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I've been thinking about this one for a long time but a bit of new info. has come my way recently. My 140HP '00 Miata has a 55mm t-body on it. A 170HP SVT Focus has a 66mm t-body on it. A 140HP Zetec has a 55mm t-body on it. A 170HP non SVT Duratec has a 54mm t-body on it. The upgrade SVT Duratec has a 60mm t-body on it.
At least to my way of thinking both Duratec t-bodies are undersized. Anyone have some method of calculating the "correct" size?
Redcoat Raceworks. Performance parts and custom fabricating for Contours, Mystiques and Cougars. Specializing in chassis and suspension parts. Custom end links, control arms, strut tower bars, engine torque braces, etc.
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The size of the t/b depends on the flow rate of air into the engine. HP, is affected by the amount of air flowing in but you can't base the size off the t/b of just HP numbers.
98.5 Black SVT with 34k miles Build #5683 Born on 4\15\1998
WR Headers GMK CAI
Blaupunkt system fiberglass tweeter pods
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been discused before, the "best" tb for n/a svt is 63mm,and n/a duratec is good with 60mm both optimized of course 
"Youth ages, immaturity is outgrown, ignorance can be educated, and drunkenness sobered, but STUPID lasts forever."-Aristophanes.
--93 pgt,headers,intake,borla=14.9 1/4mile
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Originally posted by tour96se:
been discused before, the "best" tb for n/a svt is 63mm,and n/a duratec is good with 60mm both optimized of course
Really. The "best" is a 63mm.
I disagree. For one; nailing it down to one exact size for any setup combination respectively won't work.
Then:
The SVT intake boot is ~58mm and stock MAF flow area is ~57mm.
An optimized 60mm TB has a 59mm flow area. (it's slightly above 60mm btw)
Without changing things before the TB it is not the most restrictive part. (i.e. if the TB flows 330cfm but the intake only 300cfm then the TB is large enough)
Then while taking that into consideration you also have to look at the heads and manifolds. If those can't support more airflow then the TB can support then the TB is still not the limiting factor...
2000 SVT #674
13.47 @ 102 - All Motor!
It was not broke; Yet I fixed it anyway.
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Originally posted by DemonSVT:
Originally posted by tour96se:
been discused before, the "best" tb for n/a svt is 63mm,and n/a duratec is good with 60mm both optimized of course
Really. The "best" is a 63mm.
I disagree. For one; nailing it down to one exact size for any setup combination respectively won't work.
Then:
The SVT intake boot is ~58mm and stock MAF flow area is ~57mm.
An optimized 60mm TB has a 59mm flow area. (it's slightly above 60mm btw)
Without changing things before the TB it is not the most restrictive part. (i.e. if the TB flows 330cfm but the intake only 300cfm then the TB is large enough)
Then while taking that into consideration you also have to look at the heads and manifolds. If those can't support more airflow then the TB can support then the TB is still not the limiting factor...
I'm glad we're seeing different opinions. MY thoughts on this subject....
I agree with Demon that it depends on a number of factors. My non-SVT Duratec "responded" to a straight SVT t-body swap. Then I mildly optimized with another positive "response". Next up was an extreme optimization and another turn for the better. I now have a 65mm converted to SVT linkage/fulcrum waiting to be installed-will it be too big? Hmmm.
Each time I optimized, I took careful measurements so I could calculate the area of airflow. On this point I shall disagree with Demon that an optimized SVT t-body will flow "XX" amount. It all depends on how much of the shaft has been removed/streamlined, whether countersunk or button head screws are used, whether the threaded portion of the screws are cut perfectly flush with the shaft, is the shaft the same thickness from side to side, how thick the plate is, etc.
So we have addressed the quantity of airflow, but how about quality? What affect does it have?
Redcoat Raceworks. Performance parts and custom fabricating for Contours, Mystiques and Cougars. Specializing in chassis and suspension parts. Custom end links, control arms, strut tower bars, engine torque braces, etc.
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Originally posted by Bradness:
On this point I shall disagree with Demon that an optimized SVT t-body will flow "XX" amount. It all depends on how much of the shaft has been removed/streamlined, whether countersunk or button head screws are used, whether the threaded portion of the screws are cut perfectly flush with the shaft, is the shaft the same thickness from side to side, how thick the plate is, etc.
The difference between one optimization to another are extremely minor in the grand scale of maximum airflow.
Also in an optimization I would take several things for a given. (or really it "wouldn't be optimized")
Countersunk screws, half the throttle rod, and knife edged frontal areas.
So that sets the overall flow area at a fairly exact amount for any optimized TB of the same size.
You make it sound like the difference between one TB and another would be significant when that's just not the case. The difference between the most detailed job and the least would be about 2% Most would be within 1% of each other. You know me, I'd take that extra 2% but by itself it's not a tremendous amount. That's 2% flow area and not 2% power by the way.
Originally posted by Bradness:
So we have addressed the quantity of airflow, but how about quality? What affect does it have?
More efficient - a MAF with a small air horn.
