Inline vs. V6
 

I have a W124 model, the 1991 300CE, an inline 6 engine. I've always wondered what are the advantages or disadvantages of the inline 6 versus the V6. I realize that the size and shape of the engines are different, but I'm more interested in learning the performance differences.

Does one engine have a strength in one area but weaknesses in others? Is there a difference in performance at all?

Thanks.

Phil

Another factor is the length of the crankshaft. On an inline 6-cyl, the crankshaft tends to be longer, and therefore subject to more torsional stresses. This limits the max RPM's that an engine can be run at. This doesn't seem to be as big a factor on engines being built now though. They must have the metalurgy, as well as crankshaft balancing, developed well enough now. The old 6-es of the Fords, Chevies, ect, would tear themselves apart if you tried to run them above about 4000rpm (I know about this first hand:D ) I don't know this for sure, but I'm thinking that Datsun led the way of higher revving straight sixes with the 240Z's.

an inline6:
- is perfectly balanced for forces and moments, without the need for counterbalancers
- has evenly spaced exhaust pulses; aside from the smooth sound it makes, this makes it more conducive to exhaust tuning, e.g., to take advantage of scavenging
- the long engine block, head and gasket make the differences in thermal expansion between these dissimilar materials more pronounced, contributing to gasket failures
- is easier to use a turbocharger with, because the exhaust ports are all on one side


a v6:
- has a shorter, more robust crankshaft
- is easier to use a supercharger with, the Vee of the block providing a natural location to put it

[QUOTE]Originally posted by bobbyv
[B]an inline6:
- is perfectly balanced for forces and moments, without the need for counterbalancers

I'd debate that point. Ever had an inline-6 apart? Ever seen the counter weights on one? Ever seen the effects of an over-revved inline-6? (Twisted and bent crankshaft, cracked pistons, ect.)

He's referring to belt driven counter-rotating balance shafts not crankshaft counter weights. The one disadvantage of an inline-6 is the LONG crank which can bend under extreme load but it's silky smoothness more than makes up for that...at least on the street :)

i believe that the counterweights on an inline6 crankshaft are there to counteract the reciprocating mass of their corresponding connecting rods (the center-of-mass of a con-rod goes through a circular motion as it reciprocates).

however, the "self-balanced" nature of the inline6 has to do with more with the movement of the pistons balancing each other out, in terms of forces and moments. No additional counterbalance is required to compensate for this.

It's a good thing you asked this question here and not on a truck board...I've seen some nasty fights develop over the advantages and disadvantages between a straight-6 and a V-8.

It is generally believed that an inline engine will produce more low-end torque than a similar-sized V-engine, but that a V-engine will provide more high-end torque than the inline engine. I actually prefer inline engines for their simplicity (an inline-6 typically has 40% fewer moving parts than a V-8), and I have yet to hear of an inline diesel engine breaking a crankshaft (except for some guy in a truck tractor who abused his engine, but it still got to the shop running on the aftmost 3 cylinders!). :)

I would like to see somebody bend an inline 6 crankshaft. They have seven main bearings, whereas there are only four main bearing journals in a V6, and 5 in a V8.

I think something should be said about torsional rigidity in the crank. A V6 would have a shorter crank and therefore would have the potential to be stronger.

I disagree that most v6s are 60 degree. Nissan, Alfa and Chrysler made 60 degrees, but almost everyone else including Mercedes are 90 degree.

The problem with the 60 degree is that the bore is limitted by the block angle. You can only go so big until the banks of cylinders start getting in the way of each other. You have cooling issues and if you like big flat combustion chambers (lots of valves) there are limits.

I think the inlines are easier and cheaper to manufacture. Fewer number of cams and a simpler valve train. The 60 degree v6 is smoother than an inline IMHO.

