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

 

[Deividas]

Sorry if my question is dumb, but why flat-plane V8 needs two different intake plenums?

 

[BrianPetersen]

It doesn't "Need" it ... but it is a tuning variable, akin to the difference between a "dual plane" manifold for a traditional cross-plane V8, and a "single plane" or "tunnel ram" manifold.

With a dual plane manifold on a cross-plane-crank V8, the intake strokes being drawn from each "plane" are evenly spaced 180 degrees apart. To do the same thing with a flat-plane-crank V8, it is simply the left bank kept separate from the right bank.

 

[Lou Scannon]

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

Maybe not a bad thing, thinking about "torque backup" aka "torque rise"...

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[NormPeterson]

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

I suspect that's the rule rather than the exception, at least for semi-serious and up dragstrip cars still equipped with a manual transmission. You might be able to find out what launch rpms people are actually using in the discussions found in the dragstrip sections of the various automotive-enthusiast message boards.


Norm

 

[gruntguru]

"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."

That's OK unless clutch slip is being used to regulate torque to the wheels. Higher rpm means higher energy loss in the clutch - reducing the flywheel energy available to propel the car.

je suis charlie

 

[140Airpower]

In F1 and other high-buck series in the early to mid 1960s many engines were designed with "downdraft" intake ports with the intake coming in between the wide-spaced cams. Despite the breathing advantage of unshrouding the long side this trend ended with the coming of the narrow valve angle Cosworth DFV.

A highly beneficial feature of downdraft engines was the ability to put the exhausts in the V, allowing more compact packaging and, critically for V8s, the ability to conveniently cross exhaust tubes to the opposite bank. This allowed bent cranks to be used with no power penalty from exhaust pulse timing effects in the headers. Some engines took advantage, getting rid of the vibration problems of flat cranks.
There still remained the disadvantages of a heavier bent crank vs a flat crank and, especially then, a weaker bent crank. So, you saw some of these downdraft V8s with flat cranks as well as some with bent cranks. I think the most iconic of the downdraft, bent crank engines was the Ford 4-cam Indy engine.

 

[Panther140]

The higher RPM you launch at, the faster all of the parts in the drivetrain will spin for that inertial torque to be transmitted. But spinning those parts up to that higher speed requires higher amounts of inertial torque. Not only that, but the energy required to accelerate the car to the speed which is needed to facilitate that higher rotational speed of the drivetrain would require an extreme amount of inertial torque AND extremely strong components to hold that much force. Even if you could get the force required, AND have the drivetrain durability to withstand it, you would STILL need traction.

The only way to actually implement your theory of using extremely high revs to get that inertial torque is to have an extremely low gear ratio for the gear that you are launching in, one which is impractical in real world racing.

And on the subject of engine torque backup: Given a specific amount of inertial torque, crossplane V8s attain that at an engine speed which is closer to their engine's peak torque. That means that their torque backup is used to maintain that rotational torque. Flatplane crank engines lose much higher amounts of rotational torque by the time the engine speed falls to peak torque RPM. That means that they then have to regain more that inertial torque, for which the speed was already poorly optimized to the physics of making wheels (with mass) spin and moving a car (with mass) forward to match the engines speed.

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

 

[tbuelna]

The torque curve shown in that video was interesting. It had an abrupt vertical jump around 3000rpm, and was fairly flat up to redline. The flat torque curve from 3000rpm to redline is nice for a high performance engine. But the lack of torque below 3000rpm, and the spike in torque at that rpm, would not be suitable for most other vehicles. Most auto engines spend most of the time operating below 3000rpm.

The GT350 V8 engine in the video did sound very nice though.

 

[NormPeterson]

The higher RPM you launch at, the faster all of the parts in the drivetrain will spin for that inertial torque to be transmitted. But spinning those parts up to that higher speed requires higher amounts of inertial torque. Not only that, but the energy required to accelerate the car to the speed which is needed to facilitate that higher rotational speed of the drivetrain would require an extreme amount of inertial torque AND extremely strong components to hold that much force. Even if you could get the force required, AND have the drivetrain durability to withstand it, you would STILL need traction.

