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

 

[NormPeterson]

On a road course or even an autocross layout, minimizing the torque "lost" to accelerating rotational inertias trumps all else. At least at the pointy ends of those sports.

I can understand why you might want to take some of the suddenness out of a wheelstand (I used to ride a 350cc 2-stroke triple that had light-switch behavior between being off the pipe and on it), but I doubt that to be a concern with the GT350. Nor will drag racing be all that important, though there will be those who will at least try it out there.


Norm

 

[Lou Scannon]

I've heard it argued, though not with back to back data or numerical analysis, that more rotating inertia can reduce 1/4 mile times, assuming a given launch rpm (and, I suppose, that the extra stored energy at launch goes into acceleration, rather than tire smoke or clutch heating). I remain skeptical until I see the data or analysis.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[Compositepro]

Well, I would expect that the less torque pulsation there is at the wheels, the less likely the tires are to start slipping. I have no idea how important a factor it is in real life.

 

[gruntguru]

More flywheel makes a consistent launch easier but it would probably be a negative otherwise.

je suis charlie

 

[Lou Scannon]

Maybe so, but, by far the biggest torque impulse is the launch, unless the available energy (i.e. delta energy from increased rotating inertia) is converted to heat upstream of the tires, in the form of slip, clutch, e.g.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[kjoiner]

Hello,

I saw a video on the GT350 over the weekend that mentions packaging constraints being a main driver for the crankshaft configuration. One constraint mentioned was not being able to use separate plenums for the cylinder banks. I'll see if I can find a link to the video and post it.

Kyle

 

[LionelHutz]

For drag racing, the theory is that during launch and gear changes, the higher inertia engine has more energy to dump to the wheels when the engine rpm is pulled down. It's supposed to cause a surge of acceleration each time making you faster. It also ignores the time the engine has to accelerate the inertia.

For road racing, the theory is that less engine inertia allows the engine rpm to change quicker, which means quicker changes and faster acceleration/deceleration of the car.

The physics of acceleration don't change between the drag and road tracs, so one of the theories is wrong. I've run across a few people who reduced rotating engine inertia via a much lighter clutch/flywheel and then went down the 1/4 mile faster. So the reality is that having more engine inertia just makes it easier to consistently launch a clutched car.

 

[NormPeterson]

The physics are the same, but you aren't chasing the same effects/benefits. Greater rotational inertia helps prevent bogging the engine down out of its powerband and is more useful in relatively heavy cars powered by smallish displacement engines. Especially so when the racing surface has been prepared for additional tire grip. With car weights commonly well in excess of 3000 lbs, a 5L engine is definitely "small". Since drag races between generally similar cars tend to be won or lost inside the first couple hundred feet and maybe two gears, it's more important to work with the launch side of the physics.

A closed-course racer, either road course or "oval" doesn't need to care nearly as much about 1st gear launch traction, either because the starts are rolling starts or because the wheels are allowed to freely spin (sacrificing a small amount there in exchange for better through-the-gears acceleration everywhere else). The classic illustration here would be the big NASCAR "sedans" leaving their pit space following a stop for fuel and fresh tires. The only "surface preparation" for closed course events is whatever tire rubber has been left on the pavement during practice, qualifying, and from what any supporting series leaves. Some of which may not even be on the line you need to be on to prevent being passed.


Norm

 

[LionelHutz]

More inertia can only be a benefit at launch and gear changes when the clutch is slipping. Less inertia will accelerate the car faster the rest of the time when the clutch is engaged.

Hemi - more engine inertia would help the launches using the same launch rpm, but in reality you would launch at a higher rpm with less inertia.

This engine has to be kept above 3500rpm to stay in the powerband. The engine torque increases by over 100 ft-lbs going from 3000 to 3400 rpm.

 

[Lou Scannon]

The argument that I've understood, which I haven't analysed myself quantitatively to assess its validity, is that a drag racer can in essence, "invest" in stored energy in the flywheel prior to the start of the race, and draw down the investment during the race. Of course the counter argument of needing to continually accelerate the rotating masses during the race needs to be taken into account, when assessing the value of increased rotating mass in front of the clutch (or other coupling) between the engine and the remainder of the drivetrain.
Hence, I remain skeptical.
Of course, one can conceive of a high tech, ultra light, ultra high speed flywheel connected via a high ratio reduction gear to the drivetrain proper, that is spun up to the moon prior to the start of the race, and totally eclipses the engine in power delivery during the race; but that is not the configuration of a drag race vehicle as we know them today.
I guess the reduction gear needed for the task wouldn't be exactly lightweight, but maybe an electrical coupling could provide the needed speed reduction at a lighter weight.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[gruntguru]

KERS.

je suis charlie

 

[Panther140]

Slightly higher rotating inertia reduces amplitude of torque pulsations through the drivetrain. Torque spikes are smoothed out, which is good for clutch/transmission/axles/driveshaft/Ujoints. It also smooths out the powerband, which is good for traction.

