Why Did Ford Decide to do a Flat-Plane Crank Differently? 1

[JCReynolds79]

Most V8 engine layouts are Cross-Plane (sometimes called Cruciform) due to the 4 common crank pins being positioned in two planes, 90 degrees apart. Usually the two end pins are in one plane, 180 degrees apart and the two inner pins are on a plane perpendicular to the outer’s. Figure 1 shows a typical V8 cross-plane crankshaft.

## Figure 1 - V8 Crossplane Crankshaft ##

Flat-Plane V8s are commonly only used in high-performance engines, such as the likes of Ferrari. As its name suggests, a Flat plane V8 crankshaft has all its crank pins in a single plane. It looks very similar to an Inline-4 crankshaft, albeit with longer crank pins to accommodate two big ends. Figure 2 shows a typical Flat Plane V8 crankshaft.

## Figure 2 - V8 Flat-Plane Crankshaft ##

There are two main benefits of a Flat-Plane:

[ol 1]
[li]Due to good inherent primary (1st order) balance (no primary shaking forces or couples) there is no requirement for large counterweights (a cross-plane has a rotating primary couple that must be balanced out with counterweights - hence the typical shape of the large end counterweights on cross-plane V8 cranks). This means reduced weight, reduced inertia, reduced package volume, which all equal increased engine acceleration and lower CoG possible.

[/li]
[li]Exhaust Pulse Tuning - due to the layout, firing order is alternating from bank to bank, so each bank sees equally spaced pulses of exhaust gas pressure. This means exhaust tuning can be utilised to make the engine perform better. The cross-plane layout means each bank has unequal pulse distribution.[/li]
[/ol]


So the question I am puzzling over, why did Ford decide to make use of a flat-plane crank layout in the new 2016 Mustang GT350R but (seemingly) throw away all the benefits gained by doing it differently?

## Figure 3 - Ford GT350R V8 Flat-Plane Crankshaft ##

I’ll explain “differently”. The GT350R crankshaft is shown above in figure 3. The very first thing I noticed was that is had an “up - down - up - down” configuration of the crank pins instead of the usual “up - down - down - up” layout as illustrated in Figure 2. Straight away I wondered why they had done that as I suspected (before I had a chance to do any calcs) that it was going to introduce some imbalance. This suspicion was further strengthened by the obvious larger counterweights, opposing each other at either end of the crankshaft, giving away that there was some inherent unbalanced (primary) couple.

So I did some calcs and confirmed that the unusual layout of this flat-plane V8 crankshaft did indeed have some unbalance.

The typical U - D - D - U layout leaves only an unbalance secondary, horizontal shaking force and a relatively small secondary couple in the vertical plane, all due to the reciprocating components.

The U - D - U - D layout however, even just looking at the rotating masses alone, has a primary rotating couple. So before even considering the reciprocating masses you have to add 2 large, opposing counterweights at either end of the crank to just make the crank balance. Then when you consider the reciprocating masses, you get the same secondary imbalances as the U - D - D - U but also more primary couple imbalance.

Then end result is a flat-plane crankshaft with the mass/inertia penalty of the cross-plane crankshaft. So why did they do it?

That is actually my unanswered question...unless I haven’t considered some other great benefit, I can’t see why they did it...apart from perhaps, marketing? Maybe being able to say the GT350R is different from all the rest because it has an exotic “5.2l V8 with flat-plane crank” (quoted from the Ford website).

I would really like to know more behind the decision.

Regards,

Jon Reynolds

 

[140Airpower]

To elaborate a little, a purpose designed race engine optimized to give maximum power should have its power peak very close to or at the mechanically limited rpm. If its breathing is adequate for that then there is no reason to shoot for less hp. This means you cannot run in a range of speeds around the hp peak. You have to run from below up to the max hp. The breadth of the range is set by the available gearing. At the bottom of the range it should be making the maximum power possible. Due to the way the VE/friction combination works, the max BMEP/max torque occurs below the hp peak. You make that point the bottom of your operating range. You invariably want to end up with an engine/gearing system that is best used between the torgue and hp peaks.

 

[gruntguru]

"This is precisely why the torque peak is crucial, or more precisely, the torgue and hp peak spread. You specify gear ratios to be able to run the engine in that range AND if you have more gears, you can narrow that band to give higher maximums."

