better tripper than a step. OTH, if the power boost is from better fuel distribution and atomization, then putting the tripper back at the carb / manifold interface is the better.

It may take a dyno to tell any difference ... I don't think Hobot assonseatdynotester can tell.

Slick

What is the range of velocities in a 30mm port on a standard commando doing between 3000 and 7000 RPM. A race bike with a close box usually stays in the 5000 to 6500 RPM range. Do they reach a sonic situation ? I've always had my doubts about flow bench testing of cylinder heads..

Yep sir its takes rather more than just boundary layer fuel mixing to shock/scare the daylights out of a life long G force seeker like me. While the rest of you Cdo workers are thinking more more more, my P11 and Peel in prime made my sane part scream ENOUGH to let up.

I am not selling anything but how to get what I was on still tingling form the power highs a decade later, so only pointing out what ya may be missing out on in old school combo. Five expedient-nesscessary ONLY things combined to keep in my 1st clunker Combat I just wanted to be done with and rid of after my modern and track school cycles spoiled. Small port std head, 34 mm step and crude cut gasekt, 2-1-mega after end plate 12.ga flaired and and and and and and and and and and and and and and and and and and and and ditto ditto ditto ditto ditto ditto, Bob Paton robust Rump link, Byran Tryee compliant Breast support and hobot springy Head control so do not think much about brake traction nor give front tire traction much attention or care so only have to focus on not having Peel run right out from under and what may be in the road over crests and around very sharp decreaser taking care not to crowd 100 hp+ hot rods nor leave then in dust too quickly while the can still see me so they do not try to take Peel rates and lines and likely die before Peel really kicks up heels in solo delights.

None of what made Peel work is acceptable or recommenced go fast paths by anyone on this forum or a few others - so wish me less cascades of failures and will resurrect Peel power in our live times to objectively judge any way ya like. Peel got passed a total of 3x in severe contests over 3 seasons as had so much lead on anythng after a few easy turns they only caught up when Peel let off from over 130 mph d/t blind crests I learned about school bus stop and farm truck crossing hazards so they passed me in blind oncomming lane for sense of how wild serious they squids were going about 140 by that point, & one 1200 RS BMW took Peel after 90 mph about 5 miles outside my village in longish open and a Ducati Monster in same place when Peel out dragged it eariler and busted 3rd gear cog teeth off doing it so ran in 4th only after 40 mph to stay ahead till poor ole Duci could work its gears and get over 100 before slowing up for more fun turns. Fun Fun Fun till fate took Peel T-bird keys away.

acotrel said:

Do they reach a sonic situation ?

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What does this question actually mean ?
Are you really asking if the flow goes supersonic ??
And does it really matter if it does ???

I have no idea, never investigated that side of things.
Will be interesting if this discussion goes anywhere with this.
For a lot of bikes with quiet mufflers, intake roar is a bigger problem than the exhausts.
Does that mean they have already gone supersonic in the inlet tract ?

acotrel said:

I've always had my doubts about flow bench testing of cylinder heads..

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Is there anything you are positive about, O doubting Thomas.... ?

A similar discussion on this forum someone asked about combining 30mm intake ports with 32mm carbs. I never saw anyone address the question but it seems like it would create the same situation as with the BSA. I wish it would be that easy. I have a set to try with a combat head and stock cam. Has any one played with that combination :?:

Sonic intake?
A calculation of 372.5 cc (750 Commando) of air being drawn through a 32mm (Combat) or 30mm (Commando) orifice stop/starting past a valve head for one piston stroke of a four cycle engine at (say) 7000 RPM should do it for any given molecule.

Has any manufacturer or aftermarketeer also offset (any direction) and angled a mismatched carby/manifold so the annulus becomes a crescent step to: 1. enhace pulse blocking, and/or 2. create a greater turbulence inducer?
Ta.

Thats not as simple a calc as it seems, its a pulsed flow.
And without knowing the timescale precisely, or the delay in drawing pulses,
or % cylinder filling, the calcs could be miles out....

Rohan said:

Thats not as simple a calc as it seems, its a pulsed flow.
And without knowing the timescale precisely, or the delay in drawing pulses,
or % cylinder filling, the calcs could be miles out....

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Practically, use a flow meter and look for greater than 340 metres per second.
Ta.

Do not think Commando can intake fast enough flow to worry about super sonic choke flow but could suffer from subsonic choke if too narrow and streamlined ports or a step or ridge in the wrong place. Peel 920 @ 8000 rpm with 32 Combat ports, drag cam and 10 PSI boost clac out to a bit over 70% of sound barrier shock wave choke, which implies could handle more boost getting to 8grand if I ever dare.


http://www.raetech.com/Restrictors/Rest ... nction.php

Calculator For Figuring Choke Point Of Ports
http://www.wallaceracing.com/chokepoint.php

Danno said:

Before I assembled the carbs to the motor, I matched the junctions carb-to-manifold and manifold-to-intake port. I was shocked at how mismatched these junctions were. Most of the work was at both ends of the manifolds, although there was a little grinding/sanding/polishing to be done a the carb throats to make everything the same. Was I wasting my time, or will this make it run better, stronger, smoother?