Not all inline-6's have 7 mains. I suppose any of the later ones do, partly explaining the performance from many of them now. But "back then" most only had 5 mains. That was why the old Rambler 6's could out perform alot of other makes, they did have 7 mains. (Anybody older than me remember the Hudson's? 305 CID I think they were, with dual carbs that outran almost anything else for several years in the early 50's.)
Of course I'm comparing apples and oranges here. I'm sure mosty of you are referring to anything but american inline 6's of the 50's and 60's. But that's what I grew up with, and learned to rebuild engines on.:D
Oh, BTW, I did bend a crank on my '54 Ford pickup with a 223 straight 6 back in the mid-70's. You couldn't see it with your eye, but when I took it in to have it reground, they called back and told me I needed a new one. They couldn't do anything with the one I had. Both bent AND twisted. Trying to do too many stop light drags with the poor old thing:o

You have me scratching my head on how you can only have 5 mains in an inline 6 cylinder engine. You have front and rear main bearings, and one main between each cylinder for 7 mains. Cutting back to every other cylinder (which would be a VERY fragile arrangement), would drop back to 4 mains on the crank. :confused:

Oops! Sorry. You're right. 4 would be the right number. I got screwed up thinking about 4-bangers with 3 mains. My bad!:o

4 main 6's: That was the main change (other than displacement) from the W113 230sl to the 250sl. The 230sl had 4 mains whereas the 250 got one per piston.

smoothness: there are two components to smoothness, even firing order and balance. An even firing order is designed in by making the bank angle 360/# cyls or 720/# cyls as it has been mentioned. A slightly uneven firing order isn't the end of the world. Many racing V-6's have this due to simple crankshafts and odball vee angles. Older GM v-6's also had an uneven firing order.

Balance is trickier. For any vee engine, it has primary balance when the bank angles are at 90 degrees. This is because of the forces acting on the crankshaft. A pair of cylinders sharing a crank throw 90 degrees out of phase apply sin(angle) and cosine(angle) forces to the crankshaft. When added, these form a rotating force vector of constant magnitude. This circular rotating force vector can be exactly cancelled by a counterweight on the crankshaft. And that is how you get to perfect primary balance.

If the vee angle is not 90 degrees, the resultant force vector will be elliptical and will not be cancelled completely by the counterweights.

Once you've made a 90 degree V-6, to get even firing order, most manufacturers offset each cylinder's crank throw 15 degrees on either side of the crank throw. This makes the pistons get to TDC in an even manner restoring even firing order.

Sholin

im no physycist, but i recall commentary on inline 6's having more torque due to the longer crankshaft length.

I had a 1999 328i, best inline 6 I have ever driven. It idles so smoothly that sometimes I had to check just to make sure the motor is running. When you punch it, it winds up all the way to the redline effortlessly. Great car and great engine;)

My friend's 2001 BMW 330i also has a straight-6 with 225 HP. It has an almost flat torque response across all RPMs and it winds up there beautifully as well - snaps your head at any RPM. Sounds fantastic as well - there must be something to those inline 6's that keeps German car makers making them...;)

The new M3 also has an inline 6 I believe...

I'd never heard of that 360/cyl, but it makes sense. What about the five-banger? = 72 deg...is that what M-B was up to, widening the engine a bit for a smoother ride than a v-6 with more power than a straight four and more compact than a straight 6?

Say, what about radial engines, were they ever made for a land vehicle?

Then there was the old chevy Corvair, didn't it have a flat six, configured sort of like a BMW motorcycle?

ah, i was wondering when the flat motors would arise.
when i think flat, i think porsche, and air cooled. no H2O probs.

[]----O----[]

i also have a V4 honda, 90 deg V, 180 crank, with gear driven cams. quite a unique engine, spins to 12.5K. 3/4 liter with approx 105 HP. it will kill my 3.5 liter bimmer, but it weighs less than 500 lbs!

some of the new subaru's are turbo flat 4's, like the WRX.

ive owned 3 inline MB's. two sixes, and one five. liked them all.

My Audi 5's wind up pretty tight and are pretty smooth. Better than a 4, not quite as smooth as a 6. Have a nice unique exhaust note too.:cool:
Subarus' flat four always sound like they're idling on only 3 cylinders when the exhaust system starts getting a bit loud. Kinda wierd. First time I heard it, I thought it was just particular that car with a prob, but since then, every one I've heard sounds the same.

I have to agree with Flash and Zeus that the I6 that BMW makes is pretty darn good. I've always enjoyed the smooth power of my brothers 328is. Revs pretty good too... So then I go compare this to current MB engines. A 90 deg V6 with SOHC. woopee. Am I supposed to be impressed by that. I guess I'm still disappointed they went with the V instead of sticking with the I.

This is all academic anyway... we all know nothing beats a Wankel! :D Out of curiosity, has anyone driven the new mazda rx-8 yet?

andrew

I have really enjoyed reading this post;
I understand that a 120degree V6 is pretty wide, so why not make a flat 6 like Porsche?
In addition the flat 6's are very smooth.

