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In Reply to: Turbo & Exhaust Theory posted by Azure on September 23, 2000 at 18:35:06:
I'm lead to believe the purpose of a turbo is to basically "pack in" as much air as possible into the cylinder, therefore yielding more powerful combustion, but wouldn't doing so upset the fuel-air ratio inside the cylinder, meaning I would have to add more fuel to maintain the proper ratio, thereby decreasing fuel economy?
And on a similar note, having an exhaust that's too big for your engine decreases hp, but why? I mean all the exhaust does is allow exhaust gases to escape the cylinder more quickly, so why is having "back pressure" so important? What is back pressure doing that's aiding the engine?
I'm told that once you add a means of forced induction into an engine, (like a turbo), the bigger the exhaust the better, why is back pressure suddenly a non-issue in a forced induction system?
You're best bet is to transplant the whole 745 engine, intake, and brain into the 535. The 745 engine has some really neat turbo-specific internals, like sodium-filled exhaust valves, bigger exhaust manifold studs, low compression, etc. You could find a good used engine complete for under $4k, just keep looking. You'll need to reroute the intercooler and exhaust, but that's easy compared to turbocharging a non-turbo engine.
Yes, you are correct. The purpose of the turbo is to "force" air into the system, hence the name forced induction. The air-flow meter will automatically tell the engine computer to add the correct amount of fuel. The meter sits on the suction side of the turbo, so the more air it sucks in, the farther the flap moves. The 745i meter is designed for at least 250hp of airflow. When measuring HP increase, the fuel and airflow must increase in a direct proportion to the HP increase. For instance, (these numbers are arbitrary) say 200HP for your engine needs 100 cubic feet per minute of airflow. If you want 300 HP (50% more) from mechanical modifications (I'm not talking about a chip), you'll need 50% more airflow, or 150CFM total.
Also, to get more power, yes, you will have to lose fuel economy, but only when you're asking the engine for more power. Cruising takes about 10HP, accelerating at full throttle obviously takes all the power the engine has. If you take it easy on the throttle, you're not asking the engine for more power, so it's not using more fuel. See?
Exhaust design and tuning can be considered more art and craft than pure science. Exhaust doesn't need "backpressure", it needs flow. If you increase the diameter of your header pipes, you'll kill low-end torque. Why? Because the exhaust gas speed is important to evacuating cylinders. If you increase the size of the pipes, the gasses will slow down. This causes poor cylinder evacuation. Now if you're looking for all high-end power, then you'd want bigger header pipes, but still not too big. At 5000-6000 RPM, full throttle, your engine is producing about as much flow as it can. That point also corresponds with peak HP. To get the max amount of exhaust gas flow out, you'd need the diameter of pipe that keeps the gasses flowing their fastest. That means that too small of a pipe will "choke" the flow, and too large of a pipe will cause the gasses to flow too slowly. Both have the same end result--less than optimal power.
Take the E30 M3 engine for example. You can get at least 5 different headers for racing purposes. Each one has a different primary (pipe) diameter, and different collector diamaters to tune your engine for a specific power output and range. The 2.5L Evo3 engines with high-lift, high-overlap cams will make all of their power above 5000RPM, so you'd want a fairly large pipe to handle 320 peak HP at 7800RPM. For street use, this Evo3 header would just about kill any driveability below 4000RPM. (The racing tachometers on the Evo3 cars didn't start until 4000RPM!!) The flow would just be so slow that the exhaust would be have a turbulent flow inside the pipes. Turbulent flow is very slow, chaotic flow. Laminar flow (what you really want) is more linear, orderly gas flow. The turbulent flow hinders good cylinder evacuation.
The reason you don't care about exhaust size after a turbo (a supercharger is different) is because the turbo creates a lot of backpressure between the exhaust ports and the turbine. Anything after the turbine is strictly for emissions control (catalyst) and noise control (muffler). You just want a decent sized pipe (3" or smaller is just fine for less than 400HP) into a large, quiet muffler. No sense in letting the world (e.g. cops) know you've got 300+ HP on tap!!
The exhaust side of the turbo (turbine) is a narrow, snail-shell shaped housing that directs exhaust gasses over a small wheel. That wheel will spin up to 120k RPMs (depending on turbo design), turning a shaft which is connected to a compressor wheel (the intake side of the turbo). The compressor does just what it says. At those high RPMs, you get a very efficient compressed air charge. Whenever you compress a gas, it creates heat. The huge intercooler of the 745i exchanges the heat in the compressed air charge to the outside air--an air-to-air intercooler. You can effectively cool a 10psi 250F charge to about 150F with an efficient intercooler setup. Cooler air means more dense air, more oxygen per volume of air, more stuff to burn in the cylinders, and therefore greater power.
So there you have it. Feel free to write if you have further questions. Read Maximum Boost by Corky Bell. Great turbo thoery and instllation book.
JamesM
james_535@yahoo.com
1988 M3 Henna
1983 535i -> soon to be turbo
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