Crankshaft weights

[tadd]

Can anyone point me to a reference that calls out the weights of the various crankshafts, CW 66 mm, T 66 mm, CW 70.4 mm?

Kenikh and I were discussing if the 70.4 was just a offset ground 66 since the journal size is 5 mm different and the stroke change is 4.4 mm. So if a big crecent of metal is removed from the journal, one might guess that the 70.4 crank should be a bit lighter than the 66 mm crank.

Anyone care to elaborate on this?

Tadd

[Scott Clarke]

Tadd-
I think that the 70.4mm crank has wider journals, compensating for their smaller diameter. The reason the journals are smaller in diameter is so that the longer stroke crank would still fit in an unmodified case.
-Scott

[tadd]

so I can answer 2/3 of my question soon. The non-counterweighted 66 mm crank is 27# by my bathroom scale (which matches the missues oncologist's scale).

Anyone want to fill in the CWed 66 mm crankshaft weight?



Scott:
Left my copy of Waynes book at work. I will check on that since I didn't even pay attention to that point.

tadd

[Scott Clarke]

Tadd-
I can give you a rough weight this weekend.
-Scott

[davidppp]

Hello again.

Yes, IMHO the 66 can be stroked to the 2.4/2.7 by offset grinding ...if you have plenty of time!

Its easy to put them side by sside to see this..

The 70.5 is much weaker because of the loss of overlap between the main and big end journals.

The 66mm is much heavier than needed and could be reengineered.


Would anyone be interested in a batch of racing-quality light 66mm cranks using modern fea etc techniques?

Kind regards
David

[Walko]

what would be the cost and how much weight saving and are they 66CW

Michael

[kenikh]

Originally posted by Walko
what would be the cost and how much weight saving and are they 66CW

Michael

Ditto to the question.

[davidppp]

Hello, Michael.

We have not actually done a pukka design job here..but some rough stuff suggested at least 25%..

It could be made with counterweights on...but..why?

Without these IMHO would be the way to go..with lighter narrow joutrnal rods and pistons the case pounding should not be significant between builds, for a 9000 rpm circuit engine, and insignificant in road use.

Kind regards
David

[kenikh]

Originally posted by davidppp


It could be made with counterweights on...but..why?

Because with counterweights you have the meat to do a zero balance to any rods and pistons you choose. I won't use a non C/W crank for that reason.

[tadd]

Davidppp:
Wouldn't it be more correct to say that the 66 crank was way over designed rather than the 70.4 was weaker?

Porsche did use 'selected stock' cranks with 906 Ti rods in the 2.14 turbo RSR car that beat out 500+ Hp. Of course the mag cases could only last for 6 hour races... . Again, Porsche used the 66 for their racing 2.2-2.4s using steel rods that were 727g each with a +/-3g callout (Porsche 911 story p.187). Those were some mighty beefy rods!

It wasn't till the 70.4 mm cranked 2.5 that they couldn't keep flywheels on (and broke some cranks)... BUT... the 450g Ti rods were never homogentaed for the 2.5s which I bet would have helped if not solved the problem.

Which then brings up the RSR, which won all sorts of races and that can't be done by breaking cranks. So what gives?

tadd

[davidppp]

Hello, Tadd.

As you say its relative.

But it is true to say that all the race motors which used 70.4mm rods broke cranks, especially the 2.8, and the factory tried hard to find the solution..

My bet would be on resonance..

My pal is a pukka metallurgist who has extensive experience with solving the problems of high speed reliabilty..

So if there is enough demand, the 70.4mm Solution is probably available..

[Plavan]

Bringing this back to life.
Just in from the same digital scale

66mm CW std/std crank= 30 pounds
66mm T crank= 27.5 pounds

[Henry Schmidt]

The 2.0/2.2 crank is not the same casting as the 2.4/2.7 crank.
The webs are completely different thickness.
The 70.4 crank used in the 2.8 and 3.0 RSR were special racing cranks. They had wider flyweights and narrow rod similar to the 3.0 production crank.
They (RSR) also came from the factory with titanium rods. The same rods that were used in the 917, 908 and later the 935.
Many of the ti rods were replaced with after market rods and that's when the engine started seeing crank problems again.

[Henry Schmidt]

Machining the 66 mm crank to use a 3.0 style rod /journal with a large fillet sounds like a great idea if buying custom rods is within the budget.

Smaller journal=lighter crank.

[Steve@Rennsport]

Originally posted by Henry Schmidt
The 2.0/2.2 crank is not the same casting as the 2.4/2.7 crank.
The webs are completely different thickness.
The 70.4 crank used in the 2.8 and 3.0 RSR were special racing cranks. They had wider flyweights and narrow rod similar to the 3.0 production crank.
They (RSR) also came from the factory with titanium rods. The same rods that were used in the 917, 908 and later the 935.
Many of the ti rods were replaced with after market rods and that's when the engine started seeing crank problems again.

Henry:

FYI,..The 3.0 RSR crank had different fillets that solved the breakage; they still had a propensity to spit the flywheels off at 8K and over. Too bad one can no longer buy Glyco RSR bearings with the correct shape for these excellent cranks.

2.8 & 3.0 RSR engines all came with steel rods. No Titanium until the 935's came along. I raced these cars from '76 on and had both,...

Here are a set of RSR rods

[Henry Schmidt]

I'll stand corrected.
Oh well. You can't be right all the time.
Funny though, I took a 3.0 RSR apart that was sporting a set of factory looking titanium rods.
I also sold a set of ti rods for an RSR to the owner of a 74 RSR living in Arizona.
He seemed to think they belonged.
I wonder if there was a sprint engine?

[Steve@Rennsport]

LOL,..I'd bet that you saw a set of 935 rods which look the same, except for their polished finish. I have a set here somewhere and if I can find them, I'll shoot you a picture. These are IMHO, life-limited parts and these had PLENTY of time on them before we removed them from service.

Quite a few of these Ti rods found their way into various GTU & GTO engines during that time since the RX-7's and BMW's (3.5 CSL & M-1 cars) gave the Porsche's fits,...

Since we were largely "self-sponsored" during that time, we used a few sets of 935 Ti rods in our 3.5 RSR motor sourced from local 935 teams and thats how we found that these do NOT last forever.

The factory was not racing RSR's but they did support many well-heeled privateers and sprint motors were not uncommon. We had two motors; one was a 20 hr version (custom cams and very high CR) and one was a 40-50 hour one.

[Steve@Rennsport]

The radii are the same.

[356RS]

I Love It............Great Infomation guys.