Killing of the V6... anyone else sad?

DickieG · Unread post by **DickieG** » 02 Oct 2010, 22:23

Toby_HDi wrote:
DickieG wrote:V6's are great but for smoothness straight six is the way to go
I have heard this, unfortunately I have never sampled a straight 6. I would like to though.

I'll let you have go and you'll soon see what I mean, no vibration at all.

red_dwarfers · Unread post by **red_dwarfers** » 03 Oct 2010, 17:54

I had a Ford 6.2Ltr straight 6 diesel in my tractor if that counts?
It did sound rather smooth for a '69 agricultural vehicle

I really regret selling it, I miss it so very much

Unread post by **CitroJim** » 03 Oct 2010, 18:05

DickieG wrote: I'll let you have go and you'll soon see what I mean, no vibration at all.

And I can bear witness to an engine that is running but you are barely aware.

The 4-stroke straight six and the 2-stroke triple are naturally perfectly balanced engines with even firing intervals. Nothing is smoother*, except perhaps a V12 but then they're just two straight sixes lying side-by-side at the end of the day...

One of the most naturally unbalanced engines is a 4-stroke triple and a V6 is not much better, being two triples lying side-by-side. Special tricks are used in the V6, such as off-set crank pins, to make them acceptably smooth.

*Piston engine-wise. A Wankel is very smooth but it does not have pistons in the accepted sense.

addo · Unread post by **addo** » 03 Oct 2010, 22:40

A 90° V8 is inherently balanced, as is a straight eight (IIRC).

The smoothest, quietest (as-built) engines I've heard are flatty sixes, with the Buick eight coming close behind.

Sid_the_Squid · Unread post by **Sid_the_Squid** » 04 Oct 2010, 10:15

According to Wiki a V8 has no better inherent balance than a straight 4 which as we know is not a fully balanced engine, certainly not as balanced as a straight 6 or a V12.

Keeping on topic, I also shed a tear at the demise of large engine car.

Source
http://en.wikipedia.org/wiki/Engine_bal ... _cylinders

addo · Unread post by **addo** » 04 Oct 2010, 10:45

A quick whizz through Google failed to yield, but back in the day mech engineering students were taken through a (theoretical) analysis of the forces/vectors of various common motors.

From memory the counterweights of the V-8 were a large part in its overall smoothness.

evilally · Unread post by **evilally** » 04 Oct 2010, 16:29

What also contributes to an engine's "smoothness" is the number of power strokes per crank rotation. A cylinder in a 4 stroke engine only produces 1 power stroke in 4 (average 0.5 power strokes per crank rotation). A straight 4 is producing 2 useful power strokes per crank rotation, whereas a V8 is producing twice that much. A rotor in a rotary engine is producing 3 power strokes per rotation which helps it deliver it power very smoothly.

Unread post by **CitroJim** » 05 Oct 2010, 19:37

addo wrote: From memory the counterweights of the V-8 were a large part in its overall smoothness.

Indeed yes and with them come a big disadvantage.

On V8s, there was something about flat-plane cranks and vibration as well. I can't recall now if a flat-plane one is better or worse than a twisted one and for what reasons but I do believe they cause more vibration but deliver better response as they can be much lighter. Those heavy counterweights in a V8 with a twisted plane crank really dent the ability of the engine to rev quickly and respond to the throttle and maintain acceptable smoothness.

The ES9 60degree V6 crank has practically no counterweighting at all according to the diagrams. Balance is achieved with the offset crank pins to achieve even firing. Its throttle response pays homage to that!!!

There is a lot more to engine balance than just the number of power strokes par-se. They need to be evenly spaced and close together. An I4 have them 180 degrees apart whereas a straight 6 has them 120 degrees apart.

That's by no means the end of it either. Smoothness of engines is a deep science and whole books have been written about it. When you get down to analysing rocking couples you're in deep

Vee engines naturally end up with uneven firing angles and the harshness is worse as the Vee angle gets more acute, hence why a boxer is smooth as it's effectively a 180 degree Vee. A Vee is not a terribly smooth engine fundamentally but has the huge advantage of packing a lot of cubes and cylinders into a small space. The uneven firing angles is what gives Vee engines their characteristic burbly exhaust. Early Vee-twin motorbike engines were a problem and needed special magnetos to generate uneven sparks for them. Until those magnetos came about even firing magnetos were used and set at a compromise.

Vee-twin motorbikes are horrible to this day. They sound gorgeous but they're like riding a road drill. Harleys especially. How the yanks can ride them interstate is beyond me. Riding one for about a mile was enough for me...

The old PRV V6 was a 90 degree engine (actually a V8 with two cylinders lopped off) and it used a balance shaft running above the front bank cam to achieve silky smoothness. In the 60 degree ES9 the crankpins are offset relative to each other to make the engine an even firing one and thus be able to dispense with a balance shaft. The ES9 crank is quite a work of art, its construction includes flying buttresses!

lexi · Unread post by **lexi** » 05 Oct 2010, 19:37

I've had 2 straight sixes. 71 k reg 2.5pi Triumph and Patrol 4.2 diesel Lovely engines.

Patrol had some beat when you put your ear to the snorkel

.

V engines are compact........luv em.

Unread post by **CitroJim** » 05 Oct 2010, 19:40

lexi wrote: V engines are compact........luv em.

I luv 'em too lexi. Without them there is no way we could enjoy a Xantia V6