How necessary is balancing?

I have read many places which recommend getting an engine balanced when replacing parts. However, the machine shop I’ve been using (they seem to be very good) and someone I know who has rebuilt a number of engines said to not worry about it unless I’m going to be racing at the track (which I don’t expect to). The machine shop dude will do it but said that it doesn’t really matter with a 4-cylinder not being raced.

So I wanted to get some input. Is there something in particular about these engines that really benefits from balancing?

Thanks.

 

It’s extremely important to balance engines that are horizontally opposed (boxer engines, V6, V8, etc) because the piston strokes have opposing forces that cancel each other out. It’s kind of like a tug of war with both sides being equally strong, so it remains stable. If they are not balanced, one piston exerts higher force on another. That force also gets amplified at higher RPMs, which spells catastrophic failure if one side “wins” that tug of war.

In a inline 4 cylinder engine, the mechanics are a bit different. The pistons still exert forces on each other, but the rotating assembly is always out of balance to begin with and has a force that always exerts one way. The balance shaft exists for that very reason and its job is to add an opposing force to cancel that imbalance and make the engine run smoother. However, the crank is shorter and the piston strokes have much less impact on each other, so the worst thing that happens in most cases is more vibrations. However, vibrations can shake things apart pretty easily and it all comes down to RPMs. The higher you rev the engine, the higher those forces go. So if you’re full race mode, a balanced, smoother running engine is ALWAYS more stable.

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Thanks for the details!

I’m leaning toward getting it balanced because I don’t know with certainty what I’ll want in the future, and I certainly don’t want to have to rebuild it if I do decide to put more work into it and eventually take it to a track.

With that said, if I weren’t to balance it, what kind of around town driving would make it problematic? I wouldn’t consider myself dangerous but I can be fairly aggressive, and I will redline occasionally.

 

The parts are likely not that far out of balance. The crank should be close to balanced from the factory and most manufacturers sell pistons and rods in "matched" sets where they shuffle around the finished parts to create sets that are very close to balanced.

Balancing the components is useful in extreme use cases but otherwise is a holdover from the early days of hotrodding when frankly most of the parts were junk.

On this build balancing by the machine shop is like the last 0.1% of the balancing

 

That’s very useful, thanks!

Do you think me using OEM rods with Manley pistons makes any difference in terms of ‘matched’ sets and them being balanced?

 

Most performance engine parts are precision machined and go through quality inspections. Being “matched” just means the manufacturer ensures they are all within 2 grams of each other in weight. That won’t make any noticeable difference in the normal operation 2-5k rpm range, but at 8k RPM that 2 grams is about 8 lbs of force. 8,000 RPM = 266 times a second that 8 lbs of force is “wearing” on your internals. If you start talking ounces, it can be over 100 lbs of force. That’s why balancing matters.


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Lightweight components is the only way to get those forces down. Your now getting into a whole shitload of harmonic imbalance issues which an online four has a ton of

 

Maybe I should get it balanced...? This is why I’m hesitant either way. I’m not sure if those who encourage balancing are being ‘correct’ but perhaps it’s not necessary, and those who don’t are being practical. I just don’t know...!

I’ll be using Manley Platinum 87.5 pistons along with OEM rods and bearings.

 

There are guys that are pushing over 700 HP and revving to 9k RPMs. Those guys, yes absolutely get it balanced. If you’re running stock valve springs and cams and aren’t shooting for the stars, the Manley pistons are likely better balanced than the OEM Mazda pistons were when you bought the car out of the box so I wouldn’t worry too much about it if you are trying to limit your costs. ALWAYS measure and weigh EVERYTHING using quality scales and calipers just to verify, but it largely depends on your individual plans for the car. If you want to “built it right” and not worry about it, most machine shops will charge very little to make sure everything is balanced to within 1 gram. I don’t remember what I paid, but it was a few extra bucks added to the valve job and porting they did for my head.

Edit: On a side note, the stock rods are pretty weak. I would upgrade those with the pistons if you can afford it. That would be MUCH more of a substantial upgrade than engine balancing.

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IMO if you're building a motor always get it balanced. Having a fully balanced motor will give you peace of mind for longevity even if you're driving it on the street. The less wear or possibility of knock the better.

Even most crankshafts still have some variation from the factory. You can have a machine shop balance and polish the crankshaft which will also reduce vibration. So consider that if you're gonna balance other components.

 

Thanks! I really appreciate everyone’s input.

I was trying to find ways to make this project fit in my budget, and the machine shop dude seemed more concerned about the OEM pistons than the OEM rods. I have no opinion because this is my first build and I know only what I’m told

Hopefully the OEM rods will be ok. I probably won’t go above 320hp. I can’t get any other rods now, but balancing is less than $200 and I might be able to make that work.

 

It wasn't really that expensive for me the get the entire rotating assembly balanced. Flywheel, crank,pistons, rods. I rev to 7600 and it's smooth as butter up there. I'd recommend it. Your cars still going to vibe a lot at 3k without the balance shaft but that's not where the fun is anyways.

 

The piston ring lands are weak and are prone to cracking and the rods themselves are just not very strong. Both OEM pistons and rods are fine for 320 HP, but it’s the torque that destroys the rods. What’s THE MOST important thing is a good tune. A good tune will prevent probably 90% of all bottom end issues at power that low.


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Do you mean a custom tune, or would a tune from something like VersaTune be good?

 

A custom tune

I would really hesitate to build this with stock rods. 99% of zzb is the stock rods letting go. The guys that havebcraced ringlands are more of an edge case of trying to get every last honour if a smaller turbo

 

Thanks! I’m not sure what I can do at this point. I’m keeping aside about $300 for things that come up while I’m assembling and installing, so even if I used all of that I couldn’t get Manley rods. Hopefully the OEM rods will be ok

 

I can't recommend staying with oem rods. Mine let go at 375 whp on CST4. Of course they had 155k miles on them but still. It vented the block pretty well. Maybe you can get some used rods on facebook for cheaper?

Just found these lol. Maybe they are ok. https://www.ebay.com/itm/New-Forged...-/254776637517?_trksid=p2349624.m46890.l49292

 

Nevermind they are not for DISI

 

You car runs now right? I would just wait until you have the money for rods. Your build will end up being no stronger than stock

 

No it does not run, and I need to get it running by Jan. The existing engine broke a hole in the block (at least partially due to the rod breaking, ironically... ). I think it was due to super knock.

So I bought a used engine that was in very good condition except for that it had severe rod knock. The crank couldn’t be salvaged but everything else is very good, so hopefully the crank in the broken engine is in ok condition to transfer.