Multi Throttles

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 get.

Individual Throttle Bodies – What and Why?

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 Supercars.

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 themselves.

“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 management system.

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 [where relevant] ,” he says.

“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 street car.”

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.”

Torana Throttle Conversion C-N-J 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. The 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. Tuning 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. “I 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. “It 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 respectively.

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 tuned!

Contacts: RPW www.rpw.com.au Nizpro +61 3 9761 1522 www.nizpro.com.au C-N-J Motorpsort +617 3290 3966 www.cnjmotorsport.com.au