Inline 5 vs. V-6 and I-6 [850][95 T-5R] ... posted by Alex  on Monday,
26 February 2001, at 9:31 a.m.
 

                 Ok, so I've read a lot of small comments on this message board about how the 5 cylinders are smoother than their 6 cylinder counterparts. Why is this? It sounds pretty counterintuitive to me just because of the whole odd number thing. the only explanation that I could come up with is that on a six cylinder, you have opposing torque being applied at each end of  the engine (ie. cylinder 1 is going up while cylinder 6 is going down). Does this make any sense? please clarify this for me, it's been bugging me for quite a while. Thanks!

                 Alex

                 No 5 cyl of any manufacturer is smoother than an I-6 or a 60 deg V6. 5 cyl's fire every 72 degrees. 4 cyl's are also unbalanced. 90 deg. V6's are also
                 unbalanced unless the crank journals are offset to compensate for the inherent odd firing order.
                 My 965's engine is as smooth as silk.

                 I'll assume then that you've not seen/worked on or owned an 850 or newer Volvo 5 cyl car? They're just as smooth as the six cyl engine.
 

Impossible. But the engine vibration is damped by the mounts. Additional body structural damping helps as well. Sound deadening material is used to cut down on the noise. You must be used to it by now.
                 BTW, I would like to get a T5-R to replace my 744Ti. I expect you would recommend it?

I don't know if I would recommend it. Actually, I'm not quite sure I'd buy another volvo after this one goes (which should be soon as it's for sale if you're interested). they're quirky unique cars which is why I like it, and I like working on it, but compared to some of the other cars I'm looking at, I just couldn't justify buying another. but hey, whatever floats your boat right? everyone here seems to love them. I guess mine is a love hate relationship and the love is great but the hate is just causing me too many problems which I just don't have the time or money to deal with. help you at all? good luck

                 Alex
Oh, and thanks to everyone for answering my I-5 vs. I/V6 you guys were all most helpful

                 Alex,
I don't recall anyone claiming 5 cylinders as the smoothest. It is pretty much agreed in the auto world that inline sixes are pretty much the smoothest and most reliable of engines around. Just look at BMW for example and their use of inline sixes...

                --
Tony G, '94 850GLT, '82 240DL (mint) July '99 "Rolling" cover

                 Oops, ok, I can be absent minded sometimes. maybe what I was refering to was the inline 4 engine. same idea of even, balanced number of cylinders vs. an odd number. why is the I-5 smoother than an I-4? Thanks a lot. Oh, and why is it smoother than a V-6? Sorry about all these silly questions.

                 Alex

                 The I-4 engine fires every 180 degrees. For instance, #1 is firing when #4 is
                 filling. Likewise it suffers from free forces of the second order (inertial force
                 dependant on rotating mass,crank radius, rod/throw ratio and 2xcrank angle);
                 translation, the engine wants to shake up and down. Some manufacturers use
                 counterbalance shafts to help with these forces. But ultimately it is not a very
                 smooth engine.
                 I-5 engines fire every 144 degrees. That is the main reason why they are
                 smoother. However I-5's suffer from free moments of the first and second
                 order. This means the engine wants to rotate about the centerline of the car and
                 also rotate about the vertical axis. If large enough the free moments will rock
                 the body of the car from side-to-side, full size pick-up trucks do this alot when
                 you hit the gas in neutral. This is why many people (myself included) buy a
                 replacement torque bushing for the upper engine attachment. The free moments
                 rip the bushing apart as it ages and becomes brittle.

                 I-6 engines (flat 6 in Porsches too) fire every 120 degrees and do not suffer
                 from any inertial problems. It is an inherently balanced design.

                 V-6 engines come in two angles, 60 and 90 degrees. The 90 degree variant is
                 made on the same assembly line as V-8 engines. 90 degree engines fire at
                 150/90 degrees. This means 1 cylinder fires at 150 degrees of crank rotation
                 then the next 90 degrees later (repeat twice). Like the I-5 it suffers from first
                 and second order free moments. So it should run about as smoothly as an I-5.
                 Aside: I think Mercedes uses a counterbalance shaft to absorb/reduce the
                 effects of these free moments. The 60 degree v-6 fires every 120 degrees.
                 Based on this it should give power delivery similarly to the I-6. But this v-6
                 suffers from first and second order free moments (much bigger than all of the
                 other mentioned previously).

                 As a final note: my fathers 15 year old BMW with over 400,000 miles is
                 smoother than the I-4 Saab I bought when the saab was new. Of course the 850
                 was smoother than the BMW but not much.

