Inline vs. V6 - Page 2 - PeachParts Mercedes-Benz Forum

Zeus · #16 07-15-2003, 02:32 PM

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...

Botnst · #17 07-15-2003, 08:07 PM

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?

#18 07-15-2003, 08:52 PM

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.

rickg · #19 07-15-2003, 11:28 PM

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.

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.

AWS · #20 07-16-2003, 12:32 AM

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!

Out of curiosity, has anyone driven the new mazda rx-8 yet?

andrew

scargo70 · #21 08-04-2003, 12:47 PM

Quote:

Originally posted by PaulC
Part of the answer relies upon certain design elements of a particular V6. Generally speaking, an engine that has an inline cylinder configuration will usually be smoother in operation than a vee engine of the same displacement with the same number of cylinders.

The smoothness of a vee engine is impacted by the angle of the vee. For the best possible result in smoothness, the angle should be equal to the following formula: 720 degrees/ number of cylinders. As such, an optimal vee angle for a V8 would be 720/8 or 90 degrees, which is the vee angle for most V8's. For a six, 720/6 or 120 degrees. This is a pretty wide angle, which makes for a wide engine that would be difficult to package under the hood of most cars. Most V6 engines sold today have a 60 degree vee, which is not optimal, but allows the engine to fit in the car. An exception is the 3800 V6 sold by GM and used in many front-drive applications. This engine, for ease of production on engine manufacturing lines already set up for V8 production, has a 90 degree vee. GM has partially countered the vibration inherent in this engine with an unusual crankshaft design.

There. I've told you more than I know.

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.

Kestas · #22 08-04-2003, 01:12 PM

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.

blackmercedes · #23 08-04-2003, 01:33 PM

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.

Botnst · #24 08-04-2003, 01:53 PM

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

blackmercedes · #25 08-04-2003, 02:31 PM

Quote:

Originally posted by Botnst
...and great stroke length.

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.

scargo70 · #26 08-04-2003, 04:48 PM

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

blackmercedes · #27 08-04-2003, 05:05 PM

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.

scargo70 · #28 08-04-2003, 06:05 PM

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

blackmercedes · #29 08-04-2003, 07:22 PM

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.

pentoman · #30 08-04-2003, 07:28 PM

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.