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Resident LED Guru
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Discussion Starter · #1 ·
I decided to do a write up on various induction and exhaust methods that iv researched. Here it goes.

I started by doing my research on intake systems, both NA and forced induction. And also came across exhaust tuning by accident. I’ll start with forced induction as that’s the easy part. Then ill have a look at NA and finally exhaust tuning, which are all interlinked.

I learned that turbos are good for high boost, but suffer from lag. Superchargers are split into 3 categories. Roots, centrifugal, and twin-screw. Roots is good in midrange and produces boost fairly low down. Centrifugal works like a turbo and suffers from lag, but gives good boost. The twin-screw combines the good features and more.

The twin screw supercharger compresses the air before it enters the intake manifold like a centrifugal charger does. Therefore the air in the manifold is more dense than normal. The roots charger forces the air into the intake manifold at normal density, and the compression comes from the air pressure building up. This is the reason that roots chargers suffer minor lag at low rpms. But this also restricts the amount of air that can be compressed at higher rpms. The twin-screw works like the roots charger, but as the air is already compressed it gives instant boost. It also keeps producing boost like a centrifugal charger until the airflow is limited by cam timing, or an engine limiter. Centrifugal chargers use air speed to compress the air, which is why it suffers from lag at lower rpms.

Turbochargers are the most efficient form of producing power with most types of engine. The turbocharger uses wasted energy (hot exhaust gases) to power a turbine. This turns the heat energy into moving energy. This is why is more efficient than running an engine NA (Naturally Aspirated) Superchargers are less efficient again as they actually use some of the energy produced by the engine to power the charger. They are usually powered by a belt driven off the crank. Either part of the auxiliary (fan) belt driven components, or the timing belt.

I actually started by researching by wanting to fit a twin throttle system to my car. I figured that the bigger the throttle body the better the power output. This isn’t the case though. Or course a larger area for the air to enter will allow a higher possible power output, however you still need to take other factors into account.

The intake manifold design has to be taken into account. The cam timing and where it’s going to be used also have to be taken into account. The length of the runners affects the point in the engine range where maximum power is produced. And diameter of the runners affects the permissible maximum power. There are loads of different designs of manifold that can be used. Ill go into a bit of detail on these.

To start with, there are Individual Runner Throttle Bodies (ITBs). These are fuel injected and are most effective for midrange and top end power. The throttle response is instantaneous due to the throttle plates being located close to the intake ports. They usually vent into open space which can cause a ram effect (like supercharging) as high as 7psi. I’m not going to go into why this happens here. Just believe from all the literature that I’ve read that it’s true. Sometimes they will vent into a large plenum chamber to get the same effect. The advantages of this method are that you can use a MAF (Mass Air Flow sensor) instead of a MAP (Manifold Air Pressure sensor) which is good for if your car comes with a MAF originally. It also means that you can put the air intake somewhere that’s located outside the heated engine bay, or even just into a cooler part of the engine bay.

Next is Individual Runner Carburetion. This is also known as twin carbs or side draught carbs. And from the name you will probably guess that it uses carburettors instead of fuel injection. It is almost identical to throttle bodies, except that the runner dimensions are slightly compromised to incorporate the carburettor venturi area. This means that although you still get maximum midrange, the top end is reduced slightly (over throttle bodies)

Port Injected Plenum manifold is what’s used in the Mondeo. The intake runners all vent into a large plenum chamber (like a big tank or air). The manifold has to be deigned quite specifically for this to work. If the runners don’t have the same area right the way through then is can be disastrous for power. This is often a problem due to the fact that the runners are curved. The ford design directs the runners of cylinders that are 360 degrees apart towards each other. This allows shorter runners to be used. The Throttle response from this design is very slow as the throttle (just a single throttle at that) is located above the plenum, so has to fill a large area before the extra power from full throttle can be produced.

There are a few more designs that I can across, but I figured that they are kind of irrelevant, so I’m going to skip them.

When it comes to forced induction, the most commonly used type is the Port Injected Plenum Manifold. The throttle response will be slightly quicker if there is boost ready to go in, but without any boost it won’t make much difference. A large throttle body is good as it allows the pressurised air a less restricted entrance.

