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Turbos Continued
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Air/Fuel Ratio tuning: Rich v. Lean, why lean makes more power but is more dangerous
When discussing engine tuning the 'Air/Fuel Ratio' (AFR) is one of the main topics. Proper AFR calibration is critical to performance and durability of the engine and it's components. The AFR defines the ratio of the amount of air consumed by the engine compared to the amount of fuel.
A 'Stoichiometric' AFR has the correct amount of air and fuel to produce a chemically complete combustion event. For gasoline engines, the stoichiometric , A/F ratio is 14.7:1, which means 14.7 parts of air to one part of fuel. The stoichiometric AFR depends on fuel type-- for alcohol it is 6.4:1 and 14.5:1 for diesel.
So what is meant by a rich or lean AFR? A lower AFR number contains less air than the 14.7:1 stoichiometric AFR, therefore it is a richer mixture. Conversely, a higher AFR number contains more air and therefore it is a leaner mixture.
For Example:
15.0:1 = Lean
14.7:1 = Stoichiometric
13.0:1 = Rich
Leaner AFR results in higher temperatures as the mixture is combusted. Generally, normally-aspirated spark-ignition (SI) gasoline engines produce maximum power just slightly rich of stoichiometric. However, in practice it is kept between 12:1 and 13:1 in order to keep exhaust gas temperatures in check and to account for variances in fuel quality. This is a realistic full-load AFR on a normally-aspirated engine but can be dangerously lean with a highly-boosted engine.
Let's take a closer look. As the air-fuel mixture is ignited by the spark plug, a flame front propagates from the spark plug. The now-burning mixture raises the cylinder pressure and temperature, peaking at some point in the combustion process.
The turbocharger increases the density of the air resulting in a denser mixture. The denser mixture raises the peak cylinder pressure, therefore increasing the probability of knock. As the AFR is leaned out, the temperature of the burning gases increases, which also increases the probability of knock. This is why it is imperative to run richer AFR on a boosted engine at full load. Doing so will reduce the likelihood of knock, and will also keep temperatures under control.
There are actually three ways to reduce the probability of knock at full load on a turbocharged engine: reduce boost, adjust the AFR to richer mixture, and retard ignition timing. These three parameters need to be optimized together to yield the highest reliable power.
Wheel trim topic coverage
Trim is a common term used when talking about or describing turbochargers. For example, you may hear someone say "I have a GT2871R ' 56 Trim ' turbocharger. What is 'Trim?' Trim is a term to express the relationship between the inducer* and exducer* of both turbine and compressor wheels. More accurately, it is an area ratio. * The inducer diameter is defined as the diameter where the air enters the wheel, whereas the exducer diameter is defined as the diameter where the air exits the wheel.
Based on aerodynamics and air entry paths, the inducer for a compressor wheel is the smaller diameter. For turbine wheels, the inducer it is the larger diameter
Example #1: GT2871R turbocharger has a compressor wheel with the below dimensions. What is the trim of the compressor wheel?
Inducer diameter = 53.1mm
Exducer diameter = 71.0mm
Example #2: GT2871R turbocharger (part # 743347-1) has a compressor wheel with an exducer diameter of 71.0mm and a trim of 48. What is the inducer diameter of the compressor wheel?
Exducer diameter = 71.0mm
Trim = 48
The trim of a wheel, whether compressor or turbine, affects performance by shifting the airflow capacity. All other factors held constant, a higher trim wheel will flow more than a smaller trim wheel.
However, it is important to note that very often all other factors are not held constant. So just because a wheel is a larger trim does not necessarily mean that it will flow more.
Understanding housing sizing: A/R
A/R (Area/Radius) describes a geometric characteristic of all compressor and turbine housings. Technically, it is defined as:
the inlet (or, for compressor housings, the discharge) cross-sectional area divided by the radius from the turbo centerline to the centroid of that area
The A/R parameter has different effects on the compressor and turbine performance, as outlined below.
Compressor A/R - Compressor performance is comparatively insensitive to changes in A/R. Larger A/R housings are sometimes used to optimize performance of low boost applications, and smaller A/R are used for high boost applications. However, as this influence of A/R on compressor performance is minor, there are not A/R options available for compressor housings.
