Difference between a sraight six and a V-six - PeachParts Mercedes-Benz Forum

#1 08-12-2002, 12:56 AM

Hello all:
The recent (and very interesting) thread about the sound of a straight six got me wondering: what's the difference between a straight six and a V six besides the obvious? Are there any performance and mechanical advantages of one over the other? Thanks!

speedy300Dturbo · #2 08-12-2002, 01:37 AM

Inline sixes are better.

Ask a BMW engineer why they've never used V6's before.

jjrodger · #3 08-12-2002, 03:53 AM

I'm not sure inline sixes are simply "better": it depends on your criteria. A V-6 is more compact than the I-6; it can be made less expensively as part of a modular series of engines (like the Mercedes-Benz V-6, which is three quarters of a V-8, or the VW VR6, which is a cylinder more than the VR5). A V-6 is safer in an accident because it can be better accommodated in the engine bay (and so is less likely to intrude into the passenger cell or whack as pedestrian). A V-6 can be mounted in longitudinal or transverse fashion and used with front- or rear-wheel drive (although Volvo has a transverse front-wheel drive I-6).

The Warden · #4 08-12-2002, 05:44 AM

Most V-6 engines have 4 crankshaft main bearings, with two connecting rod sets between each main bearing. Most inline-6 engines have 7 main bearings, with one connecting rod in between each. For this reason, the bottom end of an inline-6 is more stout, and is therefore less prone to breakage.

Also, there's the ol' trucker school of thought that says that an inline engine has more low-end torque than a V engine. Many people of this school of thought actually believe that an inline-6 engine will have more low end power than a similar-sized V-8. If you look at the power difference (or near-lack thereof) between a Cummins B5.9l (359ci or 5.9l inline-6) and an International-Harvester T444e (444ci or 7.3l V-8), this will help illustrate the idea of an inline-6 as compared to a V-8. Chances are, the higher low-end torque numbers are due to the increased rotating mass (as an example, the crankshaft alone on the Cummins B-series straight-6 weighs 125lbs

).

From a truck guy's standpoint, I've always seen V-configured engines as a good engine to have for high speed, and inline engines as a good engine to have for low-end power. This is ironic as my F-250 has a V-8 while my Benz has an inline-5. It ought to be the other way (straight-8, my truck would have a loooooong hood

)...

Eberhard Weilke · #5 08-12-2002, 06:58 AM

...because they have the rather unfavorable angle of 90° because they produce them at the same plant as the V8 in Stuttgart-Bad Cannstatt.

There is nothing wrong with a V 6 with 60°, like the Italiens used to build them.

With 90° you need counterbalance shafts to get the whole thing the quietness of a lousy straight four.

As mentioned before: Ask BMW engineers why they don't build V6 like they do it in Stuttgart.

Kind regards
Eberhard
(who just happend to look at pictures of Bad Cannstatt of the days they had a railroad-shop at the place where they build these strange engines today)

bobbyv · #6 08-12-2002, 12:41 PM

an inline6 is perfectly balanced on all forces and moments (twisting forces). This characteristic is shared with the flat-6 and the 60-degree V12 (although this can be achieved with a V8 via counterbalancers)

the 60-degree V6 and 90-degree V6 need counterbalancers to smooth out the vibrations, moreso in the 90-degree one.

the inline-6 has an even firing order, i.e., the exhaust pulses are regularly spaced. The 60-degree and 90-degree V-sixes will have pairs of exhaust pulses bunched nearer each other, and will therefore have a rougher sound relative to the inline-6.

the V6 has a shorter crankshaft, which makes it easier to make it robust than an inline6. Witness the F1 turbo V6 engines a long time ago. BMW's first M3 engine was not an inline6, but an inline4, because it was more robust than its current inline6 then at high rpm.

an inline-6 has a longer block. When subjected to a change in temperature, the length of an inline-6 will increase more than a V6 - this is probably the reason behind the head gasket problems of the M103 and M104 (i.e., the block and the gasket expand at different rates, but this is more pronounced on an inline6)

there are also packaging differences:
* an inline-6 has a hot side (exhaust side) and a cold side. The heat-sensitive components are placed on the cold side.

