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;)