what size throttle body for adequate air flow?

[KZjim]

Is there a formula for figuring out throttle body size/air flow?... i'm planning a 1356cc in-line 4 cylinder Kawi.... with a gpz1100 16 valve head....

[Jack]

Search for this one. There is a ton of info on the site.

Jack

[KZjim]

I have searched quite a few threads and havent found what im looking for.. i have done the calculation from the manual # of TB's/size of TB=hp, which is great.. The info i really want is how much air i need to move to support a given engine displacement. My engine will displace 1356cc's will the stock GPz 41mm TB's be enough?

[Bernard Fife]

KZjim,

The air flow requirement (like the fuel flow requirement) depends much more on the horsepower an engine produces than the displacement.

For example, a 3.0 liter 6 cylinder from a 60s sedan and a 3.0 litre V10 in a formula one car have very different airflow needs!

However, as a basic esimate, you might consider the engine's displacement as 100% filled at it's peak rpm, and use that as a 'safe' estimate. So in your case, 1356cc is about 83 cubic inches, and suppose it revs to 10,000 rpm. That means you would be moving:

83 cid * 10,000/2 = 415,000 cubic inches of air per minute (at 100% VE).

1 cubic foot is 12³ cubic inches = 1728 cubic inches/cubic foot

so in the above example you need about 240 cubic feet per minute.

While a lot of aftermarket throttle bodies are rated in cfm, you are probably looking for a diameter. This invloves some guesswork. As a comparison, my TBI throttle body, with four 1.750" bores, was claimed to flow 900 cfm. This means that 9.62 square inches of throttle area is supporting 900 cfm. So we should be able to support 240 cfm with 240/900*9.62 = 2.57 square inches. This would suggest that four throttles of 0.90" (23mm) in diameter *should* work.

This is obviously way too small to be reasonable. The reason is that the V8 throttle body draws air fairly smoothly and continuously through the four bores (two intake strokes are always occurring in a V8). In the case of indivdual runners, only 25% of the time is an intake stroke, and some of the air is blown back when the intake opens. So lets assume it is flowing air effectively 20% of the time. Then we need:

2.57/20% = 12.85 square inches = 4 throttles of 1.77 inches diameter

= 4 by 46mm throttles ought to do it.

Yours are a bit smaller than this, and whether they will work depends both on your estimates for VE and rpm, as well as on the assumptions about airflow. If this were mine, it's 'close enough' that I would try them before buying something else.

BTW, you can tell from a datalog if your TB's are too small, the MAP reading will decrease as rpm increases at WOT.

Lance.

[KZjim]

Lance,
Thanks for the great info, thats what I was looking for, I'll start with the 41mm units and go from there....

[Bow]

However, as a basic esimate, you might consider the engine's displacement as 100% filled at it's peak rpm, and use that as a 'safe' estimate. So in your case, 1356cc is about 83 cubic inches, and suppose it revs to 10,000 rpm. That means you would be moving:

83 cid * 10,000/2 = 415,000 cubic inches of air per minute (at 100% VE).

1 cubic foot is 12³ cubic inches = 1728 cubic inches/cubic foot

so in the above example you need about 240 cubic feet per minute.

OK.. I did this far and I'm stuck...

Engine: 2389 cc [145.47 ci]
Max RPM: 7200
Cubic Inches of air per minute (at 100% VE): 525,734.59
Engine CFM: 303.67

Throttle Bodies: 42mm each [1.65 in]
Throttle Body Area: 2.15 sq.in. [each]
Total Throttle Body Area: 8.59 sq.in. [2.15 * 4]

While a lot of aftermarket throttle bodies are rated in cfm, you are probably looking for a diameter. This invloves some guesswork. As a comparison, my TBI throttle body, with four 1.750" bores, was claimed to flow 900 cfm. This means that 9.62 square inches of throttle area is supporting 900 cfm. So we should be able to support 240 cfm with 240/900*9.62 = 2.57 square inches.

I got to:

303.67/900*9.62= 3.25 sq.in.

I am stuck on how to get this:

This would suggest that four throttles of 0.90" (23mm) in diameter *should* work.

Can anyone help me out?

Thanks,

Bow

[ransom]

If you're running four throttles and need 3.25 sq. in. then each one
needs to be 3.25/4 or ~0.81 sq. in.

area is pi times the radius squared, so 0.81 = 3.14 * r^2.

Then r^2 = 0.81/3.14, which is about 0.26 (rounding up), and r = sq
root of 0.26 which is just over 0.5". So the radius is 0.5" and the
diameter is one inch, or 25.4mm.

Of course, this is before Lance's 20% calculation for using ITBs
instead of a single TB, but this way you can run through the calcs
yourself, which is satisfying when it works out. At least, I think so.
But I'm a big geek

Hope this helps,
Jesse Ransom



Posted by email.

[whittlebeast]

Are you thinking that you need/want 4 throttle blades or one single throttle blade. If you are going to 4 ITB's the I would lean in the direction of the factory motorcycle setups. About 42mm. If you are going for one big hole I would look at the Honda Accord piece. Remember that small angle changes at first tip-in will make big changes in torque output. Going to a ever changing throttle arm radious cable linkage will help drivability. Plan on using MAPdot AE tuning on any setup of this type. Also plan on RPM based AE, as traditional AE will just not work.

Hope this helps

AW

[Bow]

Thanks guys..

Yes Jesse, I do get a big, goofy grin when I figure it out on paper... ;)



AW,

Thanks you also.

Yes, I am planning on running 4 Throttle Bodies, the Engine I am building it on already has 1 big TB... how boring... ;)

I picked up a set of 42mm 2002 GSX-R 750 throttle bodies and I wanted to make sure that they would be big enough to handle the task at hand.

Thanks again gents!

[Bow]

Hey all...

I was playing with Excel, trying to put all of this knowledge into an easy to use form...

I am pretty sure I messed it up somewhere in the 20% rule...

can anyone check it over?