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Mueller |
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914 Freak! ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() Group: Members Posts: 17,146 Joined: 4-January 03 From: Antioch, CA Member No.: 87 Region Association: None ![]() ![]() |
Anyone ever do back to back testing with stock and then a bore'd out throttle body??
stock dia: 45.10mm average overbore diameter??? I'm guessing 50mm, it looks like I could bore mine out to 53.50mm...hmmmm (IMG:style_emoticons/default/smile.gif) motor to have higher than stock compression, custom R.A.T. cam and Megasquirt...... |
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SirAndy |
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Resident German ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() Group: Admin Posts: 41,688 Joined: 21-January 03 From: Oakland, Kalifornia Member No.: 179 Region Association: Northern California ![]() |
ok, back to this issue. i have never seen a aftermarket "overbore" throttlebody but brad tells me they just use a stock TB and bore it out to a larger diameter.
running the numbers on the stock TB and looking at the shape of the original TB, it becomes quite obvious why this "modification" is not only NOT giving you more HP, fact is it will decrease your HP! and here is why: CFM (cubic feet per minute) is the amount of air your engine will "pump" through at a certain rpm. using the stock 2.0L (us-spec) engine as a reference, that would be ~170 CFM @ 4900 rpm (or 4829 Liter/min). this is the theoretical max if your engine had a VE (volumetric efficiency) of 100%. normal street engines have a VE of around 85%, meaning the 2.0L actually only pumps about 128 cfm. this is due to a lot of factors, like friction and turbulences and restrictive air intake and exhaust. (as well as valve size, of course). now, CFM is calculated as Area (in sqft) * Feet per Minute (velocity), where Area is the diameter of your intake, in our case, the throttle body and "Feet per Minute" is the speed (velocity) of the Air. sooo, overboring the stock TB will give us a bigger Area, thus, we get more CFM, right? Wrong! the stock TB has a special shape that introduces a "venturi" effect, which increases the velocity of the air going through the intake which results in a actual increase in CFM. if this shape is "bored out" to gain a bigger diameter, you'll loose the venturi effect, and at best, you'll end up with no gain, but most likley, you will even loose a few CFM. sooooo, what we need now is someone who has access to a flow-bench so we can confirm (or not) the theoretical numbers we have and come up with the correct size Throttle Body and intake runners and valves-sizes and exhaust system for various T4 engine-combos. anyone? Andy |
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