Less efficient - a MAF with a center bar post.
More efficient - a smooth tapering intake pipe.
Less efficient - a intake tube with pleats or ribs.
More efficient - an Extrude Honed manifold
Less efficient - a stock manifold
More efficient - a LIM without restrictive rods and plates
Less efficient - a LIM without restrictive rods and plates below 2500rpm (had to throw that in before someone whined about it  )
I'm sure you get my point by now.
I could have listed the head ports, exhaust valves, pre-cats, et cetera.
Everything must work together because any time you remove the most restrictive piece another one then just becomes the most restrictive.
Restriction can cause both airflow "quantity" and "quality" to drop. For the most part the more turbulent the air the less efficiently it will move. Also the lower the cylinder filling ability will be.
Turbulence before the TB is not as bad as it inside the manifold. Restriction is more of a factor before the TB. Well at least to the point where the TB is more restrictive then the pieces before it.
Now turbulence before the MAF is very bad but that's for other reasons; consistent airflow readings.
2000 SVT #674
13.47 @ 102 - All Motor!
It was not broke; Yet I fixed it anyway.
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Firstly, area is always measured in square something (inches, feet, centimetres, etc.) so saying it flows 59mm is incorrect. Possibly you meant that an optimized t-body flows equal to an open tube of 59mm in ID. My optimized t-bodies have an open area of 4.0 square inches when the throttle plate is fully open.
Now I won't argue about how many per cent can be gained by a "super-duper" vs. a run-of-the-mill optimization. Personally, I optimize for both quantity and quality gains PLUS a proper seal. I've found that using countersunk screws DOES NOT promote good sealing as they tend to force the plate to one or the other side of the casting. I'd rather lose a small amount of flow area with button head screws and not have the plate jamming when closed (happened more than once).
So quality = efficiency......? What about flow at part throttle?
Redcoat Raceworks. Performance parts and custom fabricating for Contours, Mystiques and Cougars. Specializing in chassis and suspension parts. Custom end links, control arms, strut tower bars, engine torque braces, etc.
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Originally posted by Bradness:
Firstly, area is always measured in square something (inches, feet, centimetres, etc.) so saying it flows 59mm is incorrect.
Possibly you meant that an optimized t-body flows equal to an open tube of 59mm in ID.
I've found that using countersunk screws DOES NOT promote good sealing as they tend to force the plate to one or the other side of the casting.
I see myself stating it has a flow area equivalent (not a flow rating or number) to a stated mm size to make all the different pieces directly comparable. 
The size of the intake boot is actual inner diameter and the MAF is effective flow area. They are even labeled as such to try and avoid confusion. Obviously the MAF's not 57m...
The only time I stated actual flow numbers was the 330cfm info for the 60mm that is based on the old information generated by KKM and then picked a number lower then that (300cfm) to prove my point.
You need to properly prep the plate for the screws. Very simple really. I've had no trouble with it at all in any of the TB's I've done over the years.
2000 SVT #674
13.47 @ 102 - All Motor!
It was not broke; Yet I fixed it anyway.
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Demon: Would you care to extrapolate on what exactly "properly prepare the plate" means?
If you look at my first post I asked if anyone knew of a "method" to calculate the correct t-body size. No one has posted (here) such methodology, although in days past a NECO member from Leo Capaldis Race Crew posted a partial mathematical calculation based on the ideal air velocity at WOT.
It still seems to me that Duratecs are using undersized t-bodies that Ford probably found in the corporate parts bin when someone said "That will fit, we can save .02, it should be OK, so let's use it."
Redcoat Raceworks. Performance parts and custom fabricating for Contours, Mystiques and Cougars. Specializing in chassis and suspension parts. Custom end links, control arms, strut tower bars, engine torque braces, etc.
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You have to prep the plates for counter sunk screws. A simple centering reamer bit does that.
I've never have any issues with the plates altering their lateral position. I've done this countless times.
Also about a formula. While you could base one on the maximum CFM draw of the engine based on size at 100% VE that would not be correct at all.
This formula would have to take into account more variables then just that and even then it's only based on optimum conditions.
That was the reason I posted the all the information about other factors being more restrictive then the TB. If other factors are more restrictive then the TB then enlarging it will only hurt velocity at the inlet of the manifolds.
In dyno and real world data it shows that a 65mm TB loses power verses an optimized 60mm TB on even moderately modified SVTs.
For that matter some dynos have shown no gains swapping to an SVT TB on an otherwise lightly modified engine.
The variables are too great to have a simple equation to be used as a basis for choosing a TB. Even if you did have one it would likely have a high range of error comparatively. (even a 3% error is +/- 2mm in TB size)
As for your "quality" of airflow. That is why an optimized TB shows improvement. It's not because the actual flow area is increased.
There are books on how to model, size, and design throttle bodies. You should try PMing Rara. He's sure to have knowledge or friends that know about it.
2000 SVT #674
13.47 @ 102 - All Motor!
It was not broke; Yet I fixed it anyway.
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