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Curious if anyone is running (or has ran) the early forged steel 6 counterweight crank. I have one as well as a later cast 12 counterweight crank and trying to decide between the 2 for an upcoming build on a street/strip car.
I'm thinking the lighter, forged steel crank would be the choice but I've read where the 12 weights can be an advantage as well.
What's your thoughts between the 2?

Last edited by Countn'Carbs; 01/30/18 01:44 PM.
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Fully counterweighted is better. If you're concerned about weight, I've lightened the fully counterweighted cast cranks by 12 lbs. in quite a few drag engines. Examples of lightening are shown in Leo's book.



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I have been told that depending on what level you are building your engine that they tend to want to turn into a corkscrew. I'm having a blown 292 being built by Gaerte racing engines and they are having a billet crank custom made by Crower.

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I had a sissal built short block that belonged to a friend. That motor ran into the 10s at the dragstrip. Used a later fully counterweighted crank. I have been told by some pretty sharp inline engine builders that while the early 6 counterweight crank is forged, it is not all that strong.

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Interesting ...I was digging around tonight and it turns out I also have a Nodular Cast Iron crank (6 weights '64-'66) that Leo talks about in his first book about maybe being "the sleeper" of the 3 choices due to it being lighter and stronger due to the wide crank arms of the shaft. I haven't checked what the second edition says yet though.
BUT...seems like the later 12 weight is the choice of many (if not all) which says a lot.

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I would use 12 weight crank and not look back. We had had really good luck with them. I know someone that used a 6 weight in a turbo performance engine and it will not rev good. We are guessing its twisting or harmonics or both so I'd stay away.

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Mighty6, do you know at what rpm or rpm range the suspected twisting occurs? Please share if you can.

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Strokersix, not really sure on the 6 weight crankshaft if that's what you are asking. My guess 5000-6000 rpms because they were never designed to spin that high. Our 12 weight crank starts at 6600ish and revs to 75-7600 because we can see it on the data logger.

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Thanks. If I understand your observation, the 6 weight seems to have torsional trouble somewhere between 5000-6000 rpm. The 12 weight seems to have trouble 6600-7600 rpm. Correct?

What data are you logging that shows this? Crank trigger?

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Yes, that would be my guess because the two engines were built the same and only difference was the crank. The 292s are problematic for harmonic issues anyway. Yes, we log all the engine functions.

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Thanks for sharing first hand experience.

My stroker is a trimmed 12 weight crank and light weight pistons. 2.0 crankpins and destroked to 4.062 to fit in a 250 block.

I don't think my crank stiffness changed much from stock but crank and piston mass are certainly less. I would expect the net effect to be to raise the natural frequency. Combined with your observations gives me some confidence. No data to back this up other than my engine is still together and my flywheel bolts are tight.

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I can't tell you the actual stiffness but I wrote a small .xls app that calculates actual journal overlap area (which affects stiffness). The traditional calculation [(main journal + rod journal - stroke) ÷ 2] only gives the length of the shared material.
Stock 292 2.30" main, 2.10" rod, 4.12" stroke, yes?
Stock overlap area .0725 in.2
New crank mod 2.30" main, 2.00" rod, 4.062" stroke. Area: .0561 in.2

By comparison, the stock 250 crank is .3202 in.2.

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It would be interesting to measure some actual cranks. Seems it would be easy to make a static test rig. Add to future project list. Thinking torsional stiffness but I suppose you could cantilever off the rear flange and measure bending stiffness of the whole crank too.

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L6 engines generally resonate at frequencies between 200 and 300 Hz (source: Heldt). These orders occur throughout, and well above the maximum RPM of the 292 crankshaft.
The engine speeds for these orders can be calculated from the resonant frequency, where “Hz” is the crankshaft’s resonant frequency, and N is the order number, using this formula:
RPM = Hz × 60 ÷ N
Easy to try, results may be difficult to evaluate:
1. suspend either crank by a loop of wire/ rope etc. through a flange hole.
2. strike a counterweight with a brass or lead hammer (not leave a mark).
3. with microphone pointed at it.
4. wired to an oscilloscope.
5. if a single strong trace appears from several trials, it will show the harmonic frequency.
6. the 2 cranks should show different numbers, for an L6 probably between 200 and 300 (V6, V8 will be much higher).
7. the higher frequency is stiffer and will experience a specific harmonic order at a higher RPM.
8. the actual frequency determines where a dangerous order (the 2nd, 2-1/2, and 3rd) will occur.
9. for example, if 237 Hz is detected, the (worst) 3rd order is 237 × 60 ÷ 3 = 4,740 RPM. The 2nd order is 237 × 60 ÷ 2 = 7,170 RPM. The 2-1/2 order is 237 × 60 ÷ 2-1/2 = 5,688 RPM.
10. the other dangerous orders for an L6 are the 6th at 2,370 and 9th at 1,580, but they're much weaker. The 1st is very strong but far away at 14,220.