It's all in the packaging. Not all manufacturers have room to put a horizontally opposed engine between the wheels, or want to go through the great lengths to lower the center of gravity in the car.

Torque has nothing to do with the configuration of the engine. It is a function of the bore:stroke relationship, cam profile, intake tuning, and exhaust. Look at the M3 inline six. It makes nothing down low. We could build a screamer over-square inline six that made no power below 5000rpm, or an undersquare torque monster that could tow a Kenworth at idle.

The inline six has an inherent balance that the V-6 does not thanks to crankshaft weighting and firing. The boxer six and the inline six have the right relationship between firings and rotation in space. The V-6 engine does not, but balance shafts (not to be confused with crank weights) can offset some of it. MB's M112 V-6's are pretty smooth puppies.

Produding a V-6 engine is not any less expensive than an inline six. The main reason for producing one is packaging space. It is possible to lower hood lines dramatically. The packaging aspect is the number one reason the V-6 is so prevalent. FWD packaging made the straight six a rare bird.

Isn't torque just the product of the rotating mass's moment and its angular velocity?

I'll bet the best torque in a reciprocating gas engine came from those old Dusenburgs with their giant in-line engines and great stroke length. Must have been a hell of a clutch, but that's what the ace up front does while we sit in the back drinking bootlegged scotch....

Botnst

You've hit the nail on the head. Piston speed, stroke length, cam profile (timing) all contibute to the character of the engine. It is not a function of the inline engine. By designing an inline engine with low weight pistons, an extreme oversquare design and radical cam profiles, we can have a high-revving engine that makes little power or torque in the lower rev range.

Or, we can design an engine with a longer stroke (higher piston speed at high rpm from the longer stroke means lower absoute rpm) and larger counterweights. Add a heavier flywheel, smoother cam profiles and longer intake runners and we've got a low revving grunt motor.

I am neophyte when it comes to engine design that is why i get a kick out of this thread..hope it never ends:)
but if subaru can package a flat 4 or 6 why dont other car makers due the same.
The subarus that i have had have been amazing smooth cars to drive, infact my lastest Outback ( a flat 4) was so smooth that excluding its lack of power could be mistaken for a six.

Also are they any good, readable books about engine designs out there

thanks

I also am a fan of the Subie boxer. And the 165hp four is only 3 ponies shy of the 2.6L M112 six in the C240 Mercedes. The Subaru four is as smooth, has lots of torque (not a high revving engine) and provides pretty good fuel economy considering it's displacement and that it's hauling the AWD system around all the time.

i agree that the Sube wasnt a highreving engine but when it was reved, didnt sound as harsh as other fours.
What makes a engine high revving.
For example I have a 1970 911s 2.2L which you can redline almost all day long while my 300te 4matic sounds beat over 4500 and doesnt want to rev any higher?

Also can someone explain oversquared?

Also any ideas about books i can read

Oversquare means that the bore (piston diameter essentially, but not precisely) is larger than the stroke (the amount the piston rises and falls). This means that the displacement is attained with slower piston speeds, allowing the engine to rev high.

The BMW M3 engine is a classic example, as are most Honda V-TEC engines.

Also, on those rev machines, the engine components tend to be of light weight. Years ago I built a small block Chev 355ci engine that could rev to nearly 10,000 rpm, and do 8000 realiably. Thanks to aluminum pistons and special rods, the moving mass was greatly reduced. The crankshaft was forged steel and carefully ground for the engine.

The problem with oversquare engines is the sheer size of the piston. The flame has a great area to cover and the spark plug can be quite a distance from the edge of the piston. Also, highly oversquare engines, as they are designed for high rpm applications, tend to have small combustion chambers (high compression ratios) and that means detonation problems. You need the gasoline to ignite at a higher compression point, and that is why premium gas is needed. It burns with less volatility than the regular gas. Also, thos esmall combustion chambers often mean reduced valve lift for the amount of fuel required, and even piston cut-outs might not be enough. The valve surface gets bigger,and this means a fuel "dump" can occur. That is why high rpm engines tend to have two intake valves, as it increases the total valve area without reducing the swirl effect of a smaller valve. Brilliant.

