I just read this that the Shelby GT 500 has one.
I know what a flat plane crank is.
I just read this that the Shelby GT 500 has one.
I know what a flat plane crank is.
I think it's just a "normal" V8 crankshaft.
^correct^
The question is, what advantages are there between the two? Why aren't all V's flat plane?
could you make a flat plane inline four? Or six?
In reply to _ :
Flatplane V8's are harder to balance but rev faster.
Inline motors are always flat plane to my knowledge. Yamaha tried a cross plane crank for a few years in the R1 but I think have since gone back to a flat plane.
In reply to Patientzero :
Those cross plane R1s sound really cool though.
In reply to _ :
My understanding is that a flat plane crank make for a higher revving engine, the powerband is higher in the rev range. Cross plane has a torquier low end and revs less.
Flat plane cranks also make some amazing sounds.
Patientzero said:In reply to _ :
Flatplane V8's are harder to balance but rev faster.
Inline motors are always flat plane to my knowledge. Yamaha tried a cross plane crank for a few years in the R1 but I think have since gone back to a flat plane.
I6's don't have a flat crank- the firing throws are 120 deg.
The balance part is- flat crank is like having two I4's 90 deg from each other, bringing their second order vibration. The 90 deg crank is actually balanced- and it will spin quite quickly- but the exhaust can't be balanced for flow as easily.
edit- without really good headers, the gain from even firing I4's is pretty lost. And there's not much space under a Mustang hood to fit really good headers. I'm not really sure why they went to the flat crank the the first place...
alfadriver said:I6's don't have a flat crank- the firing throws are 120 deg.
True, I screwed that up.
In reply to b13990 :
Then it would not be a 4-stroke anymore.
Daylan C said:In reply to Patientzero :
Those cross plane R1s sound really cool though.
Especially with a Two Brothers exhaust.
With a flat-plane crank, you end up with two cylinders firing simultaneously every 180 degrees
A cross-plane crank staggers the firing events so that you get a cylinder firing every 90 degrees instead, which is smoother and torquier, at low crank speeds in particular. This is not the natural action on 8 cylinders spread over 720 degrees, though, so the crank must be balanced by weights.
Another name for a cross-plane crank is a "split" crank- you are splitting the natural tendency of the system to fire two cylinders every 180 degrees into two separate events.
Also because of those degrees, flat-plane cranks have vibrational difficulties in V8s that aren't in 90 degree angles- it can be rectified to a point by harmonic balancers (there is a flat plane for the LS) but it only goes so far.
Another issue with flat plane cranks is- they all sound similar thanks to their firing events.
b13990 said:With a flat-plane crank, you end up with two cylinders firing simultaneously every 180 degrees
A cross-plane crank staggers the firing events so that you get a cylinder firing every 90 degrees instead, which is smoother and torquier, at low crank speeds in particular. This is not the natural action on 8 cylinders spread over 720 degrees, though, so the crank must be balanced by weights.
Another name for a cross-plane crank is a "split" crank- you are splitting the natural tendency of the system to fire two cylinders every 180 degrees into two separate events.
You mean you have two pistons at TDC at the same time on one bank. Cross plane v8's will still have two cylinders at TDC, just on opposite banks. That's just a result of getting an even firing order. Regardless of the crank orientation, the firing angle is 90 deg per firing event- so the torque pulses are the same at the crank output.
Flat V8's are different, but there have only been a handful of those total. 90 deg v8's have been the way to go for the gross, gross majority of V8s.
In reply to alfadriver :
Yes, I thought it through and I think that's right. What I said might be true of a flat V8, but with a 90-degree V-block you won't ever actually get simultaneous firing events. What you will get is, like you said, 2 pistons at the top of their cylinders at the same time, in the same bank... if you don't bust the crank into 2 planes.
Thank you (and YouTube slow motion) for helping me think this through.
The Lotus 918 twin turbo V8 was a flat plane crank. If you listen to it idle, it sounds like two 4 cylinder 910 engines running together.
_ said:The question is, what advantages are there between the two? Why aren't all V's flat plane?
could you make a flat plane inline four? Or six?
Cross pane V8s don't shake horribly like a flat plane would, which is why 99.9% of V8s are cross plane.
Asking about fours or sixes is irrelevant, because the question is ignoring why cross planes exist.
Inline sixes are inherently balanced front to rear, because the 153624 firing order is perfectly interlaced in firing order and piston mass.
.
