Where can I learn about ITB's?

[Wrecked944]

I became a convert to the Church Of Individual Throttle Bodies back when I had PMO's on my SC. The throttle response in that car was just soooo sweet. It was like sex on wheels.

I have also read with pleasure the threads about Pelicans making their own throttle bodies using motorcycle parts.

But I am curious. Where can I actually learn the mechanics of how ITB's work? I see references to various "sizes" like 46mm and 50mm being better or worse for some motors . But I don't understand it. I also have read about things like "slide valves" in BA's book vs "butterfly valves" - but it means little to me. So is there a place where I can learn about how all this stuff worksa dn why some things are "better" than others? And what is the state of the art? Like, how do F1 car throttle bodies work? And what is up with the whole "variable length trumpet" thing that Ferraris' supposedly have?

[David]

Graham A. Bell may have something in his book "4 stroke tuning".

[randywebb]

generally, at high rpm you want ot get things out of the air stream -- when the open butterfly valve is hanging there (like a vertical plate) it still causes turbulence and reduces flow - hence a vlave that slides back (out of the way)

the bigger the motor and the higher the rpm the wider the TB dia. to flow enough air... but a big bore is not good for low rpm operation (street, or AX).

[mnmike]

from bmw, muwaha

yeah i dunno though, must be some good literature out there..

[randywebb]

BTW - I said ar above, BUT I really should have said the aerosol mixture of air and gas micro-droplets...

This is important b/c if the speed slows enough then the gasoline drops will 'fall' out of the air stream - not good. Some flowers use the same principle to make unwanted pollen (or other species) drop out of the air stream and not 'impregnate' them with foreign sperm (!)

So, if you really want to get a deep understanding, you need some phsics and some fluid dynamics books... most don't want that deep an understanding - it is a lot of work.

[Wrecked944]

Yeah, I definitely get the impression there is some voodoo involved in ITB design. And yet, to the untrained eye, ITBs appear to be little more than metal tubes with a "throttle" device and a hole in the bottom for a fuel injector. And folks like CWM charge zillions of dollars for them. So I am having trouble wrapping my head around it.

[Mike Bonkalski]

You are right, ITB's are basically nothing more the metal tubes with a port for fuel injectors and a butterfly plate to control air flow.

The reason why TWM or Jenvey can charge a lot for them is a small market and they have to make their money some how. Seriously, how many Porsche's do you know of that run EFI with ITB's - I'm in that group as are a few others on this board, but the numbers are small even for Porsche standards. Typically the throttles/velocity stacks for Porsche applications run as much as a set of decent used webers.

That being said, with the right tools, mechanical know-how, and some basic measurments it would be pretty easy to fab up a set. If you need a source for basics like throttle plates, throttle shafts, air horns, linkage parts, etc... let me know and I can get you some info.

[jpnovak]

HMM there is a lot to be said about how ITBs work or why they work. I will start by throwing out a few reasonable guidelines to get you started on your search.

1. ITBs just give you a matched intake length. One key to making power is to make each cylinder behave the same. Think air fuel mixture, spark and more importantly air flow. This is difficult to do with a common plenum/runner intake setup. Some manufacturers do better than others and a lot of engineering is done on intake runners.

2. ITBs can give you a tunable intake runner length. Power can be gained by matching the intake length to a particular harmonic that occurs for a given rpm. usually, longer intakes can give you more low range (rpm) tuning and shorter tunes for higher rpms. Frequency is the inverse of wavelenth. Faster rpms give you shorter wavelengths. Variable intake length gives the best of both worlds. Think Vario-ram.

3. ITBs can be designed with a slight taper. Decreasing the diameter from one end of the intake to the valve head smooths flow into the combustion chamber. I have seen numbers that say a 1.5 degree convergence taper is about right.

4. As Randy mentioned fuel suspension is important. ITBs are usually straight down into the head. The fewer bends the air and fuel have to take the better chance of it staying as an aerosol for better combustion. remember the fuel starts as droplets from the injector or carb and then should evaporate and mix with the incoming air as they move toward the combustion chamber. A more homogenous mixture gives better burn and more power.

5. ITBs can give you better placement of components. As Randy mentioned there is no way to eliminate the turbulance caused by a butterfly valve (except a slide valve). ITBs can influence placement of the valve. placement at a "node" in the tuned frequency reduces pulsations the change scavaging and help cylinder charge packing.

This also applies to injector placement. lower injector placement is best for emmissions and low speed torque. higher injector placement is best for high rpm where air velocity is too fast to allow good mixing and atomization.

6. The size of the intake diameter will depend on the specifics of your particular application. The larger the diameter the slower the gas speed. This will influence the rate of cylinder filling at low rpms and determine the amount of volumetric flow at high rpms. You will want a diameter that is small enough to give good cylinder filling (torque) at low rpm and still flow enough air to let the engine "breathe" at high rpms.

John Leutjen has some good calculations in the archives abou head port sizes for a given engine speed. IIRC, 100m/s intake speed should occur at peak torque values. Anything faster and a Porsche head does not make much power.

Putting it all together: Use your compression ratio and cam choice to determine an rpm range for peak power. Now match the valve diameter, port shape, head port diameters to get optimal intake gas speed. Now match your ITBs in both diameter, taper and length to make your engine as happy as possible. A happy engine is a fast engine.

Remember a combustion engine is just an air pump. The easier air gets in and out the more power it can make.

Now, there is a lot more detail on each of these points. There are arguments for and against what I have said. Use this as base reading to give you topics to search on. The information is out there I have only given a direction to go find it.

[randywebb]

Good summary, Jamie.

Remember, you can't do resonance "ram" tuning using wave superposition. Take a look at the latter 993s, and cars after that. So your basic & first decision is to optimize for a wide rpm range or narrow.

[Wrecked944]

Originally posted by randywebb
Good summary, Jamie.

Remember, you can't do resonance "ram" tuning using wave superposition. Take a look at the latter 993s, and cars after that. So your basic & first decision is to optimize for a wide rpm range or narrow.

Is this why new Porsches use VarioRam - to try to approximate the best of boths worlds? I saw the following while searching for info on Ferrari variable length intake manifolds...

Borrowed straight from Formula One developments, highlights of the F140 engine include a continuously variable intake manifold and variable valve timing. Never combined on road car before the Enzo, these systems maximize fuel consumption and torque across the range....With 5998cc, the Tipo L140 returned to a four valve cylinder head and featured a variable length induction system that extended the length of the induction pipes from 340 to 430mm via hydraulic actuators.

I'd be real curious to see an exploded view of Ferrari's F1 intake manifold...

[randywebb]

"Is this why new Porsches use VarioRam - to try to approximate the best of boths worlds?"

- Exactly.

Vario-cam is also good. What would be even better, would be to have perfect control of the valve opening at any instant. The closest I guess was the use of solenoids operated valves in F1, IIRC.