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The Exhaust Stroke
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| The exhaust or Blow stroke. |
This is where blow comes from. Nothing too exciting goes on here. As the piston once again starts to go up from BDC, the exhaust valve opens and the burnt exhaust gas is forced out of the cylinder by the rising piston into the exhaust manifold, through the Catalytic Converter, down the exhaust pipe, through the muffler and out. By the time the piston is back at TDC the exhaust valve is almost closed and the intake valve is starting to open. The cycle is about to start again.
A 4 Stroke engine has one power stroke in each cylinder for very two revolutions of the crank. Since a camshaft has one bump on each lobe, it is driven at ½ the engine speed so the cam will open the valve at every other revolution of the crank as necessary.
Imagine this stuff happening at 7000 rpm! The cycle would be repeating itself at about a rate of 60 times a second in each cylinder. At that speed it is easy to imagine a more uninterrupted flow of power coming from this seemingly herky-jerky system.
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| This is the PV diagram of the Otto Cycle. It plots cylinder pressure to volume to degrees of crank rotation. It is used in figuring out the thermodynamics of the 4 stroke cycle. If you are an engineering student you will study this in your thermal class. |
Modifications
Now when you are going to attempt to modify your motor for more power, you must remember that you can never get something for nothing. The factory with its rooms full of engineers usually does a remarkable job of designing an engine that can meet the many
demands made of a modern powerplant. Remember most of the guys that work for the manufacturer are not dummies, they just have to play in a game with many rules!
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| A highly modded engine like this twin turbo Nissan VG30DETT makes more power than stock, a lot more. It was not built to meet noise, emissions, quietness or smoothness requirements like the stock engine. It certainly was not cheap. The factory engineers are not dummies, they just have to contend with much more constraints than simply making power. |
A stock engine’s design goals are not solely to make the maximum amount of power possible. Stock engines must run smoothly and quietly, produce its power over a wide rpm range, last a long time, must get maximum fuel economy, must run without blowing up with fuel ranging from 86 to 91 octane, must make as little pollution as possible and most of all, they must be cheap to mass produce. That’s a lot of musts.
Since you the enthusiast driver is willing to tweak the compromises a bit to get the speed to feed your addiction, you can do some things that give up some of the above to get more power. To go faster you must always burn more fuel, once again remembering that you cannot get something for nothing! Usually you will have to sacrifice some things to get more power. The usual sacrifices are fuel economy, emissions, having to have a more narrow power band and perhaps shorter engine life. In the following months we will talk about the modifications you can do from headers to turbos, and how they work to turn you the customer, into a knowledgeable consumer.
Stay tuned to this channel for more!
