WEEK­END WORK­SHOP

GRANT MUNRO FROM ROTORSPORT RAC­ING & ENGINE BAL­ANC­ING SHOWS US EX­ACTLY WHAT GOES ON WHEN BAL­ANC­ING THE IN­TER­NALS OF THE ROTARY ENGINE

NZ Performance Car - - Contents - WORDS AND PHO­TOS: MAR­CUS GIB­SON

When you think about the speed in rpm at which your engine in­ter­nals are mov­ing, both in a ro­ta­tional and re­cip­ro­cat­ing (for pis­ton en­gines) or ec­cen­tric (for rotary en­gines) mo­tion, while you’re hard up the loud pedal, it’s some­what mind­bog­gling to think they can see up­wards, and some­times in ex­cess of, 10,000rpm, es­pe­cially when we’re talk­ing rotary en­gines. The bal­ance of th­ese com­po­nents is in­te­gral to al­low­ing th­ese types of rpm and not have your engine com­pletely de­stroy it­self — think of a wash­ing ma­chine on fast spin, with a cin­der block in­side.

When deal­ing with high rpm, even the slight­est vari­a­tion in weight be­tween any parts in the ro­tat­ing assem­bly can cause vi­bra­tions. Any vi­bra­tion in the engine is do­ing dam­age. So ex­actly how do you en­sure your in­ter­nals are bal­anced per­fectly, and why should you have some­thing bal­anced even if it’s from a fac­tory engine? We spoke to Grant Munro of Rotorsport Rac­ing & Engine Bal­anc­ing to learn the art. Grant has been bal­anc­ing en­gines since his teenage days grow­ing up in his dad’s shop. Bal­anc­ing in­ter­nals is a very spe­cial­ized task that re­quires tools you won’t find in most back­yard work­shops. In fact, many top engine builders send their stuff to Grant. The en­tire assem­bly sits on two cra­dles with hy­per­sen­si­tive sen­sor plates on which it spins. The soft­ware then cal­cu­lates the vari­a­tions in weight and can tell you ex­actly where on the assem­bly the weight needs to be re­moved or added. Just how sen­si­tive? We’re talk­ing .01 (one hun­dredth) of a gram.

The first stage in bal­anc­ing be­gins with the ro­tors them­selves. Grant tells us it’s not un­com­mon to find a ‘match­ing’ pair be­ing up to 50-80 grams out de­pend­ing on the fac­tory weight of the ro­tors. Mod­ern RX-8 ro­tors aren’t bal­anced at all from fac­tory.

The process be­gins by dy­nam­i­cally balac­ing the ro­tors. To do this, each ro­tor is weighted side-to-side, be­fore stat­i­cally balac­ing each one on a spe­cial jig. The heav­ier of the two ro­tors is then light­ened by re­mov­ing ma­te­rial evenly from the sur­face on the side of the ro­tor be­tween the side seals and the oil-con­trol seals. This is com­monly done with a drill — hence the in­dents you’ll see on any ro­tor near each apex. The CWT bal­ancer will tell you ex­actly where the ma­te­rial needs to be re­moved from in or­der to achieve a bal­anced ro­tor. This process can be very time-con­sum­ing, as you’re con­stantly hav­ing to set the bal­ancer up, then re­move the ro­tor to ma­chine it, and then recheck. It’s this process that soaks up the hours, es­pe­cially when you’re work­ing to Grant’s .02g tol­er­ance.

Once you have two identical ro­tors, the next stage is bal­anc­ing the rest of the ro­tat­ing assem­bly to suit the weight of the ro­tors. This is done by set­ting up the ec­cen­tric shaft with ev­ery­thing from the front pul­ley, to the oil-pump drive, and front and rear coun­ter­weights. The ro­tors them­selves sit this part out, and are sub­sti­tuted with a pair of cir­cu­lar weights known as bob weights. The rea­son the ro­tors them­selves are not used is be­cause they are ac­tu­ally miss­ing some of the mass that they will have once your engine is as­sem­bled and run­ning. Each bob weight al­lows Grant to eas­ily add weight to com­pen­sate for the seals, springs, and even down to things like the oil that will be in­side a ro­tor when run­ning. The bob weights sim­ply slide in place and are held there by a small grub screw in the oil pas­sage. This assem­bly is then spun up, and the com­puter cal­cu­lates the weight dif­fer­ence be­tween front and rear.

The ad­just­ment comes from adding or re­mov­ing weight from the front and rear coun­ter­weights. The front coun­ter­weight co­in­cides with the rear ro­tor, and vice versa. Un­like bal­anc­ing the ro­tors — where Grant picked the light­est and sim­ply brought the heav­ier one down to suit — in this case, weight some­times needs to be added. In the case of our 13B assem­bly, the bal­ancer showed that we needed 10g added to the front. The pro­gram will tell you ex­actly where on the du­ra­tion of the coun­ter­weight to add this. Grant uses a su­per-heavy metal known as Mal­lory, which he im­ports from the USA in a rod form, spe­cific to the pur­pose. It weighs 50g per inch, which is twice the den­sity of a mild-steel vari­ant. A hole is drilled in the side of the coun­ter­weight and the Mal­lory slug is pressed in. As the force act­ing against it when the engine is run­ning is cen­trifu­gal, there is no way the Mal­lory can dis­lodge and fire out like a bul­let.

Once back on the bal­ancer and re-spun, the rear coun­ter­weight needed a slight cor­rec­tion, this time to lose weight, which is a much more straight­for­ward exercise. Grant sim­ply sanded the edge of the coun­ter­weight at the point that the CWT had iden­ti­fied. Back on the bal­ancer for a third time, it re­quired a lit­tle more re­moved from a dif­fer­ent spot. This time it was a greater amount of mass, so it was drilled from the outer edge of the coun­ter­weight. As the bal­ancer is set up on the mill, this was done in place. The ex­cess weight is re­moved from the out­side edge, as this is the area that will have the great­est im­pact. The closer to the cen­tre of the assem­bly, the more ma­te­rial you would have to re­move to achieve the same re­sult. We had to re­peat this process a few times to bring the assem­bly in­side the .02g tol­er­ance Grant works to, although up to .038g is an ac­cept­able tol­er­ance.

The fi­nal stage was bal­anc­ing the flywheel and pres­sure plate. We in­stantly jumped from .02g to 2.9g af­ter adding th­ese items. This time, weight is re­moved from the pres­sure plate to bring it back down, by drilling into the face. In this case, be­ing an al­loy Quar­ter Master, it re­quired a fair amount of ma­te­rial to be re­moved to bring it back within tol­er­ance. So there you have it — the com­plete ro­tat­ing assem­bly, ready for the engine builder to piece to­gether with the knowl­edge that it will hap­pily spin away at ex­treme rpm with­out fear of it tear­ing the engine from the mounts and de­stroy­ing the in­ter­nals. Yes, it’s an added cost at the engine-build stage, but if it’s not done, or worse, done in­cor­rectly, it could cost you your engine in a very short space of time and land you a big re­pair bill. Do it once, do it right.

6 This process can be very time­con­sum­ing, as you’re con­stantly hav­ing to set the bal­ancer up, then re­move the ro­tor to ma­chine it and, then recheck. It’s this process that soaks up the hours, es­pe­cially when you’re work­ing to Grant’s .02 tol­er­ance

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