Inline fours are a bastard for vibrations for two reasons. For one, the inertial mass and friction loads goes through a cycle twice per revolution. An interesting statistic, before pondering this further, is that 1% of all energy produced in the world is to overcome piston ring friction. This includes nuclear, hydroelectric, solar, etc. Piston rings are a huge problem. That said, twice per revolution in an inline four, the piston rings are at zero friction. Two cylinders at TDC and two cylinders at BDC. So the rotating inertia of the crank and flywheel and damper and big ends of the rods and everything else that rotates has two times per revolution where they don't have as much force to fight against. This makes I4s shaky. The other issue with inline fours is related: two at TDC, two at BDC. 90 degrees away from that, all four cylinders are at 90(a/b)TDC. The problem with that is due to rod angle, 90 from TDC is not halfway down the bore, it's quite a ways further down. This means that the reciprocating mass's boinging up and down is uneven with respect to the boinging up and boinging down, so there is a large vertical shaking force as the center of reciprocating mass (the CG of the pistons and top of the rods) goes from centered in the stroke to lower than centered in the stroke, twice per revolution. You really notice it in engines larger than 2 liters, and I forget who invented the counter-rotating balance shafts, but they were a genius: By having two shafts rotating in opposite directions, you could time their imbalance to negate each other in the lateral direction and compound each other in the vertical direction, and by timing them to the crankshaft, you could time this compounding to offset the piston masses' vertical shake. Which is why Honda K24s sing like kittens and Iron Dukes vibrate like paintshakers, even though the K24 has a much much MUCH longer stroke and a much worse rod/stroke ratio, which makes the rod angle problem worse. (Iron Dukes have a 2:1 rod/stroke ratio, one of the highest in a production car engine and probably THE highest in the last 50 years)
Now. V8s. A flat plane V8 shakes like two inline fours for the reasons above. Much worse, actually, because twice as many shakes. Cross plane cranks eliminate most of the issues because they break up the firing order on each bank while maintaining an even firing order at the crankshaft.
Flat plane cranks tend to be vastly lighter. A cross plane crank has counterweights between each journal in two different planes. A flat plane crank just needs enough steel to connect in one plane. For that reason, you have a much lighter rotating assembly. Most of the benefits happen in vee engines or engines where more than 6 cylinders are present. Many 4 cylinder engines are already flat plane, and some (usually exotics) V8s are flat plane. It's why a Ferrari sounds like a Ferrari.
A flat plane crank (in a V8) effectively does for the exhaust what 180-degree headers try to duplicate, but also has other benefits.
Pros:
Cons:
There is nothing exotic about a flat plane crank. In an I-4 it makes little difference as you only have 4 cylinders anyway. They fire every 180 degrees, so it makes little difference to vibes and harmonics if the crank is flat or cross. They tune out some of the vibes with the firing order. In some engines, it is impossible to use a flat plane crank because of the orientation. Like a V4 or I6.
Most manufacturers choose a cross plane crank for V8s for smoothness, reliability, and because they aren't necessarily seeking the performance benefits that come with a flat plane crank.
The answer to the original question, I think, is that Fords marketing department is using an unneccesary word to make people think they are building something special, when it's just an average, everyday, pedestrian, normal, not fancy thing. But most people won't know that.
Streetwiseguy said:The answer to the original question, I think, is that Fords marketing department is using an unneccesary word to make people think they are building something special, when it's just an average, everyday, pedestrian, normal, not fancy thing. But most people won't know that.
Perhaps. I think it's actually useful in this case though, because the GT350 that came before it had a flat plane crank, so people might otherwise be expecting the GT500 to have the same thing.
In reply to codrus :
I thought the GT350 was supposed to be a scalpel (well, maybe a flaying knife) while the GT500 was the 16 pound sledge.
I'm confused. If a flat plane crank is hard to balance, why would they allow such high revs? Doesn't a smooth balanced engine allow for higher revs?
rustybugkiller said:I'm confused. If a flat plane crank is hard to balance, why would they allow such high revs? Doesn't a smooth balanced engine allow for higher revs?
It's not that they "allow" high revs. It's that they tend to be fairly small, and motorsport-biased, which means they require high revs.
The rest of it is exhaust tuning - crossplane crank V8s have uneven firing order per bank, which screws things up with the headers unless you can run a bundle of snakes exhaust that crosses two cylinders' exhaust from one bank over to the other.
Don't confuse crank balance with the issues with being a flat plane. You can make a flat plane crankshaft perfectly balanced but it will still shake your eyeballs out of their sockets because of the piston weight effects, which happen at a frequency that you can't fix with crank balance.
rustybugkiller said:I'm confused. If a flat plane crank is hard to balance, why would they allow such high revs? Doesn't a smooth balanced engine allow for higher revs?
It's also kind of a weird vibration- it's not something that shakes the engine apart- it's a second order vibration (which is twice engine speed) that you feel. The same issue follows I4 super bike motors, and those spin to high heaven. Short stroke engines lessen this vibration, as well- which is the path most high speed engines follow.
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