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Trevor
Dodge Dakota
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11/01/2005
15:19:04
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 Subject: Math of Throttle Body Swap
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Has anyone done the math to see how much air the 3.9 can take? How many cfm do the throttle body's flow and how many can the motor take? And is this a justified calculation?
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N56629
Dodge Dakota
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11/01/2005
15:37:19
| RE: Math of Throttle Body Swap
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I don't think anyone cares. After all don't we put on CAIs capable of flowing 1200-1500 cfm and think we need every bit of it?
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Wadak
GenIII
11/01/2005
23:44:20
| RE: Math of Throttle Body Swap
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As I understand it a 318 running at 6000 rpm would need 700cfm. At the rpm the 3.9 would need 525 cfm.
92 and 97 Daks both with many mods.
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Wadak
GenIII
11/04/2005
02:03:37
| RE: Math of Throttle Body Swap
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By looking at the F and B wer site their 48mm TB flows 660 cfm. Their 50 mm TB flows 760 cfm and the 52mm TB flows 860 cfm. I seems that increase inlet size increases flow by 100 cfm. Taking this idea in reverse a fastman 46mm TB would flow 560 cfm. A 44mm TB would flow 460 cfm and a 42 mm TB would flow 360 cfm. I once read where a 318 would need 700 cfm at 6000 rpm. Take 700 rpm by 8= 87.5 cfm per cylinder. 87.5 x 6= 525 cfm. A 46mm TB should be enough in most cases to provide the maximum airflow. The 48 mm seems to be the best size for the v6 without torque loss. The best way to increase the results of the increase of your 48mm TB is to go with a cold air intake, port matching the intake manifold to the cylinder heads (port amtching alone can inrease Hp 8 to 12 hp according to Larry fro J&J Auto). Another step is to go with a low restriction muffler such as a flowmaster, or dynomax. You could add headers or higher flow exhaust manifolds such as the 1992, 1993 exhaust manifolds (the 92/93 exhaust manifolds flow close to headers in performance and may be good for 10 hp). Going with a good set of low resistance spark plug wires and spark plugs gapped out about 5 to 10 thousands over factory setting will help add additional performance. Adding an MSD ignition system can add an additional 10 hp. I hope this information helps those of you who are considering modifiying your truck. Along with horse power increase a gain mileage can be realized if you take it easy on the gas pedal. Don't for get the free mods you ca do also. Happy Modding and good luck.
92 and 97 Daks both with many mods.
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Wadak
GenIII
11/04/2005
02:24:01
| RE: Math of Throttle Body Swap
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A 2mm increase from 48mm increases flow 100cfm going down 2mm in size reduces flow by 100 cfm. A 46mm would flow a maximum of 560 cfm. A 44 mm throttle body would flow a maximum of 44 cfm. Remember the F and B throttle bodies are the best on the market for the 3.9, 5.2 and 5.9 magnum engines. All other makes will flow less. The numbers I have come with are my own working F and B flow rates in reverse. This is just an estimate of flow rates for the different sizes.
92 and 97 Daks both with many mods.
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N56629
Dodge Dakota
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11/04/2005
06:48:52
| RE: Math of Throttle Body Swap
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That's a pretty rough estimate. The flow is more closely proportional to the square of the radius than it is to the diameter. In other words if you doubled the diameter you would have four times the surface area, not two times.
"Remember the F and B throttle bodies are the best on the market for the 3.9, 5.2 and 5.9 magnum engines. All other makes will flow less."
Why would two venturis of equal size and shape flow different amounts of air?
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Wadak
GenIII
11/05/2005
00:55:00
| RE: Math of Throttle Body Swap
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I understand the mathematics, I talking about numbers that have been measured on a flow bench and extrapolating from there.
92 and 97 Daks both with many mods.
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N56629
Dodge Dakota
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11/05/2005
09:32:46
| RE: Math of Throttle Body Swap
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In order to compare throttle bodies you would have to run them all on the same flowbench, same day and all to the same standard. I don't know of a single person that has done that. Even if two manufacturers gave their unbias results and they differed I wouldn't be able to determine one was better than the other.
The only differences between say a Fastman and an F&B of the same size would be the flow characteristics going into the venturi. The stock tb has that huge cavity for the IAT which causes turbulance and that generally reduces flow.
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95V6
GenII
11/05/2005
12:44:58
| RE: Math of Throttle Body Swap
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Just a question for my education: why bigger TB openning is supposed to rob some power in low rpm? I understand that air flow speed is less with bigger opening, but I thought that fuel-injected engine is not sensitive to air speed, unlike carb.
95 3.9L 4WD V6, K&N, Gibson exh, JBA hdrs, JET stage II, MSD 6A, Rancho shocks and more..
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N56629
Dodge Dakota
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11/05/2005
15:16:13
| RE: Math of Throttle Body Swap
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Less sensitive would be more accurate. I guess you could cut the entire top off a kegger just to illustrate the point. It does run pretty well without a tb but you have no control. You can make a very simple throttle control by making a sliding plate to go over the openings on the manifold.
Here's a pretty good article on Induction Systems. It explains the basics better than I can.
I think I'm going to put a v6 tb back on my v8 and see how much I can "chirp" my tires. I wonder if all that cold air I'll be sucking in during the winter will make up for the reduction in flow.
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Trevor
Dodge Dakota
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11/05/2005
20:43:49
| RE: Math of Throttle Body Swap
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Just to compare, what is the throttle body size on a stock 3.9 and what cfm does it flow?
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95V6
GenII
11/05/2005
23:51:47
| RE: Math of Throttle Body Swap
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N56629, thanks for the pointer, it's a good article.
Trevor, F&B claims that stock V6 flows just under 600cfm.
95 3.9L 4WD V6, K&N, Gibson exh, JBA hdrs, JET stage II, MSD 6A, Rancho shocks and more..
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