Crankshaft Balancing?

[4point6]

Wondering what everyone's thoughts are on crankshaft balancing for a stroker. I have heard "yes do it" and "don't need to cause its an inline 6". I plan on using 258 rods with dished 2229 pistons.

[1bolt]

I've never understood why the I6 is supposed to be naturally balanced, to me it's one of the roughest running engines I've ever owned. Including everything from 1.8L toyota 4 cyl's to 500 cube diesel engines. Chrysler and GM big blocks, small ford V8's of every generation since 1965 (289, 302 and 4.6 modular). The Inline sixes I've owned are easilly the LEAST smooth, the I6 in my first mustang was a real thumper, the two 4.2's I've owned were the thumpyest vibration filled motors of any new or all worn out. And the 4.0's are all a close second. The smoothest I've driven in terms of perception was the ultra rev happy 4.6 SBF DOHC 4 valve.

The only thing I've driven rougher than my I6's was a Detroit deisel in Freightliner Semi

[jsawduste]

Well I cannot make comparisons to how rough an I6 vibrates and I can show by a few examples.

1. On common is is the harmonic balance rotating on the rubber ring. This has been an issue with the Hesco crank triggers uses the HB instead of the flywheel.

2. Those whom have switched to the MORE (for instance) engine mounts have noticed an increase in vibrations. Primarily because of the harder durometer of the isolator within the mount itself.

3. While the I6 crank is a far bit longer then a typical V8 it is however well supported. Perhaps a non issue but that long crank can lend itself to quite a bit of whip.

4. Hesco among others takes great pains to control; engine harmonics in any of there builds that will see higher then normal RPM.

Having said this. The only way to insure you are doing the balance job correctly. Is to have not only the crank. But the HB, flywheel and clutch pressure plate (if used) dynamically balanced as a whole.

Remember we are changing the weight of the rods/pistons and pins from what was likely a factory weight. So it stands to reason that the crank may need to rebalanced on that fact alone.

When my engine was balanced not only was it done dynamically but even the pressure plate was indexes to the flywheel. To ensure it was reinstalled in the same orientation upon final asm.

The engine runs smooth at idle despite and very noticeable lope. At speed there are no transmitted harmonics from the engine itself.

FWIW a very wise idea when building up your stroker.

[Shark]

this all sounds like your trying to build some sort of swiss watch. this aint no BMW this stroker is going in. i know that the added vibes and friction will hurt HP/TQ numbers but i feel that if you get a crank out of a motor that wasnt abused youre doin OK.

[seanyb505]

I think Im going to have it all balanced just for peace of mind.

[Shark]

for my first stroker im going to save money where possible. that includes leaving the crank alone and hoping for the best.

[mecompco]

The only way to insure you are doing the balance job correctly. Is to have not only the crank. But the HB, flywheel and clutch pressure plate (if used) dynamically balanced as a whole.

Stupid question: How is this accomplished--with the parts installed in the engine? Thanks!

[Shark]

im assuming thats done without the parts installed in the motor.

[John]

Yes take the rotating assembly to a reputable shop and tell them the rpm range you will be operating in, I saw a Harley v twin balanced for high speed operation that was horrible in its operating range, Harmonics are funny things.
John

[tirod]

Frankly, engine balancing and natural vibration from harmonics are two completely separate issues. Even a slapped together V8 has better harmonics by it's design than an inline six. It's a natural result of the eight cylinders in a 90 V firing and layout vs an inline firing at 120 degrees. Inline sixes will always have more.

For the low rpm application, checking that the engine balance falls in the (wide) range of factory specs is good enough. Doing the half gram per part or rod end, etc is a bit much, and is not necessarily money well spent on a simple econo motor.I see it as beyond the point of diminishing return. I note that nosigma doesn't run his 6 much past 5750 rpm because of crank vibration issues. I remember the Ak Miller days of hot rodding my 200 Ford six, and that limit existed then, too. I don't think closely balancing the motor will help much when the crank itself has torsional limits and comes apart when overloaded.

Got 181k on my 90 xj, and since a cobra kit in not sitting in my nonexistent shop, I'm being practical with my limited funding and dreaming of a proper rebuilt stroker (which will have to happen in the next 50-70k the way this one is running.)

[1bolt]

Got 181k on my 90 xj, and since a cobra kit in not sitting in my nonexistent shop, I'm being practical with my limited funding and dreaming of a proper rebuilt stroker (which will have to happen in the next 50-70k the way this one is running.)

Interesting I wonder how much the I6's "harmonic" at 5750RPM's could be mitigated by lightening and Balancing? I mean really removing some material from the throws and grinding down stress risers everywhere else? I wonder if Knife edging a crank could reduce the high RPM stresses somewhat. Then shot peening and cryogenically treating it.... Wonder if you can do both with any additive benefit?

I couldn't agree more with the Cobra Kit comment... I started Hod rodding my XJ because that's what I've got, I can have fun with it, or I can not have fun with it. Expensive to maintain, expensive to insure more traditional "hot rod" or sports car projects are out of the question for me for a while.

Making the most of the I6 is something that starts to get under your skin, as with anything it's partly realistic needs and partly "but what can I do within these constraints".

With any luck some day I'll be back to working on and Hot rodding classic mustangs which are just as much in my blood as Jeeps... But WAY more expensive. Before then I hope to have a couple very hot I6's under my belt... All of them done on the cheap. And putting down serious power.

[John]

It is a camshaft harmonic at 5600rpm that can break the timing chain, not the crankshaft that is the issue on our motors, knife blading, removing stress risers will not address this, but removing those stress risers is a good idea for a built to the wall motor anytime.
John

[jeepinwi]

I did not have my rotating assembly balanced, and it was no rougher than any 4.0 i have driven. I now have the engine apart because of a leaking head gasket. It going back together with a new harmonic balancer, so we'll see how smooth it is once back together.

[John]

1bolt if you are interested in high revving Jeep I6 and the issues limiting the design, check out this thread. http://www.hesco.us/forum/forum_posts.a ... W=5600+rpm
John

[1bolt]

Interesting I missed that thread somehow, I've read others there that were bogged down with Dino (Cheromaniac) and Lee arguing...

You know when I said harmonic I must have been thinking of the issue the crank has that Lee in a different thread described as "tearing it self apart"... But now I can't find that thread on there. even searching the old archived posts. I read the HESCO boards enough that I should have known it was a Cam harmonic and not a Crank one.

Suffice to say that our I6 isn't really meant to spin much past 5000 RPM's, and even solving the Cam harmonic there are other issues.