Turbine A/R - Turbine performance is greatly affected by changing the A/R of the housing, as it is used to adjust the flow capacity of the turbine. Using a smaller A/R will increase the exhaust gas velocity into the turbine wheel. This provides increased turbine power at lower engine speeds, resulting in a quicker boost rise. However, a small A/R also causes the flow to enter the wheel more tangentially, which reduces the ultimate flow capacity of the turbine wheel. This will tend to increase exhaust backpressure and hence reduce the engine's ability to "breathe" effectively at high RPM, adversely affecting peak engine power.
Conversely, using a larger A/R will lower exhaust gas velocity, and delay boost rise. The flow in a larger A/R housing enters the wheel in a more radial fashion, increasing the wheel's effective flow capacity, resulting in lower backpressure and better power at higher engine speeds.
When deciding between A/R options, be realistic with the intended vehicle use and use the A/R to bias the performance toward the desired powerband characteristic.
Here's a simplistic look at comparing turbine housing geometry with different applications. By comparing different turbine housing A/R, it is often possible to determine the intended use of the system. Imagine two 3.5L engines both using GT30R turbochargers. The only difference between the two engines is a different turbine housing A/R; otherwise the two engines are identical:
1. Engine #1 has turbine housing with an A/R of 0.63
2. Engine #2 has a turbine housing with an A/R of 1.06.
What can we infer about the intended use and the turbocharger matching for each engine?
Engine#1: This engine is using a smaller A/R turbine housing (0.63) thus biased more towards low-end torque and optimal boost response. Many would describe this as being more "fun" to drive on the street, as normal daily driving habits tend to favor transient response. However, at higher engine speeds, this smaller A/R housing will result in high backpressure, which can result in a loss of top end power. This type of engine performance is desirable for street applications where the low speed boost response and transient conditions are more important than top end power.
Engine #2: This engine is using a larger A/R turbine housing (1.06) and is biased towards peak horsepower, while sacrificing transient response and torque at very low engine speeds. The larger A/R turbine housing will continue to minimize backpressure at high rpm, to the benefit of engine peak power. On the other hand, this will also raise the engine speed at which the turbo can provide boost, increasing time to boost. The performance of Engine #2 is more desirable for racing applications than Engine #1 where the engine will be operating at high engine speeds most of the time.
For more advance reading you can visit
Turbos For Expert users
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Downpipes & Exhaust
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Downpipes and exhaust systems are the manner of moving exhaust gases out of the car. With or without a turbo you will want a way to move these gases from the exhaust side of the head out the back of the car. With a turbo system you will need a downpipe to take the exhaust gases from the exhaust side of the turbo housing to the exhaust system.
Downpipes
A downpipe is usually nothing more than a pipe that moves the exhaust gases from the turbo housing to the exhaust system. Depending on the type of turbo you went with most people will have either 2.5 or 3 inch downpipe. You can get downpipes from most turbo shops. On the site there are plenty of members who make custom downpipes. You can pm
Kracker or
Fade2Black to make you a custom downpipe.
Exhaust Systems
Exhaust systems are not as important as you think. You do want one that provides some back pressure but is also free flowing. Some people don’t put an exhaust system on and just ride around open downpipe. Although it sounds cool be prepared to be targeted by the police. In all honesty going to a local muffler shop and having them do a custom 3in exhaust is the most common solution for turbo exhaust. However, here are two links where you can buy exhaust systems and exhaust mufflers. Visit
Exhaust Systems or visit
Exhaust Mufflers. Btw please be aware you need to contact clubrsx to talk with a member of the staff for more details about the exhaust systems or mufflers before you make a purchase.
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Intercooler Piping
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Intercooler piping is the piping that runs from the compressor side of the turbo to the manifold. It’s what delivers the compressed air into the manifold. There are three types of ic piping. One is for tubular turbo manifolds, one of log style manifolds, and the other is custom. Unless you are a professional at cutting and measuring piping I would not suggest the custom even though it is the cheapest. If you want to make a custom piping kit you will need to talk to clubrsx because it’s a very detailed process.
