Throttle Body Calculations questions.

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cOoTeR
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Throttle Body Calculations questions.

Post by cOoTeR » Sun Dec 17, 2017 1:04 pm


I was looking into the benefits and down sides of the SX having smaller throttle bodies than the H2 and came across a throttle body caculator. It can be found at http://www.hipermath.com/math_center/ra ... lculations and uses the formula I've attached to this post.
Putting in the SX's Bore (2.99213 inches), Stroke (2.16535 inches), Number of cylinders and Best Power RPM (11,000) results in "44.1276 millimeters (1.7373 inches) is the best size for a street going car. It will provide the best power throughout the rpm range.

44.1276 * 1.15 = 50.74674 millimeters (1.997895 inches) is better for racing. 50.74674 is 15% bigger than the street going version and will provide more horsepower at higher rpm ranges. The drawback is it may cause poor idle and/or make it necessary to increase the idle rpm to make the engine idle smoothly. That, along with less power at lower rpms, is the reason most people do not drive cars with racing engines on the street."

Now I don't know what affect forced induction, having 4 throttle bodies and being a bike will have on this calculation. But it is interesting to me that the ideal size for the street is 4mm larger than the stock throttle bodies and the ideal racing size is about 1mm larger than what comes on the H2.

If the number of throttle bodies, forced induction and being a motorcycle don't have an effect on the calculation I wonder if the throttle bodies can be ported to 44mm and get better power? I also wonder if adding the 50mm throttle bodies would make it more like the H2? I would expect better results by also doing exhaust and tune.

44mm throttle bodies are on the new zx14r which I found interesting so I put its numbers in the calculator. B=3.30709 inches, S=2.55906 inches and I used the 11,000 rpms for this because I couldn't find where the 2018 14r makes peak power. (I think 11 k is pretty close to its redline though). But the results were,
53.0215 millimeters (2.0875 inches) is the best size for a street going car. It will provide the best power throughout the rpm range.

53.0215 * 1.15 = 60.974725 millimeters (2.400625 inches) is better for racing. Changing the peak power rpm to 10k the sizes changed to 50.554 and 58.1371

Oddly the calculations put the zx14r's throttle body size further away from what the calculation figures is the "Ideal" size.

Im sure Kawasaki did their r&d and chose these sizes to get the best balance of performance, gas mileage, rideability and longevity. But there is always room to make changes to move the balance to where one might want it.

Hopefully some of the really smart people can look at this and shed some light on how the number of throttle bodies, the forced induction, being on a bike might affect these calculations. Maybe the ram air affect of the motorcycles is better than a car which allows them to go with slightly smaller throttle bodies improving the low end power.
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Re: Throttle Body Calculations questions.

Post by UFO » Mon Dec 18, 2017 10:09 pm


I took a look at some of those equations and am not sure how accurate any of them are, I also see no reference to the source or basis for any of the equations used in the calculations.

Having studied some fluid dynamics I know that choosing throttle body size is far more complex than the simple equation above, and adding a supercharger and pressurized airbox makes it even more complicated. I have confidence that Kawasaki runs many computation fluid dynamics (CFD) simulations to get the size and shape of all the components in the intake system.

I would not rely too much on that equation. :)
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Re: Throttle Body Calculations questions.

Post by cOoTeR » Mon Dec 18, 2017 11:30 pm


UFO wrote:
Mon Dec 18, 2017 10:09 pm
I took a look at some of those equations and am not sure how accurate any of them are, I also see no reference to the source or basis for any of the equations used in the calculations.

Having studied some fluid dynamics I know that choosing throttle body size is far more complex than the simple equation above, and adding a supercharger and pressurized airbox makes it even more complicated. I have confidence that Kawasaki runs many computation fluid dynamics (CFD) simulations to get the size and shape of all the components in the intake system.

I would not rely too much on that equation. :)
Like I said I'm no expert or engineer I'm just a anxious guy who gets bored easily. I did punch in the specs for my Camaro SS with an LS3. It came up with the stock throttle body size as ideal for street use and 93mm as ideal for teaching applications. Guys who have posted comparison dyno graphs on the camaro forum have had the best results with the 92mm throttle body.
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Re: Throttle Body Calculations questions.

Post by Lentz » Tue Jan 23, 2018 1:24 pm


Perhaps it's a simple as the Camaro has 1 TB and the H2 has 4. The formula wouldn't need to know the number of cylinders if it were just sizing the TB for one Cylinder. So clearly it's sizing the TB for the size of the engine, not just for one cylinder.
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Re: Throttle Body Calculations questions.

Post by vcyclenut » Tue Jan 23, 2018 1:40 pm


yes I think you would need to run the formula based off the size of one cylinder since it has one tb per cylinder

I agree also its an oversimplification but it still is interesting.
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Re: Throttle Body Calculations questions.

Post by Lentz » Tue Jan 23, 2018 2:57 pm


Try setting the engine displacement to the displacement of one cylinder and see what you get. Flow through the port is supposed to equal a certain mach number that Superflow has figured out. I think I remember .8 or 80% of the speed of sound. Whatever the number is apparently strikes a balance between the velocity/inertia of the air - being able to continue to fill the cylinder after BDC and the airflow. Also, I'm not an aeronautical engineer but I believe I remember that air does funny things at the speed of sound. So it's not too hard to believe that it starts doing those things as it approaches the speed of sound. Also airflow is a function of the area of the port times the velocity. So also you reach a point of diminishing returns ( or negative returns) when you make the port so big that you lose more in velocity than you pick up in area.

Mathematically, I suppose it looks like a parabola with the peak being at .8 and the derivative of dy/dx going negative after the peak. Y would be the CFM per minute with both velocity and area being a function of the area (for a round port think of the radius) and some type of fluid function probably related to drag based on the shape of the port. I couldn't write the equation. I'm not that smart.
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Re: Throttle Body Calculations questions.

Post by vcyclenut » Tue Jan 23, 2018 3:27 pm


The efficiency of the combustion chamber has to equate in there also.

I think this is one of the biggest improvements over the last several years on high performance bikes and probably motors in general.

older bikes when I tune them I usually can get some decent gains with ignition changes, most of the newest models out I get minimal gains from timing changes. I can only guess but I think the combustion chamber design is so good and manufacturing is so consistent they can set the timing to optimal without worrying about predetonation.

I would think a more efficient combustion chamber would be able to handle a larger charge and change what tb size would be optimal, but really im just guessing. Then I start to think about port shape , laminar flow, should port size match tb size , cams and lift and overlap and my brain starts to hurt

maybe always just filling the cylinder with as much charge as possible is best for the strongest BOOM, lol
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