OK-I've been test driving other cars since my '91 M5 is close to being scrapped (see my other post). I've driven an '01 M5 and an '01 540i sport, both with V-8's obviously. There is a noticable difference in power-band range and engine sound/feel than my '91 M5 straight I-6 and my old '89 Toyota Supra I-6 (3.0 liter with modified intakes, upgraded turbo and free flow exhaust). Why the difference in where the HP and torque sit in the rpm's??? We've always heard (and seen) this to be true but why? Disregard the extraneous stuff like exhaust layout, turbos, etc. There is a basic difference in engine performance. In addition, the V-12 is known for it's superior smoothness (that's why a lot of early fighter planes had V-12's-- no vibrations) while a twin V, a V-2, on a Harley Davidson bike is extremely rough and shakey. Even on sport bikes, you can see V-4's vs. I-4's with same cylinder displacement give very different dyno tables. Let's not even discuss 2-cycle vs. 4-cycle, diesel vs. gasoline, and things like Wankels and radials (true radials as in old airplane engines).
Well, my curiosity is getting the best of me right now so I hope someone out there has a good reference for this issue. I'm looking for the actual physics or mathematics behind these engine designs and why they do what they do. I'm pretty sure some mechanical engineer or gear-head out there has read a good book on this or may know of an in-depth website. I don't expect someone to bang out a reply going in-depth on this issue (highly appreciated though) but was just hoping from some good references.
Any help would be appreciated!
BTW, I really miss the sound of my E34 M5. I would like to get it rolling again..........
Well you have really opened a can of worms here, regarding engine power / torque outputs, etc, and your questions dont have a simple answer, otherwise all manufacturers would be producing the ideal engine.
you can look at several things on a car that will give a fair idea on how its going to perform
Power to weight ratio
Power output per litre of capacity in atmospheric induction, anything above 75/80 bhp per litre will tend to feel peaky
Regarding engine configuration, the basic rule the more cylinder its has the smoother it will feel as there are more firing pulses per engine revolution, but there is a practical limit to everything, this is usually dictated by physical engine bay sizes and cost.
ok, on a 4 stroke engine the engine performance is dictated by number of elements which affect the volumetric efficiency, but the bigests is the valve timing, ie when the valve open and shut, for example a racing camshaft will make the engine develops lots power at high rpm, but it will make the undriveable at low rpm ( not practical for road use ) and conservative camshaft timing will have the oposite efect, nice smooth idle, lots of power at lower part of the rev range.
Now engine designers have overcome some of the limitations of valve timing with variable valve timing, like vanos on the BMW engines or VTEC on the honda engines, but these solutions are complex and expensive, otherwise you would see every engine fitted with this type of solutions, but one problems is that variable valve timing will allow you to produce more power , but has little effect on the torque output of the engine, so they still feel peaky. The smaller the engine capacity the peakier it will apear to be.
Then you have other constraints like how much weight do you have to move, thats why motorcycle engines can afford to develop such large power outputs as the weight they have move is so small compared to a car
Regarding engine configuration most things have been tried over the years
In line 4 is most common configuration, its reasonably easy to package inside car, suffers from secondary vibrations which get worse as the engine capacity goes up which can be subdued with help of balancer shafts, few manufacturers make them above 2.0 litres
Flat four, is smoother doesnt suffer from secondary order vibrations but is not as easy to package, if you dont believe me, look under the bonnet of Subura impreza, you will see what I mean.
In line 6, smoother than the four, but obviously its a 3rd longer so its harder to package, its achilles heel torsional vibrations that build up in the crank as the rev rise, requiring excelent design of the crank and vibration dampers and other moving parts, as well as above average materials to withstand high revs.
V6 has most the advantages on the inline 6, but its not as smooth, its suffers from secondary oder vibrations, the main advantge is size and packaging, a compact V6 design will fit in most engine bays designed for a inline 4 cylinder engine.
Flat 6 is smoother, than v6 as the secondary vibrations are less, its shorter than the inline six, but has other packaging restraints and width, and accesability for maintanence.
V8, are very smooth, but have other restrains, in order to able to fully make use of exhaust pulse tuning you would have to make of a flat plane crankshaft so that each bank of cylinders acts like a inline four and looses some of its smoothness as a result, like ferrari with the 308 or 328 for example.
V12 or flat 12 are the smothest engine design but the overal size makes them hard to package more so if you are going to add decent induction and exhuast system and valve gear to make produce power for given engine size, you wont find a them smaller 5 litres usually.
I dont expect to have answered all your questions with this, and probalby raises more questions than it answers, but there any factors in engine design, and in many cases the marketing department has a say in, rather than the engineers.
Thanks Joe! More good info. My main question is actually more about the torque/HP and where it lies in the rpm band. Is this just a figment of my imagination that V8's are the torque-iest in the low end vs a I6 by engine layout? Or, does torque depend upon the timing and just raw displacement? I just hear a lot about how V8's have low end torque while our I6's carry it in higher rpms. Please educate me.
ok, lets see if I can shed more light regarding your question of power and torque in V8 and in line 6 engines.
