flat plane cranks

Ok, so this term just made the rounds again in the Ferarri thread, could someone explain it? I've heard it many, many times and know its a good, but rare thing.

Pirated from Wiki:

The flat-plane or single-plane crankshaft has crank pins at 180°. They are imperfectly balanced and thus produce vibrations unless balance shafts are used, with a counter rotating pair flanking the crankshaft to counter second order vibration transverse to the crankshaft centerline. As it does not require counterweights, the crankshaft has less mass and thus inertia, allowing higher rpm and quicker acceleration. The design was popularized in modern racing with the Coventry Climax 1.5 L (~92 cu in) V8 that evolved from a cross-plane to a flat-plane configuration. Flat-plane V8s on road cars come from Ferrari, (every V8 model they ever made, from the 1973 308 GT4, to today's F430 and California), Lotus (the Esprit V8), and TVR (the Speed Eight). This design is popular in racing engines, the most famous example being the Cosworth DFV.

So how does having to rotate two balance shafts lower the mass? Pictures?

Listen to a Ferrari V8, then listen to a Chevy V8. Ohh the sound of a flat plane crank.

Flat, two up two down.

Cross, no two in the same place.

Are most straight 4,6,and 8 engines use a flat plane crank. what engine is that 4cyl pic

That is a motorcycle crank. I think it is out of an R1, but I won't swear to that.

A flat-plane V8 is basically two four-cylinder engines with two rods attached to each journal of what looks like a normal four cylinder crank, as I understand it. That's why they sound different - they sound like two fours.

What about the balancing rods? They have mass, too. From the two cranks pictured its hard to tell how much lighter the flat plane crank is. I'm not trying to argue, I just trying to wrap my head around the concept...

Looking at the two pictures, the counterweights on the cross plane crank look to be more massive than the flat plane. It could be the flat plane crank weighs a good bit less.

The benefit of a flat plane V8 crank is that each cylinder bank has even firing.

On a twisted/90-degree V8 crank, there is one cylinder on each bank that is offset from the other three. This has implications for the exhaust system

With a flat-plane crank, each header collector sees an even exhaust pulse interval. For a 90-degree, this requires a bundle of snakes header over the trans, or under-oilpan crossovers, which is space intensive and inelegant. (But the bundle of snakes looks AWESOME)

Ok, mystery solved, sort of. If flat planes require extra balance shafts, don't they cancel out the benifit of the lighter crank? You're still spinning more mass. To sum up, Flat plane aren't used in "normal" poduction cars due to increased NVH and the extra shaft requirement...

In reply to Gearheadotaku:

That was what I was thinking...

Are you guys sure that they run balance shafts? I can see how they benefit from it, but I'm not sure it's required. After all, four cylinder engines don't have to run balance shafts.

BTW, here's an excellent rundown on the fundamentals of all sorts of engine configurations. I remember seeing it years ago: http://www.e31.net/engines_e.html

of interest in ferrari v8 engines.. the firing order is in numeric over. 1,2,3,4,5,6,7,8...

3R Toyota engines are 1234 as well.

When the Ford V8-60 was being used in Midgets against the Offy 4-cylinders there were flat-plane cranks available for the V8-60.

The old-timers I've talked to told the the Ford motor shook like hell but made more power at the top-end than a conventional crank would.

A wonder what a flathead with it's three exhaust ports per side and a 180deg crank sounds like.

Shawn

Way back when, the 3.8 Buick V6 was originally made in an 'odd fire' configuration, which was basically a V8 crank with one set of rod journals whacked off. After a few years of trying everything to get them to quit shaking like a dog passing peach pits, they went to the 'even fire' or cross plane configuration. Much less vibration.

so how badly does a dog shake when passing peach pits?

Pretty bad. I can't help it, I grew up in the country.

Gearheadotaku wrote: Ok, mystery solved, sort of. If flat planes require extra balance shafts, don't they cancel out the benifit of the lighter crank? You're still spinning more mass. To sum up, Flat plane aren't used in "normal" poduction cars due to increased NVH and the extra shaft requirement...

My guess is that the lghter crank plus the balance shaft still has less rotational inertia than a crossplane crank?

Crossplane cranks do not weigh any significantly more or less than flat-plane cranks.

Flat plane cranks, incidentally, most certainly do require counterweights. A flat plane crank is nothing more than an inline-four crank with extra wide rod journals and fours require counterweights too.

Jensenman wrote: Way back when, the 3.8 Buick V6 was originally made in an 'odd fire' configuration, which was basically a V8 crank with one set of rod journals whacked off. After a few years of trying everything to get them to quit shaking like a dog passing peach pits, they went to the 'even fire' or cross plane configuration. Much less vibration.

Less torque, too. My dad had a 231 oddfire in his Jeep, and swapped to a even-fire when he went fuel-injected. It definitely lost a little grunt down low, though it gained at least 2k rpm on the top end; he broke his tach the first time he test drove it after the swap.

Knurled wrote: Crossplane cranks do not weigh any significantly more or less than flat-plane cranks. Flat plane cranks, incidentally, most certainly do require counterweights. A flat plane crank is nothing more than an inline-four crank with extra wide rod journals and fours require counterweights too.

counterweights on the crank.. or balance shafts? Counterweights I can see

Ok....so why does my 4cyl 944 have 2 balance shafts?

Yes, i'm confused.

If only they would make flat plane cranks for LSx engines...

SillyImportRacer wrote: Ok....so why does my 4cyl 944 have 2 balance shafts? Yes, i'm confused.

Because it's an even-firing inline four and large enough that the inherent shaking is objectionable.

When pistons 1 and 4 are at TDC, pistons 2 and 3 are at BDC, and vice-versa. However, when the crankshaft is at 90deg from that point, the pistons are all more than halfway down the stroke. The crankpin is half a stroke's away from being directly under the piston, and the resulting connecting rod angle causes this effect.

This makes for a vertical shaking, since the piston is moving faster in the period 90BTDC-90ATDC than it is 90BBDC-90ABDC. The pistons at the bottom half of the stroke don't cancel out the pistons at the top half of the stroke.

That's why an even firing (flat) V8 crank engine shakes too. It's two inline fours joined at the crank.

You need two balance shafts because, by being timed to each other and rotating in opposite directions, they can eliminate that vertical shaking without adding any additional vibrations. I hasten to point this out because I remember someone claiming the 968 had a horrible engine because "it required TWO balance shafts!!" Well, yeah, that's like saying a rearend needs two gears, one by itself is useless.

You HAVE two balance shafts because Porsche wanted you to have a more pleasant driving experience than if they bolted something as buzzy as an Iron Duke under the hood. Balance shafts are not strictly necessary and are often eliminated with little drama. It's common to ditch them from Mopar 2.5s, for example.