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Heavy or light crankshaft for 4.5 build ?

Performance mods and Advanced Stroker discussion.
GoatBoy4570
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Heavy or light crankshaft for 4.5 build ?

Post by GoatBoy4570 » July 5th, 2020, 4:01 am

im working on a 2000 4.0 for my 83 Cj-7 and i'm debating about which 4.2 crank i should use:

I have both a heavy (4723) and a light (3727) crank on hand.

I would like to hear what the general consensus is these days about which to use, my Jeep gets about 50/50 on/off road use.

I would think the heavy cranks are fairly rare now so there may be people who have a opinion on running the light crank and wishing they had the heavy version, perhaps the difference isn't that noticeable.

Additionally i am planning on using a Lunati 213/220 .500ish lift cam and i have somewhat tall gears so that could favor the heavy crank more.

I guess my main concern is acceleration, does the heavy crank motor still get up and go ?

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Re: Heavy or light crankshaft for 4.5 build ?

Post by jsawduste » July 5th, 2020, 7:30 am

Decent cam but you don't need the .5 lift. Head doesn't flow enough to warrant the (potential) extra work for that amount of lift spring wise.

50/50 ? What's the trail portion ? Slow speed technical or simple fire roads ?

I run a cam that is similar but has 480 lift and a bit more duration. 37`s and 5.38`s. Gearing that is deeper than most and use the heavy crank. The transmission has fairly wide gear spacing (NV4500) and the extra crank inertia helps. Truly believe that extra inertia helps off road and on oroad.

Not knowing how you want to use the Jeep off road and the overall gearing/transmission/tire size its hard to give a good response.

GoatBoy4570
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Re: Heavy or light crankshaft for 4.5 build ?

Post by GoatBoy4570 » July 5th, 2020, 9:52 am

Logging roads mainly, a bit of trail use getting in and out of spots by rivers and lakes, some road washouts and then there's some bushwacking or trench crawling when getting round gates. So pretty easy with some challenges. Its more of a hunting rig than anything Tires are just 33's and gears are 3.31's trans is T5 w/300

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Re: Heavy or light crankshaft for 4.5 build ?

Post by GoatBoy4570 » July 5th, 2020, 10:07 am

The way i look at high lift especially with a flat tappet is that if a head flows well at a certain lift aways cam higher lift than that because the valve is only at max lift for a split second but flow is still happening quite well at say 0.100" on either side of that peak lift, if your head flows well at 0.480 you really need to go to 0.520 to get any real open curtain time at that 0.480" flow point. Your cam for example is taking full advantage of flow in the 0.450-0.460 range, high lift also helps maintain area under curve with short duration cams.

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Re: Heavy or light crankshaft for 4.5 build ?

Post by IH 392 » July 5th, 2020, 11:16 pm

The only reason they went away from the heavy crank was for cost of materials savings.
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Re: Heavy or light crankshaft for 4.5 build ?

Post by KarmaKannon1 » July 6th, 2020, 1:02 pm

From talking with Russ it sounded like there was a benefit to the heavier crank because it helped with what I assume was balance or harmonics. I think he mentioned something about how a crank in general works better if it's heavier. He'd be the guy to call and ask about that and I think he offers both heavy and lighter scat cranks so he should have some comparison info.

I believe the 3727 is the serpentine style snout and I don't know about the other crank you mentioned. Just something to keep in mind.

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Re: Heavy or light crankshaft for 4.5 build ?

Post by Tolemar » July 24th, 2020, 10:57 am

I specifically went with the heavy crank. imo the benefits out weigh the lighter crank.

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Re: Heavy or light crankshaft for 4.5 build ?

Post by Russ Pottenger » July 29th, 2020, 11:39 am

IH 392 wrote:
July 5th, 2020, 11:16 pm
The only reason they went away from the heavy crank was for cost of materials savings.
This would be my best guess also.

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Re: Heavy or light crankshaft for 4.5 build ?

Post by Number21 » August 10th, 2020, 2:08 pm

Tolemar wrote:
July 24th, 2020, 10:57 am
I specifically went with the heavy crank. imo the benefits out weigh the lighter crank.
Dumb question: What are these benefits?

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Re: Heavy or light crankshaft for 4.5 build ?

Post by Cheromaniac » August 10th, 2020, 11:44 pm

Number21 wrote:
August 10th, 2020, 2:08 pm
Tolemar wrote:
July 24th, 2020, 10:57 am
I specifically went with the heavy crank. imo the benefits out weigh the lighter crank.
Dumb question: What are these benefits?
It isn't a dumb question. The heavier crank has 12 counterweights instead of 4 and thus dampens more harmonic vibrations for a smoother running engine. The extra rotational inertia also makes the engine less likely to stall when rockcrawling at low rpm.
The lighter crank is more suitable in racing applications where weight saving is advantageous.

