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.

__________________
John Shellenberg
1998 C230 "Black Betty" 240K

http://img31.exs.cx/img31/4050/tophat6.gif

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.

__________________
190E's:
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black 2nd owner
http://www.maylane.demon.co.uk/190esmall.jpghttp://www.maylane.demon.co.uk/190esmall2.jpg

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.

__________________
John Shellenberg
1998 C230 "Black Betty" 240K

http://img31.exs.cx/img31/4050/tophat6.gif

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.

__________________
John Shellenberg
1998 C230 "Black Betty" 240K

http://img31.exs.cx/img31/4050/tophat6.gif

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 Shellenberg
1998 C230 "Black Betty" 240K

http://img31.exs.cx/img31/4050/tophat6.gif

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 Shellenberg
1998 C230 "Black Betty" 240K

http://img31.exs.cx/img31/4050/tophat6.gif

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...)

__________________
John Shellenberg
1998 C230 "Black Betty" 240K

http://img31.exs.cx/img31/4050/tophat6.gif

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.

__________________
John Shellenberg
1998 C230 "Black Betty" 240K

http://img31.exs.cx/img31/4050/tophat6.gif

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

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)

__________________
190E's:
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black 2nd owner
http://www.maylane.demon.co.uk/190esmall.jpghttp://www.maylane.demon.co.uk/190esmall2.jpg