180deg V4 better than boxer

[marcel]

Tony do you have pictures of the 908 crank? I know they changed the 908 crank design after two failures in a race. Not sure if they used a 911 or 917 style crank on the 908.
I don’t think the shorter crank will be an issue with the air-cooled barrels. The VW pistons offset are much more than that of a 911 motor, meaning: if you ad 2 more pistons to a VW it will be longer than a Porsche 911 motor.
I would not use carbs nor would I use pushrods on a motor like this.
The 917 / 908 / early 911’s all had a 66mm stroke the 956 had a 65mm stroke.
This motor would have a crank similar to 1/3of a 917 except that the crank angle would change. I would not go more than 66mm stroke.
Tell me why would the vibration on this crank be so high?
One could go for different firing orders not sure if any one of them will improve performance or vibration. One could even consider big bang firing two at a time something that’s very popular with bikes, not sure if it will work on cars

[Tony Z]

Sorry no pics of the crank. I do have one of the 130kg fibreglass body though.

Remember, there is more to balancing than just the usual balancing that we know as static or dynamic balancing.
There is also secondary types of balancing but I think this is relegated to the pages of massive ship engines.
And then, lets not forget about harmonic frequencies. Same concept as in car audio - some frequencies will be louder inside a car at the same power as others.
A conrod, crank, pistons, any part of the engine has a harmonic frequency. If this part is exposed to its harmonic freq for too long, it will destroy itself. This could be induced from a certain rpm, certain firing order, valves opening and closing at a certain speed, anything that makes a frequency can affect any other part of the engine if you are unlucky. Plus the engine as a whole will have another freq than its individual components.
Want proof. Pick up a guitar, stand next to a piano. Pluck a string, then silence the guitar. Listen to the same note still playing on the piano.
You design the engine to avoid the harmonic frequencies or you redesign the components to change their freq (add weight to make it lower, lighten to make it higher).

"big bang firing" I dont see that being a very reliable method of firing, which is prob why is has mostly been left to bikes or racing bikes. Think about how this works...
It is known for its "unsmooth" engine characteristics, but lets go one step further. Take an inline 4 in normal mode, you have a power stroke every 180deg of crank shaft rotation. So in theory the crank is always delivering power - as one power stroke comes to an end, one is starting, keeping the crank loaded in the "positive direction". Your big bang theory... this involves 2 cylinders firing at the same time, every 360deg. So for argument sake, you have a power stroke for 180deg, then the engine decelerates for 180deg, then power again, then decelerate again.... forget what this is doing to the engine bearings and how it is trying to rip the flywheel off, but think about the poor gearbox which is smashing its teeth apart.
I'll admit, you can pull my description apart and point out numerous things that I said which are not 100% correct (powerstroke for 180deg), but I am generalising to get a point across.

My guess as to why all the early Porsches had a 66mm stroke? The T4 engine (914) which had a 66mm stroke and they stuck with it? Maybe, maybe not?

You could also fit a harmonic damper on the fanbelt side of the engine to reduce vibrations.

[marcel]

You still haven’t answer my question of why an 180deg V4 would require counter weights that’s too big to fit in the casing. Porsche used two cranks on the early 911’s they used a forged counterweight steel crank on the 911S and a cheaper cast iron non counterweight crank in the 911T many that raced these cars preferred using the lighter T crank as it would spin up faster up to 9K with no problem. I’ve had both cars and I could not feel a difference in vibration. An 180deg V4’s crank could be made considerably lighter than that of a boxer.

[eben]

You still haven’t answer my question of why an 180deg V4 would require counter weights that’s too big to fit in the casing. Porsche used two cranks on the early 911’s they used a forged counterweight steel crank on the 911S and a cheaper cast iron non counterweight crank in the 911T many that raced these cars preferred using the lighter T crank as it would spin up faster up to 9K with no problem. I’ve had both cars and I could not feel a difference in vibration. An 180deg V4’s crank could be made considerably lighter than that of a boxer.

In "your " engine, both piston assemblies on same crankpin would move in same direction. That would require a large counterweight to balance it on the opposite side of the crank as its unbalanced. In a boxer they move opposite directions so a large counterweight is not needed.

Sent from my Hero using Tapatalk

[eben]

Mince your V animation wasn't typical...

[MINCE]

Cool video. Thos W engines are crazy.
Don't understand why that blue arrow rotates faster

[Tony Z]

You still haven’t answer my question

Now now, dont be cheeky, I give my knowledge out for free and for the sake of a good discussion

You still haven’t answer my question of why an 180deg V4 would require counter weights that’s too big to fit in the casing.

no way, you would need massive counterweights prob too big to fit in the block to offest 2 sets of conrods and pistons moving the the same direction.

I did answer it when I wrote it anyway.
The weight of two crank pins, two rods and 2 pistons will need big counterweights to bring it into balance. Look at V8 counterweights, they are big and they are pretty thick and heavy. The VW block wasnt designed for massive counterweights. Look at a counterweighted beetle crank. Look at the size of those counterweights. Those are only there to offset half of a crankpin, half a conrod and half a piston. To counterwight your V, you will need two counterweights with roughly double the mass or at least double the inertial effect.

Porsche used two cranks on the early 911’s they used a forged counterweight steel crank on the 911S and a cheaper cast iron non counterweight crank in the 911T many that raced these cars preferred using the lighter T crank as it would spin up faster up to 9K with no problem. I’ve had both cars and I could not feel a difference in vibration. An 180deg V4’s crank could be made considerably lighter than that of a boxer.

H engine or V?
The H already has better balance than the V. And if the crank is strong enough you wont feel the vibration caused by non-counterweighted cranks, nor will you have any issues with the crank not being counterweighted. Also, shorter stroke reduces inertia which in turn reduces the need for counterweights.
A lot of people prefer to race with non-counterweight cranks because they are lighter and thus rev quicker. But if you run those rpms for too long on the street you will effect longevity, even on a crank with really strong main bearings like I referred to above. Any form of imbalance will cause stresses to be set up. Eventually the metal with fatigue and the stress will be too much for the metal and you will have a failure.




Mince, the blue arrow is acceleration. Which is max at TDC or BDC and zero (horizontal) at 90deg. Piston velocity is exactly opposite, being max at 90deg (roughly but close enough) and zero at TDC and BDC

[eben]

Marcel is this what you are talking about?

[marcel]

Yes Eben that’s it.
I am no expert on cranks. See this from another forum on cranks http://www.rcuniverse.com/forum/m_8107130/printable.htm

All the old British big twins used 360deg cranks having both pistons at TDC
They had a lot of vibration but they were also reliable and their performance wasn’t bad either.

[eben]

I HAVE a 360deg parallel twin engine I'm working on the moment 650cc. The crank is huge and the counterweight is even "huger". It causes a pronounced rocking especially at low engine speeds.

There is enough cranking in my video to show you what I mean :


It doesn't affect normal running but does limit amount of safely usable revs.