I believe part of the reason regarding your question is probably due to fact you are in the states and the evolution of engine designs, has been different to europe due to several reasons. the biggest one has been cost of ownership and fuel consumption
Lets take BMW, people tend to think of BMW of a manufacturer of premium car with fine engineering, and advanced engine design, with a sporting soul them.
The average in line 6 engine during the 80 s would produce max power at 6000 rpm + and max torque a 4000 rpm +, especially the smaller displacement variants of 2.0 litres.
But for example Jaguar which until recently also produced in line 6 engines, these would produce max power at 5000 rpm and max torque at 3000 rpm, and if you drove one, you notice the engine caracteer is completely diferent because jaguar tuned their engines to produce torque and power at lower rpm, whats more if you drive an XJ 40 and nail the throttle to the floor, you will notice that the engine at first pulls very hard (courtessy its high torque output at low rpm ) but as the revs climb the engine looses push and actually feels strained at 5000 rpm ( if you had any mechanical simpathy you wont be pushing the engine at 5000 rpm very often).
Now lets look at the development of BMW and its inline 6 engine
M30 2500cc Max power 150 BHP 5500 rpm Max Torque 215 n/m
this engine was fitted to the E28 5 series, and was based on the big sixes and the cam was chain driven
M20 2500cc Max Power 170 BHP 5800 rpm Max torque 222n/m 4300 rpm.
This engine was fitted to the E30 and early E34, had 2 valves per cylinder, fuel injection. this engine would feel relatively peaky to drive in a 5 series due to weight of the car.
M50 2500cc Max Power 190 BHP 5800 rpm and MAX torque 250 n/m at 4200 rpm
This engine was fitted to the E36 and later E34, it was fitted with a 4 valve per cylinder, and individual coils, fuel injection, and a cam chain instead of cambelt , this engine will also tend to feel peaky, in order to make swift progress the engine revs need to be kept above 4000 rpm.
M52 2500cc Max power 190 BHP 5800 rpm and max Torque 245 n/m 3900 rpm approx
This engine is an evolution of the m50 with a modified cylinder head which includes a single Vanos on the inlet cam.
M54 2500cc Max power 190 bhp 5800 rpm max torque 245 n/m 3500 rpm
This is the latest version of the version of the engine of the engine as fitted to the e46 3 series and last E39. and is fitted with double vanos allowing the engine to produce more torque at lower rpm without loosing top end power.
Now if you compare V8 engines, you will find that american designs have been quite conservative in design untill recent years, the gm northstar engine was one of the first to introduce all the technology which the european and japanese had been using previously, because they where beaten at their own game.
but variations of small and big block american v8 from ford and gm suffered from por cylinder design,single point injection, overhead valve gear and other factor which strangled their power, you will find that in many cases max power was produced at 4000 rpm or less and and max torque at 3000 rpm or less , and relied on more engine capacity to produce power, this in part is due to the fact that the states historically had had the lowest cost for fuel and competition between the different manufacturers was fierce and they tried to outsell each other with lower costs which did not much money in the coffers to introduce new engines and new technology. So if you compared a BMW mid range saloon producing 190 bhp (2.5 litre engine ) with your average american car producing similar power figure it would probably be up against big block v6 engines displacing 3.5 litres or more, and with the benefit of the torque of the larger engine would give.
An example of low engine efficiency in a current american car is the Buick Century
Engine 3.1 V6 max power 175 bhp at 5200 rpm max torque 195 lbs/ft at 4000 rpm
On the other hand look at the cadillac XLR V8 4.6 vvt, max power 320 bhp at 6400 rpm and 310 lbs/ft at 4400 rpm, very respectable, if you ask me, but so it should be , as its GM northstar v8, with alloy block, 4 valves per cylinder, variable cam timing, multipoint injection.
I hope this helps, I am not sure all my figures are correct, but they are just mentioned to ilustrate the issue.
Thanks again. I appreciate the detailed info. Growing up, we only had V8's around the neighborhood (from muscle cars to work trucks) and they did only two things: 1) Smoke the tires from a standstill and 2) suck huge amounts of gas.
Go to www.bimmer.org and make a post on the 6 series board. My friend Ed, M6 Mechanic, frequents that board and the E31 board and knows the S38 and the V12 very well. In fact, he took bad heads of both and took a slice of each to show the V12 guys really how inferior the intake and exhaust ports were compared to the S38. Moreover, the cooling capabilites of the S38 head are so far superior to the V12 it was shocking. He did this because all the V12 guys were doing mods or extrude honed intakes and he wanted to prove the engine really has no capacity to take advantage of it. The intake port on the M5 is beautiful by the way. He may even post pics if you ask him.
On track this weekend, I always get reminded why I love this car. All the guys that rode with me thought it was a V8. They just perform better than their numbers indicate. I also quite suprised a WRX on the way home. He, he, he....
I have driven an E39 540i 6 and I really don't like the power delivery vs. our cars. It just dies past 5000rpm.