Flat/180 degree crankshafts
Discussion
On a Rover V8?
I did hear about some flat cranks being made but I doubt if the block is strong enough to cope. That would really mean 2 cylinders kicking at once as you would be mating two 4's rather than treating it as a "pure" 8.
You might have a lot of fun balancing it as well.
I'm sure some of the more knowledgeable engine boys will be on later and they can advise.
Dave
I did hear about some flat cranks being made but I doubt if the block is strong enough to cope. That would really mean 2 cylinders kicking at once as you would be mating two 4's rather than treating it as a "pure" 8.
You might have a lot of fun balancing it as well.
I'm sure some of the more knowledgeable engine boys will be on later and they can advise.
Dave
deleteall said: Two cylinders don't fire at the same time (that's a long standing myth). There is still one spark every 90 degrees of the crank, the difference is that there is a full 180 degres of rotation between sparks on each bank.
Erm - oh yes they do, I assure you!!!!!! Think about it. On a four stoke engine, each cylinder must fire every two revolutions = 720 degrees of crank rotation. On a flat plane 4 cylinder engine you will get 4 bangs per 720 degrees of rotation = one cylinder firing every 180 degrees. But on an 8 cylinder engine you have to have 8 bangs every 720 degrees of rotation = 2 bangs per 180 degrees, i.e, 2 cylinders must fire together. Simple maths!!!!!
What you said can, of course be true, for a 90 degree crank on a V8, but not for a flat plane crank.
Regards
John McKenzie
Erm - oh yes they do, I assure you!!!!!! Think about it. On a four stoke engine, each cylinder must fire every two revolutions = 720 degrees of crank rotation. On a flat plane 4 cylinder engine you will get 4 bangs per 720 degrees of rotation = one cylinder firing every 180 degrees. But on an 8 cylinder engine you have to have 8 bangs every 720 degrees of rotation = 2 bangs per 180 degrees, i.e, 2 cylinders must fire together. Simple maths!!!!!
What you said can, of course be true, for a 90 degree crank on a V8, but not for a flat plane crank.
Regards
John McKenzie
Surely it depends on the Vee angle?
Given that the Rover lump is 90 deg you're probably right, but it's not just maths!
I stand open to correction, only having a quick lunchhour browse, but assuming a 90 degree v-angle:
2 cylinders firing in 180 degrees of crank revolution will be one on each bank firing 90 degrees apart i.e. not firing together, but both will be driving the crank simultaneously (note they don't have to be on the same crankpin).
Firing pulses for a flat plane V8 are even, so the noise is not 'beaty' like a Rover V8.
The exhaust manifolds are (I have head) easier to optimise, hence more power from high revving engines
The engine is well balanced, as a 90 degree vtwin is a well balanced engine (dynamically) and the flat plane V8 is simply 4 of them bolted together.
To be quite honest I never really understood the point of a 90 degree crank for a 90 degree V8, although I must admit that the noise is execellent.
Now for something really different try the 90 degree V6 in the Alpine-Renault GTA V6 Turbo, with offset crankpins to restore some semblance of balance!
As I said above, I stand to be corrected...
Mike.
2 cylinders firing in 180 degrees of crank revolution will be one on each bank firing 90 degrees apart i.e. not firing together, but both will be driving the crank simultaneously (note they don't have to be on the same crankpin).
Firing pulses for a flat plane V8 are even, so the noise is not 'beaty' like a Rover V8.
The exhaust manifolds are (I have head) easier to optimise, hence more power from high revving engines
The engine is well balanced, as a 90 degree vtwin is a well balanced engine (dynamically) and the flat plane V8 is simply 4 of them bolted together.
To be quite honest I never really understood the point of a 90 degree crank for a 90 degree V8, although I must admit that the noise is execellent.
Now for something really different try the 90 degree V6 in the Alpine-Renault GTA V6 Turbo, with offset crankpins to restore some semblance of balance!
As I said above, I stand to be corrected...
Mike.
I disagree, if you think of it like two inline fours, when you rotate one of the engines by 90deg it becomes 1/4 of a circle ie 1/8 of a four stroke cycle out of phase
johnmckenzie said:
deleteall said: Two cylinders don't fire at the same time (that's a long standing myth). There is still one spark every 90 degrees of the crank, the difference is that there is a full 180 degres of rotation between sparks on each bank.
Erm - oh yes they do, I assure you!!!!!! Think about it. On a four stoke engine, each cylinder must fire every two revolutions = 720 degrees of crank rotation. On a flat plane 4 cylinder engine you will get 4 bangs per 720 degrees of rotation = one cylinder firing every 180 degrees. But on an 8 cylinder engine you have to have 8 bangs every 720 degrees of rotation = 2 bangs per 180 degrees, i.e, 2 cylinders must fire together. Simple maths!!!!!
What you said can, of course be true, for a 90 degree crank on a V8, but not for a flat plane crank.
Regards
John McKenzie
If you don't believe me take a 4.5 Cerb engine to bits
That's got a flat crank
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