Because of where the tubular manifolds place the turbo you need a certain intercooler piping kit. If you go with a full race or peakboost manifold you can use a full race or peakboost kit. The same goes for log styles kits. Log manifold piping kit is only good for the log manifolds. Obviously a custom kit is made to fit and will work for any manifold. Here are some links to visit for kits and custom piping information.
Purchasing Links
Reading Links
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Wastegates
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Wastegates are simple yet important devices. They allow you to set a maximum boost that can be achieved. It works by letting exhaust gases by pass the exhaust housing so that the exhaust fan doesn’t spin as fast and therefore too much boost is not created. This amount of by passed boost is controlled by the spring in the in the wastegate. When the pressures from the gases exceed the pressure created by the spring the spring gives and opens a valve it was holding shut. This allows the gases to escape and by pass the turbo housing. There are two general size of wastegates. There are the 38 and 44mm wastegates. The size you get is determined by the manifold you choose. For example the treadstone manifold uses a 38mm wastegate. Please check with the store to confirm which wastegate your manifold supports.
Also wastegate springs measure resistance pressure in bars instead of psi. So to help you out I have placed a bar to psi conversion table.
Links
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Blow Off Valves
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Blow off valves are essential to keeping a healthy turbo. They are also one of the main reason people love turbos is for the cool sounds the blow off valve makes. But it has a practical use as well. The compressor housing pushes compressor air through the intercooler piping into the intake manifold. This is good as long as the throttle body is open, but when you let off the gas the compressor air has no where to go and the pressure continues to build and can eventually start to push back against the blades of the compressor housing. To prevent this compressor surge the blowoff valve releases that pressure in the piping when the throttle body is closed. It uses a vacuum connection on the intake manifold to control when to release the pressure. Basically when the throttle body is closed and the last of the air in the manifold is sucked out it creates pressure inbalance. At this point there is more pressure in the piping than in the intake manifold. This pressure indifference is what causes the blow off valve to go into actions. Because there is more pressure pushing on the valve on the piping side than air from the manifold side, the valve opens up releasing that in the piping until the pressure is balanced once again. All blow off valves are the same it’s all dependent on what you want, and what sound you like.
Links
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Intake Manifolds
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Intake manifold may seem like a non turbo mod but it can be a very helpful mod for our turbo cars. Honda actually makes a very efficient manifold, however they did not put it on the rsx. The manifold is called the RBC manifold. Most turbo users who want to get the most out of their system spend the extra money, to get the better free flowing manifold. If you decide to go with the manifold there are two parts you should go with. First is the manifold it’s self, and the second is the hondata intake manifold gasket. The manifold gasket is made to conduct less heat, and heat is the enemy of engines and power. You may ask your self why would you want to put the money into a new manifold, and the answer is you want to provide the best air for flow your engine. Imagine with the PRB manifold (stock rsx) your car has asthama, the rbc will fix that. It allows the car to breath better, and for a better air flow and air deliverence.
Links
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Map Sensors
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Map sesnors are very important because they allow your ecu to read larger intake air pressures which are associated with boosted setups. Map sensors are pretty straight forward. Just besure you follow the installation guides because if you don’t you will destroy your mapsensor and you will have to buy a new one.
Links
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Gauges
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Gauges are a debatable modification. Some people say that gauges are a ricers toy while others vouch for their capability to give the driver access to vital information about the car. There are a lot of different gauges out there. Below I have listed some of the most important gauges. You will need to choose how many gauges you want to run and then choose a
piller pod to put the gauge in.
There are two types of boost gauges. One is an electrical gauge which measure the boost and then sends a signal to the gauge cluster that is correlated into a reading, the other is mechanical which has a hose which runs to the gauge from a vacuum source and displays the reading. Electrical is more accurate, but also more expensive. It is up to you as to which type you go with, and which manufacturer. You can see all the gauges that clubrsx sales
here. If you want more help don’t be afraid to ask, or to call the store for help.
