Ask Sarah: What’s Your Intake?

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No this is not a Toyota 3SGT BEAMS engine, this is something out of Kojima's photo archives from when he was young.  It is a Toyota 152E race engine from the mid 80's.  It has ITB's not with electronic fuel injection but slide valve mechanical constant flow fuel injection.  Real old school stuff.

So what's the advantage of ITB over a single TB?  First, there is much better throttle response with ITB.  Since they don't scavenge air from other cylinders, you can make big power on a high compression ratio, naturally aspirated engine with a lot of cam.  You can use different length runners to tune for different applications/goals.  And of course they sound pretty dope!  Yeah, I said dope…  But they're also more expensive, harder to tune, and harder to package under the hood than a single throttle body.  A simple MAP system won't work.  You could use an alpha-n (TPS meters fueling) to provide the driveability, especially with big cams, and use a MAP reference or switch to MAP for wide open throttle or boost conditions, incorporating a standalone engine management system in to handle this capability.  

The Iconic Nissan RB26 has ITB's with a large plenum leading to dual MAF's.  The ITB's were used to give the engine good throttle response.  The Nissan SR20DET 54C engine found in the Pulsar GTI-R is also known for its ITB's with plenum.

While a single throttle body maintains a strong vacuum in the manifold, there is little vacuum present downstream with the ITB and the vacuum fluctuates with each intake stroke.  This can create issues with cold starts or running accessories like the A/C or for power assisting the brakes.  Also, a normal fuel pressure regulator relies on a vacuum signal.  You can swap in a static fuel pressure regulator to provide a consistent supply of fuel and programmable fuel management system to regulate that fuel for the entire range.  Or you can tune ITB with a MAF and plenum like the Nissan RB26 or BMW I-6 M series engines use.  

An RB26 again.  Some tuners replace the ITB's with a big plenum by my favorite combo is the ITB's with a big plenum.  I just love how the RB26 sounds, I-6's have a 120 degree firing pulse that sounds very musical which gives me shivers.  Take a close look at what car this RB is in.  It's no Skyline GT-R!

You can connect the vacuum side of each throttle to a shared plenum to pull a vacuum signal that is strong enough to provide a load signal to the fuel pressure regulator and power assist the brakes, but the manifold pressure may not register any change at low throttle.  ITB are also not usually the best thing for a forced induction engines, especially when you're converting to ITB from a single throttle body.  Turbocharged engines are inherently laggier and you'd have to add a plenum to run ITB with forced induction, pretty much negating many of the benefits (ie better throttle response) of an ITB system.  Plus, fitting all of that and the plumbing required for a turbo or supercharger under the hood would require a lot of ingenuity.  

Is it just me or does this engine look gold plated?  This V12 solid lifter valvetrain with 12 individual throttle bodies was originally developed for the BMW M8 concept, before it was determined there was almost no market for this 500+ hp engine.  It's a common misconception that this engine was the basis for the McLaren V12 but though it shared some of the same design ideas, the M8 V12 was too long and too heavy to be considered.

To sum up, EFI will give you a wider powerband and better driveability.  Carbs can have more peak power due to better atomization and charge cooling through evaporation but only for a narrow powerband under truly ideal conditions that are pretty much a pipe dream- like the perfect air density for your jetting.  ITB have better throttle response but must be synced and have tuning issues.  Think ITB for full race (usually naturally aspirated) engines while a big plenum single throttle body is better for most turbo applications and other setups.