More cylinders = More HP ? (long post)

this was pulled from Fark.com

2004-05-14 05:33:41 AM computerboy


Ok, number of cylinders doesnt determine horsepower; neither does displacement (although closer). The number of cylinders is just thathow many pistons/cylinders are in the engine...nothing more. In general more displacement means more power, but not always.

Now that that is out of the way, here is some information. There are many different engine configurations v's, flat's, inline's, and rotary's tend to be the more common types today. Each has its own characteristics that make it desirable. In any engine there is a desire for it to be as balanced as possible. Inline 6, flat 6, and V 10s (ignoring rotaries since there is really only the RX7 that uses it) due to the engine design and firing sequence tend to be perfectly balanced. What do I mean by that? Well there is a force applied to the crankshaft of every engine that is 90 degrees out of phase to the pistons force (right hand rule). In these engines all of those forces just happen to cancel and they tend to be really smooth. Other engines must be balanced by using engineering tricks. So at least in theory since you are doing less work to keep the engine from fighting itself you may get more power.

Now what is horsepower? Horsepower is just a formula consisting of torque and engine RPM. That is why really high reving engines (like the S2000s) tend to have relatively high horsepower numbers, but little torque (i.e. the create less torque, but do so at a much higher RPM). Horsepower is just that...power or another way to look at it work (I'm not being technical on this...I know that there is a delta t in the math so dont bother nit picking it). How much work is an engine doing is directly related to the amount of horsepower we rate it for.

Here is an example: I have two engines both making 400HP. One is a tractor engine that does so at 1450RPM and the other is a Ferrari that does it at 8000RPM. Which one is more powerful? Neither, they both make exactly 400HP, but we all know that if I wanted to pull a really heavy load the tractor would win. On the other hand the Ferrari would always win a race. How can this be? Simple, HP is a function of torque and engine RPM so the Ferrari is generating less torque then the tractor, but doing so at a much higher RPM. Think of it this way. I have load of bricks that needs to be moved from point A to point B. In the extreme I can do one of two things: carry one brick at a time or carry them all at once. If all the bricks for each method are moved in the same time then exactly the same amount of work was done (note actually power, but reads better this way and doesn't measure the actual energy expended). Same with the engines, same work different ways of doing it and depending on the task one method may be better then the other. So torque is an actual measured value (look it up in a physics book), HP isnt, but is useful for comparing to very different engines work ability (Ill get back to this later). Torque is also directly related to how heavy a load can be moved at one time (neglecting gearing as usual for this entire post).

An engine is basically an air pump and the more air you pump the more power you generate (in general). This is where displacement comes in. Displacement is a measure of the engines volume (max of each cylinder times number of cylinders is displacement). So holding to the air pump theory more volume should mean more air pumped per unit of time and that is correct except it has a downside. The larger the cylinders become the more mass and consequently inertia they posses and thus the maximum engine RPM goes down (note that all of the really high reving engines are small in displacement). So as usual there are two schools of thought, larger cylinders and slower spinning engine or smaller cylinders and faster spinning. American cars tend to take the previous ideology while foreign manufacturers opt for the second. So while the number of cylinders and displacement affects the total power generated it doesn't determine it absolutely. An example of this is BMW back in the inline 4, twin turbo days of formula one made a 1.5 liter engine that produced 1500HP (no, that isn't a typo). Compress the air a lot and spin little cylinders really fast and you pump a lot of air. Efficiency tends to be more important in determining a vehicles achievable performance. This is measured in HP per Liter. The S2000 had the highest ratio of any production naturally aspirated car in the world at 120 HP/L. So this Lambo has 500HP and 5Liters or 100HP/L. Thats pretty efficient, especially when compared to a Viper at 500HP and 8Liters or 62.5HP/L (I'll get to why this is done later).

Where am I going with this? Well, remember the discussion about HP? How you generate that power is determined by what you want to do with it. In general higher reving engines tend to do better in racing because they weigh less and thus have less mass to motivate, while generating the same HP (note that rules can negate this, but a race held on a circuit course, smaller engines will tend to rule since the bigger engined cars cant carry the speed through the corners. Also racecars are almost always in the upper end of their RPM range so shifting all the torque to the high RPM generates more HP, thus going faster (i.e. same torque, higher RPM = more HP). Engines that must pull heavy loads tend to be bigger and spin much slower (tractor trailers, etc.) because they need the torque at lower RPM where they will be operating. Gearing can completely change this, but that requires a lot of effort into making it work properly.

So 500HP is a lot for such a small car and how much do you really need? It will already go 0-60 MPH in less then 4 seconds and tops out at about 200. An engine that big could produce way more power, but at what cost? Less reliability, gas mileage, etc.. They know that no one is going to use all of the power on a modern road (and if they do their too busy trying to control it or have the biggest grin on their face to care that its ONLY 500HP). Its actually not bad absolute performance and considering the cars market its right on target. See buying these cars is about the experience it brings, not the numbers it generates. Dodge knows that the reputation of American cars is a big engine and lots of power, but not handling. So they put a REALLY big engine in the Viper and don't have to work to get lots to power because of displacement. This is how most American cars make power and we do it because we tend to care more about straight-line speed instead of circuit speed. In this case just put the biggest engine that fits between the wheels and have at it, but if you want to turn and go fast that big engine holds you back. So you tend to get power by going to higher RPM and lighter engines. It takes more engineering work, but it can be done and that is much more inline with a Lambos idea of a sports car. Other makers go even farther, such as Ferrari and Porsche, as the Lambo has always been more about power then handling, but not to the American extreme point.

If you want to see the most extreme engines in the world, watch Formula one. The current rule set is a 3.0L V10. The last BWM engine was running at 19,000+RPM and generating about 930HP (note no turbos, supers, nitrous, or any other power adding devices). Go Montoya!