What brings RPM limits...

[edmayo]

I have heard of factory rubber damped crank pulleys (like your basic chevy) but have never seen one. Has anybody , Grady? Steve? It was to solve that harmonic problem, seems like it might.

[sithot]

I have heard of factory rubber damped crank pulleys (like your basic chevy) but have never seen one. Has anybody , Grady? Steve? It was to solve that harmonic problem, seems like it might.

FWIW: BMW did this on the R69-S (motorcycle) after experiencing similar issues. It was a steel wheel with a rubber center mounted directly on the front of the crankshaft. The front cover was bulged to clear it. The only downside was when the rubber center gave up. It usually destroyed whatever was in the way.
R50-S models also had this done as a retrofit as these were the peakiest of all the BMW engines.

Tom

[Steve@Rennsport]

I have heard of factory rubber damped crank pulleys (like your basic chevy) but have never seen one. Has anybody , Grady? Steve? It was to solve that harmonic problem, seems like it might.

Hi Ed:

I too, have heard of them but never seen one to date. These did need some type of torsional vibration damper.

LOL,....I just saw my first 7R aluminum engine case,...



Sherwood:

The RSR cranks did have wider radius fillets on the rod journals and special, matching bearings (which are long NLA) to prevent cracking and breakage.

[72/2.8]

Porsche solved the valve train limit with titanium retainers and forged, non-adjustable rocker arms (lash caps on the ends of the valves). With stock springs and installed height, there isn’t any ‘valve float’ at 9000 rpm.


Best,
Grady

Agreed, the non-adjustable rocker are spoken of as the grail, but are there any sources?
Shut my 2.8 off at 8000 and with the current setup it really peaks around 7800, although the cams should work to 8300 and that would move the peak to 8100...But I've been fraid to do that with stock style rockers.

Any thoughts?

[Steve@Rennsport]

RSR/935 non-adjustable rockers are NLA as well as VERY expensive (with all the required lash caps) if you can find a set. Its THE most reliable system to use for high RPM race engines, tho,..

Since the stock iron rockers are so heavy with somewhat unfavorable mass distribution, an excellent alternative are the early steel forged rocker arms which are hell-for-strong, rigid, lighter, and most importantly, have less mass at the respective ends.

That raises the threshold of valve float,....

OEM cast iron rockers were designed to snap in case of valve-to-piston contact but thats not always happened as anyone with bent valves from a missed shift can attest to.


One last thing,.......these engines do have some oiling issues when operated consistently above 7300 RPM. The center two rod journals (#2 & #5) do not receive sufficient oil volume & pressure to prevent rod bearing failures unless the crankshaft, engine case and center main bearing are modified to suit.

[tomkirkcis]

I am not an engine guru and do not understand all the implications of what I read, but I recall reading the the cast iron rockers and the forged rockers had different ratios. I am uncertain if it was the rsr racing rocker or the early forged production rockers that were being discussed. What exactly is the ratio? was it the racing or production rocker and how does it affect the engine preformance?

[72/2.8]

So I've found.
I have one of them....now to find eleven more.
it's built on 3.0 turbo case, with a 66mm crank
light rods and pistons.

[chris_seven]

Interesting set of problems and there must be a solution but cost could be prohibitive.

It would be possible to carry out a detailed torsional analysis of the crank, ords, pistons and flywheel to look at inrtia torques without too much expense but modelling gas torques would be a bit more difficult and would need some expensive software.

It would also be useful to run a motor on a dyno equipped with a torsional analyser and look at rhe vibration signature.

It may then be possible to design a suitable damper to help reduce the damaging vibrations. (I would guess that most of these are higher order).

Ti rods and light weight pistons would also probably help as they effectively increase crank stiffness by reducing forces.

If custom crankshafts were used, and I guess we maybe reaching the point where this is becomong viable, there are several improvements that could be considered.

Design tools have improved greatly in the last 30 years and I am sure that counterwight design could be optimised to help reduce vibration levels.

Material selection could also be enhanced and 'EN40B' Aero Release materials with full nitirding rather than Tenifer style heat treatment would give improved fatigue life and could be comined with generour fillet radii.

The clamping forces holding flywheels could be improved by increasing the number of bolts (Cosworth BDA engines now commonly use 12 bolt flywheels as opposed to early generation 6 bolt designs)

It really needs a detailed review of the design and the problem areas as a starting point.

I don't think cast steel flywheels have any significant damping, other than, perhaps due to differences in mass/inertia. Grey Cast Irons have higher damping capability than steel due to the presence of graphite flakes and this means that large castings that used to be used for machine tool bases tend transmit less vibration than fabricated components that are now relatively common but I don't think this type of material would affect torsional vibrations in an engine. I would be very interested in any data.

'Solid' 935 Rockers are available but are extremely expensive. They have been designed to work with 'Motorsport' Rocker Shafts that can still be ordered from Weissach.



We can also manufature suitable lash caps for these rockers.

[72/2.8]

That's the puppy. I have one, someday maybe I can find 11 more.
I found a 962 66mm stroke crank and mixed it with a 77 3.0 turbo case good forged rods and pistons. I'm convinced it's as solid as I can get it, but worry about the valve train

I've talked with a few 2.5 challenge guys that say they twist their motors to 8500/8700 with the early rockers and just change them more frequently?
Till I get it sorted out, I'll keep it dialed back a bit, but I sure would like to find some solution for the rocker worry

[a911s]

FWIW, I emailed Grady Clay when I built my motor, and he did not feel I would need solid rockers.