You can influence the character of an engine with intake and exhaust tuning as well. Long intake runners allow the air to slow, which is desirable for low revving engines. MB makes a neat variable intake runner that allows long paths during low rpm for greater torque and the path to shorten during high rpms for greater efficiency at high rpm. BMW makes their own version too. An open exhaust allows the cylinder to empty faster, but this in NOT always better. Most folks don't understand about cylinder head flow and the timing of exhaust valves. You need the exhaust system to allow the cyclinder to empty at the CORRECT rate, and with most street engines, some backpressure is required. You don't want the exhaust to empty too fast if there is cam overlap, but not too slow either. Now, that M3 engine could probably run with an open exhaust, but OE systems have become so good, there would be little improvement with an open exhaust, and might get you a noise ticket.

Engineers spend lots of time trying to get a certain feel to a driveline. That includes the engine, the flywheel, the transmission ratios and the final drive ratio. Mercedes' cars have typically been tuned for decent low rpm performance, but excel in midrange and low-high (like 5,000-6,000) performance. Take the old M103 W124 300E. The engine made 177hp, and on paper should be a stoplight killer for it's day. But, tuning made it a different car. Once rolling, it could take on most comers, and even outrun the 560SEL. Years back the car mags grinned when the 300E could outrun a Porsche 944.

Some cars are tiring to drive thanks to that tuning. I spent some wheel time in the E46 BMW M3. It's a hoot (except for quality problems) but might get tiring on long trips and daily commutes. The engine has no flywheel effect and needs lots of concentration when crawling in traffic. The engine makes HUGE power, but not much down low, and this means rowing the shifter constantly. However, in the right conditions, it's the king-of-the-hill. However, guys like me would probably take the 330i for real world living. Keep a fast motorcycle in the garage for those moments when you feel like tearing up the road.

Oversquare is an engine with a shorter stroke than it's bore. Bore is the diameter of the combustion chamber for each cylinder, while stroke is the depth. The cylinder moves up and down inside this combustion chamber.

If it helps, you can imagine the piston in an engine as your hand, working a bicycle pump going in and out (or up and down).

In an engine, in each engine revolution, the piston will go UP once and DOWN once. At 2000 RevolutionsPerMinute this is once every 0.03s. If you have a short stroke (say, 90mm), Moving 90mm (twice) in 0.03s won't require the piston to move too fast. If you have a larger stroke of 200mm, you have to move that larger distance (twice) still in 0.03s - so it's going to have to move much faster, which equals more momentum, friction and more stress for the longer stroke engine at 2000 rpm.

So, a shorter stroke engine can rev higher with less stress (think BMW here) The disadvantage is that the less momentum mentioned above equals less torque. Japanese high-revving banzaiboxes have short stroke, high revving engines and no torque. No variable valve timing can counter for this (as I am currently discovering in my VVT Audi V6) - it is physics. VWs tend to use long stroke engines. Low-revving and boring maybe, but they're nice and torquey.


(NB my maths is probably wrong)




FTR, it is generally acknowledged that the Subaru flat four is a naturally un-torquey unit - even with the turbos. I've only driven 1 or 2 but would probably agree.

I would disagree on the aspro 2.5 four. It's got lots of low end power and can be dragged around town in high gear all day. The turbo 2.0 has NO low end grunt, and thusly, turbo lag is a huge problem. I understand the turbo 2.5 has no such problem and the STi can suck the headlamps out of a 911 without much difficulty.

So it seems that torque is a more a function of momentum of the piston that is speed(ie hig revs).

Also if there oversquare is there undersquare?

and everyone talks of HP but isnt the blending of torque/hp more important than just HP.

Yup. There are plenty of square and slightly undersquare designs out there.

Torque and HP are linked in the formula:

HP = torque * (RPM / 5252)

So, suppose your engine makes 300lbs/ft at 1500rpm. The HP at that rpm is

= 300 * 3000/5252

= 171HP

Perhaps you noticed that horsepower and torque are equal at 5252 rpm. Neat, huh? Check some HP/Torque graphs and you'll see they always meet there. Physics at work.

From this function you could derive that making more torque at higher rpm is the secret to making higher horsepower. The key is to make sure that the torque curve does not fall off, and you'll get to big HP numbers as the rpm's rise. From the formula we see that HP will get bigger with rpm, but a falling torque curve will eventually cause HP to drop off too.

Let's expand on the formula in the previous post to clarify the relationship between HP and torque.

