GETTING HOT?
How to maximise your engine’s efficiency with our guide to ‘flow’
We all understand that the more air you can cram in to your engine, the more power it can potentially produce, but in reality, things aren’t quite as simple as that. And this is where some common misconceptions and myths can confuse us. It’s all too easy to forget about getting the spent exhaust gases back out of your engine, or simply thinking that bigger is always better. Not to mention things such as the importance of equal flow, and the massive knockon effects a single restriction can have on your entire engine setup.
While the confusion about flow and restriction is certainly most common with turbocharged motors, supercharged, nitrous injected, and naturally aspirated engines are all affected by the issue too.
While there may be many books out there explaining the complex, and quite frankly boring, scientific theories behind gas flow, we’re going to look at real-world problems and, more importantly, real-
world solutions. This feature may totally change the way you look at your engine modification plans...
Removing restriction
One thing you should always remember is that any restriction on your engine – from the air coming into the filter to exhaust gases exiting the tailpipe – will have negative effects on both performance and reliability. As a basic rule of thumb, an engine is only as good as its
“one seemingly insignificant issue has the potential to destroy your engine!”
most restrictive parts – no matter how good the other bits are.
Even the most basic restriction such as a poorly designed turbo inlet pipe or overly small air filter will cause numerous problems. At worst it will create such a powerful vacuum between the filter and compressor inlet it will suck the pipe shut under full load, stalling the engine. But any kind of restriction is bad news for performance and reliability, and can cause a chain-reaction of issues throughout the engine.
As an example, let’s look at what the aforementioned restrictive air filter can do. A restrictive filter will force the turbo compressor to work harder, which in turn will lower compressor efficiency, which will then raise inlet temps, which lowers performance and brings the engine closer to potentiallyfatal detonation. Not only that, but the turbine would have to work harder spinning the compressor wheel, reducing turbo response and increasing pre-turbo backpressure, which in turn increases exhaust temps, reduces power, and once again brings the engine closer to potentially-fatal detonation. You can see how one small, seemingly insignificant, issue has the potential to destroy your engine!
Equal flow
While there’s no denying things such as log exhaust manifolds can produce high levels of power if they are big enough, especially on turbo engines, they do have a lot of other issues that mean an equal length exhaust manifold is a huge improvement over log-style items. Race teams the world over don’t spend millions developing fancy equal-length exhaust systems for nothing!
The importance of equal flow isn’t limited to exhaust manifolds either, as an equal flow inlet manifold design is almost as important. Even intercooler efficiency can be improved by an end tank design that spreads the air flow equally across the entire core, rather than one that allows the vast majority of the air to simply travel across the centre third of the intercooler.
Equal flow exhaust manifolds
Most production engines come with log-style exhaust manifolds, and while these are perfectly fine for production cars, they have serious flow issues that can drastically affect reliability and overall performance. Unequal or log
“Race teams don’t spend millions on equal length systems for nothing!”
manifolds mean, by design, that some ports flow far better than others. This not only affects exhaust gas leaving your engine, but as the cylinders connected to the poorer flowing exhaust manifold runners will have more spent exhaust gas in them than the others, less air/fuel mix can be ingested on the next intake stroke. The result of this is reduced power from those cylinders, not to mention the fact they will be more susceptible to engine damaging detonation due to the higher temps and diluted air/fuel mix.
While this may not sound too significant, it’s no surprise that certain engines tend to fail on certain cylinders, and this is because of the unequal heat levels across the engine; something certainly not helped by unequal flowing manifolds.
The final advantage of an equal flowing exhaust manifold is a much improved exhaust scavenging effect. With a well-designed manifold, the vacuum created behind an exhaust pulse actually sucks the gas out of the other cylinders too. In a basic log manifold design, the exhaust pulses do exactly the opposite, by restricting flow from the other cylinders.
Equal flow inlet manifolds
Equal flow inlets are most effective on cars with forced induction, and just like exhaust manifolds, very few production car inlet manifolds are designed with equal flow in mind. In fact very few aftermarket inlets are either! Having said that, you will find that almost all high-end turbocharged race engines, have complex inlet manifolds that are designed for equal flow rather than simply maximum flow.
Most production, and many aftermarket
inlets, are simply an alloy box fitted with a throttle. And while many can flow huge volumes of air, just like with log exhaust manifolds, the airflow is very unequal and can cause some cylinders to run far leaner than others. This is compounded with high boost pressures, and is a big factor in why certain turbocharged engines fail on particular cylinders far more than others. Ever wondered why a YB prefers to melt piston number three first...?
Equal flow inlet manifolds are complex and expensive to make, and therefore are rarely fitted to production cars as the engine simply isn’t highly tuned enough to warrant one. But just like an exhaust manifold, an equal flow inlet can play a surprisingly large part in both performance and reliability.
While equal flow inlets are less important on naturally aspirated engines, they still have an effect, especially on highly tuned race or drag engines.
The effect of fitting equal flow inlet and exhaust manifolds
With equal flowing inlet, and particularly exhaust manifolds, we create a situation where each cylinder allows the same amount of air in and out. Therefore each cylinder produces the same amount of power, heat, and backpressure, which not only makes for a very efficient engine, but also improves reliability too. All cylinders will be subject to the same conditions, and therefore require the same fuelling and ignition settings.
This means that quite often, higher boost levels and more ignition advance can be used, as we no longer have to compensate for the risk of detonation on poorer flowing cylinders. Instead, having equal flow to all cylinders means we can extract the maximum potential from that engine.