The only way to actually implement your theory of using extremely high revs to get that inertial torque is to have an extremely low gear ratio for the gear that you are launching in, one which is impractical in real world racing.

That's as good a problem description for drag racing as any I've heard. Keep in mind that the initial inertial energy in the flywheel, balancer, and crank is acquired before the timing starts, where it does not adversely affect acceleration.

And on the subject of engine torque backup: Given a specific amount of inertial torque, crossplane V8s attain that at an engine speed which is closer to their engine's peak torque. That means that their torque backup is used to maintain that rotational torque. Flatplane crank engines lose much higher amounts of rotational torque by the time the engine speed falls to peak torque RPM. That means that they then have to regain more that inertial torque, for which the speed was already poorly optimized to the physics of making wheels (with mass) spin and moving a car (with mass) forward to match the engines speed.

???

Seems to me that a lower MOI crankshaft would lose less stored energy for a given rpm drop. That it has less at any given rpm to begin with is a separate question, but circuit racing is going to be more interested in reducing MOI effects while the clutch is completely engaged (this being most of the time) than in developing brief torque spikes via a (harsh and potentially risky) no-lift upshift technique.


Norm

 

[Panther140]

^ Right, it will lose less stored energy for a set RPM drop. However, the entire basis for what I am saying is that the RPM drops are not equal between those two engines. The flat plane engine has to be spinning faster to achieve that inertial torque. That inertial torque happens significantly less at the speed of max torque. That is bad. You want inertial torque and engine torque benefitting eachother. Launches are the period of time when engine speed, wheel speed, and vehicle speed are being coupled. The benefit of using inertial torque on launches is that it is constructive to the engines peak torque output, which means more energy to get the car up to speed, which means the car gets into the power band faster. Inertial torque + Engine torque

For launches, you want the inertial torque to be considerable around the speed where the engine makes peak torque. You also want that engine speed to be realistically close to what the car's speed can facilitate shortly after the launch is complete (when the engine is coupled to the drivetrain). Most guys don't let off much between shifts until going into 4th gear, and the gear ratio of that gear reduces how parasitic the MOI of the crank is to the car's overall acceleration.

 

[140Airpower]

I think lowest possible rotational inertia in wheels, clutch, flywheel and engine rotational components makes for the fastest acceleration in road racing.

 

[Tmoose]

Some engines have such broad, flat torque curves that "peak torque" is barely a blip on the torque curve.
Seems like Insistence on "hitting" the torque peak when shifting etc does not count for all that much.
Ed Iskenderian said it never did.
see attachment

==============

And then, specifically on the GT350 with flat crank, there are the on board confusers to be reckoned with, or do the reckoning.
"The performance software also includes launch control, which adjusts engine speed between 3000 and 4500 rpm and holds it there so you can simply drop the clutch and go."

http://www.caranddriver.com/flipboo...gt350s-chassis-and-bodywork-are-incredible#10

It will be interesting to see who wins the 60 foot time competition. The launch control, or a keen driver au natural.

 

[Panther140]

Yeah lowest moment of inertia of wheels and clutch is also important in drag racing. But, you can't sell a thoroughbread road race car to the general public very well, and/or a good chunk of Mustang customer base.

If you go to a car show and talk to the older guys that can actually afford new cars, its common to hear them appreciating cars that "drive like a big block". I think the GT350 is a great car. If I were to sell my Cobra, it would be so that I could buy a GT350. But I won't sell my cobra They don't make cars that simple anymore. Nothing makes you appreciate simple cars quite like owning a complex one for long periods of time.

 

[Panther140]

Yeah launch control is popular in drag racing. Its better to have it installed from the factory, too. If somebody attempted to install it in their garage, the wiring might be intuitive, or worse, easy to work on

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

 

[BigClive]

With questions like this about why a certain crankshaft layout is chosen and similar questions - why not just ask Ford? - it may not be a secret.