The physic-guru James Watt found that a typical horse could lift a 550 lb weight one foot in one second. This translates to 33,000lbs in one minute. That's our modern measure of a horsepower. However, you notice that it would be measured in pounds moved over one foot? Pounds-foot? That sounds like torque! You're right! You can't measure horsepower. A dyno measures torque and we use some math to show HP.

Suppose we have a circle that is one foot in diameter. We extert one pound of resistance against it. That means in one full revolution we have moved one pound through the circumference (which is 2*Pi) or (3.14159*2) = 6.2832 feet.

Now, we know that one hp is 33,000 foot-pounds, right? Take the 33,000 and divide it by the 6.2832 feet of one revolution, and we have the 5252 number. That's where the relationship comes from. Blame Watt for the formula!

We end up with: HP = torque * (RPM / 5252)

It's simple, but the design and creation of en engine's torque curve is not. Lots of variables.

John, thanks for taking the time for that well written, interesting note. You've got a knack for teaching.

I notice you have an '88 Mazda 626 with well over 200K miles. My '89 626 just tipped 205K. I've replaced the clutch twice and a transaxle once and the window gearing in the driver's door. I bought the car at 60K (commuter car previously owned by an anal retentive NASA M.E.) thinking I'd get 120K but the car will not die. I've taught two daughters to drive on it (hence the two clutches...) and it looks like I'll have to drive a stake through its heart to stop it.

Do other people have similar experiences with the 626?

Botnst

Our odo is measured in kilometers, so we're actually at about 160K-miles.

It runs perfect. It drives well (for an old FWD car...) and everything works. The struts have been replaced once at about 140K, and the fronts are weeping. It's got a new rad and alternator. One MAS. Other than that it's been routine stuff.

The engine is still strong, but the hydraulic lash adjusters make a peep if the car hasn't been driven for a few days. Noise goes away right after start up. It uses a bit of oil, but nothing to get excited about. A litre every 3000km's (2000miles) or so. Ours is the G4A-EL 4-speed electronic automatic, and it's held up really well.

The body is starting to show it's age. The Japanese makes were not double galvinizing then, and we're getting some rust spots on the wheel-wells. The interior rattles.

The air seems to be not quite as cold, but it's been REALLY hot until today (35C or close to 100F) so it could just be the extreme heat. It still cooled the car, but not the ice-cold air the C230 or Outback can make. The Outback makes good cold air, but is still not as good as the C230.

We kept the 626 even though we need only two cars. It's worth way more to us than we could get for it. I use to go to "high-risk" shopping malls, and the flip down seat allows transport of crap we wouldn't soil the other cars with. I am doing some backcountry camping later this month, and it's great to leave at the trailhead. No worries!

It's an LX with air (rare in these parts back in 1988) and automatic. Power toys, cruise and sunroof. It was $18,800 back then. The most expensive non-turbo 626 you could get.

We've got a great dealer here that is super with older Mazdas. They kill themselves when I bring the car in ($14.99 oil/filter parts and labour, why do it at home?) at the condition. The engine bay is as clean as the rest of our "fleet." I subject it to my "MB" service regiment, and it's worked well. It's always in top shape and I don't let anything go.

John,
I know this is getting away from the topic of this tread but

1. it seems that since HP is derived from the torque equation that
torque is the more important variable.

2. why does torque fall off at higher RPM?

3. i read once that HP is the a indication of the engine's abilty to do work while torque is the actual work.
what do you think?

and 4. can you refer an good book to read?

thanks

Sort of. You can't measure HP, just torque. HP measurement is a simple math calculation based on the torque values found on the dyno.

Because an engine can only rev so high. This is a function of the bore:stroke relationship among other variables.

Close enough.

Books? Can't think of one off hand other than my physics text that needs some dusting off... (Physics 243 in 1983...)

the main reason for this is decreased volumetric efficiency at higher RPM - i.e., the engine cannot breathe very well.

to "flatten" the torque curve, engine designers have resorted to the use of variable valve timing (and lift, for others), and variable induction systems. 4valve-or-5valves per cylinder also increase volumetric efficiency at the upper rpm ranges

Torque is not the actual work. Actual work is measured as the number of revolutions that a given torque is applied. Torque is measured as (lb-ft/radian) and revolutions in radians. Multiplying these two, you get ft-lbs, which is the actual amount of linear work.

In the electrical world, torque is analogous to voltage (Volts). RPM is analogous to current (Amperes).

i will check Amazon for books.

