We all hate it.
Just about every owner of a modified third generation RX-7 has been through this dialog. Performance computer upgrades that control the fuel delivery are expensive. It does seem like you should be able to open up the intake and exhaust without needing to upgrade the fuel system, especially when you plan to stay at the stock boost level. However, the fact is that you need to address the fuel system early in the modification process. Contrary to the common notion that this is tuning shop marketing hype, there are some very real and understandable reasons for this general recommendation. I hope that you will be able to understand and appreciate this necessity by the time you finish reading this page.
For more information about when the fuel system should be upgraded, please see Steve Cirian's Stages of Upgrades page.
How Upgrades Work
In our quest for understanding, it is first necessary to understand what happens when you upgrade the various engine systems. An engine makes power in proportion to how much fuel it can burn in the combustion chamber. It is pretty easy to get a lot of fuel into the combustion chamber, but it doesn't do any good unless their is oxygen available for it to burn. So, the basic limitation becomes how many oxygen molecules can be crammed into the combustion chamber. There are two very simple ways to get more oxygen into the combustion chamber. The first is simply to raise the boost pressure and the second is to lower the intake temperature. Both of these methods increase the density of a given volume of air with respect to its oxygen content. There is a third way to increase the number of oxygen molecules in the combustion chamber that is a bit harder to grasp. This third way is related to how well the whole system flows. The stock EFI system can account for changes in boost pressure and temperature, but it simply unaware of changes in flow and this is how we run into trouble.
How well something flows is not a very intuitive notion on its own, but there is an easy way to make it more concrete. We can look at the pressure drop for each component. Pressure drop is simply the difference in pressure at the inlet and outlet of some component at some flow rate. A flow rate is just a volume of air passing through something in a given amount of time, and is often measured in Cubic Feet per Minute (CFM).
It is common to think that since you are running the same boost pressure that flow through the system will not change. This notion has intuitive appeal when you consider that it is the same boost pressure pushing air through the same ports for the same amount of time no matter what intake, intercooler, or exhaust system you have. The important factor that is unaccounted for in this mental model is what is pushing back when the intake port opens. The reality of the internal combustion engine is that you never quite get all the exhaust out of the combustion chamber and that most engines are designed with some overlap where both the intake and exhaust ports are open at the same time. The basic enemy here then becomes backpressure. More backpressure means that the small volume of exhaust in the combustion chamber will be at a higher pressure, thus leaving less space for fresh air when the combustion chamber in question reaches its final intake pressure. With overlap, this also means that there will be more pressure directly opposing the intake charge from entering the combustion chamber. In reality, there is a lot going on in the combustion chamber during the intake phase, and these are merely simplified explanations of two aspects of a very complicated phenomenon. Nonetheless, the basic message here is accurate.
Backpressure is the Key
Is backpressure significant enough to make a difference? Yes, and this can be verified with some experiential proof. Consider again the idea of upgrading the exhaust system without changing the boost pressure. I think we all know from experience that this would give a noticeable increase in horsepower. What changed? The boost pressure is the same, where did that power come from? It must be from the reduction in backpressure.
Okay, we've established that a reduction in backpressure means an increase in power, but how do the normal upgrades reduce backpressure? It is very easy to see how exhaust upgrades reduce backpressure. What is not so clear is how an intake or an intercooler could have the same effect. The key to understanding here is to understand how the turbos translate flow restriction in the intake system into backpressure. Turbos create an increase in pressure between their compressor intake and output. They create a corresponding increase in backpressure on the exhaust side. These two pressure differences are related. If the turbo needs to create a pressure increase of 14 psi on the intake side, it will increase the backpressure on the exhaust side by some multiple of that number. It may seem reasonable to assume that 14 psi of boost pressure will produce 14 psi of backpressure, but the different geometries of the turbo compressor and turbine sections (and other factors) make this not the case. Corky Bell suggests that backpressure on a properly designed turbo system should be limited to about 2.5 times the boost pressure. I believe this recommendation includes the backpressure caused by the basic exhaust system, but it still suggests a multiple greater than one for the backpressure caused directly by boost pressure.
A Working Model
You have suffered through this lecture long enough. We aren't done yet, but I want to give you a break to play around a little bit and consider how all this flow stuff affects the engine system. Change the various pressure drop values to see how they affect air flow. This model is not intended to be very accurate, but rather it should help illustrate how changing the intake components can affect backpressure. Change some numbers and see how all the components relate to backpressure. The Turbine pressure drop equals the Compressor increase plus 3 psi (the flow effects of the turbo housings). The Flow Increase is calculated by the difference in backpressure over boost pressure divided by the compression ratio.
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