How To Build A Exotic High Revving V8... Crankshaft design and engine dynamics

[OlderThanMe]

So Proxlamus had a good thread going about the fact that he wanted a high revving exotic V8 and what were available choices...

 

Over the last several months I have read numerous papers and websites with often conflicting information. I decided to only trust sources that were from major auto manufacturers and people that worked for major auto manufacturers and all of their data has been without conflict so far.

 

There are several issues that will need to be addressed in designing your crankshaft.

1. What is your goal?

2. How wide and deep is your wallet? (if it’s size is a problem, do not read on)

3. What is the intended use of the engine?

4. How many miles do you plan on putting on the engine between rebuilds?

5. Full Race, wild street/mild race, or daily driver/mild street?

6. Build it cheap or build it to no limits?

 

I'll slowly go through an introduction and hopefully some more detailed information on how to design/build one of these engines from an available USDM V8 engine block, wether it be Ford, Chevy, Nissan, Dodge, Toyota, Lotus, etc...

 

 

 

First off,

SHAKING FORCES (THE BASICS)

 

Many people on the internet say that the amount of shaking forces in a flat plane V8 in comparison to a 4 cylinder engine is 1.41 times greater for the displacement (I have even said it, not knowing much better than to believe those goons). This is just completely inaccurate as it does not take into account any actual parameters of the V8 flat plane crankshaft itself. Would a super short stroke, large bore flat plane V8 have the same vibration as a long stroke, small bore flat plane V8? Obviously this is not the case.

 

 

In actuality the second order shaking forces on a flat plane V8 with a 90 degree bank angle (the famed vibration) equals (sourced from S.A.E.):

 

 

H= (4*sqrt 2*Lambda*C) (sin 2θ)

Where,

H= unbalanced horizontal inertia shaking forces

 

Lambda = ratio of crankshaft stroke radius (stroke/2) to connecting rod center-to-center length.

 

C = Centrifugal force in pounds produced if the reciprocating mass of one cylinder(half of the crankshaft on the connecting rod throw side) were rotated with crankshaft speed on the crankshaft axis. This should include the mass of oil filled oiling passages and oil layer on bearing surface.

 

Here is a visual that I sketched in SolidWorks of what needs to be designed for balancing (minus oiling passages and fillets in the connecting rod and main journals)

 

 

There are no vertical or centrifugal shaking forces involved in the first or secondary order forces with this crankshaft. There are also no rocking forces involved.

There is a fourth order event around the middle of the crankshaft which is usually minimal and not particularly worth worrying about. I have seen a F1 flat plane V8 crankshaft that was not evenly balanced(counterweights not between the main and rod bearings at every location) and it snapped in the middle. Thus I believe counterweights should be used on every possible spot to not cause problems at the center of the crankshaft. That means 8 counterweights.

 

If you think about it, a flat plane V8 is naturally balanced from one side to the other. This means that if you spin the crankshaft along its axis then no particular side will end up facing up or down. You then ask why are counterweights needed at all? Crankshaft deflection. From my readings a partially counterweighed crankshaft can twist upwards of .09 degrees which is enough to begin wearing on the bearings.

 

I will be adding more shortly...

 

BTW lets keep this thread 100% technical and no hearsay or blabbering posts...

[dr_hunt]

It's not that the crank needs balancing IMO as much as it is the odd firing order and the effect that has on crank twist. Obviously a low HP will have less of that than a higher hp engine will.

[Jerminator96]

This thread should be interesting, I'm going to have to keep an eye on it.

[drwharton85]

Very interested in this thread "more info please"

[kiwi303]

so are there any CAD designs for flat plane V8 cranks for either Chev smallblocks (308, 350) or ford smallblocks (302, 351) that can be used by a crank manufacturor to manufacture the cranks? I'm sure with the popularity of the chev smallblock SOMEONE must have at least drawn a flat plane crank even if chickening out of actually making it.

[OlderThanMe]

Sorry for no second update yet guys... I'm swamped in classes right now although I have been reading some books on oiling issues and other crank design parameters.

OTM