 

[140Airpower]

Big Clive, even if Ford gives a plausible answer there still may be a secret. And, Fords legitimate reasons are surely a compromise that other companies may differ with -since other companies do it differently.

 

[gruntguru]

Shall we start a "Torque War"?

"Seems like Insistence on "hitting" the torque peak when shifting etc does not count for all that much.
Ed Iskenderian said it never did."

Ed was right. The torque peak is totally irrelevant to shifting. Shifting should be performed so that the average power output is maximised for the period spent in each gear. (assuming negligible engine rotational inertia etc etc)

je suis charlie

 

[NormPeterson]

Yeah lowest moment of inertia of wheels and clutch is also important in drag racing. But, you can't sell a thoroughbread road race car to the general public very well, and/or a good chunk of Mustang customer base.

Which makes it all the more remarkable that Ford has developed this engine and tweaked the chassis accordingly. But perhaps less surprising than it might seem on the surface, given that Ford has recently stepped back a bit from the drag racing scene, gotten more involved in 'drifting', built the strongly road course oriented Boss/Laguna Seca versions of the S197, and that they were mentioning the European makes as 'targets' for the S550 rather than its traditional rival the Camaro.

As a road course kind of enthusiast for longer than I've held a driver's license, it's been a long wait for U.S. domestic sporty cars to truly be about more than just a powerful engine and a rumble-y exhaust note.

If you go to a car show and talk to the older guys that can actually afford new cars, its common to hear them appreciating cars that "drive like a big block". I think the GT350 is a great car. If I were to sell my Cobra, it would be so that I could buy a GT350. But I won't sell my cobra They don't make cars that simple anymore. Nothing makes you appreciate simple cars quite like owning a complex one for long periods of time.

I know what you're saying here - I'm likely as old as many of those guys, and either driven or ridden in some of the same cars. And I feel much the same about my '08 as you do about your Cobra. Wish it was a bit simpler in some respects, developed a bit further in others, some of which I'm slowly doing on my own (with a modicum of success - 1.2+ lateral g's on true street tires, datalogged) A complete Voodoo engine and 3160 transmission would be a great swap, but that would probably end up costing nearly as much as I paid for the whole car even if I did all of the conversion work myself.


To the matter of inertial torque + engine torque, maximum acceleration in a given gear, maximum acceleration at any given speed, maximum acceleration spike on upshift, and optimum dragstrip ET and MPH results probably won't all happen during the same run. That doesn't make the logic behind any of them wrong, perhaps just not applicable to every situation.


Norm

 

[Panther140]

^I like the way you think. A voodoo swap is a highly advisable investment though. By swapping, you would have more freedom to tailor the build to your needs. Owning a simple car and developing it into what YOU want can have benefits.
You can implement ideas into the car in the exact fashion that suits your needs, as long as you're capable and know what you want.

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.

http://articles.sae.org/13709/

"Flat-plane-crank engines have limitations. First, the lack of counterweight balancing typically limits cylinder displacement to about 4.5-4.6-L due to greater second-order vibration. Ford has solved that in the 5.2-L application with a new crankshaft-mounted damper system and extraordinary attention to NVH abatement during the design and prototype phases. According to Nair, the engine program (which was concurrent with GT350 vehicle development) nearly wasn’t approved for production.

“This [vibration] was our biggest engineering challenge even after we had the first prototype,” Nair noted. “Things were breaking and the technical guys were worried. Whether or not we continued down the flat-plane-crank path for GT350 came down to a critical prototype drive we had scheduled. After that drive, we all went into a meeting room for the debriefing. And we unanimously concluded that we simply had to have this motor! We were determined to solve the issues.” "


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

 

[140Airpower]

Gruntguru, ".....Shifting should be performed so that the average power output is maximised for the period spent in each gear. (assuming negligible engine rotational inertia etc etc)...."

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.