3.9 liter or 3.8? - Mustang Evolution

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Old 02-28-2005, 07:11 PM   #1
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3.9 liter or 3.8?

My friend claims that his stock 2004 mustang is much faster than mine with his 3.9 liter engine even though my 2003 has a MAC cold air intake, BBK throttle body, and a Motorvator throttle body spacer. Is his faster or is he full of Sh*t. I read online that the only reason its a little bigger is that Ford redesigned the intake manifold or something like that but they weren't sure how much HP that makes. Do you guys know anything about this?
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Old 02-28-2005, 07:12 PM   #2
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He's full of crap. The differences aren't enough to make the 04 any faster with it's 3.9
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Old 02-28-2005, 07:14 PM   #3
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I thought the 3.9's were one of the finest tuned engines for the 6 cylinder mustangs but it still doesn't matter. As long as you're both manual, considering equal driving skill you should pull away because you're modded. Why don't you just race to find out though?
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Old 02-28-2005, 07:16 PM   #4
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same motors, just rounded the size differently
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Old 02-28-2005, 07:20 PM   #5
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Actually he might get beat by the stock cause of his throttlebody. I heard that you can actually lose HP from putting a new one on too soon on 99-04's.
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Old 02-28-2005, 07:23 PM   #6
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Quote:
Originally Posted by Kiljosh
Actually he might get beat by the stock cause of his throttlebody. I heard that you can actually lose HP from putting a new one on too soon on 99-04's.
maybe it causes turbulence?
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Old 02-28-2005, 07:30 PM   #7
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the motors are identical minus a little crank rework... I belive they just yanked parts from the windstar to save money.... so there is no difference in performance, if he dont like that then we could agree with him if we both wanted to be wrong.
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Old 02-28-2005, 09:43 PM   #8
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The only thing that could affect the size of an engine (displacement) is a change in the bore or the stroke or both. The intake has nothing to do with either, nor does a cam, throttle body, spark plug, type of oil or any other thing anyone can think of.

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Old 02-28-2005, 10:21 PM   #9
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SpectorV has it exactly right. They are just saving money by using the same crank that another production car uses. There is only a few CI difference. I believe it's less than 10 cubic inches difference so they rounded up rather than down. No performance gain, just a different number on the sale sticker.
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Old 02-28-2005, 11:16 PM   #10
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The engine comes from the new Freestar minivan where as the 3.8 came from the Windstar. They did away with the Windstar and brought in the Freestar. Ford decided to up the displacement just a minute amount to market it different, and the v6 had the same engine as that for 1/2 model year. The 2004 1/2s are the first with the 3.9L

The reason larger throttle bodies can lose HP (supposedly) on 99+s is because the split port manifolds already have a 60mm throttle body which is more than enough for 190bhp. Going larger increases volume but decreases velocity. (due to pressure rules) Decreasing the velocity in an N/A motor loses HP. Furthermore, the mouth of the intake plenum is matched for a 60mm throttle body. Increasing the TB and not the plenum causes there to be a couple mm difference. This is a wall for the air. You still have a 60mm opening at the manifold side, but a larger diameter for the TB. So you lose the velocity thru the TB and then hit a wall and lose the volume you increased with the larger TB. NOT TO MENTION a wall disrupts air flow and causes turbulance which also can make you lose HP. This is all speculation as I have not seen any dynos of 99+ with just a TB difference.
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