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Discussion Starter #1
Is there an aftermarket 2 stage TB available for the ZTEC? I notice the typical mod is a single stage 65mm unit. However, physics tells us that the smaller diameter 1st stage will provide torque at lower RPM, as well as better gas milage, so that's what I'd like to go with if it's available.

Dave

2000 SE Sedan w. 5 spd. Zetec
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if your talking about twin butterfly tb's...nope, don't have em, but with work you can run dual throttle bodies.
 

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torque at lower RPM, as well as better gas milage, so that's what I'd like to go with if it's available.
Hey your in luck you already got a T.B. tuned for that. The stock one. If you are considering a progressive linkage two butterfly(one small, one large) T.B. the after market doesn't have what you are thinking of. I'm guessing your going on the pricipal that a smaller diameter, passing the same volume, will increase velocity? The average Focus doesn't pick-up much actual power from a larger T.B. its more of a throttle response mod. You might be able to gain a little torque and MPG with a smaller unit, but you would kill any mid to upper rpm power,which is aready not ZTEC's stong suit. I don't really know why, better engine managment I suppose, but progressive linkage doesn't really exsist on newer cars and I've never seen it used withFuel Injection. Last time I saw that was on a 4 barrel holley carb, tiny little primaries for Tq and mileage, and huge secondaries that opened when you put your foot down.
 

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Discussion Starter #4
Hank said:
I don't really know why, better engine managment I suppose, but progressive linkage doesn't really exsist on newer cars and I've never seen it used withFuel Injection. Last time I saw that was on a 4 barrel holley carb, tiny little primaries for Tq and mileage, and huge secondaries that opened when you put your foot down.
Two Butterfly Progressive linkage was used with all of VW's CIS (Continuous Injection System) Fuel Injected cars- I've owned several, the most recent being a '91 Passat, which had an electronically controlled, as opposed to purely mechanical CIS. (Rabbit and Jetta from '77 to '87 had mechanical CIS, prior to introduction of the troublesome electronic "Digifont"). The early Passat re-introduction in '90 maintained the CIS with two stage TB, and a degree of electronic control that was less troublesome than the Digifont used on the cheaper cars. This setup worked very well on the big 2900 lb. Passat, offering plenty of torque out of a 2 litre 16 valve engine to get the big car moving quickly, provided great highway performance, as well as a real world 35 mpg. With similar engine specs, I do find the Focus Zetec to be a little more lively in city driving, but it is also 400 lbs. lighter, and you're lucky to get 30 mpg. out of it. There are probably many other factors, but I attribute VW's progressive fuel injection as being the major thing.

I wonder for the sake of science if a person could get a TB from a junked VW and adapt it to the Zetec intake? Probably wouldn't be easy!

Wave
 

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Hey KicknZX3 I don't think he's talking about dual butterfly's......I've only seen the Camaro Z28 use that. I thought that exhaust velocity picks up when the air goes into a bigger tubing and then has to go through a smaller tubing. Thus more torque? I could be completely wrong on this but this is how I imagine it for some reason.
 

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jibberish18: New Monte Carlo has duel butterfly TB (small side-by-side butterflies)....I know the topic is talking about something different...stacked TBs seem like the smaller one would just bottleneck after the larger one. If you want to increase air velocity, mount the 70mm TB to the end of an intake tube that narrows to 63-65mm at the intake manifold...that seems to be the best way in my mind (or 65mm TB narrowing to 60-62mm). I don't think the engine will pull more air than it would with a regular setup this way though (even though the air is moving faster from the TB to the manifold). It'll maintain it's regular combustion rate unless you do other engine work, or go FI.
 

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Well when I was watching Horsepower TV one time they had a pair of very long tube headers and they were explaining how the headers went from larger tubes down to smaller tubes for better torque I think. That is why I mentioned this with the throttle bodies.
 

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I can see that possibly being true as some back pressure is good for the engine's performance...I think I heard that somewhere else (other than hpTV) about some back pressure making tq and high flow making hp. I could be wrong though, and I hate jacking this thread...sry waveydavey.
 

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O.K. you got me[paranoid] Of course VW had to have a funky set-up in their history[:D] . With the Z-28 the plates are the same size and open at the same time, my Isuzu trooper is set up like that as well. It is dual butterfly, but it is not a progressive set-up.
That engines need back pressure is a myth. What they need is exhaust gas velocity to produce a scavenging effect in the cylinderhead, drawing out the burnt gasses. If you put too large of dia. tubing in your exhaust, you lose tq. due to the lack of proper velocity, not lack of backpressure. Putting a muffler, or any other back pressure creating device, helps create a scavenging effect in an improperly sized exhaust system. In street systems it is a trade off. You have to have a muffler on the street, so you need to run larger dia tubing to compensate for the increase in back pressure. If you remove that muffler and retain the large dia tubing you will lose low end tq. and some hp, leading many to believe you "need" some backpressure in the system.
Those tapered headers take a large volume of exhaust gas intially, then taper the tubing size to promote higher gas velocity, which in turn helps draw exhaust out of the system. Take a look at any racing series you want, unless mufflers are mandated by the sanctioning body, you won't find a race car with any restrictions in the exhaust.
The T.B. is serving a different function, it restricts how much air the engine can pull in, it is possible to go too large. You would have to size the T.B. to the point where it's size exceeded the engines ability to draw air at the proper velocity. Going to a smaller size is in effect choking the engine off. The vacuum created by the engine pulls the air in, on the exhaust side there is nothing helping the exhaust escape other than the exhaust stroke of the piston, so the correct exhaust sizing and design is required to help draw the exhaust out of the cylinder.
 
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