Originally posted by eight
What makes the most power and what makes the most effiency are two different things.
you're partly correct...
the thermal efficiency of an IC engine can be expressed as power out divided by the power in.
Nth= Wout/Win
in our case, work out is the amount of energy an engine produces and work in is the amount of chemical energy we put into the engine via fuel. if you increase one variable without changing the other then it will change the efficiency. Wout is directly proportional to Nth and Win is inversely proportional to Nth. so, as Wout increases efficiency increases. if the jets in a carb and the pulse width and fuel pressure in a efi engine arent changed, then Win doesnt change., but we can play with timing to run more efficiently and make more power. in a given engine with no mechanical modification and tuning only, max efficiency and max power are synonymous.
now, if you want max power from an engine regardless of modifications necessary you have to increase BMEP, brake mean effective pressure. BMEP is the average pressure that works against the piston top throughout the 4 stroke cycle. during power stroke this is positive in relation to motion. during the three other three stokes, the pressure in the cylinder is working against motion. so, you want to increase pressure during the power stoke and decrease it during the intake and exhast strokes (i.e.- better breathing=less pumping losses). decreasing pressure during the compression stroke would be more counter productive than anything else, so even though it works against power output, it more than makes up for itself during the power stroke. BMEP is what creates torque and HP is simply torque multiplied by a time and distance, how quickly the torque can move a given force a certain distance in the least amount of time. you can increase torque one of four ways. by increasing dynamic compression ratio, increasing stroke (the distance BMEP works), increasing the area of the piston top (pressure x area = force), or increasing the RPMs (which will decrease the time that BMEP is working). since the last three involve major modifications and would result in pretty much a different engine and for the sake of brevity, ill only talk about increasing dynamic compression ratio.
you can change DCR one of two ways. first, increasing the volumetric compression ratio. this is what you see referred to as 10:1, 11:1 and so on. this is a geometric calculation but does not mean you'll get that much compression during operation. for example, if you had a 10:1 CR and the intake runner was 1/4" in diameter, there is no way the cylinder would be filled when the intake valve closes. you would be starting the compression stroke with a negative pressure in the cylinder, not atmospheric. to increase DCR you must either increase CR or get as much air into the cylinder during the intake stroke as possible. this is one of the easiest modifications to do outside of proper tuning. anything that makes the engine breath better will help with DCR. the term that drives DCR is volumetric efficiency. this is the amount of air that actually fills the cylinder compared to the amount of air that is needed to fill the cylinder to atmospheric pressure. if an engine completely fills the cylinder to ambient pressure before the intake valve closes, then it is said to have 100% VE. forced induction engines can exceed 100% because the manifold pressure is above that of atmospheric. with 15 psi boost on pump gas, 200% VE is easily achieveable. a 350 sbc with 200% VE will flow as much air as a n/a 700 cu engine. therefor, it will be able to burn the same amount of fuel and produce the same amount of torque. horsepower will differ b/c the geometry and speed of the engine will differ. this is why they say that forced induction effectivly increases your displacement by a factor of two. typical small block engines operate between 80-100% VE with the latter being in the high performance arena where low end performance is sacrificed. some extremely well built high performance naturally aspirated engines can exceed 100% by taking advantage of the ramming effect brought on by high intake air velocities. this essentially results in a (slighty) higher than ambient pressure in the cylinder, possibly up 105% VE. when you increase VE you have more air in the cylinder for fuel to react with, so you need more fuel. in order to get more fuel you have to change the jetting on a carbed engine and the fuel pressure and/or pulse width on an efi engine. this decreases economy but not efficiency if the engine is performing where it should.
really, the only way to make an engine of said displacement get better gas mileage is to make it run at max efficiency, which im pretty sure its close to doing in the case of the 5.3l vortec. the next step is to reduce pumping losses, because an engine really is just a big air pump. better breathing is the answer to this. the PCM will be able to modify fuel and spark curves for any cheap modification you will make. frictional losses play a big part in power loss as well. the only easy and cheap way to do this is follow grayson's advice and run sythetic oil. there are many ways to do this internally, none of which are cheap or easy. gaining proper alignment and clearancing, teflon coatings and proper lubrication are all expensive and labor intensive operations. all this applies to making the engine more efficient. if you were trying to make it more economical you would need to reduce the displacement, therefor reducing the fuel requirment and potential power output. for the answer on how to do this, refer to my previous post.
in conclusion: efficiency and power are one in the same if you take into account the actual definition of efficiency. your statement would be better worded, "What makes the most power and what makes the best economy are two different things."
efficiency=power
Reply With Quote
i was wondering if anyone else had a stong preference. at the moment i am kinda just considering mail ordering one and putting it on myself unless i can find a better deal.
Originally Posted by Shaggy
It does however have four O2 sensors, one before and one after the cat on each side. I have heard that they flow well and there is no reason to **** with them. True dual would be wicked easy to do though, because it is true dual right up until it empties into the 55 gallon drum muffler. Then some chrome "boom tubes"!!
