Why the ITB's so high on EFI builds?
 

For those of you who have put ITB's with EFI on your 911, why do you have them so high? Is it due to throttle geometry, clearances of something or other, ICV clearance, or what?

I can't get the thought out of my head of putting ITB's nearly right on the intake port with no intake tract like an E46 M3 or sportbikes for my setup.

I'd imagine the throttle response would be unbelievable.....

High butterfly is good for WOT on N/A motors.

Another thing that is only useful on N/A motors is tapered bore.

Both take advantage of tuned intake pulses (reversion?) to over-fill the intake at a specific range in the power curve - and are pretty much irrelevant for forced induction, to my way of thinking...

Yes, in the absence of bolt-to-head throttle bodies, the shortest manifolds you find (Rothsport, for one, have some very low-profile ones), together with the shortest throttle bodies you can find (Janvey make some in 40mm parallel bore an inch or so high).

Together with a stout plenum that won't oil-can under boost, you'd be all set... Might need to re-tune the EFI slightly.

Must.. Resist.... Spending.... More.... Money..... :)

Tall throttles help contain intake reversion in engines with long-duration camshafts.

I'm salivating at the thought of ITB's....... :)

Hmmm, never thought of that. So, do ITB's change the characteristics of long duration cams and their "lopiness"??? Meaning, do ITB's lessen lopiness, increase, or neither?

Longer stack produces peak torque at a lower rpm, shorter stack produces peak torque at a higher rpm. Since ITBs are a fixed length, you are effectively optimized at a single rpm, and not optimized above or below that rpm. Variable length would be utopia - hence Varioram.

Read about intake reversion wave and ram tuning to understand what's going on inside the stack.

Back in the day some can - am cars had velocity stacks of two different length, claimed to spread out the torque curve. Don't know about how much it helped - looks cool tho

http://petry.org/pics/drboblola2.jpg

Right, and intake reversion creates "lopiness" in the sound of the engine.

So, do ITB's alter lopiness coming from a common plenumed intake or not, being's all other engine parameters unchanged?

When I mean lopiness, I mean "pop-pit-tee....pop-pit-tee....pop-pit-tee" when an engine idles with long-duration cams.

Yes. Generally single throat per cylinder can handle much more cam overlap than a common plenum. An engine with a "lopey" cam will idle smoother with ITBs vs a common plenum.




Also, it is common in the motorcycle world to have multiple velocity stack heights on the same engine to broaden the power curve.

That was my theory; less lopiness since you're not cross contaminating cylinders like a common plenum.

The idle lope in race cams is due to narrow lobe center, long duration cams. Intake reversion only becomes a factor closer to peak torque in the RPM range. We deal with this with throttle placement and intake stack lengths.

Common-plenum, single-throttle intakes are extremely sensitive to lobe center placements and will not tolerate narrow LC cams due to reversion issues. 112 degrees is MAX and 114 degrees is much preferred for these intake systems.

ITB's tolerate this much better, even with a resonance-plenum intake so the limit is really about the engine's configuration and purpose.

This is really a very complex subject and I've just barely touched on this,...:)

Ok, I have some studying to do then.... :)

As Steve says lots of overlap isn't handles well by street common plenum single throttle intakes because of exhaust reversion

The narrow lobe centers are one tool used used to get more valve overlap which is useful in a motor that has tuned headers to generate hp at specific rpms(usually high) another is lope shape/height
http://forums.pelicanparts.com/uploa...1361811389.jpg

street cams(dotted) vs race cams (solid), not the extra overlap in the race cams, this is where hp comes from. The overlap time where booth valves are oped at the same time is sometimes called the 5th cycle as the effects derived from the headers suck additional fual/air mix into the combustion chamber. Modern variable cam timing engines can have the best of both worlds, little overlap for low speed drive-ability/fuel economy/emissions and increased overlap for high rpm hp
http://forums.pelicanparts.com/uploa...1361811436.gif
http://forums.pelicanparts.com/uploa...1361811762.gif

993 vram used long tubes for low rpm and short tubes w/ plenum resonance for high speed hp, the difference between working and non working hp is ~50hp, ie when the tunes stay short at high rpm you lose ~50hp

You mean stay long they lose ~50hp?

yes

transitions are @ 5160 and 5920
http://forums.pelicanparts.com/uploa...1361836217.jpg

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