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Heavy crank vs lightweight crank

 
Old 04-01-2013, 04:27 PM
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Smile Heavy crank vs lightweight crank

All things being equal, what characteristics would a heavier crankshaft have in an engine compared to a lighter crankshaft of the same design?

Application: Jeep 4.0L stroker engines.

The Jeep 4.0L has a 3.875" bore and 3.411" stroke. Take an older AMC 4.2L crankshaft and stick it into the 4.0L block and you have a 3.875" bore, 3.895" stroke 4.5L inline-6. Bore 0.030" over it's a 4.6L, bore 0.060" over it's a 4.7L...

The AMC 4.2L 3.895" crankshaft comes in 2 main designs, a heavy 12 counterweight 66lb version in the 70's and early 80's, whilst the lighter 4 counterweight 46lb version is from 81-86 and newer.

Some say the heavier crankshaft is good for rock crawling, low end torque and has an easier time staying balanced. Some say the lighter crankshaft is good for racing engines, boost and high revvs.

What do you say?

Last edited by SynergyV8; 04-01-2013 at 06:03 PM. Reason: Listed stroke wrong
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Old 04-02-2013, 10:05 AM
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I had to make a decision similar to this as I planned out my new engine. My plans were for 8,000 RPM and the choice was between a $2,000.00 Callies crank, or a $900.00 Eagle crank. One about 43 lbs. and the other 55 lbs. I decided for the 55 lb. crank and to spend the money on aluminum flywheel. In the end I'll be within a couple pounds of the more expensive crankshaft.

So, are you planning on high RPM usage, or heavy duty usage? While a 66 lb. crankshaft is heavy, I think the lighter crank can do the trick if it is something like nodular iron, or forged. Are you planning on turbo charging the engine, turning high RPMs, or subjecting it to large torsional loads? Call a couple crankshaft firms and get their opinion. Maybe all you need to do is get the lighter crankshaft checked for micro stress cracks magnafluxed. You can also take a grinder to the heaver crank to knife edge the leading edge of the counter weights to lighten it up a bit. Or, spend a few hundred dollars to have a good machine shop take a few pounds off of the counter weights.

Do any of the Jeep forums have any feedback?
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Old 04-03-2013, 06:48 PM
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Originally Posted by Gregory View Post
Do any of the Jeep forums have any feedback?
The JP forums are littered with opinions but very few facts so I brought the question here.

This engine will rarely see above 3000rpms, let alone red line. It's for my wife's Jeep Cherokee. 3" lift and 33's, daily driven every day. I ended up choosing the heavier 66lb 12 counterweight crankshaft mostly due to cost and availability, but wanted to hear the tech that some of the greater minds on this website are known for.
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Old 04-03-2013, 09:49 PM
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If your jeep is an auto get the lightest flywheel and rotating assembly you can. If it's a stick you will want higher moment of inertia to help you crawl rocks, make it easier to start/stop.
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Old 04-04-2013, 04:54 PM
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A good example is the older Honda B18C GSR and Type R. The GSR was more of the street version with a NON fully counterweighted crankshaft. The GSR had a rev limit of ~8200 but was not designed to be subjected to sustained 8k plus rpms. The Type R was the track version with a fully counterweighted crankshaft that weighed 4lbs more. The Type R had a ~9k rev limit and was designed to be subjected to 8k plus rpms all day long. Long story short, more counterweights equal more weight, but better engine balance with less harmonics.
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Old 04-04-2013, 07:09 PM
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Originally Posted by SuperSport01 View Post
A good example is the older Honda B18C GSR and Type R. The GSR was more of the street version with a NON fully counterweighted crankshaft. The GSR had a rev limit of ~8200 but was not designed to be subjected to sustained 8k plus rpms. The Type R was the track version with a fully counterweighted crankshaft that weighed 4lbs more. The Type R had a ~9k rev limit and was designed to be subjected to 8k plus rpms all day long. Long story short, more counterweights equal more weight, but better engine balance with less harmonics.
That's true, but these are jeep cranks were talking about. I had an XJ once, the stock rev limit is 5000rpm.

Assuming the whole rotating assembly is properly balanced, there should not be a difference between the light and heavy crank at the rpms he will be seeing. The only difference is going to be related to moment of inertia, with a clutch, trying to crawl rocks, you want that extra inertia.
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Old 04-05-2013, 02:16 AM
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Originally Posted by 409CISecondGen View Post
If your jeep is an auto get the lightest flywheel and rotating assembly you can. If it's a stick you will want higher moment of inertia to help you crawl rocks, make it easier to start/stop.
i'm a "wheeler" and this guy ^ kinda has it right.

if it's an auto tranny rig you will want the lighter rotating mass because the torque converter is the torque multiplier and there is no need to store kinetic energy via the higher mass of the rotating assembly, it's just a waist of fuel to try to spin up to rpm again. that is why the later years went lighter to save fuel in the epa mpg.

if it is a manual tranny it's a balance of vehicle weight, axle gearing, tire diameter and mass, and number of gears [speeds] of the tranny, and finally the power of the engine.

a little four banger with a 80lb flywheel will move a truck around... but not very quickly.

a big block big ci engine could get away with a 30lb flywheel and it would get around quick.

I have a feeling that it's an auto tranny and with bigger tires you would probably have been better off with the lighter crank and ended up with better throttle response driving around.

btw, the hybrid 4.0 block/ 4.2 crank is very popular in the off road jeep world. good move on a street jeep too!
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