Want max power and response from your engine? Want
to impress your mates when you pop the bonnet? Well, individual throttle bodies
should be on your shopping list – especially if you’re building a naturally
aspirated engine and you’re desperate for all the breathing efficiency you can
Individual Throttle Bodies – What
So what are individual throttle bodies and how are
they installed on an engine?
An individual throttle body setup comprises a
throttle butterfly for each cylinder – a four-cylinder engine will have quad
throttle bodies and a six-pot will have six throttle bodies. These individual
throttles are typically mounted on the engine on a short runner intake manifold
where each airflow passage is kept separate – there’s no shared plenum chamber
downstream of the throttle valves.
In all mass-produced applications, individual
throttle bodies are configured to draw air from a plenum chamber which is
connected to an otherwise conventional intake system (ie there’s an airbox and,
where fitted, airflow meter). In some race applications, there’s no plenum and
you’ll find the throttle bodies are connected to individual ram-tubes that are
designed to maximise intake airflow. However, in other race applications, you
may find the ram-tubes are sealed by a large forward-facing scoop that
effectively forms a large volume plenum. This is a common approach in V8
So what are the advantages – and disadvantages -
of individual throttle bodies?
David Thomas, head of Racing Performance Works
(RPW), is an expert in Mitsubishi, Proton and Hyundai vehicles and has developed
several individual throttle packages to suit. According to Mr Thomas, the
biggest advantage of individual throttles is they don’t scavenge air from other
cylinders and there’s a virtually unlimited supply of air to the engine.
“In a naturally aspired engine with a high
compression ratio and big cam there is great potential for power,” he says.
“Throttle response is also amazing. In most
instances, the actual throttle opening – the total opening across all cylinders
– is higher for your given throttle input. I’m just picking figures but you
might find that an individual throttle system has the same airflow at 15 percent
throttle input as the standard throttle at 50 percent throttle input. But that
will depend a lot on the size of the throttles.”
Mr Thomas suggests it’s very important to select a
throttle body that isn’t too large for the application.
“If the throttle bodies are too big you’ll
find that drivability deteriorates. And, really, in the majority of cases, you’ll
also find you’ll lose some bottom-end torque - that’s why we don’t suggest
individual throttles in a car with an automatic transmission.”
Another industry expert with experience
tuning engines with individual throttles is Simon Gischus, head of Nizpro. Mr
Gischus is very familiar with OE Nissan individual throttle systems (as found in
the RB26DETT and GTi-R SR20DET) and suspects that their biggest advantage is
improved throttle response.
“But, really, I don’t think they’re such a big
deal,” says Mr Gischus. “When Nissan used a 2.6-litre engine in a car weighing
about 1 ½ tonnes - and they put on individual throttle bodies - they were kidding
“I don’t think they’re all about increasing peak
power because each runner still has a throttle shaft taking up space in the
bore. I imagine you can probably go to a bigger diameter throttle to compensate
for the restriction of the throttle shafts but then you can run into dramas with
intake air speed.
“That’s why we have slide-throttles – they don’t
have a throttle shaft in the way.”
Brisbane-based C-N-J Motorsport has
tuned individual throttle systems. Bill, co-owner of C-N-J, has some fresh thoughts on the subject.
“Generally, individual throttle bodies will give
you better throttle response and full throttle power,” he says.
“We tuned an individual throttle system on a
customer’s Holden Torana which has a pretty big cam and, with an Autronic
management system, it drives very well.
“In the first few degrees of throttle movement the
throttles open quite a lot of area and the engine picks up really nicely. But
they are a bit sensitive to tune. To tune it properly, you really need to do it
on the basis of throttle position and revs.
“With enough time tuning it on the dyno and
testing it on the road, you can make it drive really smoothly.”
Installation and Tuning
There are some unique challenges when installing
individual throttle bodies.
Most problems stem from the lack of a dependable
vacuum source. Unlike a single throttle setup (which can generate a
strong vacuum inside the manifold), there is minimal vacuum downstream of each
individual throttle body. To make matters worse from a tuning point of view,
this vacuum fluctuates dramatically with each intake stroke.
