engine balancing - really?

I've been reading and working on auto mods since I bought a '66 Mustang in the early '70's - now I have a '90 XJ, which is a lot more similar than I would have first thought (inline six, unibody, aging electrics, etc.)

Every engine buildup I have read or done included engine balancing - getting the crank dynamically balanced, matching rods, and pistons. I also remember seeing engine photos taken through transparent oil pans that showed how much oil collects on the crank when running. Recently I was told an eye witness account from my boss - an interesting story. He was at an aquaintance's balancing shop, who demonstrated how two spoonfuls of oil completely threw off the high dollar balance job on a crank in the balancing rig. His statement was he wished people just wouldn't waste their money on it - it didn't make any power and kept him from more important work.

Considering how closely race engine builders are fitting crank scrapers, and how street engines don't use them but do have 4-5 quarts of oil flowing out the bearings at street speeds, I'm not sure I disagree.

Has anyone ever seen a real and scientific test to prove engine balancing is worth it for street engines? Maybe the factory balance is enough - not the machine shop's month end balance sheet?

In modern engines where the quality control of the components is very close, in a street engine (especially in a 4.0 where the crank harmonic limits absolute rpm to under 6K rpm) balancing is of little importance. I would weigh the reciprocating components as separate batches...if they are even within 10 grams or so, call it good. If the index is correct on the crank all should be good for a street motor.

I balanced the assembly in the mighty 2.5 because I turn it to very high RPM (not much of a crank harmonic problem).... and in my opinion, it would be very difficult to trap enough oil on the crank surface to invalidate a good balance job.

Doc Dyno added a windage extension to his pan on his stroker motor..should help some...

I thought that I6's were naturally balanced engines and didn't need as much balancing as say a V6, or I4

Depending on the make, an I6 engine is internally balanced - meaning that you should not need to add balance weights to the flywheel/flexplate or harmonic damper in order to achieve a balanced reciprocating assembly (in contrast, quite a few other engines are externally balanced, meaning that you'd have to balance them with the flywheel/flexplate and/or HD attached, depending on where the external balance weights are.)

I'd not be averse to balancing the RA of an engine that is going to see, or live at, high crankshaft speeds - that's just attention to detail. However, the typical I6 engine won't get bumped up past, say, 5000rpm - and will spend most of its life working under 4000. As long as the part weights match reasonably well (piston/rod/bearing assemblies within, say, 10 grammes or so - or individual parts matched to within 3-5 grammes) I don't see any reason to go farther, unless you're really trying to eke out every last little bit of power/torque, or you're someone like Barney Navarro (who ran an AMC I6 at Indy, I believe.)

The harmonic issue referred to seems to come up around 5800-6700rpm (reports vary,) but it goes away above that range, and I seem to recall BN running his up around 7500 without incident. It's a fairly narrow band caused by "natural harmonics" present in the long crankshaft, and I honestly think no amount of balancing will cure it - just don't stay in that range. If you're going to get up to speed, run right through the 6000-6500 range, and don't stay anywhere long. It is quite possible to snap your crankshaft!

I'd not recommend this with a production engine tho - just because QC may be good, I don't see it being that good. Why buy trouble when we get enough for free?

All engines require some sort of balancing - that's why there's material that can be readily removed on most parts ("balance pads," they're called.) The detail applied during balance is usually up to the builder/buyer, the intended application and service life of the engine, and what you're willing to pay...

5-90

get it balanced, for the cost it's worth the piece of mind.

RyleyF said:

I thought that I6's were naturally balanced engines and didn't need as much balancing as say a V6, or I4

Click to expand...

Harmonic balance versus individual reciprocating assembly component balance are two different things.

Harmonically an I6 is balanced but say, if each of the six connecting rods and/or pistons or whatever are not "identical" in mass you will have out of balance component vibrations occur.

I had a built 4.0L motor a few years back with head work, cam, intake, exhaust, injectors...and didnt ballance the rotating assymbly. It ran great and didnt have any problems BUT when I built my stroker I went and had everything balanced. The harmonic dampner, crank, rods, pistons, and flywheel. It made a HUGE difference. The motor runs super smooth with no vibes anywhere in the rev range. The old motor had some slight vibes around 4700rpm and just felt rough compared to the stroker which pulls all the way to 5200rpm smoothly with out hesitation. I wont build another motor with out balancing the rotating assymbly.

AARON