I've been seeing a lot of posts about general forced induction questions and I thought I'd make a thread that broke down a few concepts to help people along in their quest for power. These are pretty basic and meant for the FI noob.

1. Forced induction is meant to increase horsepower by adding more air to the intake system. With more air you need more fuel, so the basic concept is more air and more fuel equals more power. Pretty simple. But there are other things that must be considered. Exhaust is a big one. Flow of the exhaust has a large affect on the amount of power the car can make. It's all about breathing. Example...go for a run and take deep breathes in and shorts breathes out. How does that work for you? Probably not very well, same concept with a car. A good rule of thumb is forever HP you intend to make you need 2.2CFM of exhaust flow to maintain zero backpressure in your exhaust system. So if I want 400HP, you need an exhaust that will flow approximately 880CFM (a 2.5" open pipe flows 560CFM). Now you can still make 400 without said exhaust, but you will need to make more power do hit that number.

2. Turbo vs. supercharging. This has been asked so many times my eyes hurt when I see the question asked again. Both will do the job, they just do it different ways. A centrifugal SC (comptech/vortech) spins off a pulley that has a belt running to the crank pulley. The higher the engine revs the faster the crank pulley spins which makes the compressor of the SC spin faster. The faster the compressor wheel spins the more air is drawn in, compressed, and sent to the intake manifold. By decreasing the size of the pulley hooked to the compressor wheel you can run more boost because you are making the wheel spin faster than with a larger diameter pulley. A turbo has a compressor wheel just like the SC does, but rather than having a pulley and a belt, it's bolted to an exhaust manifold. A turbine wheel is connected to the compressor wheel, as throttle pressure is applied to the engine the airflow in the manifold increases. This spins the turbine wheel, which in turn spins the compressor wheel. The advantage is you are not using power from the crank to spin the wheel, and the turbo spins or "spools" faster because you are not limited by how fast the crank in spinning. Which is better? Depends on who you talk to. Some like the linear power of the SC, some like the faster spool up of the turbo. On equally tuned setups, SC and turbo will make the same PEAK HP at a given boost pressure. However the turbo will make more power in the midrange of the engine.

3. Fuel. As everyone should know by now, if we add more air we must add more fuel. The S2000 has stock injectors that flow 360cc/min or 34lbs/hr of fuel. These numbers are calculated at a base pressure of 43.5psi. So without buying new injectors that flow more, or adding a high dollar engine control unit how do we increase fuel? Most of the kits sold today use something called a FMU or Fuel Management Unit. So what is it? It's a fuel pressure regulator that increases fuel pressure as boost pressure increases. Here is what one looks like.



Looking at it you see three connections. The two on the bottom are your fuel supply and return lines, the smaller nipple on top is for your vacuum source. The basic concept is that as boost pressure increase, pressure is applied to a diaphragm in the unit which closes the opening for the fuel return line. This makes your fuel pressure at the fuel rail increase. So if our injectors flow 320cc/min at 43.5 then at say 50psi we now get 383cc/min. Now there are limitations. You cannot just run 100psi of fuel pressure and call it a day. You'll be stressing out your fuel system, from the injectors to the fuel lines to the fuel rail. So you'll have to increase your injector size to compensate. However, you need someway to control the injectors because the stock computer only knows about the stock 360 injectors. OK we'll take about fuel more in a bit let's move on for now.

For now I'll let this all sink in, I'll be posting more over the next few days. Tomorrow I'll go into BOVs/by-pass values and wastegates. I'll also touch on engine control and fuel a bit more Please feel free to post questions and comments. This is to learn from and I am by no means an expert.

 

The blow off valve/by-pass valve.

On a forced injection system you are adding air pressure to you engine to increase horsepower. This is great when the engine can use that pressure. But what happens when you are under load then get off the throttle? The throttle plate shuts leaving the air in the intake system no place to go except back to the compressor wheel. A BOV allows this extra air to be released so it doesn't cause the compressor wheel to surge. Surge on the compressor wheel puts wear on compressor bearings, which after time will lead to failure. So how exactly does it work? Let's take a look shall we.



This is a diagram for a GReddy BOV. It's a pretty standard design, almost identical to the Type-S, Vortech, and many other popular BOVs. The only connection is a vacuum line from the intake manifold to the vacuum nipple on the BOV. Under vacuum condition (such as idle and decel) the vacuum pressure from the manifold opens the large valve and vents any pressure in the intake track. Under boost, there is pressure applied to the diaphragm of the valve which keeps the BOV valve shut. So it's holding boost when you want it to (full throttle), and once you get off the gas, it opens the valve and releases the extra pressure you don't want.

The only difference between a blow off valve and a by-pass valve is that with a by-pass valve the extra boost pressure that is released is sent back into the intake track before the turbo/sc where the BOV just vents it to the atmosphere.

 

Thanks for the info. More please .

 

This is nice work Scorpion. Keep it up!

 

You tha man!

 

We want more, we want more

Advantages/disadvantages between venting to atmosphere as opposed to recycling.

 

Nicely done, I'll be adding to the FAQ

 

"Looking at it you see three connections. The two on the bottom are your fuel supply and return lines, the smaller nipple on top is for your vacuum source"

Looks great, but this seems a bit unclear. The fuel return line is plumbed into the FMU and it acts as an additional restriction on top of the existing fuel pressure regulator. The fuel supply line from the pump to the rail is 100% unmolested. However, you could consider it a supply line to the FMU from the FPR and a return line from the FMU to the fuel tank.

Tim

 

I'll have a shot at that. On cars with Mass AirFlow sensors (not ours) they become confused it they see the air go through but it doesn't get to the engine. Not sure of specifics but it's not relevant to us anyway.

The 'advantage' of venting to air is that fully sick blow off valve sound. The 'advantage' of plumb back is a reduction of that dumbass sound. It all depends what you like.

Also, some authorities have a requirement that says once air enters your system via a filter it should not be released unfiltered except via the exhaust. I believe this comes from the old days when people used draw through carburettors on turbos and vented air/fuel mix to atmosphere. Very dangerous in an engine bay. I think these days the only possible relevance is to do with oil contamination from the filter or turbo but it is a rule in some places nonetheless.

Other than that there is very little difference.

 

Anyone can let us know how a boost selenoid controls the waste gate and what are the limitations on max/min. boost depending on the WG spring?
Let me say my WG spring allows me to run 8psi max without any BC, what is the min and max boost I can run with it?