Miata Turbo Forum - Boost cars, acquire cats.

- Engine Performance (https://www.miataturbo.net/engine-performance-56/)

- - Engine balancing practices (https://www.miataturbo.net/engine-performance-56/engine-balancing-practices-95270/)

Quote:

Originally Posted by Savington (Post 1453482)

This is not a crossplane V8 crank, the opposing pistons balance themselves. For the same reason, you do not need bobweights when balancing a BP crank (or any other flatplane 4cyl crank).

If what you are saying is true, then just slapping rods or pistons into a motor would shake it to death with massive imbalance. That is not what happens.

The only witness marks you would look for on a balanced rotating assembly would be alignment marks for crank>flywheel and flywheel>pressure plate. Dampers should be zero-balanced independently. The pistons should be matched to holes independent of the balance.

My opinion of balancing with flywheel/clutch is that it's too pedantic for 99% of customers who request it. If you ever are required to change the clutch or flywheel, you lose that balance. You also have a spinning disc internal to both which will switch clamped positions every single time you engage/disengage the clutch, and that becomes part of the system when the clutch is engaged, so I see little use in including the clutch/flywheel in the system when the disc is just going to dick it all up in the end. Flywheels and PPs are balanced independently, that's plenty for 99% of builds (the 1% are not discussing their motor plans in a public forum).

To recap:
-Damper is zero-balanced independently, not by your machine shop
-Crank is balanced independent of piston/rod weights, with or without flywheel/clutch
-Pistons are matched to each other to <1g
-Rods are weighed big end to small end including wrist pin weight and adjusted independently (big ends matching, small ends matching)
-Pistons are matched to cylinders based on p2w clearance, not balance

so just because I'm a little slow and still in food coma. We should only have the crank, pistons, rods balanced at our engine builders for 90% of us?

I asked my machine shop and they answered as Andrew did, thankyou for your help.

Dann

Quote:

Originally Posted by engineered2win (Post 1453516)

That is wrong. There are many difference processes manufacturing a connecting rod: cast, forged, machined from billet, powdered metal.

Maybe you should put your glasses on, I wrote all are ASSumed forged. Not that forging is the only process- but 99% of the cars on the road are running forged rods, and assumed to be. Even Carrillos start with a forging. Out of curiosity, who makes cast rods?

Quote:

Originally Posted by tetraruby (Post 1453640)

Mazda, for one.

I assumed when he made the statement about forged rods, he was referring to built BP engines..........what the thread is about.

Quote:

Originally Posted by z31maniac (Post 1453658)

I assumed when he made the statement about forged rods, he was referring to built BP engines..........what the thread is about.

I remain surprised how often I see people saying they're going to use 323 GTX cast rods in their build. None of our regulars of course but CR and m-net folks.

Your shop said +/- 5 g? That seems like a lot of variance for a performance build. However, the factory doesn't seem to do much better. In this "rods only" motor the oem pistons were out 3+ grams... and the pistons don't have much room to remove weight. Even after tapering the pins, I still had to re-work the pin boss and skirts quite a bit to get the weights close. Hard to do without weakening them excessively or leaving stress risers. https://cimg6.ibsrv.net/gimg/www.mia...9b3811f66c.jpg
If you can't get them perfect, another strategy is to try to match the total wt of the pistons going up against the pistons going down (1+4)=(2+3).
BTW, why does a little engine like this use pins that weigh 8g? ... that's v-8 pin territory. I wonder if anyone offers a tapered pin for a miata?

Quote:

Originally Posted by tetraruby (Post 1453640)

99% of the cars on the road are running forged rods, and assumed to be.

This is a very poor assumption

Does my builder need a damper to balance the rotating assembly or should i be good just giving him the crank, rods, and pistons? Im getting two stories is why i ask.

I think the consensus is that he should not need it as it should balanced to itself therefore will not affect the balance of the complete assembly.

Quote:

Originally Posted by Savington (Post 1453719)

This is a very poor assumption

I appreciate the fact that you guys challenge me- it's forced me to do some homework and open my eyes. My apologies for the misinformation. I learned something new. Ty.

I found a google book link that breaks down a little info about the subject of rod manufacturing processes:

https://books.google.com/books?id=Sq...ged%3F&f=false

Quote:

Originally Posted by tetraruby (Post 1454080)

Submit now.

https://pics.me.me/wow-owen-hihon-i-...-17212727.png?

So in the end, we do not have consensus. Some say that there is no benefit to adding weight to the counterweights to accommodate the added mass of aftermarket rods and pistons (Pat and Steve), whereas others say this has no value (Andrew).

Not sure how to really know. I will say that accommodating higher mass rods would be more important when used in conjunction with a lightened flywheel.

Maybe NOT accommodating the increase in rod mass is part of why heavier harmonic balancers help hold modified engines together.

Again, not sure how to determine relative worth of modifying counterweights.

And there is this

Racing Rods - Builders Have Many Options From Which to Choose - Engine Builder Magazine

This answered my question on what to balance. The super summary is an internally balanced rotating assembly is better for parts longevity and high performance applications. The internal balance is just Crank, Rods, Pistons, Rings, and Pins. Then balance flywheel seperatly