* the valley in the V of a V6 is a natural place to put a supercharger. The valley is bigger on a 90degree V6 than a 60degree V6.

MB's main motivation for going for a 90-degree V6 as against a 60-degree V6 (or an inline-6) is engine modularity. If they need a real answer to BMW, they will need to re-develop their inline6 - for me at least ...

#7 08-12-2002, 02:18 PM

Guys,
Thanks for all the responses. Pretty interesting stuff.

jjrodger · #8 08-12-2002, 06:00 PM

Whoever said the Mercedes V-6 sucks might want to read this post:

http://www.peachparts.com/shopforum/showthread.php?threadid=1582&highlight=M112

The M112 V-6 won a Ward's "Best Engine" award at some time, too (although so has the Beemer plant).

I've got a M112 V-6 in my car, and it's great. As for the availability of torque, my meagre 2.4-litre V-6 develops a flat 170 lb/ft all the way between 3,000 and 5,000 rpm; the larger capacity MB V-6s have a similarly flat torque curve, they just develop more. My motor is a bit keener to rev, though, because it has a very short stroke.

Eberhard Weilke · #9 08-12-2002, 06:31 PM

No question. They invested a lot of for finding beautiful engineering solutions.

No question. The engine does its job.

But I will never, ever understand why they stopped improving the I-6 to start with a V 6 with 90° angle. And why the engineers accepted that angle just because it could be made on the same machines as the V 8.

Many people in Germany who had to buy this engineering marvel didn't like it at all. It didn't even save fuel.

Road test at most of the german magazines didn't like the engine compared to the M 110, M 103 or M 104.

No matter, how much engineering you put into that engine, it was the wrong start from the beginning.

It would be like building passanger cars with three axles just to be able to assamble them in the truck plant.

V 6 with 90° and counterbalance shafts is wrong. Period.

Kind regards
Eberhard

bobbyv · #10 08-12-2002, 11:44 PM

same sentiment here as Herr Eberhard. The inline-6 appeals to the engineer in me.

imagine, even GM went back to the inline6, and a wonderful one at that, with 4.2li and 270hp ...

jjrodger · #11 08-13-2002, 02:26 AM

I couldn't deny that the I-6 is a more "pure" engine than a V-6, especially one with a 90-degree angle between the cylinders. As such, an I-6 will always appeal to the engine engineer more than the V-6. But to the car engineer? The V-6 is easier to build and easier to package (lower, shorter bonnet; front- or rear-wheel drive; longitudinal or transverse mounting), not to mention safer in a smash. It gives the car engineer so many more possibilities.

As for the BMW engine being a better engine, I have one thing to say: remember the Nickalsil cylinder bores? Great for longevity.

engatwork · #12 08-13-2002, 06:17 AM

my .02 cents worth - The reason for the demise of the inline 8 cylinder way back when was because of the additional cost associated with having more bearing/journal surfaces. Engines don't get any smoother than an inline 8 as far as everything I have read but they went with the V8 because of lower production cost. I imagine that lower production costs were factored into the decision to go with the V6.

Zeus · #13 08-13-2002, 11:57 AM

Well, as a DIYer, it is easier to do a head job on an inline-6 than a V6! One head, one camshaft (although some inline 6s have 2), one gasket. I love my M103!

Ken300D · #14 08-13-2002, 12:34 PM

I really like the idea of the straight 5-cylinder on my 300D compared to the little problems a V-5 might have.

Ken300D

bobbyv · #15 08-13-2002, 02:32 PM

Quote:

Originally posted by Ken300D
I really like the idea of the straight 5-cylinder on my 300D compared to the little problems a V-5 might have.

Ken300D

VW has such an engine - the VR5. This is the VR6 less one cylinder. Which is for me, the wierdest configuration out there.