Therefore both torque and RPM should fall off with increasing RPMs.

Why is a flat torque curve important?
With most HP curves seem to have a upward slope while the torque "curve" is usually flat.

Not to be disrespectful (this being a website for us mercedes benz fan) but I too think the subaru flat 4 is a remarkable engine and so is their AWD system.
But then again they have had many years to finetune that engine and system.
In the same way that Porsche has had with their flat 6-essential unchanged in 30plus years.

Hey blackmercedes, that is really good stuff..the first time I've gotten the torque/power thing.

Re the subaru flat four - I have an old 911 turbo (flat six) and was wondering how they compare. I've moved to a rural area and there are lots of impreza turbos around - probably make more sense on the narrow, twisty roads. Wondering if I should trade in the 911...

A flat torque curve usually means that there is lots of torque available at low rpm, which means lots of "grunt" and acceleration. If you draw a graph of the equation I posted above, HP will increase with a totally flat torque curve thanks to increasing rpm. At some point the engine cannot breathe in any more air or fuel and the torque curve starts to fall off. Then, even rising rpm cannot save the HP, and it too begins to fall off.

Suppose we have 300lbs-ft of torque at 1500rpm and 300lbs-ft of torque at 4000rpm.

HP = torque * (RPM / 5252)
= 300*(0.2856)
=85.7 @ 1500 rpm

HP = torque * (RPM / 5252)
= 300*(0.7616)
= 228 @ 4000 rpm

Completely flat torque curve, dramatically rising HP curve.

Engine RPM is limited by many things. There are mechanical limitations that include the ability of the valve springs to close the valves, the speed the piston can move up and down, and so on. There are fule/air related limitations that involve the amount of mixture that can be moved into the chamber and the amount of exhaust that can be removed.

Horsepower is the ability of the engine to do work over time. The time thing is very important in the definition. Torque is the amount of force, and hp is the amount of work over time.

Hmm.. The more I read, the more I wonder if we talk about different Subaru's :confused:

In the UK the general opinion is that the 'gas mileage' of the Subaru flat four is a serious weak point of the engine - and that's what i've always heard from owners, too. I've no idea why though. Flicking through some magazines, overall test averages show about 19 and a half MPG for the basic 208bhp Turbo and 24MPG for the 101bhp 1.8GL.
The same magazine averaged from 22 to 26 MPG in 3 E36 BMW M3 tests.

The STi has the 2.0, not the 2.5, and remains much maligned for its lack of torque. The 911 is still a rather faster car.

Against its own rival(s?) the low centre-of-gravity of the flat four lends a handling bonus, but can't stop it now being beaten in every area by the (straight-four) Mitsubishi Evo series.

Give me 6 or more cylinders, and I don't care how they're aligned (okay maybe not vertically in the engine bat) ;) :)

Interesting. Must be different markets. The North American Sti has a 2.5L engine with no turbo lag and serious sub 5.0 0-60 runs. A Canadian car mag duplicated a bunch of 0-100km/h (62mph) runs in 4.7/4.8 seconds.

The 911 has a higher top speed, but I would be VERY hesitant to bet real money on the 911 against an Sti in any contest but.

from the formula of BlackMercedes, you can see that the HP varies linearly with torque or RPM, depending on which you are holding constant.

in the ideal case, where you have a perfectly flat torque "curve", i.e., the torque is constant from 0 rpm to redline, the engine power will be linear with RPM - it will also be a straight line starting at 0 from 0 rpm to the max power at redline.

in the real world, an engine designer could only hope for a "torque plateau", spread over as wide an RPM band as possible, over an RPM range where the engine will be spending most of its time.

With this "flat torque curve", the torque stays relatively constant over a wide range of RPM. This is the engine's "sweet spot", where HP varies linearly with RPM. From a driver's perspective, you get a very proportional and predictive response from the engine, as you increase RPM.

However, the fly in the ointment is aero drag, which increases with the square of the speed, so even though an engine may have a linear power curve all the way to redline, the performance of the car will be less than linear.

Another benefit of a flat torque curve is that shifting becomes a no brainer. It is easier to drive th ecar around town as it's making decent HP at lower rpm. Then, when letting 'er run, you don't have to be as precise about shifting at the torque peak. That's not to say you can get max performance by shifting at any 'ol rpm, but overall driving experience is better.

