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Old 14-Nov-2006, 08:50
loony888 loony888 is offline
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even though the swept volume will stay the same, tuning the engine can, and does need more fuel/air to make more power. after all an engine is really only a pump and the more air you can get in the more power you can make, note here i said air, not fuel, getting fuel in is the easy part, and only there to ignite the air, getting more air in is the challenge and without air, burning fuel is a waste of time. changing lift and duration of cams, going to larger valves and different port shapes and sizes is purely to help maximise flow so more air can make it's way inside. turbo'ed or supercharged engines are more efficient at this than naturally aspirated engines but none will achieve 100% capacity, most naturally aspirated engines can achieve at best 70-75% capacity, and mass production and machine line tolerances mean even less for std engines. obviously, once you have done all this work the thing will run lean so more fuel is required to achieve an optimum air/fuel ratio and thus optimum horsepower (this is after all the point of the exercise) so there must be more fuel and air being consumed by each cylinder to make more power. the waste from this has to go somewhere so a matching exhaust with the ability to allow the waste gases to travel freely away (let's not get into scavenging here)is needed. add to this high revs allowing less and less time to fill/compress/burn/empty each cylinder and it all gets too much for the std exhaust to deal with. larger bores are one way to do this, with the added benefit of being quieter, but clever design, like the "spaghetti" systems work very well too.

more discussion........
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Old 14-Nov-2006, 09:57
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phil_h phil_h is offline
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Exhaust tuning is very simple.
Hot gas at combustion pressure will pass through any exhaust port into any exhaust pipe really easily, but the faster the better to reduce heat transfer into the engine.
When it is in the pipe, getting it out the other end is not a real problem by itself, cos its hot and expanding, so any size pipe will do.
Its the sound pressure waves that have to be managed in the pipe - and you simply have to choose what frequency you are going to tune for ie what engine revs you want to optimise.
You then work out the exhaust volume you need in order to get the returning pressure wave keeping the fresh change in the cylinder at that rpm.
You then simply lay out the plumbing.

So, if you want high rpm power, you need a high volume exhaust, and if you want low rpm power, you need a low volume exhaust.
Engine tuners have been doing this for almost 100 years.
Go get a copy of 'Tuning for speed' by Phil Irving.
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Old 14-Nov-2006, 18:41
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Smacker Smacker is offline
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Many Thanks some most excellent explanations

Steve
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