AEM Wideband EUGO Gauge
One of the most common gauges used by boosted car is the air to fuel ratio gauge. This gauge measure the ratio of the amount of air vs the amount of gas going into the motor. This gauge is important just to give you an idea of if your running rich or lean. To little gas and you won’t get the power you want, and to much gas you can hurt your motor by having to hot of an explosion. However most standard air to fuel gauges don’t have a wide enough range to boosted applications. This problem is overcome by the wideband eugo gauge. It gives the user the ability to measure lower a/f ratio values. You can get more information about the gauge, and purchase the gauge here
AEM Wideband EUGO Gauge
Boost Gauge
This is without a doubt the most common gauge of all, however that doesn’t make it the most useful. This gauge is really more for appearance than anything else. It’s good for seeing your boost levels however, your boost is controlled by your boost controller. If want some examples see here
Boost Gauge Showoff Thread
Fuel Pressure Gauge
Fuel Pressure is not one you see often. Most people ignore this one because they can get an idea of how their fuel system is acting by their a/f gauge. That is not true because you could have an air leak which will affect you’re a/f gauge, but your fuel system is fine. It’s totally up to the user to choose which gauges. They want. If you use your gauges for practicality then a fuel pressure gauge is important, if they are just for looks then not so much.
Oil Pressure Gauge
Last but not least is the Oil Pressure Gauge. This gauge is important because it’s important to keep a right amount of oil in your car. Without enough oil you motor will seize up. Because the turbo uses oil as well you often need more than the 5 quarts that is suggested by the user manual. As a result the gauge is a good way to measure how much oil is in the system.
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Vacuum Box
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An important part of a turbo system is having an adequate vacuum source for all your devices like boost gauge, boost controller, wastegate, map sensor, bov, etc… Because of this it’s hard to find adequate vacuum sources for all these devices. So there is a simpler way to handle all of this. The easiest way is to run one hose from the intake manifold to a vacuum box and from the vacuum box to the other devices. All you need to make it work is hoses, the block, and t fitting. You can get all of that here at the
Forced Induction Accessories Page You will have to determine on your own how many slots you need and if you need to block any off. If you have any question about Vacuum Box’s please use the chat threads because these answer are usually pretty short and easy to resolve, so theres no need to waste server space with a new thread.
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Transmission
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The weakest part of this whole endeavor will be the part you probably planed on moding the least. The transmissions in the rsx were not mean to hold powers over the 300 to 350 range, 400 really pushes the trannies to their limits. Because of this you need to be prepared to do tranny work. Until recently there was no way except to do constant gear replacements. However, Bullhead gears recently came out with a gear set they say is strong enough to hold up to the pressures demanded by a boosted rsx. The best person to speak with on the forums is fade2black. He can also be reached at the crsx store. He has rebuilt enough transmissions that he could probably do it in his sleep. If you have any questions you would get the best results by directing your questions to him
Bullhead Gears
As I stated above the only recourse we had was to do yearly gear rebuilds on our transmissions. However, clubrsx has introduced Bullhead Gears. They were the first to use these gears, that are imported all the way from Australia. If you want more information on the bullhead gears please visit the two links below, or you may contact fade2black as he tested them personally.
Transmission Rebuilds and work
If you do end up needing transmission work done you might consider having clubrsx do they work. They do a very good job, and have tons of experience working on rsx transmissions. They are also very well priced. If you have any questions just visit
here for more information.
Limited Slip Differentials
One of the important aspects of boosting a car is dealing with how to get the power to the ground. One problem you will face is that with the stock differential you will often spin out of control if you just try to punch it, especially in a lower gear. One way to prevent this is to swap out the stock differential with a limited slip differential. Again this work can be done through the clubrsx store if you wish and they even sell the lsd’s. Just contact fade2black for more information.
Cluthces
Clutches are a very important part of the build. You don't really have a choice to upgrade the clutch. If you leave the stock clutch in it won't last very long at all. Based on what horsepower level you want determines what clutch you should go with. The most common clutch upgrades go with either stage 3 or stage 4. For the really high horsepower applications you can go with the twin disc clutches, but those put a lot of stress on the transmission. Visit the store for more informaiton.
Clubrsx Store - Clutch Section
Flywheel
One thing most people upgrade as well is the flywheel. The stock flywheel is very bulky and heavy. Most people opt to go with a lighter flywheel. This will give you smoother shifting and help with your shifting time, which is important when racing. Notice i said help with your shifting time. The shifting time is still mostly affected by the capabilities and expierences of the driver.
02-07-2009, 02:47 AM
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