Combination crankshaft - flywheel lignhtened or not for a 2.0 and 2.2 ?

[964-RSR]

Hello,

crankshaft 66mm exists with or without counterweight and flywheel can be lightened or not.

Do you know :

1 - if the 66 mm crankshaft without counterweignt is solid enough to run 8000 rpm in a 2.0L engine ?

2 - what is the best combination crankshaft / flywheel for rallye, race ?

3 - what is the advantage or disavantage of crankshaft counterweight and lightened flywheel ?

Thank you

[tadd]

This has been a question of mine for a long while and there are a few posts somewhere :-). Bought a T crank in 2005.

The non counterweighted cranks have been used to 7.5k in track cars. Bearings take a beating when used with stock weight rods along with the case. But...

Harmonics is a tought subject, but we know the counter weights are added to the crank to equal the mass of the rod journal and rod big end. So if one were to regrind a 66 crank to GT3 journals that's a big chunk of weight lost. Add in that most aftermarket steel rods are around 550g vs the stock 720g and that's a very large change. Corrspondingly there should be a significant reduction in the mass of the counter weight. Ti rods are worth another 100g each.

As for a low MOI motor, heck yes . I grew up on bikes and I'm building my Porsche with Ti rods and wrist pins with a fidanza aluminum flywheel and 5.5" tilton clutch.

Something I was told by a 'grey beard' I really respect is that he has given up on any mods to the bottom end (other than cross drill) cause the bottom end is just so strong.

Just have to adjust how one drives.

t

[Brant]

A lot of race motors are built with Non counterweighted crank shafts
I can't say from personal experience what the maximum redline of a race motor with out counterweights would be... but I'd guess they can be built to run close to 8K.... maybe a little less?

The advantage of the counterweight is smoothness and thus balance at high rpm everything is taking a beating without the counterweights
I would assume the case would have a much shorter life
perhaps all of the rotating components would have a much shorter life?
certainly the bearings.

I use a counterweight for longevity
You can save rotating weight with rods and flywheel/clutch

lots of flywheel options out there
somewhat depends upon your budget and class rules
here are a 3 different ones I have personally used
You can go very light with a race flywheel and compensate for counterweights (which was my strategy)

[964-RSR]

Thank you for your advices

I will keep my crank counterweight, use flywheel lightened and my aluminium pressure plate from 911 S 2.0 MFI.

[chris_seven]

I am not sure that I agree with the comments about lateral balance being the reason for the counterweights on a 911 crankshaft.

A flat-6 is a fully balanced configuration which is in perfect primary and secondary balance. The three cylinders on each side of the crankcase tend to have an end-to-end rocking motion, like a pair of straight-3 engines, but in the boxer engine configuration, the imbalances on each side cancel each other, resulting in a perfectly smooth engine.

If this is a true statement there is no reason to add counterweights to a 911 crankshaft for reasons of lateral balance. In fact the increased mass will only add to the inertia and hence lateral loads on main bearings. It will also influence the engines ability to change speed.

I have thought, for some time now, that the counterweights in high rpm 911 engines were developed to conteract the high order gas torques (4th/5th) that occur in these engines.

These gas torques can produce a significant torsional vibration and if this becomes too close to a resonance can seriously affect the crank's fatigue life.

Because it is a torsional vibration it won't really be measurable by a typical vibration senso (accelerometer) and the first notice could well be a blown up engine.

The only way to test out this idea is to find an engine dyno fitted with a device that can measure torque variation within a revolution - virtually all engine dynos only measure mean torque- such as a Bently Nevada Key Phasor, but these are very expensive.

It could be software modelled but again the cost would be very high. Torsional Analysis programmes for auto engines are about $50 000 and it would take 2-3 weeks to establish the model.

Some real data would be good.

[PFM]

Brant,

Nice photos. Do you have the weights of each by chance? I am working on a 914-6 so those look very good to me. The last aluminum flywheel uses what clutch? The 5" Tilton?

Thanks,

PFM

[Brant]

Originally Posted by PFM

Brant,

Nice photos. Do you have the weights of each by chance? I am working on a 914-6 so those look very good to me. The last aluminum flywheel uses what clutch? The 5" Tilton?

Thanks,

PFM

they get lighter as you go down the page of pictures

- On the windowed and lightened stock flywheel I was using the aluminum pressure plate and clutch disc for that (total around 20lbs)

both the steel (black middle pictures) and bottom aluminum flywheels are using a tilton 5.5 clutch pack.