As long as the crank is lighter than a cross plane crank, it is smart for them to do this.

I have also heard that balancing flat plane V8s with displacement over about 4.5L is harder to do. I do not understand exactly why that is said. Does anybody have ideas as to why that is said?



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

 

[tbuelna]

Looking at the pictures in the first post, it looks like the FP crank might have slightly smaller diameter bearing journals than the cruciform crank. This would reduce friction losses in the main/rod bearings.

 

[NormPeterson]

The argument that I've understood, which I haven't analysed myself quantitatively to assess its validity, is that a drag racer can in essence, "invest" in stored energy in the flywheel prior to the start of the race, and draw down the investment during the race. Of course the counter argument of needing to continually accelerate the rotating masses during the race needs to be taken into account, when assessing the value of increased rotating mass in front of the clutch (or other coupling) between the engine and the remainder of the drivetrain.

If you can't keep the revs up high enough on the launch - IOW, the engine bogs before adequate road speed is reached - it won't matter how much more advantageous the light-MOI setup is everywhere else. The race has already been lost. It at least used to be common for 40-lb flywheels to be fitted to small-block Chevy engines, and the numerically high axle gear ratios also frequently fitted would perhaps "cover for" the lower vehicle accelerations once the engine is operating in its best powerband and the tires fully "hooked up". It's really just a matter of optimizing the entire combination for a clearly and very narrowly defined performance measurement.

On a small time step level, rotational momentum converts to additional torque as the rpms drop, and how much this amounts to also depends on the rate of rotational deceleration. You might for a very brief period of time "gain" 100 ft-lbs this way, which might just be enough. Consider that clutch engagement isn't necessarily instantaneous from no grip to no slip, and that there likely is some torsional "wind-up" in driveshafts, axles/half shafts, and tires that provide some delay and (slightly later) energy returns of their own.


Norm

 

[NormPeterson]

I have also heard that balancing flat plane V8s with displacement over about 4.5L is harder to do. I do not understand exactly why that is said. Does anybody have ideas as to why that is said?

Guessing the magnitudes of the forces and moments and the phase relationships among those components between banks that are still separated by 90° has something to do with it. Dividing the con-rods into two masses with no MOI of their own is at best an approximation.


Norm

 

[Tmoose]

"I have also heard that balancing flat plane V8s with displacement over about 4.5L is harder to do. I do not understand exactly why that is said. Does anybody have ideas as to why that is said?"

It used to be said that inline 4s with displacement over about 2 liters had too much secondary vibration to be acceptable in passenger cars, without resorting to the complication and expense of a pair of counter-rotating secondary balance shafts. The pistons etc got too heavy for the final result to be ignored or "hidden" with squishy engine mounts

As a flat crank V8 is somewhat equivalent to two inline 4s, the 4.5 l limit you describe may have been referring to secondary vibration.

 

[Panther140]

I'm glad there was at least some merit to the idea that was put in my head. Are there any flat plane crank V8s which use a traditional flat plane crank without balancing weights?

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

 

[LionelHutz]

http://www.uucmotorwerks.com/flywheel/LWF_DRAG_TEST.pdf

Stored energy is proportional to rpm^2 so after lowering the inertia of the engine, increase the launch rpm to get back to the same stored energy.

 

[Panther140]

That is true, but there are more factors at play here. Launching at that high of an engine speed would require an excellent and gradual clutch engagement, and the power impulses at that high of an RPM are not be staggered as well for maintaining traction.

Not only that, but you would have to launch the car when the engine is well past peak torque..

 

[kjoiner]

Hello,

I found the video. If you fast forward to around 7:30 in the video, there is an explanation as to why the GT350 flat plane crank is different than other flat plane cranks.

https://youtu.be/GbbNlkP-ZSM

Kyle