Why Do Individual Throttles Respond Better?

[prophecymiller]

This is something I've wondered for a while now...Why do individual throttle bodies for each cylinder give better response than a single throttle? Anyone know what kind of gains are to be had in our application with such a setup?(pretty far out of course, but just curious:D)

Another question that just dawned on me...would there be any gas mileage gains to be had? Basically, would individual throttles reduce pumping losses at cruise possibly?

[drcustom]

Are you thinking about setups on things like sportbikes?

I don't really know what I'm talking about, but two things come to mind worth considering:

1) Think of the air moving inside a plenum..it essentially has to change directions each time a different cylinder valve opens...so there's some loss/lag created there.

2) The length of the runner is much shorter (at least in cases I can think of) when you have individual TB's. That's always a tradeoff though...shorter runners = more low end power but at a high end loss...so the 'short' version is that individual TB's may give you better throttle response but not necessarily as much top end.

There's a pressure wave effect that is gained from a plenum: When the valves in one cylinder close, the air still has momentum going into the chamber. When that happens there will be a slight compression of that air piling up on itself, and that pressure will impact the rest of the plenum. I think I read something a while back that said this effect can be positive or negative...at some RPM's the wave will work against the air moving into another cylinder, while at other RPM's the pressure will help the air move into the next cylinder. Conversely, isolated runners with independent tb's just have air stopping and starting (at some speeds I assume there would be some residual effect) but overall you wouldn't see any of the gain or loss...but I still think it would have the properties of a shorter runner so overall you'd probably end up with less top end (again, this is all just a guess).

[Cloud]

From what I can tell the biggest benefit is the throttle response is pretty much instant. On a turbo car you pretty much negate that benefit immediately just by having a natural time delay in boost. Faster throttle response does actually add up to entire seconds on a race track so this is why you tend to see them on most race-type vehicles including sport bikes.

The increased response time comes from reduced piping volume. Air must accelerate when the throttle opens and even a pressure wave at the speed of sound has a longer time to travel a longer length of pipe. It has very little distance to travel with an ITB setup. This is also a similar principle as why huge intercoolers with giant piping volume starts to alter throttle response, you have to accelerate and deccelerate a very large column of air each time you change throttle position.

Additionally shorter runners mean higher rpm power band and when you are seeking maximum rpm and maximum power you want as little runner length as possible. Bikes turning 16k would likely be significantly hampered if they had to breathe through a plenum just based on the additional runner length involved with adding one.

Some setups actually are faster with a properly tuned runner and plenum setup as it improves area under the torque curve typically even if it drops off a bit up top comparatively.

I am not entirely sold on the ITB setup getting better mileage or less pumping losses but it is possible.

[drcustom]

Additionally shorter runners mean higher rpm power band and when you are seeking maximum rpm and maximum power you want as little runner length as possible. Bikes turning 16k would likely be significantly hampered if they had to breathe through a plenum just based on the additional runner length involved with adding one.

Some setups actually are faster with a properly tuned runner and plenum setup as it improves area under the torque curve typically even if it drops off a bit up top comparatively.

The first part of that goes against my ideas of plenum length (but I did wonder about sportbikes).

Is that because a longer plenum runner length is good for more top end but still not as good as ITB's?

[prophecymiller]

So ITB's with a plenum are basically no different than a single TB/plenum? This is what I was thinking, but some setups still use them...such as a stock RB26. Any ideas as to why Nissan would use them?

I was wondering about less pumping loss due to a greater surface area for the throttling....but then again, the ITB's would just be closed more than a single smaller TB right? So I guess that is a no go:p

So the best setup for an NA motor would be ITB's with dedicated filters for each runner? Or just open bellmouths...

Now like Drcustom mentions, are there no benefits of harmonics tuning with an ITB setup since they typically don't use a plenum?

[drcustom]

Wow, I hate to admit it but I was thinking backwards...shorter length = higher RPM torque. Adding length to the runners is a way of shifting the peak torque to lower RPM's.

I suppose the closer the TB(or ITB's) are to the valves the faster your throttle response would be...but there's nothing to say you couldn't still have runners and a plenum behind the TB's.

This would probably be overkill and may not really offer many gains...but on the drawing board I suppose you could have ITB's sitting at the head with runners to shift the torque lower and also a plenum to tune.

Now that I've got that picture in my mind I actually think a single TB would reduce pumping...the pressure drop over larger volume of air would be less (imagine trying to suck the air out of a bottle vs. trying to suck the air out of an entire sealed room).

[Cloud]

Plenum tuning and runner length tuning are separate but do have interaction. Something to consider in tuning your intake is that each section has an effect on the pressure wave.

I still think the primary advantages of ITB's are throttle response (both on and off) and to get the intake length shorter. Ultimately the shortest intake path you could have is the length of the runner in the head plus the throttle body length. If designed poorly this could still be too long for very high rpm operation.

Ideally you would use ITB's with open bellmouth, correct. As has been shown you can use open bellmouth in a plenum as well though. The plenum adds length, volume, a mixing chamber and the potential for more resonance tuning. I have a worse grasp on plenum tuning than runner tuning but generally speaking the effects of resonance in the intake or exhaust are additive. So yes, runner tuning works without a plenum. And it can be that the flow losses from the added plenum and intake length can offset any gains at very high rpm. It can also be that at lower rpm ITB setups have very little to gain and plenums may be genuinely more advantageous.

Generally speaking you have larger total throttling area with ITB's but it is about the same since the instantaneous flow requirements of each cylinder are higher they need the extra area. You are dealing with peak flows rather than average like at the front of the plenum. This is the same reason we have much larger runner diameters if you added them all up compared to the single throttle body's diameter, same goes for exhaust piping, instantaneous flow areas are much different than average flow area requirements.