According to Mr Thomas, it is possible to run an
engine with individual throttle bodies using a MAP sensor – but it’s a difficult
and inaccurate way to do it.
“It is certainly possible to connect the vacuum
side of each throttle to a shared vacuum chamber – this will improve the
strength of the vacuum signal to the MAP sensor but there will still be big
pressure fluctuations which make it very difficult to tune.
“Depending on the size of the throttles, you might
also find that manifold pressure might not vary much after you’ve reached about
25 percent throttle. That makes it even harder to tune on the basis of MAP.
“In all of our individual throttle
systems, we offer an optional idle speed control system which uses a bypass in
each throttle body. This gives things like air conditioning idle compensation
and cold start idle-up,” says Mr Thomas.
The lack of a dependable vacuum source also means
you won’t be able to use a normal fuel pressure regulator which receives an
engine vacuum (or vacuum/boost) signal. Mr Thomas solves this problem by using a
static set fuel pressure regulator to deliver around 60 psi fuel pressure across
the load range. This is one of the reasons he suggests using a programmable
Yet another vacuum-related problem can be providing
power assistance for the brakes. But Mr Thomas says he can typically draw adequate
vacuum for the brakes from only one of the engine’s individual throttle bodies.
This is achievable because manifold vacuum is greatest the moment you step off
the throttle and apply the brakes.
Mr Gischus has similar views in relation to tuning
with individual throttle bodies. In the past, Mr Gischus has removed the
factory individual throttle system on highly modified RB26DETTs because there’s
not enough vacuum to accurately tune on the basis of MAP.
“Really, the only way to tune a cam’d engine with
individual throttles is using TPS versus rpm and boost
“If you’re prepared to wear some extra intake
restriction, you could use an airflow meter feeding a plenum before the throttles
- but that makes it harder to make big horsepower. I guess in a street car you
really need to have the throttles connected to an air cleaner anyway so,
generally, that means you’ll need some kind of plenum. So it could work in a
Interestingly, Mr Gischus points out that OE
Nissan individual throttle systems have a small vacuum passage in each bore
which connect to a shared log-type chamber. This chamber stores a vacuum signal
that’s strong enough to assist the brakes and provide a load signal to the fuel
pressure regulator. Idle speed is controlled by a stepper motor bypass.
Bill from C-N-J Motorsport is another believer in
tuning individual throttle bodies using throttle position and rpm.
“Really, there’s nowhere to pick up a strong
vacuum source with individual throttle bodies because the power pulses are so
far apart and distinct,” he says.
“Because of that, the best thing is to tune using
TPS versus rpm. With an aftermarket management system, it’s also best to use
sequential injection because that tends to give crisper response. I also find
that high quality throttle linkages with individual throttle valve adjustment
give the best results.”
Motorsport recently tuned an individual throttle body system on a Holden 308ci
(5-litre) V8. Vehicle owner, Alex X, says he wanted to give the engine a
performance rebuild after leaded replacement fuel was phased out and individual
throttles were a great way of adding some power.
TWM throttle system was purchased from the US and, having the same mounting
flange as 48mm IDF down-draught carbs, he was able to source an aftermarket
manifold designed to fit Webers to the locally built 308. Alex installed the
combo together with custom linkages and a custom vacuum chamber (a 40mm diameter
aluminium tube about 300mm long) to feed the brake booster and a Holden
Commodore VK EFI fuel pressure regulator.
the injected 308 is achieved with an Autronic programmable management which
teams with a VT Commodore distributor, Bosch injectors and high-pressure fuel
system. Alex says he was never chasing huge power but the engine easily
generates 266hp (198kW) at the wheels with very progressive torque.
didn’t have the opportunity to get the car dyno’s before the new throttles went
on but it’s like chalk and cheese,” says Alex.
feels sharper and is much more responsive.”
OE Individual Throttle Systems
BMW is the car manufacturer with the most
experience fitting individual throttle bodies to production cars.