An area that’s commonly overlooked is running ITBs with a turbo (and even more so with supercharging). This method is actually much more effective than the standard practise. You can run the same amount of boost and get more power due to the fact that there is less restriction. You need to use a large plenum for this though. I believe the extra cost is the main reason for overlooking this method. As most people will see it, you can get the same power if you increase boost. The main advantages of this method are much faster throttle response, improved filling efficiency. And the one that I think is most important is increased off boost torque. This in theory should allow the turbo to spool faster as there will be higher exhaust flow (needed to run the turbo)

So that’s pretty much as far as I got
 

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Turbo Boost Me!
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1,018 Posts
Where is the bit about exhausts?? Am I blind or isnt there anything about it??

I will add a little bit, Here is a bit about Back Pressure

Mainly back-pressure will hurt you in the form of reversion back into the engine during camshaft lobe overlap. So if the intake and exhaust valves are open a little at the same time (ment for scavenging on a naturally aspirated engine) you'll get the higher pressure exhaust gasses into the combustion chamber since they're higher pressure than the boost pressure. Although you don't want too much more boost than back pressure either cuz then you'll blow raw fuel stright out into the exhaust during overlap.

For street cars there's usually a happy medium since higher back pressure usally means your turbine is sized such that you get satisfactory boost response on the street. But on a race engine you can get closer to a 1:1 boost:back-pressure ratio since the engine is screaming nearly all the time and you're not as worried about boost response from an idle condition.

On a turbocharged car, if you add more back-pressure after the turbo(turbo exit forward) then you're going to reduce your boost response. You want to highest possible pressure differential possible across the turbine to spool the turbine up quickly (high pressure on the engine side and low pressure on the tail pipe side).

Also, on the system as a whole, naturally aspirated or turbocharged, a higher back pressure is going to increase pumping losses as the engine has to work harder on the exhaust stroke to pump all of the exhaust out of the cylinder against the back pressure.
 

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Resident LED Guru
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2,912 Posts
Discussion Starter · #3 ·
oops. lol. i knew there was something missing.

Thats not really all i have got. Iv got loads more to do with headers and tuned runner lengths(itake and exhaust) etc. Will post up more in a few days. This could turn out to be a good serious performance info thread so taht people can learn the basics of serious performance
 

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Just me
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19,673 Posts
Avinitlarge said:
Also, on the system as a whole, naturally aspirated or turbocharged, a higher back pressure is going to increase pumping losses as the engine has to work harder on the exhaust stroke to pump all of the exhaust out of the cylinder against the back pressure.
But, am I right in thinking that a completely straight through exhaust is no good, ie, some back pressure needed?
 

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Turbo Boost Me!
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1,018 Posts
Dansmondeo said:
Avinitlarge said:
Also, on the system as a whole, naturally aspirated or turbocharged, a higher back pressure is going to increase pumping losses as the engine has to work harder on the exhaust stroke to pump all of the exhaust out of the cylinder against the back pressure.
But, am I right in thinking that a completely straight through exhaust is no good, ie, some back pressure needed?
True, On a N/A engine you still need a little back pressure but with a turbo, The less Back Pressure, The better
 

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Just me
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19,673 Posts
I know this is going away from Serious Performance a bit, but on a TD then is it still better with no boxes fitted? I have removed one and its been fine with no problems as far as I can tell and on the rollers I go some good figures.
 

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Turbo Boost Me!
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1,018 Posts
The turbo acts as a silencer, It also helps create back pressure in the exhaust manifold so it the same principle as a petrol.

If you lower the back pressure too much on a worn engine or turbo you usually get smoke

If the crank case pressure is higher than the back pressure the turbo seals move out of place and oil leaks into the exhaust. There is a way round it and that by improving crank case ventalation, The best way is by adding extra breathers on the block, Whist I had my engine out I thought I might aswell add extra breather, I added 2 extra breathers on my block, 1 on the back of the block below the starter and one on the front next to the crank sensor
 
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