The rpm for peak torque bears no relationship to the rpm for peak power. While I don't disagree with what you "invariably want to end up with" you can't assume that is what you have actually got so rules about operating between the torque and power peaks are by no means universal - ie not a rule at all.

je suis charlie

 

[140Airpower]

To this assertion, "...The rpm for peak torque bears no relationship to the rpm for peak power...", I'll make this reply:
An auto IC engine is a tuned system that makes NO torque at zero rpm. The torque must follow a curve from some low value at some low rpm to a peak at some higher rpm where it will begin to diminish as rpms increase. I can support a claim that torque does not disappear suddenly as rpm rises past the peak, but diminishes gradually at a generally increasing rate of decline.

Since power is a product of torque and rpm, power will increase from what it is at the torque peak as rpm rises until the rise in rpm is matched by the rate of fall in torque. That point is the power peak.

The simplest thing you can say about the relationship between the torque peak and power peak is that the power peak must occur at an rpm above the torque peak. That fact alone completely negates your statement that there is no relationship.

The rest of what I said assumes you are able to fully design your engine/gearing system so that you are able to put your power peak at the mechanical limit and your torque peak at the lowest range determined by gearing. If you are not fully able to design the system because of rules or some existing starting design or constraints, then other effects can occur. For a common example, if the engine can't breath well up to its mechanical limit (restrictor?) such that there is a safe margin above the power peak, then revving somewhat past the power peak could be an advantage. But, then re-camming to raise the torque peak rpm will usually also increase max power. You would ideally still have the torque peak at the lowest rpm accommodated by gearing.

 

[JCReynolds79]

Back on subject! I think there were other issues (non performance related) that lead to fords counter-weighted flatplane crank. I do think the main problem was that balancing a flat-plane V8 that's bigger than 4.5L is not easy. This one is 5.2L. As somebody mentioned earlier, second order vibrations might start to get vicious.

Same magnitude secondary horizontal shaking forces exist in both FP set ups. So I don't think this is purely connected to engine size as they didn't solve that problem.

Regards,

Jon Reynolds

 

[NormPeterson]

It's a lot like 4 cylinder engines that do not feature balance shafts - typically pretty smooth up to about 2 liters but get progressively rougher as displacement is increased much beyond that. Just that with a FPC V8 you have two of them on a common crankshaft and can't fully use what's happening on one bank to offset what's happening on the other. The change from U-D-D-U to U-D-U-D isn't going to solve that, and might actually make it worse in some respects.


Norm

 

[Panther140]

In essence, is this similar to having 4 V-twin engines coupled at their cranks, and putting them 180 degrees out of phase of their neighboring V twin?

In the future, that concept may be helpful as NWO emissions gestapo crack down on industry. Have the V-twins run and be balanced independently. The engine would basically be 4 separate V twins connected in line at their cranks. If you could do that, you could then connect them with clutches, instead of having them share a solid crank. The tricky part is adjusting phase appropriately so that the secondary forces don’t constructively interfere/overlap with each-other. That seems like a potentially beneficial platform for cylinder deactivation.

 

[flavioct]

You are looking at finger instead of looking at tthe moon.
The cost of a cross plane crankshaft is 2 times more than a flat plane crankshaft.

 

[Lou Scannon]

I'd like to see the cost numbers on Ford's new crank vs the cross plane cranks from the current high end Mopar and GM V8s. I didn't think this market segment was in a cutthroat cost cutting mode.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[Panther140]

I tried to think of this from a cost-cutting perspective, but the money spent developing this rules that out unless they can start using it in their trucks as well..

I think they are early investors in the future market of where the Camaros and mustangs will be. T

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[flavioct]

If a flat plane is cheaper, it weight less, is strongher, is more racing oriented (look at all V8 racing engines Cosworth Ferrari etc, it will never reach 12.000 rpm and so no problem for destructive vibration, wich one is better?

 

[NormPeterson]

Better for what purpose?


Norm

 

[140Airpower]

I think that the production cost savings of a flat plane crank is much more than outweighed by the cost of isolating the vibrations from the passenger compartment and the cost of beefing up components that are adversely affected. Most components attached to the engine will suffer shorter service life if not modified.
Cosworth stated that when they shifted from V10s to V8s in F1, despite their long experience with FP cranks, they were surprised at how destructive the vibrations were to the engine accessories.