The first individual throttle engine available to
the public (though in very limited numbers) was the late ‘70s/early ‘80s E26 M1
road/race car. The M1 was equipped with an early generation 3.5-litre S38 in-line
six with a DOHC, 24-valve head, Kugelfischer/Bosch fuel injection, forged steel
crank, dry sump and lots of other goodies – it’s a very serious engine. As seen
in this photo, the engine’s individual throttle bodies were equipped with
ram-tubes approximately 15cm long. Production versions of this engine (with a
relatively mild 9:1 compression ratio) generated 207kW at 6500 rpm and 330Nm at
5000 rpm together with what was then regarded as impressive driveability. We
believe that Group 4 race versions were tuned to over 350kW and employed
slide-type individual throttles.
In ’86 BMW released the E30 M3 which we believe
remains the most powerful mass produced atmo four-cylinder to come from Bavaria.
The E30 M3’s 2.3-litre four is equipped with a DOHC, 16-valve heads and four
throttle bodies fed by a large volume plenum. Output is 147kW at 6750 rpm.
Next comes the updated 3.6-litre S38 six, as found
in the 1988 to 1991 E35 M5. This engine has multi-valve breathing, solid lifters
and individual throttles to make 232kW at 6900 rpm while an updated version
(from ’92 to ’96) boasts an extra 200cc and 18kW.
BMW M-Power strikes again with the Euro-spec E36
M3 from 1992 to 2000. The E36 M3’s S50 engine incorporates a DOHC head with
solid lifters, single VANOS and individual throttle bodies. This 3.0-litre
engine generates a healthy 210kW. Late model European versions of the E36 M3 are
fitted with an even more potent engine – a double VANOS S52 3.2-litre making
236kW at 7400 rpm. This engine is carried over in S54 spec in the ’01 E46 M3.
Almost as powerful as BMW’s mighty V12s is the S62
4.9-litre V8 as fitted to the 1999 E39 BMW M5 and Z8. Boasting DOHC, 32-valve
induction along with double VANOS, solid lifters and individual throttle bodies,
this engine cranks out a mighty 294kW and 500Nm. The rev limit is set at 7000 –
not bad for a big V8!
Outside of BMW circles, Japanese manufacturers
adopted individual throttle bodies from the late ‘80s/early ‘90s.
The ’89 Nissan Skyline R32 GT-R is equipped with
the company’s showpiece 2.6-litre straight-six engine boasting twin
turbochargers and, of course, six individual throttle bodies. These throttles
feed from a large volume plenum chamber which is pressurised by the twin
turbocharger system. Twin airflow meters provide the ECU with a load input.
Claimed output for the GT-R is conservatively quoted at 206kW at 6800 rpm and
355Nm at 4400 rpm.
The GT-R was followed in late ’91 by the N14
Pulsar GTi-R. The GTi-R uses the famous SR20DET engine equipped with high-spec
internals, a top-mount intercooler, large capacity turbocharger and four
individual throttle bodies – as seen here. Like the GT-R, the GTi-R’s intake
system comprises an aluminium plenum chamber fed by the turbo system. Engine
management is also airflow meter based. Power is 162kW at 6400 rpm and torque is
284Nm at 4800 rpm.
One of the most popular individual throttle setups
can be found on the Japanese Toyota 4A-GE 20-valve engine. Released in ’92, the
early generation 20-valve engine (identified by its silver valve cover) uses a
10.5:1 compression ratio, lightweight rods/pistons, a DOHC
five-valve-per-cylinder head and an elegant quad-throttle system. As seen in
this photo, the quad throttles draw air from a large plenum that’s fed by an
airflow metered airbox. Output is 118kW and 162Nm available at 7400 and 5200 rpm
Later model examples (identified by their black
valve cover) are revised with an 11:1 compression ratio, slightly altered
throttles, a high-flow plenum chamber and induction pipe and adopts a MAP load
sensor. Output is bumped up to 121kW at 7800 rpm and 156Nm at 5600 rpm.
All of these OE individual throttle body systems
are a great source of relatively cheap second-hand components and provide value
setup information for any street application of individual throttle bodies. In
an upcoming article we’ll look at the specs and prices of Japanese import
throttle systems and check out what’s available in the local aftermarket – stay