93BlackFD3S

Can you guys tell me if you have any NVH when you free rev past 6k rpm? I am getting a little bit of vibration that I don't think I had before when the engine was stock. I rebuilt the engine and had the rotating assembly balanced with a bunch of other mods such as porting, poly motor mounts, clutch and flywheel etc... Wondering if it's just the stiffer mounts (FFE w/street bushings) or if my ACT flywheel isn't balanced well.

Please make sure you mention if you have aftermarket mounts.

DaveW

Generic comments... Any change in stiffness can make an imbalance more (or less) noticeable, and any imbalance will tend to be more noticeable at higher RPM's. So there might be a tiny bit of out-of-balance in the assembly - note that it is impossible to have absolutely zero imbalance, but you obviously try to minimize it.

Howard Coleman

iTrader: (6)

agree w Dave as it will be more noticeable w solid mounts. however an internal imbalance situation in your motor is generally linear w rpm and you mention 6k. doubt it is the flywheel. hate to say it but in my opinion the most probable culprit is the "balance" job. there are only a few that can properly balance a rotary although many provide the service...

unless you plan to spend a lot of time on the other side of 9 it is not necessary and in many cases a backward step. varying amounts of oil fill your rotor when the engine is running so you could have perfectly balanced rotors and due to differing amounts of oil have them imbalanced. i do hope the answer lies elsewhere.

R-R-Rx7

iTrader: (24)

The FFE with the 68A inserts will not add any vibrations.

fendamonky

iTrader: (6)

Depending on who you paid to balance your rotating assembly, I'd say that's your culprit. I paid Chips to balance my RA years ago during a very expensive rebuild. My engine shook so bad it rattled my oil lines loose, destroying the motor in about 4,000 miles. I was so pissed that it happened after just dumping ~$15k into the build that I just put the entire car on mothballs.. that was 11 years ago, I'm just now getting back to putting her back on the road. Turns out, the "balance" was never even close to accurate.

R-R-Rx7

iTrader: (24)

The machined “lightened” stuff does not mean balancing even though people very often confuse the 2 and i dont mean that about you. He even markets them as such. I have never seen any actual balance marks from chips so i suppose they only look “pretty “ lol.
but for what it’s worth, Stock balance is plenty for 95% of the people out there. Not really needed unless the engine gets revved at high rpm.

fendamonky

iTrader: (6)

Yup, lightening and balancing are two separate services. I paid for both, which is what pissed me off so much.

The shop that eventually took over making my engine right pointed out that there almost wasn't enough material left in the sides of the rotors to actually balance them. I almost needed to scrap them completely and start over with a fresh set.

DaleClark

iTrader: (55)

IMHO it's not worth balancing an engine during a build unless you are building a 100% dedicated high-RPM track car. Besides that you just invite more problems.

Balancing a rotating assembly is a black art and there's only a handful of people that can do it RIGHT. Also the chance of components getting lost or damaged in shipping is VERY high.

If you have a bad rotor, just get another rotor that's within one letter of your existing one and you are done. Have a C weight rotor? Get a B, C, or D rotor and you are done.

Dale

Pete_89T2

iTrader: (4)

Howard nailed it here, the fact is there's no way ANY balancer shop can know with 100% certainty how much oil weight should be assumed, and where in the rotor that assumed oil weight should be distributed in the rotor during the balancing process. Too many variables at play here - oil temp, viscosity, brand, oil pressure/flow characteristics of engine build, etc... So unless you're building a competitive race engine that will spend the majority of its working life above 8000+RPMs, the risk of doing the balancing job often outweighs any benefit - just get matching letter rotors and the correct matching rotating components applicable to your build (e.g., flywheel, counter weight, etc.) and call it a good enough.

Agree what Dale said above is perfectly acceptable WRT rotor weight code letters per the Mazda FSM, but if you want to be extra retentive in optimizing balance in a street build, getting matching letter code rotors for your stack (e.g., acquire both "C" code rotors in his example) is better than accepting a "B" or "D" code for the bad rotor replacement. Of course this choice is a performance, cost & schedule trade-off, and matching the letter codes optimizes the performance (i.e.. balance) aspect only - actually acquiring 2x matching code rotors might blow your budget or build schedule.

arghx

iTrader: (3)

I think you guys are overthinking it. It's the motor mounts. I've never had non stock motor mounts that don't increase vibration. Brand new stock ones cost a lot of money and are worth every penny, as long as you're not planning to do hard launches.

93BlackFD3S

I forgot to mention I also have the banzai trans brace with the poly mount but I had that before the rebuild. It's not a strong vibration by any means and probably less than you'd have with most piston engines, I just feel like it wasn't there before. Poly mounts are most likely the culprits but the "balance" job definitely makes me nervous. The guy that balanced them has been around for decades in the rotary world and is known to be one of the best so I trust he knows how to do it, I just hope he didn't make a mistake.

Thanks everyone for your input, you guys are definitely right tho, not worth balancing and then wondering if it did more harm than good...