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Breathing Deeply

Modifying the inlet manifold system on EFI engines

by Julian Edgar

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Most EFI cars use a complex intake manifold incorporating a plenum chamber and tuned-length runners for each cylinder. The design and construction of this inlet system can have huge implications for performance.

Throttle Body

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The throttle body controls the amount of air that can enter the engine. Most cars use a single throttle placed at the beginning of the plenum chamber but some cars use dual or even triple throttles. In the latter case, the individual throttle blades are often contained within the one casting. In multiple throttle engines the blades are usually opened in sequence, with the second (and/or third) starting to open after a pre-determined opening angle of the first blade. The exception to this is those engines that use one throttle blade per cylinder, like the BMW M3 shown here. These throttles are normally located close to the cylinder head and open simultaneously.

If the engine power has been lifted considerably, the throttle may be posing an undue restriction even when it is fully open. The easiest way to measure whether this is the case is to check the manifold pressure at full power. In a turbo car, measuring the boost pressure either side of the throttle will show if there is a restriction - the two figures should be nearly identical. In a naturally aspirated car equipped with a Manifold Absolute Pressure (MAP) sensor and programmable engine management, the manifold pressure will be able to be read off on the laptop PC screen. It should be very close to atmospheric pressure. In a car running standard factory engine management, direct measurement will need to be made. This can be done using a manometer or other sensitive pressure gauge. The manometer sensing tube should pass through an in-cabin tap so that the manometer can be isolated from the plenum chamber except when full throttle is used. Otherwise, manifold vacuum will suck all of the water out of the instrument!

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If the throttle body is causing a pressure drop it should be replaced with the unit from another engine. This VL Commodore Nissan six is using a larger Ford Falcon throttle body. Note that the opening into the plenum chamber should also be enlarged to match the new throttle. Attaching the new throttle body will normally require the welding of an alloy plate to the plenum chamber.

Custom Intake Manifold Design

An engine constantly starts and stops its intake airflow as the intake valves open and shut. When the piston descends, it creates a low pressure in the combustion chamber that causes a negative wave to race along the intake port and manifold runner. In a naturally aspirated car, the wave is below atmospheric pressure while in a forced aspirated car the wave is at a pressure of less than manifold pressure. When the wave reaches the plenum chamber, it is reflected back towards the engine - but this time as a positive wave. This returning wave has the potential to help ram more air into the combustion chamber, but only if it reaches the valve when it is again open! The trick is to design the system so that these positive reflections arrive at the intake valves at the right time - helping to push air into the engine.

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The gains available form a well-tuned intake system should not be underestimated. Pioneering work done by Jaguar on their mechanically injected racing engines showed that it was possible to gain more than 100 per cent volumetric efficiencies (VE) by using very long intake runners. This means that the cylinders actually breathed in more than their swept volume! Many manufacturers have since developed efficient tuned intake systems that give very high volumetric efficiencies. As an example, Ford in Australia developed a superb dual-length intake manifold for their straight six, a system that achieves a VE of 100 per cent at the peak torque rpm of 3000. Power and torque gains of up to 20 per cent have been seen in some engines - and that's as much as is achieved by low-boost turbocharging!

The three variables that can be changed in designing an intake system are runner length, runner diameter, and plenum chamber volume. Jaguar found with their experiments that the longer the intake runners, the better were the peak torque outputs, and other manufacturers have seen similar results. However, very long runners cause an overly great pressure drop and are not properly tuned in length for peak power, so explaining the changeover characteristics used in some sophisticated intake systems.

A starting point for working out the length and diameter of intake runners can be gained from the following equations. In a Helmholtz Resonance system (one with runners connected to a common plenum), US-based engineering guru David Vizard suggests that a runner length of 17.8cm at 10,000 rpm makes a good starting point. (In this context, "runner length" refers to the distance from the inlet valve to the plenum chamber.) Add to this length 4.3cm for each 1000 rpm less that the system is being tuned for. Tuning for peak torque (not peak power) is the norm, and so if the engine were being tuned for 4000 rpm, a runner length of 43.6cm would be required. You can see that for an averagely-sized engine bay, the longer the runner that can be fitted in, the better!

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One equation for runner diameter is to multiply the engine volume in litres by the engine's volumetric efficiency, then by the tuned rpm, then divide this sum by 3330. The final figure is square rooted, giving the runner diameter in inches. As an example, a 5 litre engine with an 80 per cent efficiency (expressed as 0.8) and tuned for 3000 rpm, will have a runner diameter of 1.9 inches, or 48 mm. The volume of the plenum should be around 80 per cent of the volume of the cylinders to which it is connected.

There are also several computer programs around that can help you design a new intake system. These include Engine Analyzer, which can calculate runner length and diameters to take advantage of not only the first reflected pulse, but also the second and third! Note, though, that a lot of information about every detail of the engine needs to be inputted before this program can start work. Controlled Induction and Controlled Induction Junior can also be used in this area.

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However, as with extractor design, nothing beats dyno experimentation of different intake designs. The complexities of an internal combustion engine are very difficult to model, and with factors such as engine bay room, pressure drop if intake runners are overly long, and individual engine-to-engine variations, the equations are only a rough guide. If making prototype intakes, use mild steel tube and MIG welding - so much the better for mocking-up a trial intake than TIG'd alloy!