- the steel middle flywheel was around 14lbs total with clutch/pp mounted

- and the bottom aluminum flywheel only another 1 or 2 lighter

I'm running a very tall "F" first gear and the combination of tilton with tall gearing make it a bit weird to start from rest. the tilton feels a lot different than stock without any allowed slipping. It should be good for the tranny that way though.

[tadd]

Chris:
Gas torques are variable, right... Since the fuel loading varies with throttle and rpm. Would the counter weight then be calculated as an average or just peak load?

Don't get me wrong, those engineers were smart folks, but all they had were slide rules and graph paper (good enough to go to the moon!). I haven't gone van gooh and cut off an ear and weighed it, but I have traced it and did a pencil volume and it's really close to the journal/rod rotating weight. It makes sense that our flat 6 is inherently balanced, but does that not assume an infinitely ridged crank (and case), and that our firing order has isnt 180 out. Maybe I'm being too simple minded, but it seems smart to me, that if you can minimize forces at each throw then your correcting at the source rather than across two feet of crank that winds like a spring.

Given the simulation or measuring costs this seems like a problem for try it and see what the bearings look like at the mantience build (which a track car would have done anyways). The only cost to find out would be the reground T crank ($100 plus regrind, harden and x-drill $600).

BTW, a non CW crank (170g each throw iirc), Ti rods (450g), Ti pins (70 g) and your new pistons (325g) would be awesome!

t

t

[Steve@Rennsport]

FWIW,.....we've done several high-RPM 2.2 race engines using the non-CW crankshaft without issues to 8.8K+. These used custom Pauter Engineering rods and some NASCAR racing bearings.

We see a little more fretting on the main bearing webs, but thats about it. For race applications where maximum performance is the main priority, I see no issues. Its not my choice for street engines where maximum engine longevity is required.

They do spin up quicker.

[Walt Fricke]

I'm thinking I read somewhere that the non-CW cranks are cast, not forged? They were in the Ts? Cost saving? Urban Porsche myth? Or is cast plenty strong?

Walt

[Walt Fricke]

Brant - are you still running the 901 gearbox? If so, are you really going to be able to use that tall 1st on the track?

I had a tall (60 mph @ 8,000 rpm) 1st in my 911s 915 box. Thought I'd wax everyone's behind coming out of slow corners (Spectator and Farmhouse and Pueblo #7) with all that extra delivered torque. And it was quick. But I didn't see the hoped for results. Some comparison of RPMs at various points persuaded me that what I gained with the gear I lost with the extra upshift. Only place it helped was really slow starts (and then only if I remembered to upshift).

I'm now back to stock. Didn't hurt at Hallett (which has at least one slow corner), and much easier loading onto the trailer. In anticipation of eventually using the 5.5" setup. The alternative is either 1) a convenient winch, or 2) a lot of faith in a fast run into the trailer with no modulation. My winch is functional and useful when the engine won't work (all too common, alas), but is a bit of a pain for everyday track use.

Walt

[Brant]

Walt,

I'm still using a 901
I'm running F/J/S/O/V
I think I'm loosing a little bit on shifting too often

but very close ratio's
I've been running this box for 4 years now (already had to rebuild it once)
I'm honestly shifting too much...

but the F and J are great for slow or fast rolling starts
and the F actually works well in slow corners
The box is incredibly versatile for varied tracks

I just put in a new motor with a higher redline that I haven't driven in anger yet
I planned the box with the higher redline in mind, but will see what its actually like in reality this year.

the clutch is a pain in the pits. Lots of stalls. and were committed to hand pushing onto the trailer

you've got to come over to some more vintage stuff soon!

[Steve@Rennsport]

Originally Posted by Walt Fricke

I'm thinking I read somewhere that the non-CW cranks are cast, not forged? They were in the Ts? Cost saving? Urban Porsche myth? Or is cast plenty strong?

Walt

Hi Walt,

ALL 911 cranks, CW or non-CW are forged. None of them produced after 1965 ever received a cast crank unless it was aftermarket.

[Walt Fricke]

Steve - must be a Fricke memory myth.

But an evil thought occurs: if the counterweights were carefully calculated to deal with stock rod/piston weights, and we run rather lighter stuff, does that cause a problem - overcounterweighted? Maybe only in theory? Or down in the noise for practical matters? You don't hear of hot rod cranks breaking right and left.