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Thanks for sharing panic - I love the engineering lessons in your posts. Leading to a couple of questions:

(1) Is the resonance frequency and corresponding critical RPM range(s) more important than journal overlap?

I found your tech paper instructive on this topic. But my memory was leaving me confused by the conclusions drawn about SBC cranks.

I went digging through my library and sure enough in the old Grumpy Jenkins speed manual he discusses at length the retrofitting of the earlier cranks to new blocks:

"In our drag racing engines we use the early smallbearing
327 cranks and a spacer sleeve is required
between the late block bearing saddle and the small
journal bearing. This crank is an especially stiff forging
and the smaller bearing diameter reduces bearing
speed. Adapting this early crank to the late case is
much simpler than it sounds. It is, in fact, extremely
easy. To sleeve down the block we install the late
(large diameter) bearing shells in the saddles and pin
them in place with small roll pins as can be seen in
the photos. These pinned bearings/sleeves are then
align honed to the Chevy recommended saddle diameter
for the early (small diameter) bearings."

(2) If the later cranks with larger journals/overlap are stiffer - was Jenkins trading crank flex for reduced bearing speed?

(3) Was this trade-off specific to his drag application - several seconds at very high RPM?

Curious . . .

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2. yes, they are, but the V8 crank (and block) are inherently much stiffer (less subject to bending and misalignment) than an L6.
3. IMHO yes, very smart guy, the choice for NASCAR might be the other way

I don't think that it's an accident that some of the most successful turbo engines developing extreme power (1,000+) with the original block and crank have very stiff cranks:
1. Buick OHV 231 V6 (GNX etc.) - 4.24" bore pitch
2. Toyota 2JZ-GTE - it's an L6, but the crank (and block) is extremely short - 3.622" bore pitch with 3.386" bore!
3. Nissan GTR 3.8 V6 - 4.409" bore pitch

Crankshaft length is critical to stiffness (although this can be somewhat offset with very large journals), which is why L8 engines were never successful with any power - too flexible.
For comparo, the length of a crank (#1 to last main bearing) is roughly the bore pitch X number of cylinders in one bank, + 1 rod width if a "V" engine.
You can see immediately that a V4 would be best (but very limited displacement), V6 next, then L4, V8, L6, V12 etc.

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Excellent post.
One suggestion, for 3/4; you can use FrequenSee or similar app on your smartphone instead of a microphone and oscilloscope.


Originally Posted By: panic
L6 engines generally resonate at frequencies between 200 and 300 Hz (source: Heldt). These orders occur throughout, and well above the maximum RPM of the 292 crankshaft.
The engine speeds for these orders can be calculated from the resonant frequency, where “Hz” is the crankshaft’s resonant frequency, and N is the order number, using this formula:
RPM = Hz × 60 ÷ N
Easy to try, results may be difficult to evaluate:
1. suspend either crank by a loop of wire/ rope etc. through a flange hole.
2. strike a counterweight with a brass or lead hammer (not leave a mark).
3. with microphone pointed at it.
4. wired to an oscilloscope.
5. if a single strong trace appears from several trials, it will show the harmonic frequency.
6. the 2 cranks should show different numbers, for an L6 probably between 200 and 300 (V6, V8 will be much higher).
7. the higher frequency is stiffer and will experience a specific harmonic order at a higher RPM.
8. the actual frequency determines where a dangerous order (the 2nd, 2-1/2, and 3rd) will occur.
9. for example, if 237 Hz is detected, the (worst) 3rd order is 237 × 60 ÷ 3 = 4,740 RPM. The 2nd order is 237 × 60 ÷ 2 = 7,170 RPM. The 2-1/2 order is 237 × 60 ÷ 2-1/2 = 5,688 RPM.
10. the other dangerous orders for an L6 are the 6th at 2,370 and 9th at 1,580, but they're much weaker. The 1st is very strong but far away at 14,220.


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Thanks.
Alternative: if you have a tone generator, attach a few long thin wires to the suspended crank, and point the speaker to it. Dial across the 200-300Hz range slowly, and watch the wires. When they begin to buzz, that's your frequency on the dial.


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