crankshaft desighn question

Just a hypothecical question is all as I am sure "they" have figured out a reason for doing it this way but on typical V8 engines, how come the first two journals are 90* apart then the next two are 180* apart then another 90*? Why would it not be better to be 180* on the first two so the balance would cancel each other out, then 90* then another 180* so the last two more or less cancel each other out, seem it would balance a lot easier that way and you could have a lighter crank right? What am I missin?

Flat-plane versus cross-plane. This might explain it for you:

https://en.wikipedia.org/wiki/V8_engine

Had one on my dyno, made pretty good power, but they have to be a small displacement engine (around 300 cid) or they will start to vibrate. (according to the builder)

this one was 318 cid and had a real slight vibration.

https://www.youtube.com/watch?v=78Zc6qQBTng

basic concepts of a flat plane crankshaft from the engine builder that you see on the dyno.
https://www.youtube.com/watch?v=tybQr44WOn0


Joe

I think that you two have missed the point. He's not talking about a flat crank. He's talking about a flat crank with a 90degree twist in the middle. Starting from the front of the crank with engine at TDC you have rod journal at 0, next rod journal at 180, next rod journal at 90, fourth rod at 270 degrees.

I believe it'd have worse vibration than a 180degree flat crank.

A four cylinder engine has a flat crank and it has an inherent vibration. That's why engines much over 2 liters have balance shafts. The inherent vibration comes from the fact that unless one has an infinitely long connecting rod, the piston motion is not a perfect sine wave. So the forces of the ascending and descending piston do not exactly cancel each other out.
Bolting two four cylinders together at 90 degrees adds the same vibration but now it is from both four cylinders, added together. This is the flat crank V8. Neat thing about that is you can tune each bank to get some real exhaust scavenging effects, and the crank can be lighter. Bad thing is it has inherent vibration that gets larger as engine parts get heavier. Thus the 5 liter "limit".

I believe that on Dave's engine there would be vertical vibration plus a rocking couple. In short that engine would jump around like a drop of water on a hot skillet.

This stuff is out there in textbooks.

R.

yeah Im not thinking of a flat plane.

It just seems the two front journals (front 4 cylinders) would balance each other and the back two journals would balance each other meaning no or very little counter weights, more like two V4 engines.

If you can figure out a way to make a better crankshaft that will balance without counterweights then you'll be a rich man. The OEM's would love to have a new crank design that ran smoother and was cheaper to build.

I remember back in the late '80's or early '90's, Might of been Buddy Ingersoll, use to run a Buick with a flat crank in comp Eliminator.
Sounded like it was fartin' when it ran. I believe it ran pretty well if I remember correctly. Back when he ran the turbo pro stock.
Jeff

It ran so well, they invited him to not bring the car back. Other pro stockers said he was cheating because he was way faster than they were, and they were cheating, so they knew he had to be to.

Im not sure if the buick V6's ever had a flat plane crank but they did have what they called an ODD FIRE crank,And for sure it sounded diffrent but man did they make some power.It was in 1992 and i was helping a comp team and helping at the shop and Mr Cook as he was called had a 230 ish odd fire buick d/ed made over 600 on the dyno,qualified number 1 at houston and blew up bad,fixed it over nite and made eliminations,they have an over lapping jounal and thats the weak point.I saw a full effort odd fire comp d/d make 3.1hp per inch.Comp was great back then heaps of innovation and crazy stuff.

Have a nice day

O.K. my memory is coming back.
It must have been Ingersolls pro stock that had the flat crank.
And if I remember correctly, two cylinders would fire at the same time, so the engine had four firing positions per revolution instead of eight.

Warren Johnson experimented with the flat crank. Warren said the idea was to have tuning capabilities like 2 separate engines. I don't think it made it past testing.

The offset pins were the even fire crank Buick started in the '80s.

I had a couple odd fire motors. Smooth was not a word you would use to describe them.

I remember hearing two weird sounding 1968 Hemi Cuda at the 1971 NHRA Winternationals, one had a set of 180 degree headers running either in S.S or Modified eliminator, the other one had a 180 degree crankshaft in it according to the announcer running in Modified eliminator, both sounded like they had 6 cylinders in them, but revved a lot quicker

Have a nice day

Thank you, Poly.

Time to pick nits: The Taurus SHO V8 was a 60 degree V8, as was its son, the Volvo V8 which could be had for a couple of years in the largest XC model. It was created by adding a pair of cylinders to the V6 SHO engine that had been built by Yamaha. I guess it was economical to use the same bank angle. Most bank angle choices are dictated by economics.

A 60 degree V6 is usually built with a crankshaft with the three crankpins at 120 degree angles. It is very unusual to find a 60 degree V6 with split crankpins like the later Buick or the 4.3 Chevy or the 3.9 Mopar engines. Here's what is in Wikipedia about the 60 degree V6: "In the 60 degree design, the connecting rods are attached to individual crankpins, which are angularly displaced at 120 degree intervals. This geometry results in an even firing interval, eliminating primary vibration and reducing secondary vibration to acceptable levels." My V6 SHO has a very simple crank with three throws and runs smoothly to 8000rpm or thereabouts.

R.

I'd explain, but then I'd be wrong.