Single or mulitple throttle plates? A theory question

independent butterflies give better throttle response. there is less air that has to change direction.

look at the mazda renesis. it has 3 plenum valves that act as dynamic restrictors, so that at lower rpm you aren't burdened with the poor response of a large plenum.

 

Motorcycle engines that are CC for CC much higher output than a car engine use individual throttle bodies for each cylinder.


Russell

 

that's correct, it's the total TB area which makes the difference.....your eliminating the upstream (of valve) bottle neck, and more of those you can eliminate, the less restriction you have on flow and resulting power potential

ITBs have proved a power producing method for small and larger capacity engines....they are less effective on boosted applications, but still get used even then (i.e. F40)

 

Think of it like this: With a single throttle (or a single throttle per side) it is going to take a while for air to rush in and charge up the plenum. Thus there is a delay from gas pedal goes down to engine comes alive. In addition, now consider laying back on the throttle. Here, the engine will take a few rotations to pump the intake down to vacuum levels and thus power does not leave as quickly. Sort-of like driving the engine through a rubber band.

Now, consider indivitual throttle bodies. There is less than one cyclinder didplacement of air after the throttle plate, so when the throttle opens, you get power on the very next intake vent on that cylinder. Same goes for removing of the throttle.

 

Total area of the ITBs compared to a single TB is not a good comparision. Not all the cylinders have the intake valve open at the same time. The area for a single TB should be much smaller than the total area of all the ITB's. Throttle response is the primary benifit of a ITB.

Cheers Jim

 

the ITBs act like the webers i replaced on my car. what a compliment to the 308 motor. The hardest part was to somehow collect manifold vacuum from all 8 cylinders for a stable MAP sensor reading to the ECU. we made up this little cigar shaped catch can for 4 cylinders. someday i will add the other 4 cylenders from the other bank, but for now - this does nicely for a stable signal.
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true ITB is as said, known for the immediate throttle response advantage, with the throttle plate located closer to the intake valve.....however as any one valve is open, the ITB for that intake provides a potentially lower restriction then a typical single TB with manifold to draw through, and does contribute to better power potential

the trade off with ITBs is the increased complexity, and throttle linkages involved

 

that's a nice clean looking setup....who made your ITB system?

 

From a design point of view for a given engine it is quite easy to make an ITB system more restrictive than a TB or to make a TB system more restrictive than an ITB system, it is just a matter of sizing the butterflies to favor which system you want to win.

The large butterfly found in a TB system will have a higher flow per square inch capability because the throttle shaft is a smaller portion of the available bore diameter. A large part of the flow losses through a butterfly are caused by the throttle shaft and the screws that hold the butterfly to it. On a 1.25 inch carb with a round throttle shaft it is possibe to modify the throttle shaft and screws and get a ~12% improvement in flow. On a larger butterfly the same mods are not as effective but still very worthwhile.

Cheers Jim

 

While it may seem intuitive that ITB's (with separate butterflies for each that are close to the intake valve) should produce better throttle response and greater power, that's not necessarily the case. Far from it.

Here's an analogy that most folks can understand: one would think that on a typical V8 a set of four down draft webers should produce more power than a basic four barrel Holley carb, right? After all, the webers have separate ITBs and butterflies close down to the intake valve, right? And, the webers should do a better job of atomizing the fuel and keeping it in suspension at the correct AFR right down the (short) TB, right? On the contrary, the Holley will make almost exactly the same hp/tq. Why? One reason is that the intake pulses are actually better accommodated by a common plenum - in other words, the common plenum allows for better filling of the cylinder with the right AFR because the reverse pulse created by a closing intake valve helps to "charge" a different opening intake valve on the common plenum.

The above isn't just my opinion, there is indeed a fair amount of hard data out there which has been accumulated over the years. We're not the first to consider the common plenum v. ITB debate....that debate has been going on for many, many years amongst hard core gearheads and engine tuners!

 

Agreed, here.

Although the books I have show the 4-Weber (1 per hole) system having greater upper midrange, it has no advantage in peak power or peak torque (compared to the Holey).

The common plenum is taking the 8 pulses and averaging them into a common constant draw. It is not difficult to set up a carb to deal with a constant draw. It is more difficult to set up a carb for a single pulse.

However, this common plenum STILL delays the throttle response due to the plenum volume (albeit small). Also note, this is exactly the kind of thing that does NOT show up on a dyno--throttle modulation response does not show up on the dyno.

Also note: when you get into the finer details, perceived throttle response is a part of the ECU tuning. The ECU detects the throttle being opened and adds more fuel even before the air moves, this fuel flies across the intake and clings to the walls, evaporating over a few intake cycles. So by pre-richening the fuel mixture, the engine takes off better. Then later when the throttle is closed, the ECU subtracts fuel even before the air quits flowing so the walls can evaporate back to the target mixture. Overall, its a big chess game getting the A/F mixture correct over all sorts of throttle, engine speed, and atmospheric conditions. Carbs played all these games, too, using idle, part throttle, accelerators, and high speed enrichment games. Get them right and the car feel great. Get them wrong and its a turd.

 

Mitch, yup, exactly right - you know what you're talking about. Tough to beat EFI/ECU for fine tuning the AFR throughout the whole rev/load range, including the "anticipatory" tuning actions you mention above.

Related, the direct injection of any good EFI system (high pressure, injector centered in the flow and near the inlet valve, etc.) doesn't suffer the problem of the atomized charge bumping all over the plenum walls and falling out of suspension which is, of course, worsened with a common plenum.

Good insights!