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Danno - you have been very quiet since you started this thread! I think your question was answered in the first reply:

From what I have seen -up to about 1mm mismatch doesn't seem to make much difference on a 32mm port. Smaller higher velocity ports need a good match. Jim

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ugh, Flow fysics for funny farm fun, as Peel and BSA research found bigish carb into smallish port without smoothing nor taper worked surpringly well so just accademic philosphy quessing if not testing which way works best in a particular engine d/t port dia, lenght, bends, cam, valve size and bore stroke ratio. Both my cases of lip steps into head helping were in short path intakes if that matters. Ever wonder why some hi end frisbies have sharp stepped rims instead of smooth tapper leading edges? Those with time on their hands could try the rough intruding gasket trick to see if anything noticed. I've wondered what effect the two flow exposed inner bolts of 2-1 manfiolds did, helping or hurting in its Y shape & shortness.

I had some personal communications with Kernel65 who reminded me many auto carbs have a secondary venturi downstream of the fuel induction venturi. After some further thought, I now have an explanation for the venturi that the Beezer boys used in lieu of a step.

As most know, the venturi section speeds up the flow while simultaneously reducing the pressure in the converging section, while the reverse occurs in the diverging section (Bernoulli Law). The flow that exits the venturi tube has, neglecting the very slight friction loss in the tube, the same energy and streamlines as the flow entering the tube. In short, what goes in, comes out virtually unchanged. Fuel droplets, however, being heavier than air molecules, do not follow the streamlines of the airflow, but will cross streamlines and hence disperse. This lends evidence that the BSA power boost is attributed to more efficient combustion resulting from increased fuel atomization and vaporization, rather than increased mass delivery.

In auto carbs and the typical auto manifold, the secondary venturi is a furlong from the twists and bends of the valve guides, and can be there for no other reason than to assist in breaking up and dispersing the fuel by the mechanism described above. Moreover, in the auto carb, the secondary venturi is in the center of the flow and thus in no place to affect the boundary layer energy.

Slick

Aw slick intake path areas that sling fuel droplets out of the main flow d/t mass not swinging the bends as sharply as lighter air molecules does not happen in the straighter parts or before bends. The fuel sling out issues only concerns short intervals of sluggish response as once throttle set at next postiton new fuel piles up and shifts equilibrium back to about neutral, ie: as much fuel landing as leaving back into flow into chamber. Ventui after main carb throat is more to avoid over or under feeding ports in a single manifold that fits in available space or to meet flow restricted race even rules. Do not know of more polite way to say the steps or lips on the scale of BSA and others with offset faces on purpose is d/t streaming extra flow into chambers not extra evaporating fuel off surfaces better - although that could well be an added bonus. If looking deeper into fuel drop dispersal and richness conditions inside of chamber to make most power its similar to water injection wanting some drops to make it into chamber, while fully evaporated fuel or water is rather better for efficient mileage at less than WOT. There is some minor cooling of fully evaporating fuel before chamber but that is not as significant as being able to pack more in. I had to delve into this for Peel against the grain combo to exist off pump gas and not need water injection till making some boost on fast opening throttle then at at throttle opening it settles back to nil boost and no water to prevent detonation. Only one way to know if applies to a particular engine or not as not universally beneficial or most intake would be stepped and lipped before head ports.

Rohan when velocities approach the speed of sound the dynamics are considerably different to those at low velocities. I doubt that normal flow benches can replicate the situation. Most of the noise from bike is impulse noise which does not register on many sound meters. I suggest that extrapolating results from a flow bench might be illusionary. Over to you Professor.

acotrel said:

Rohan when velocities approach the speed of sound the dynamics are considerably different to those at low velocities.

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And you know this from your flow bench testing ???

My guzzi made more noise from the (unsilenced) inlets than the zorsts.
They most certainly had to do something about it, later models had airboxes purely for that reason.
Nortons went to the plastic Mk A airboxes.

Please don't pontificate on subjects you know little of....

hobot said:

Do not think Commando can intake fast enough flow to worry about super sonic choke flow p

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And this is based on what ??

Measuring speeds in ports, or seat-of-the-pants 'engineering' (guesswork) ?

I put my ear to the back end of my carbie and I could hear the ocean.
Perhaps you try it with the bike running to see if there really is a sonic boom!
Ta. <-

That 'sonic boom' would be your eardrum being sucked out of your soul ?

The howl that comes out under full throttle could be all sonic booms ??