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

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j0nr
6
Joined: 04 Jan 2016, 13:22

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

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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.

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## 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.

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

There are two main benefits of a Flat-Plane:
  1. 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.
  • 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.
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?

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## 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.

Tommy Cookers
616
Joined: 17 Feb 2012, 16:55

Re: Why Did Ford Decide to do a Flat-Plane Crank Differently

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your crossplane crank picture shows far more counterbalance weighting and inertia than your GT350R flat crank has
hence a thicker and heavier crank to maintain natural torsional etc frequency, also giving greater friction
ie the GT350R design is better (than the crossplane) for a high rpm engine

compared to the expected flat crank design - surely there's a unique noise ?

EDIT it has 4-3-1 unequal length exhaust manifolds for shortness etc for quickest catalyst light-off
maybe this factor is linked to the unusual crankshaft layout ?
Last edited by Tommy Cookers on 05 Jan 2016, 00:35, edited 2 times in total.

j0nr
6
Joined: 04 Jan 2016, 13:22

Re: Why Did Ford Decide to do a Flat-Plane Crank Differently

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Tommy, its not how big they are collectively, its about how they are distributed. The flat-plane crank counterweights are evenly and symmetrically distributed along the crank, whereas the GT350 CW's are not. Large, opposed CW's at either end to balance out primary rotating couple. (As in any Cross plane)

There is no such rotating couple in UDDU flat plane, hence no opposed end to end CW's. That's a major advantage of a flatplane, minimal mass/inertia.

OO7
OO7
171
Joined: 06 Apr 2010, 17:49

Re: Why Did Ford Decide to do a Flat-Plane Crank Differently

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Tommy Cookers wrote:your crossplane crank picture shows far more counterbalance weighting and inertia than your GT350R flat crank has
hence a thicker and heavier crank to maintain natural torsional etc frequency, also giving greater friction
ie the GT350R design is better (than the crossplane) for a high rpm engine

compared to the expected flat crank design - surely there's a unique noise ?

EDIT it has 4-3-1 unequal length exhaust manifolds for shortness etc for quickest catalyst light-off
maybe this factor is linked to the unusual crankshaft layout ?
Yes there is Tommy. A few weeks a go I read about the switch to a flat plane crank and then watch a promotional video of the car. I couldn't understand at the time why it had a bit of a cross-plane burble to it. :D

j0nr
6
Joined: 04 Jan 2016, 13:22

Re: Why Did Ford Decide to do a Flat-Plane Crank Differently

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OK the images I chose were merely illustrations of the two different layouts. Not necessarily designed for same engine size etc.

The fundamental point I am making is that one usually goes with a flat plane to reduce mass/inertia of rotating components plus gains in pulse tuning at the cost of a secondary horizontal shaking force.

The reduction in mass is due to no out of balance couple in the system, I.e. dynamic imbalance which a cross plane has until you add the extra heavy end counterweights.

Due to the odd layout of the Ford FP crank, they have not gained any of these usual benefits. Hence my question, why did they bother ?

Tommy Cookers
616
Joined: 17 Feb 2012, 16:55

Re: Why Did Ford Decide to do a Flat-Plane Crank Differently

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the GR350R has gained most of the benefits (and of course the disbenefits) of the usual flat crank

compared to usual flat crank the disbenefit of the GT350R crank is an extra (but small) primary couple (ie smaller than a crossplane crank's)
this much smaller couple will demand (as I said) a much smaller amount of counterbalance weighting

consider the GT350R is roughly analogous to two conventional 180 deg twins placed end-to-end
then a crossplane crank is analogous to such a twin straddled by another with cylinders abnormally far apart - the moment is greater

as counterweighting is not a full treatment of the (crank bending moment and main bearing load) effects it's trying to treat
for performance the flat crank will always be better, because of the number of effects reducing crank diameter/friction
though performance flat cranks have failed by overdoing the 'size zero' counterweight etc concept ie the original Guzzi V8
it might be interesting to look at the 60s Indy Ford and the Lancia D50 crankshaft to see what counterweighting they used
and the first 60s F1 1.5 litre BRM and Coventry Climax before they went flat crank

remember despite the fuss about inevitable moment imbalance, the crossplane V8 gives far less vibration than a flat crank V8
so the GT350R can afford to have some of the crossplane's moment imbalance
the vibration from this will be trivial relative to the force imbalance inherent with any flat crank V8
Last edited by Tommy Cookers on 05 Jan 2016, 18:51, edited 3 times in total.

xxChrisxx
44
Joined: 18 Sep 2009, 19:22

Re: Why Did Ford Decide to do a Flat-Plane Crank Differently

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No, no, no. This won't do at all OP!

A thread that contains interesting facts, that's actually technical. You must be new here.


I knows it's been said, but its most likely a brand character thing. A big yank V8 suddenly sounding 'like a ferrari' would just be wrong. Its a very interesting way to achieve it though.