Remember that 300lbs-ft is what the driver can feel. That is the force that causes the car to move. HP is a measure of how long it can do it. That's why super peaky high rpm engines can accelerate cars so well. Remember the fictional car that makes 300lbs-ft at 1500rpm and at 4000rpm. It matters not what rpm you are at, you have the same force available to you.

This is where gearing becomes important. People stopped thinking about gearing when automatic became the number one choice, but it's not so simple. Making torque at high rpm is usefull because you can use gearing.

This summer in the Yukon we had a tour of a sternwheeler paddleboat from the goldrush era. A neat machine that took HUGE advantage of gearing. It took THOUSANDS of foot-lbs to twist that giant wheel through the water. How did they do it? A giant engine? Sort of. The thing didn't make tons of torque (as literally required) but used torque multiplication through gearing.

Suppose we go back to our 300 ft-lb MB engine. Could it run that boat? You bet! The boat ran the wheel at only 22 rpm. Take you engine that makes 300 lb-ft at 4000rpm and gear it down to 22 rpm. Thanks to the wonders of gearing, we can exert over 54,000 ft-lbs on that paddle-wheel. Of course, in cars we want speed and gearing that has the engine spinning at 4000rpm and the final drive at only 12 rpm wouldn't work too well. The lesson is that we can always use rpm and harness it with gearing.

That is how cruise ships use engines that are small (huge, but not considering the boat size) to run props that are so enormous it's insane.

not to beat a "dead" horse but is safe to say that
1. torque is the engines ability to do work i.e. the ability to turn the drive shaft and hence the wheels

2. and HP is a reflection of how "quickly" the engine can do this work.

For example: is an engine has a 300ft/lb of torque from 1500rpm to 6000rpm, as the rpm rises the engine can do its work/torque quicker.
Is my analogy on target?

Also for a given torgue/hp curve where does the tranny gearing paly a role?

Although the popular press is always talking about HP, wouldnt it be more useful to talk about torque?

Horsepower is an expression of torque and means something when talking about a car's absolute ability to accelerate.

You analogy is right. Torque is how much work the engine can do. It's how much it can lift. The horsepower is how much it can lift, how fast.

Gearing plays a strong role. The higher you can have the engine rpm the more room you have for gear reduction. Gear reduction is a torque multiplier. Suppose we have that mythical Mb that makes 300lb-ft at 4000rpm. If we have such an unbelievably steep gear that the wheels are only turning 10rpm while the engine is spinning 4000, we have:

4000rpm/10rpm = 400 multiplication factor.

Take your 300 lbs-ft and multiply it by 400. That's 120,000 lbs-feet of force available. That's an amazing amount of force!! You are going very slow, but you couls push a huge vehicle along. This is the principle that boats and huge tractors (semi's) use. Super steep gears that allow enormous torque multiplication.

But, from the math, can you see why it's so important for an engine to make rpm? AND make that torque at high rpm? It allows greater gearing flexibility and torque multiplication.

Take our above example. Suppose that we operate that engine at 1500rpm.

1500rpm/10rpm = 150 multiplication factor.

Now we have only 45,000lbs-ft of torque available. We want the engine to be spinning fast and the final drive to be spinning slow if we're going to have huge force amounts available.

Why do you think that little Honda S2000 can rip off sub 6.0 sec 0-60 runs with only 240hp and a super peaky torque curve? RPM baby. Motocycles take advantage of the same thing that big trucks do. RPM and gearing. It is always prefereable to have an engine that makes high torque at high rpm and use gearing than an engine that makes high torque at low rpm. Of course, many engineers have spent tons of time trying to figure out how to have both. Variable intake runners, variable valve timing, variable exhaust baffling, etc.

We want the engine to be spinning fast and the final drive to be spinning slow if we're going to have huge force amounts available.

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But doesnt this makes a car like this tiresome to drive eg the Honda S2000.

It would seem to me that you want your power(torque/hp) down low so you dont have to rev the engine to get going or keep going. I guess this is the difference between Big V8 and smaller engines.

But doesnt this makes a car like this tiresome to drive eg the Honda S2000.

It would seem to me that you want your power(torque/hp) down low so you dont have to rev the engine to get going or keep going. I guess this is the difference between Big V8 and smaller engines. [/B][/QUOTE]

It can make the car tiresome to drive, and that is why engineers have tried to make engines that produce torque at low rpm as well as high.