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kota on 20s
Dodge Dakota
7/10/2001
04:11:08
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 Subject: 104mm TB?
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Message:
i was just wondering how the mesurements work on TB's. the 5.2 and 5.9 have twin blade 52mm which would make it a total of 104mm right? if so why are the 4.7's single blade so small? even 70mm is 34mm smaller. can some one clarify where is my thinking going the wrong way?
ERic
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Dan Gruber
Gen III
7/10/2001
08:49:39
| RE: 104mm TB?
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I've wondered about that as well. Here's my take on it, given my admittedly limited engine knowledge:
The 4.7 has a larger bore than stroke (3.66" bore, 3.4" stroke). Theoretically, that makes an engine more likely to produce less torque, but rev higher, making more power through revs. Practical experience with the 4.7 shows that is fairly true. To boost torque, high airspeed is required to ensure good mixing of the air and fuel at low engine speeds. Good mixing equals a more complete burn, which translates into more energy created by the exploding fuel-air mixture and lower emissions. High airspeed is created by limiting the diameter of the air intake (in this case the TB), thus creating a stronger vaccum to fill the cylinders. Imagine sucking air through a 12 inch straw versus sucking air through a 12 inch paper towel tube. The total volume of air inhaled will be the same (the capacity of your lungs), but sucking the air through a straw will cause the air to move faster. This boosts torque, but ultimately limits power. This would also explain why 4.7's seem to gain a relatively large amount of power (without a corresponding increase in torque) from larger TBs, free-flowing intakes, and free-flowing exhausts.
Make sense?
Dan
2000 CC SLT 4.7 4X4 Auto 3.55 LSD
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kota on 20s
Dodge Dakota
7/10/2001
12:17:23
| RE: 104mm TB?
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Message:
i see what you are saying, but is the 5.2 and 5.9 TB 104mm?
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Dan Gruber
Gen III
7/10/2001
12:40:55
| RE: 104mm TB?
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What I didn't put in there because I got distracted was that DC probably figured that the 4.7 needed a smaller throttle body to boost low end torque.
As far as the actual measurement, the twin 52mm TB does not equal one 104mm TB. As an illustration, try this: draw two 52mm diameter circles side by side on a piece of paper. Now draw one 104mm circle over your twin 52mm circles. You'll notice that there is a lot of extra space on the 104mm circle. That extra space (whatever it equates to) is the difference between the twin 52mm TB and one 104mm TB. The 104mm would have a HUGE amount of airflow possible, much more than the twin 52mm setup on the dodge. I wasn't a math major in college, so I'm not even going to attempt to figure out the actual difference between the two, but you get the idea.
Is that right or am I on crack?
Dan
2000 CC SLT 4.7 4X4 Auto 3.55 LSD
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kota on 20s
Dodge Dakota
7/10/2001
12:44:59
| RE: 104mm TB?
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Message:
ok i thought their was something wrong with my wave of thinking. now i understand, thank you for clairifying that for me.
Eric
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Kyle M.
Dodge Dakota
7/10/2001
13:14:13
| RE: 104mm TB?
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Message:
Here is some math, hopefully it works:
Area is equal to Pi(Radius)^2
So Dual 52mm Throttle Bodies Equals:
52/2 = 26 then 26^2 = 676 then 676*Pi = ~ 2123.72
Then multiply by 2 and you get: (Area) = 4247.44mm2 for the dual 52mm Throttle Body.
Now here is the 104mm Throttle Body:
104/2 = 52 then 52^2 = 2704 then 2704*pi = ~8494.87mm2
As you can see the 104mm Throttle Body produces just about twice the area as the twin 52mm's.
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bernd
Gen III
7/10/2001
16:56:36
| RE: 104mm TB?
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Its a boundry layer/flow resistance thing... at least for a naturally aspirated application. The two smaller barrels have more flow resistance than one large barrel.
The air molecules nearest the barrel walls is "attached" to the wall of the barrel and has 0 velocity (stagnant) while the air in the center of the barrel has maximum velocity. The more air at the wall of the barrels, the lower the total velocity of the air. With the one big hole the central velocity is higher (flow resistance due to boundry layer is lower) than on the two small holes...A simple version.
1997 Dodge Dakota SLT - V6
Supercharged @ 10#
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kota on 20s
Dodge Dakota
7/10/2001
17:56:36
| RE: 104mm TB?
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Message:
so the holly 4 barrel will have more resestence than the stock TB? or are the holes bigger than the stock TB?
Eric
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Dan Gruber
Gen III
7/10/2001
20:04:07
| RE: 104mm TB?
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Thanks to Kyle M's math, here's another way to look at the numbers: TB area per liter of engine displacement.
4.7 (w/65mm TB)= 706mm^2 per liter
4.7 (w/68mm TB)= 773mm^2 per liter
5.2 (w/twin 52mm TB)= 816mm^2 per liter
5.9 (w/twin 52mm TB)= 719mm^2 per liter
On a 4.7, to equal the TB area per liter of the 5.2 (the best of the bunch), the 4.7 would need a 69.9mm TB. I guess that explains why the posts I've read about the test 70mm TB's for the 4.7 all say that it really wakes up the engine. And it shows how much Dodge has choked the 4.7 and 5.9.
Dan
2000 CC SLT 4.7 4X4 Auto 3.55 LSD
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Jeffster
Dodge Dakota
7/10/2001
20:08:57
| RE: 104mm TB?
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Message:
No it won't have more resistance because there is more surface area for the air to pass through.
Kyle you did your math wrong.
Surface area = Pie multiplied by the radius squared.
For the twin 52 mm's it's 3.14 X 26 = 81.64 multiplied by itself is 6,665 square mm's or 66.65 square cm's. Multiplied by 2 is 133.3 sq cm's.
A single 104mm Barrel is
3.14 X 52mm = 133.3. multiplied by itself is 17,769 mm's or 177.7 cm's which is substantialy larger.
I am not sure what the stock intake is on a 4.7 but I have a feeling it's pretty close when added up against the dual 52's on the 5.2.
Basicly if you double the diamiter you increase the surface area close to 3 times it's previuos size not just double it.
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Travis B.
Dodge Dakota
7/10/2001
20:49:40
| RE: 104mm TB?
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More calculations.
Area= Pi X r^2 not (Pi X r)^2
Bernd's point about boundary layer area is a relationship of the surface area of the bore or circumference.
Circumference = Pi X D
For 2 by 52mm holes, Area= 4244mm^2
For 1 by 70mm hole Area= 3846mm^2
Surface area where Velocity = Zero
for 2 52mm = 326mm
for 1 70mm = 219mm
This means that the single 70mm, although it has 10% less total cross-sectional area to flow air, it also has 30% less surface area to cause frictional losses. These percentages don't correlate directly to flow ratings, but they are something to consider. The frictional losses get significantly worse as the length of the restriction gets longer and as the surface gets rougher.
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Kyle M.
Dodge Dakota
7/11/2001
13:44:35
| RE: 104mm TB?
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I used the Pie * Radius Squared Jeffster. Travis's Calcs also are close to mine. I think maybe you calculated wrong.
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Jeffster
Dodge Dakota
7/11/2001
19:22:33
| RE: 104mm TB?
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I think your right Kyle. I had misread the formula. If it wasn't for my shop classes my GPA would have really sucked.
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