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Re: Heavy or light crankshaft for 4.5 build ?

Post by Number21 » August 12th, 2020, 2:01 am

Interesting, I just assumed the lighter was better to have less rotational mass. Do aftermarket cranks address this? They tend to advertise lighter = better.


What is better on the street? Like normal traffic, not rock crawling, not racing. I'm trying to put together a stroker that is as "OEM" as possible.

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Re: Heavy or light crankshaft for 4.5 build ?

Post by GoatBoy4570 » August 12th, 2020, 9:21 am

Ive thought about this since posting the question a month ago , in the meantime i came across a question on an unbalanced flywheel and the vibration that that imposed and it led me to do some calculations and thinking about rotating weight in or attached to an engine.

I think the flywheel plays a much greater role in terms of storing and releasing energy than the crank does, and i think crankshaft counterweighting plays a huge role in controlling torsional oscillation/frequency.

So weight/mass when rotating is dependant on rpm and distance from centerline and a doubling in weight creates a quadrupling of the lbs.ft. forces involved, its exponential.

Hence a flywheel with lots of mass at it perimeter makes a great energy storage device and a crankshaft with its weight not nearly as far from the centerline ... not so much, not nearly as efficient in this regard compared to the flywheel.

So that leave the question; Heavy or Light crank ?

I think a heavy crank has some real benefits in terms of engine performance, i think a properly counterweighted engine is easter on bearings and all components within the engine, t-chian, valve train, pistons etc... damping and controlling crankshaft frequency is critical and i think AMC did the 4.2 and 4.0 a huge disservice when they lightened up the crank.

Driving on the street, acceleration wise i think you will notice way more from flywheel mass, and that crank weight is more of a negligible secondary factor.

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Re: Heavy or light crankshaft for 4.5 build ?

Post by jsawduste » August 13th, 2020, 6:10 am

Its simple physics.

As you have already figured out. The further the mass is from the centerline the delta of its effects increase exponentially.

The mass of the crank itself will change the harmonic frequency range either up or down depending on a variety of factors beyond simply counting on weight.

One would be simple torsional rigidity. There are some engines where the actual combustion event is programmed to change as a factor of crank twist.

Point being is that there is no hard and fast rule that says adding mass will create a better or worse harmonic. We can generalize and accept that in our particular case the difference in harmonics between a 4cwt and a 12 cwt is minor in the RPM range the engine typically see`s.

What we do see is a change in the amount of energy required to change the state the crank is in. That is, it takes more to move it and once moving it takes lot more to slow it down.

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Re: Heavy or light crankshaft for 4.5 build ?

Post by GoatBoy4570 » August 13th, 2020, 8:18 am

jsawduste wrote:
August 13th, 2020, 6:10 am
Point being is that there is no hard and fast rule that says adding mass will create a better or worse harmonic. We can generalize and accept that in our particular case the difference in harmonics between a 4cwt and a 12 cwt is minor in the RPM range the engine typically see`s.
I would argue that there must be a hard and fast rule regarding placement of mass and a corresponding frequency response with any rotating assembly, you say it yourself when you describe an engine that controls this phenomenon in the combustion process, that engine design knows precisely when torsional vibration is occurring and is modifying/utilizing that wasted energy with combustion timing ?

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Re: Heavy or light crankshaft for 4.5 build ?

Post by jsawduste » August 13th, 2020, 8:58 am

No, really don't want to overthink this subject nor do I wish to argue.

You're thinking that both crankshafts are made of the same material and exhibit comparable dynamic functions. I do not know if they are made of the same material nor do I know if the added mass further away from the centerline has any effect on the torsional properties.

Chrysler, Mopar, Daimler, FCA whatever you want to call them now decided that they could make the cranks lighter with no ill effects on NVH or reliability.

The combustion process is another aspect far removed from the subject at hand.

FWIW the primary damaging harmonics has been identified as actually being in the camshaft. With a resonant frequency that seems to peak in the 5300 +/- rpm range.

I also vehemently disagree with you're personal analysis on connecting rods. Unless you know the metallurgical properties a simple eyeball test leaves plenty to desire. What if one rod was made of 1018 and the other of 4340 to just throw a couple numbers out there ? I`ll take a skinny 4340 over a fat 1018.

How about main caps ? Early solid vs. later model scalloped designs. Would you say the solid is stronger than the scalloped by simply looking at them ?

I`d at the very least would want to do a hardness test and even that wouldn't tell me a whole bunch.

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