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Old Mon, May-02-2005, 11:45:17 PM   #1
m3 hammerdown
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Default Forced Induction for Dummies

OK guys multiple people asked for a simple version of some of the things that are happening in a forced induction setup. You MUST keep in mind that we are leaving this simple to start and many variables may not be initially addressed until people ask specific questions. If anyone with knowledge wants to chime in, contribute further, or notify me of something I have said that is wrong feel free. Hopefully everyone will be respectful and listen to responses between all people with an open mind. This thread is not for criticism.

We are going to take the knee-bone's-connected-to-the-thigh-bone approach here. Keep in mind all engines are basically giant air pumps and the goal is to get air and fuel in and out as efficiently as possible. The middle point of this process being the cylinder, we will talk in terms of flow before and after combustion. For the example I am going to use a centrifugal supercharger such as a Vortech. Other roots style blowers or other variants (especially turbos) may very in function slightly.

1. An air molecule is floating along on a nice spring day until it is suddenly and violently sucked into the intake tract in FRONT of the Supercharger / blower (I chose a S/C for the example). A good high flow air filter will make this suction in FRONT of the impeller easier and more efficient right off the bat. It is then compressed by being passed through the impeller of the supercharger (think very nasty fan blades here) and launched along with MANY other air molecules out the discharge side of the blower toward the throttle body. Several things have already happened, most notably the air temperature has risen significantly as a byproduct of being compressed (ambient air can go from 85 F to 280 F in a split second). This becomes significant later as increased levels of boost may reach the point of diminishing returns and detonation and engine reliability become very big concerns. It is also why cold air or water intercoolers are so great! (they may also be before or after the supercharger, but most commonly after).

2. The air has now been compressed into the intake tract. It can already be measured as "BOOST." This boost is going to encounter some obstacles that may increase boost or backpressure on its way to the cylinder. The first may be the route to the throttle body (or in some cases bodIES). Curves, ruffles etc... that disrupt this trip detract from efficiency also (notice K&N's 57/63 trashed that accordion tubing that BMW uses!). Larger throttle bodies may allow easier passage to the manifold and hence less backpressure. Similarly as the air enters the manifold the size, texture, and contour of the passages has an effect on its travel. You may notice that different cars use many varieties of manifolds. The reason is there is no correct solution for every car. In fact runner length and velocity are used to shape the power characteristics of the engine's performance. Forced induction cars tend to benefit from shorter induction tracts and therefore less runner length with better flow, as they make their own velocity and torque mechanically by way of the impeller jamming air backward to the motor. (Ask for specifics I am moving on from this).

3. Along this part of the process fuel injectors will add our spray of gas to the airflow. Forced induction cars will typically need injectors that are much more capable of high and longer duty cycles (spray harder and longer when needed) than stock. There is a general "ideal target" for Air/fuel ratio in forced induction cars, however each is its own animal and the real balancing act is to get as close to lean and mean as you can without melting a hole the size of a quarter through your piston....I know, I have one on my coffee table as an ashtray but it held together long enough to go 9.50 in the 1/4 mile at 147 LOL! Also A/F can have a different impact on performance at different rpm ranges so programmable fuel curves are a nice luxury when youre building your ignition/fuel profile. A/F ratio may not have one preferred constant rate at all rpms. You may want more fuel in some areas compared to others to address flat spots in the power curve.


4. Air then enters the head, passing the valve on the intake side. AT this point we can get into cam science for days but I'll deal with that in a second and keep it as WAY simple as possible. The intake valve is also a major area that can be "tricked out" by knowledgable tuners. Port work and larger and lighter valves can make flow much better. The valve movements are controlled by the camshaft (s). This is where the discussion would get way over most weekend tuner's heads. It is VERY hard to simplify.

5. The cam opens the intake and exhaust valves to let air and fuel in and out of the cylinder. A normally aspirated car must **** (pull) this mix into the cylinder during the piston stroke downward. A forced induction car essentially starts to cram it in there as soon as the intake valve opens. Now the cam controls how high (lift) the valve opens off the seat, how long (duration) it stays open, and exactly when (cam timing.....but not ignition timing thats different) this occurs in relation to everything else that is going on. In addition, the cam lobes also must time the movement of the intake valve to the exhaust valve itself (this is calculated in when they are ground in the first place). In a normally aspirated car there is a performace trick to getting more power. There is a slight OVERLAP when the intake valve starts to open while the exhaust valve has not yet fully closed. The pulse of exhaust gas going OUT of the cylinder actually creates a vacuum behind it that helps to **** (pull) new air/fuel mixture INTO the cylinder behind it. BUT, in a forced induction car this is a tradeoff between that process and a loss of boost out of the still-open exhaust valve. Therefore as a general rule of thumb less overlap is often preferred by cam grinders designing for forced induction cars. (I feel like I just gave birth.........moving on).

6. As the piston rises to compress all of this air and fuel into a small space (the combustion chamber) it achieve as certain compression rate at the top of the stroke (for example 11:1). Compression alone is only a small part of the equation. The combustion chamber size based on piston and head work is the whole story. I have had to explain this to many customers in the past when I was building forced induction motors. You can take a naturally aspirated car that makes 11:1 at X number of cc's combustion chamber size and compare it to a forced induction car that also ultimately achieves 11:1 compression but with a combustion chamber size totaling X+20 cc's and guess what......your making a lot more power at the same compression level because you are simply compressing a LOT more air and fuel in a larger chamber to the same overall pressure.............thus the BANG is BIGGER so to speak. It was very common when I raced NMRA for forced induction racers to build "lower compression motors" with the intent of running high boost from a power adder for this very reason. You can achieve whatever "mechanically managable" level of compression you want with more raw material in the chamber. Think of it the opposite way.......if your chamber characteristics stay exactly the same as stock but you force a little more air and fuel into the chamber with ...say..a supercharger than the car would normaly **** (pull) in on its own with a piston stroke you effectively raise combustion very quickly and can ultimately mechanically cope with less boost before the wrong BOOM happens!