Brant - you haven't broken off the end of the mainshaft with the 1st on it? A result of perfect, light right foot? When I realized my speed out of Spectator was the same using 1st and 2d (RPMs in 2d at upshift point on track were the same regardless of gear used in corner), I stopped using 1st except for starts, so I haven't really missed the tall 1st.

An 8,000 rpm motor is nice in terms of gearbox flexibility: the 5th doesn't top out even at Brainard (I think fastest straight I've driven) or Sebring, but is useful at HPR's longest straight and on the two long Miller straights. And 2, 3, and 4 are OK for Hallett.

How come I haven't seen you at the annual PCA race at HPR? As light as you have gotten that car (and with all metal body panels no less) it should get around well.

[tadd]

Walt, Steve:
That 'fact' is in Bruce andersons book (p50, ISBN 0-87938-269-4). Frere says nothing about construction, just that it weighs 6kg less than the counter weighted. The very next sentence says that cast ROCKERS were used... So who knows.

It never made any sense to me that there would be a special crank for the warmed over model given the scale of economies of using a standard forging.

One other interesting tid bit is that the T crank came along at the same time as the 906E really started development... It had almost vestigial counter weights to go with the light weight Ti rods.

NASCAR are 2" journals, right?

It must be officially said that Steve has done everything! Man is amazing .

t

[Flieger]

I must admit, like Chris, I have often wondered why the counter-weights are there if the flat-6 is "perfectly" balanced.

[Brant]

Originally Posted by Walt Fricke

Brant - you haven't broken off the end of the mainshaft with the 1st on it? A result of perfect, light right foot? When I realized my speed out of Spectator was the same using 1st and 2d (RPMs in 2d at upshift point on track were the same regardless of gear used in corner), I stopped using 1st except for starts, so I haven't really missed the tall 1st.

An 8,000 rpm motor is nice in terms of gearbox flexibility: the 5th doesn't top out even at Brainard (I think fastest straight I've driven) or Sebring, but is useful at HPR's longest straight and on the two long Miller straights. And 2, 3, and 4 are OK for Hallett.

How come I haven't seen you at the annual PCA race at HPR? As light as you have gotten that car (and with all metal body panels no less) it should get around well.

the F 1st is good for 200 - 225hp
I don't have a lot of hp.. just a measely 2ltr.
so no problems yet with 1st gear... and less weight with the 901

I've thought about coming back to a club race, but frankly I don't want to get tagged.. and the club races are a little physical from my perspective

plus a metal car with stock suspension points and no aero is just not a competetive GT class car in PCA. PCA doesn't really have a class for vintage cars. I'd have to cut all of the panels off and rebuild the whole car to be competetive.

PCA has kinda moved on past small bore stuff and doesn't have a class for old cars. The club isn't really interested much in air cooled, at least the air cooled stuff doesn't run competetively with the water cooled monsters. I doubt many 914's or 356's show up these days. I'd be a fish out of water with your guys. (or is that a land animal in water?)

[BURN-BROS]

Originally Posted by Flieger

I must admit, like Chris, I have often wondered why the counter-weights are there if the flat-6 is "perfectly" balanced.

I have talked to a Formula one Engineer that is familiar with the porsche cranks. He commented that on paper the counterweighted cranks need more counterweight. But due to the boxer layout it does not seem to pose a problem.

Porsche also drilled 6 holes radially along the axis to lighten the throws.

906 crank

[Walt Fricke]

Brant

Agree not too many 356s race in PCA these days.

But otherwise the days of the GT air cooled motors are hardly over. I hold my own just fine, and there are guys who are distinctly faster than I am with air cooled motors in my GT4 class. The fastest GT4 at Sebring this year was Tim McKenzie, who ran times in his 914 which would have put him about in the middle of the Cup Cars. Chris Musante, with an aircooled motor in a 1971 chassis (somewhat modified, to be sure), regularly declares his GT3 class car to be GT2 so he can be in the run group with Cup cars, and he runs right up front with them.

But you are right, there is no PCA class specifically for cars running to the race rules of yesteryear. We allow the old wine to be in new bottles.

[chris_seven]

Originally Posted by BURN-BROS

I have talked to a Formula one Engineer that is familiar with the porsche cranks. He commented that on paper the counterweighted cranks need more counterweight. But due to the boxer layout it does not seem to pose a problem.

Aaron, did he say why?