Brian Coat
99
Joined: 16 Jun 2012, 18:42

Re: Why Did Ford Decide to do a Flat-Plane Crank Differently

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Ford did offer some comments about why here ...

http://www.edmunds.com/ford/mustang/201 ... oo-v8.html

It does not answer all the questions but it gives a flavo(u)r.

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bdr529
59
Joined: 08 Apr 2011, 19:49
Location: Canada

Re: Why Did Ford Decide to do a Flat-Plane Crank Differently

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there was an article in Hot Rod magazine last year about the engine and why they used the flat plain crank.
I don't think I have the magazine any more, but I do remember it said that essentially it was for a higher revving engine
I was at the race at Mosport this summer and can attest to the sound being different to the other mustang there.
especially when they were climbing up the back straight, it may not sound like a Ferrari but certainly not like the mustang's of old

Nice touch by Ford to paint one in the old Comstock Racing colours (green stripes)


Jolle
132
Joined: 29 Jan 2014, 22:58
Location: Dordrecht

Re: Why Did Ford Decide to do a Flat-Plane Crank Differently

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Apart from the obvious plus benefit from the even degrees between explosions, is that it might be cheaper plus they can sell it as "a racing crank"

j0nr
6
Joined: 04 Jan 2016, 13:22

Re: Why Did Ford Decide to do a Flat-Plane Crank Differently

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All interesting points, but I fear the topic is straying a bit. I did not want to debate whether Ford should have gone with a FP crank or not or whether a FP is better than a CP or not.

I am only curious about the decision making of Ford to choose the layout they did. From a fundamental design POV, the basic imbalance of each layout is different. Main bearing relief aside.

Fundamentally, a Cross Plane 90deg V8 has no shaking forces and a primary (goes with engine speed) rotating couple. So you design your CWs to at the very least cancel out the rotating couple, hence why all CP V8s have larger, opposed end CWs. These large CWs are always present (if you want balance) and lead to a heavier crankshaft.

Fundamentally, a Flat Plane 90deg V8, UDDU layout, only has a second order, horizontal shaking force. No couples (well a very small, negligible second order, vertical couple due to bank offset), so no need for larger end CWs. This is where the mass/inertia saving comes in allowing higher RPM. Any CWs on a FP V8 UDDU are merely for main bearing relief.

So then the odd Flat Plane 90deg V8, UDUD layout that Ford adopted...It too has the characteristic second order shaking force of the UDDU, BUT by going UDUD they have also introduced a rotating couple, similar, although not as big, as the CP V8, so they now MUST add heavier, opposed CWs either end to cancel it out. Again, this is for fundamental balance, not main bearing relief.

My interest is solely from a design POV as it is an odd layout to choose if making the decision to flat plane. I just want to know the reasoning behind the decision...not whether its the best one or the right one.

There must have been some design meeting where someone said "let's do a flat plane" and then someone said "I know, let's try UDUD instead. I think it will be good to do it this way because........."

j0nr
6
Joined: 04 Jan 2016, 13:22

Re: Why Did Ford Decide to do a Flat-Plane Crank Differently

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At last, some light shed on the exact issue I have been trying to explain/get to the bottom of:

http://www.edmunds.com/ford/mustang/201 ... plane.html

So...if I am reading into it right...its all about MARKETING! The sound it makes seems to have driven the decision making on the odd layout.

Annoyed I didn't find that in my searchings.

Tommy Cookers
616
Joined: 17 Feb 2012, 16:55

Re: Why Did Ford Decide to do a Flat-Plane Crank Differently

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edmund's info from Ford suggests it allows better compact inlet and exhaust systems for this car (than would the usual flat crank)
this seems plausible

Ford also evaluated the 3rd possible flat crank design

there might be some 8 crankpin design with great balance and good manifolding ? (viable if a high bore:stroke ratio is acceptable)

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dren
226
Joined: 03 Mar 2010, 14:14

Re: Why Did Ford Decide to do a Flat-Plane Crank Differently

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Tommy Cookers wrote:edmund's info from Ford suggests it allows better compact inlet and exhaust systems for this car (than would the usual flat crank)
this seems plausible
That seems plausible, but have you seen a Mustang? The hood is massive. Surely that wasn't the sole reason? They could fit a lot more under the hood, unless Ford is planning to use this lump in other race cars?
Honda!

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strad
117
Joined: 02 Jan 2010, 01:57

Re: Why Did Ford Decide to do a Flat-Plane Crank Differently

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have you?
It's a tight fit in that engine compartment.
If you're speaking simply of height don't forget Ford has to think about the sight line over the hood
While you and I might be tall enough for it not to matter, Ford has to think about short people too. :wink:
To achieve anything, you must be prepared to dabble on the boundary of disaster.”
Sir Stirling Moss