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As mentioned, many manufacturers use variable intake systems. These change runner length or plenum volume, depending on engine rpm or load. This allows the intake to have more than one tuned rpm - giving better cylinder filling at both peak torque and peak power, for example. The system can be variably tuned in a number of ways, including (especially on six cylinder engines) connecting twin plenums at high rpm but having them remain separate smaller tuned volumes at lower revs. The introduction of a second plenum into the system at a particular rpm is another approach taken. However, the most common method is to have the induction air pass through long runners at low revs and then swap to short runners at high rpm.

Intake Manifold Layout

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Given that the intake runners need to be long and the plenum chamber generally large, some thought needs to be given to how everything will be fitted in under the bonnet. Different engine configurations need different approaches, with 'V' engines the hardest. Because of potential underbonnet clearance problems and the large number of intake runners required, the runners for a V8 are very limited in length if they are to remain straight. As a result of this, many manufacturers bend their V8 intake runners through 180°, placing the plenum at the base of the runners in the valley of the V. For example, both Holden and Mercedes do this. However, while this is good for mass market passenger cars that require a lot of bottom-end torque, peak power suffers because of the pressure drop caused by the long, relatively thin runner diameters.

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'Straight' engines can use curved runners that pass over the rocker cover to give a very long intake design. This approach was first used by BMW on their injected sixes in the early Seventies. However with crossflow heads, this also means that the plenum ends up on the hot (exhaust) side of the engine and so heat shielding is needed. Using runners that dip well downwards before turning around to come up to the plenum can lengthen the effective runner length while keeping the plenum chamber on the cool side of the engine. You need only look at the tricks that manufacturers go to in the pursuit of long intake runners to realise their importance!

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Where the runners enter the plenum, bell-mouths should be used to smooth the flow of air into the tube. A metal spinner can spin custom fabricated bell-mouths out of aluminium, with these then being inserted into the ends of the runners.

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Many people suggest that in a turbo car the entrance to the plenum should not be directly opposite any of the runners - it should face a blank wall so that the airflow collides with it, spreading evenly within the plenum. If the intake pipe is aimed straight down one of the runners, that cylinder may receive too much air, giving it a leaner mixture. Most manufacturers use shorter runners on the turbo versions of their cars, often with a larger plenum chamber.


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Both the runners and plenum chamber should be constructed of heavy-duty materials. This is because the pressure wave activity occurring within the system can cause fractures and cracks in lightweight designs. The runners can be made from mild steel exhaust tubing, with the plenum folded-up from sheet steel and the head flange made up like an exhaust flange. A more expensive - but better looking - approach is to fabricate the system from aluminium sheet and tube. The finished item can be polished, powder-coated or painted, depending on the materials used and the budget available.

Standard road cars always have the injectors placed close to the intake valves, but many race cars move the injectors much further back along the intake runners. It is suggested that the greater evaporation of the fuel that occurs when the spray is positioned well back from the head aids intake air cooling, so increasing its density. In fact I have seen a race engine on an engine dyno that used this approach. After running hard, its carbon fibre intake runners were so cold that condensation was streaming down them! However in a road car this distant injector location is likely to lead to poor driveability at low loads and the greater possibility of an engine fire if a backfire occurs.

Some factory manifolds use bolt-on injector mounts, making it easy to adapt these to custom intake manifolds. The Australian JD Holden Camira is one car using bolt-on injector mounts. A turned-up fitting welded to the runner can also be used, but taking this custom-everything approach can be very time consuming. Holding the injectors in place with the fuel rail is a neat and easy way of avoiding the need for multiple tapped fittings on each runner.

Modifying the Standard Intake Manifold

The standard intake manifold and plenum can be modified. Extrude Honing, a special process where an abrasive solution is forced through the intake runners, can be used to smooth and enlarge the intake system. Another approach is to cut open the plenum chamber and port the runners with a die grinder, before welding the plenum back up again. However, I have seen some expensive failures occur when the intake system has been ported by hand in this way. Gains of only a few percent in power mean that the time could have been better spent making a completely new intake system.

One of the most time-consuming tasks in making a new intake is the marking and cutting out of the new manifold plate, and then the welding of port-matched intake runners to it. This step can be avoided if the engine uses a two-piece intake manifold, where the plenum chamber and upper portion of the runners separate. In this case, a new plenum chamber (perhaps with revised throttle bodies) can be easily made and attached to the shortened runners. Airflow into the runners can be encouraged by the use of push-in spun alloy bellmouths. This type of modified standard intake will usually achieve better top-end power.

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Another approach is to adapt a carby manifold to EFI use. This will result in an intake system that uses large diameter, short runners. Where the carburettor normally sits a multiple throttle assembly can be used to regulate airflow. The injectors can be placed either in this throttle assembly or the manifold can be drilled and tapped to accept injectors on each intake runner.

If a higher torque design is needed than that provided by the new car manufacturer, you're probably not in luck! This is because nearly all road car engines use intake systems designed to bolster as much as possible bottom-end and mid-range torque. To improve in this area over the standard manifold will be extremely difficult.

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