7. As exhaust gasses race out of the cylinder through the exhaust valve they enter the header. Notice also that equal and unequal length headers are available for many performance cars. The advantage to equal length headers most of the times is this...........as the exhaust pulses leave the head the are in succession (as each cylinder fires and then dumps through the corresponding exhaust valve). In unequal length headers the pulses may effectively crash into eachother at the junction of the pipes and impede overall flow. Equal tubes and cross over pipes are all little tricks to help equalize and tune exhaust flow to improve engine performance.

8. Exhaust gasses cool very quickly as they make their way down the pipes to the mufflers. Ultimately they are shot out of the back of the car as a byproduct of this whole process and then sucked into the intake of the corvette that is losing behind you. Let him have the stale air. F#@k him if he can't take a joke!

OK I left a lot of stuff out so start asking specifics and I'l try to get back to the areas I can expand on. We could talk about combustion chambers, ignition timing etc... for days. Hope this helps.
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Last edited by m3 hammerdown; Tue, May-03-2005 at 12:06:36 AM.
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Old Mon, May-02-2005, 11:47:02 PM   #2
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this should be an FAQ
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Old Mon, May-02-2005, 11:51:24 PM   #3
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Now I have to go back and fix the 500 spelling mistakes...........this is worse than school! lol

By the way every **** the site edited says s u c k
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Old Mon, May-02-2005, 11:55:42 PM   #4
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That was very informative, thank you for posting this information
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Old Tue, May-03-2005, 12:03:17 AM   #5
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Nice post, very informative. I would like to learn more from the questions asked by more knowledgable forum members, since I do not have any on this topic.
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Old Tue, May-03-2005, 12:08:09 AM   #6
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Thanks Hammer. Now can you address what we discussed in the other thread as to how so much more power can be extracted from a given volume and boost.
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Old Tue, May-03-2005, 12:14:38 AM   #7
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Beo I haved already addressed it in theory. My question to people who doubt it was exactly the OPPOSITE. Why CAN'T it make that much power? I cannot explain in detail exactly how that car does it unless I have the specific details of the engine. Suffice to say, it seems there have been some revisions to the combustion chamber using replacement pistons and, I would imagine, a whole lot of little other tricks involving a custom ground cam, head modifications, and extensive ignition and fuel curve remapping.

My main concern in the other thread was to dispell the assumptions that were being repeatedly made. The facts are:
-equal compression ratios do not mean equal horsepower
-Boost is not a measure of horsepower, only backpressure which can vary from one car to another based on induction and cylinder mods.
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Old Tue, May-03-2005, 12:19:49 AM   #8
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I know you don't have details but perhaps you can just explain how modifying each parameter you indicated would affect the outcome. Just generalize since I want this to be an informative thread we can pin for later reference.
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Old Tue, May-03-2005, 12:23:13 AM   #9
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Excellent idea. I knew they made you a mod for some particular reason Alex



Quote:
Originally Posted by Beowoulf
I know you don't have details but perhaps you can just explain how modifying each parameter you indicated would affect the outcome. Just generalize since I want this to be an informative thread we can pin for later reference.
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Old Tue, May-03-2005, 12:38:49 AM   #10
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Lets assume we are comparing a stock M3 with X brand blower to one with the same blower and mods.

A centrifugal blower will spin only as fast as it is mechanically spun by the belt assembly. For example, lets say the blower pulley (which is attached to the impeller inside) is run directly off of the crank pulley as is most often the case in supercharging (Not turbocharging!! They are driven by exhaust gas). Lets also assume that the final drive ratio results in the impeller (nasty blades inside the supercharger) rotating at a fixed speed of 56000 rpms at 7000 rpms of engine speed. Yes, that fast, sometimes even faster depending on the pulley setup! We now have measurable boost between the supercharger and, say, the throttle body.

Now lets assume your unmodified car, with that blower setup, went from 333 hp to 393 hp (I am picking these numbers out of the air) for a net gain of exactly 60 horsepower. Lets also assume that my car with identical blower setup has a much larger throttle body, more free flowing intake manifold, larger intake valve and head porting, and more aggressive cam lift and duration.

Question 1. Do you think the stock (unmodified) car would make more horsepower than the modified one? VERY unlikely.

Question 2. Which car do you think will make more measurable boost at the same impeller speed? The one with the free flowing intake tract or the one thats like sending a leaf blower through a straw? The stock car makes more boost because it has more backpressure, while the boost in the other car is being used more efficiently for its intended purpose....getting raw material to the cylinder efficiently and making greater horsepower! Do you want more boost on your guage or do you want to stomp the guy with more boost than you in a race???????

Lets also assume that both cars have a compression ratio of 11.5:1 but the modified car has a larger combustion chamber than stock due to replacement pistons or whatever other mods may contribute. Which car is getting a larger amount of air and fuel into the cyinder? Regardless of the ratio to which it is pressurized by the piston (compression ratio), more air and fuel in the cylinder makes more power. You can have 10 pounds of internal pressure in a balloon or ten pounds of internal pressure in a blimp. Which has the capacity to hold more material and produce more kinetic results after combustion?

Therefore the assumptions behind those two myths I had discussed are dangerously inaccurate.

Generally speaking forced induction cars want to open the flow of the intake path as much as possible. The really important and more secretive part of the tuning is in the experimenting with cam grinds, valve overlap, fuel and ignition curves, and head/valve/piston work. The possibilities are endless.

I'll get back to this with some more examples later. My wife needs to finish a paper for her Master's Degree LOL.
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