I just want to run my calculations by the group and see where I'm going wrong with my assumptions. To know where I'm getting my basic assumptions and formulae go here.

I was investigating the possibilities of using individual throttle bodies for the slant and considered using motorcycle TB's for that purpose. However I wanted to make sure they were large enough.

First how much air do I have to move at most. So max redline will be set at 5,000 rpm and stock displacement is 225 c.i. So 5,000 x 225 c.i. = 1,125,000 c.i. air/minute or approx. 651.04 cubic feet air/min (CFM)

Now, from the website linked above we come away with the assumption that 9.62 sq.in. will support 900 CFM w/1 throttle body. However with ITB's you only have 20 - 25% efficiency with only one intake stroke.

So to support 651.04 CFM we solve for x:

((651.04/900) x 9.62) / 20% = x

which leads to x = 34.8 sq.in. for our total intake area.

Therefore with 6 TB's, each TB must be 34.8sq.in. / 6 = 5.8 sq. in.

now to determine the diameter of the throttle body, which is how they seem to be sold, we solve this for r

A = pi x r x r

5.8 = pi x r x r

where r = 1.359 in.

which gives us a diameter of 69 mm

so we'd need 6 69mm TB's for this to work on the slant. It'd look great, be can that be right?

Even if we go with the higher assumption of 25% efficiency we end up with a 61.73mm TB, close enough to 62mm.

The biggest MC TB's seem to be 46mm.

Who even makes a TB in those sizes? 62 or 70mm

I've heard of a single 70mm TB being more than enough for a single TB, let alone one of 6. So where to from here, where did I make my mistake? Point me in the right direction folks, or I'm gonna' see if I can't make this work with some MC throttle bodies.

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The old white brick

You just gave me a headache

I looked at the stickies on MS forum regarding T.B. sizes before to see what would work and played with the mega manual's calculators and got myself more confused as I went along. I decided on a single 5.0L muskrat T.B.

I hope you find your answers because I too thought this would be a bitchin idea!!!

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Chris'
Autobody
Restoration
Service

That sounds way too large - given that bikes tend to rev 2 to 3 times as high as a slant six, I'd expect the throttle bodies off most liter sportbikes or a Hayabusa to be just about perfect.

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair

4-stroke cycle engines require two revolutions for the cylinders to complete a cycle.

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Joshua

Max HP buildup will need something like 45 mm Weber carbs.

Lou

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Home of Slant6-powered fun machines since 1988

Your on the right track.............

With individual TBs (as well as individual carbs) each carb needs to be sized for peak flow rather than average flow.

With a 6 cylinder engine, peak flow for each cylinder is just about the same as the average flow for the whole engine.

So you would need 6 TBs that can flow 326cfm each (you're off by a factor of 2 because it takes 2 revolutions per cycle)




edited: picture link fixed

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Correct, it takes two full turns of the piston on the crank to complete its cycle, Intake, compression, power, exhaust. But, it only has one downward stroke while the valve is open to fill the cylinder with air/fuel, which is why we use a 25% efficiency for the TB's. A v-8 and our 6 are filling the intake with air rather continuously vs the ITB's filling only when the intake valve is open.

Also, I was looking at the formulas listed on the link and I just noticed that for the initial air requirements, he divides the rpm by 2 so instead of 225 x 5,000 we should be using 225 x 5000/2, why does that occur?

emsvitil, i now know where that error occured, and its taken me till typing this far to realize that, look above at the formulae posted and I see why. Still why do we divide by two?

I refigure it all and throw up work later.

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The old white brick

Oh wait, got the divide by two thing, just had to apply gray matter to it. Two revolutions per cycle, and we're calculating air requirements using RPM. Good god(s), you'd think I'd have gotten that sooner. That's why I post here, always someone to catch a mistake and show you.

Any ways, ran through the math again using that insight (/smack) and 25% efficiency and we come away with 22mm TB's which now sounds really small, however with the 20% effic. rating we come away with 24.4mm so 25mm would work. Now, does this sound right, or still too small?

Now I know Lou said that you'd need 45mm Webers. So it makes you wonder if those webers are too big and losing a bit of throttle response from having too big an initial tip in. That's just speculation, I've never had the pleasure of driving in a Slant powered ride with 6 carbs, so I just don't know.

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The old white brick

45mm Weber carbs have smaller venturis than their throttle plates. They are commonly referred to as chokes. In my Haynes Weber book I found a 3770cc using three 45 DCOE carbs and they used 33mm chokes. The big 4 liter Aston Martin 6 also used 45 DCOEs, but with 40mm chokes.

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Joshua

Cameron Tilley has been using the 40mm chokes on his high HP NA motors, I believe. He thought it could use more - like 48mm carbs...

Lou

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Home of Slant6-powered fun machines since 1988

Which makes me wonder what cubes and RPM's Cameron is running, I'm sure I could search and find out though.

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The old white brick

Or it could be a closely guarded secret

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Chris'
Autobody
Restoration
Service

of course it could.

Like what actually goes into "Secret Sauce"

or, if you have a child under the age of 5, what makes Krabby Patties so delicious?

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The old white brick

Using your figures.............

325 cfm for 6 carbs

325 / 6 = 54.2 cfm per cylinder

54.2 / 20% efficiency = 271 cfm each..............


Look at the graph I posted above.

It shows average, max (additive effective of when 2 or more cylinders need air), and individual cylinders. You'll notice that at peak flow for each cylinder, airflow requirements are about what you need for the average or max flow. (the cfm in that graph are based off lower rpms and volumetric effieciency, thus it's around 200cfm)

Flow thru the carb is at max when the input valve is opened all the way, and less when the valve is opening and closing. The area under the curve needs to equal the air requirements for that cylinder.

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Tilley NA motors around 242 ci and 5500-6100 rpm at peak power.

Lou

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Home of Slant6-powered fun machines since 1988