Good explanation. When GM came out with their 90 deg V6 in the 70's, they had to use a very strange distributor lobe to get it to run. Later, they adopted offset  crank journals to achieve a much smoother running engine. I supose those hydraulic engine mounts helped as well. 5 cyl's are unbalanced but can be made acceptable by adding enough damping.

                 I4 engines have extremely large second-order vibrations which give them the
                 shakes. As far as V engines go, there is an ideal angle for the cylinders which
                 results in maximum smoothness. For V6s, that angle is 60deg. 60deg V6s are
                 very smooth. Many manufacturers make 90deg V6s, however, which are
                 unbalanced, unless the crank journals are offset. 90deg is the 'magic' angle for a
                 V8.

                 The smoothest engines are I6 - they have absolutely no first or second order
                 harmonics to cause problems. And V12s, which are just two I6s together, are
                 smooooth too.
                --
                 Speed Racer '83 240 R, '74 164 E, '93 940 OL1, '64 544 Sport (Smithfield, NC)

                 Inline-6 vs. V6, the real deal!
                 The I-6 and V12 are the only "naturally balanced" engines configurations. The
                 Inline-6 is definetely making a comeback. Check out the all new 4.2 liter GM
                 Vortec Inline-6. It has 270hp/257lb.ft of torque. It has been getting rave
                 reviews thus far. The 2.8 liter twin-turbo Inline-6 in the Volvo S80 T6 and the
                 new 3.0 liter Inline-6 in the new BMW 330i and 530i are all marvelous
                 engines.

                 The I-6 engine is balanced end-to-end and requires no balancer shaft. The
                 vertical and transverse forces generated by the individual cylinders, no matter
                 first order or second order, are completely balanced by one another. The
                 resultant vibration is nearly zero, thus the Inline-6 is virtually a perfect
                 configuration. The Inline-6 is not the only configuration can deliver near
                 perfect refinement, but it is the most compact one among them.

                 All Boxer engines are perfectly balanced, but they are two wide and require
                 duplicate of blocks, heads and valve gears. This results in less efficiency
                 versus an Inline-6. V12 engines also achieve perfect balance, but are obviously
                 out of the reach of most mass production cars. Automotive engineers knew that
                 long ago, that's why you can see most of the best classic engines were Inline-6,
                 such as the Rolls-Royce Silver Ghost, Bentley Speed Six, Mercedes SSK,
                 many Bugattis, Jaguar XK-series and BMW's and Volvo's various models.

                 As space efficiency becomes more and more important, most car makers favour
                 the V6. The most influential V6 was perhaps Alfa Romeo's 2.5-litre 60° V6
                 used in the GTV6. It established a reputation for a V6 that it can be compact,
                 powerful, and smooth. An equivalent inline-6 would have never fit the small
                 and sloping engine compartment of that car. Compare the shape of a BMW with
                 an Alfa and you'll know the packaging advantages of a V6.

                 When comparing a 60° or 90° V6, it is important to realize that they have
                 somewhat end-to-end vibration like 3-cylinder engines. The is especially true
                 for the 90° V6. It needs a counter-rotating single balancer shaft, at crank speed,
                 to suppress the vibration. The balancer shaft is located inside the V-valley, so
                 it is not space engaging. On the other hand, the 90° V6 has a decisive advantage
                 in a production point of view. It can be machined in a V8's production line
                 because both of them are 90°. Unlike a V6, a V8 can only be optimised at 90°.
                 This saves a lot of production cost. An example is Mercedes' fairly smooth 2.4
                 to 3.2-litre V6s, which share the same architecture with their V8s, but added
                 with a balancer shaft.

                 Inline-6 engines are nearly impossible to be used in front-wheel drive cars as
                 well. Even a car as wide as Volvo S80 had to introduce the world's shortest
                 gearbox in order to make space for the 2.9-litre Inline-6 mounted transversely
                 in the engine compartment. Longitudinal mounted Inline-6 doesn't have such
                 problems, but it engages too much space in north-south direction, thus engage
                 some space which would have contributed to cockpit room.

                 However, BMW and Volvo are still loyal to Inline-6 engines. Ultimately, the
                 Inline-6 engine is more efficient yet smoother. The V6 has more energy loss
                 because it duplicates valve gears and camshafts, which increases frictional
                 loss, while the use of 2 cylinder banks leads to more heat loss. In terms of
                 production cost, although a V6 has 3 fewer main bearings, it has more valve
                 gears, which is getting more and more costly these days, with the introduction
                 of twin-cam, hydraulic tappets/finger follower, and variable valve timing. The
                 Inline-6 is going to be cheaper than equivalent V6. The volumetric size versus
                 horspower and torque of a well designed Inline-6 will always be better than a
                 well designed 60°or 90° V6.

                 SwedeSpeed

                 -DrewSRX