4 bolt 350; zz4 crank, h-beam rods, forged pistons

how absolutely necessary is it? what will happen if I don't balance?

 

Not 100% neccesary but if you do you can safely spin higher RPMs and the motor will also last longer because it wont have all the vibrations beating it to death.

 

my instructor told me if the motor isn't intended for circle track or high rpm cruising that I can get away with not balancing it... although it's recommended on all engines obviously.

 

If you really dont want to spend the few hundred to balance it just keep an eye on that tach and dont let it go to high or stay there for extended periods or you could end up killing it fast!


Just a daily/street car i presume?

 

It is only necessary if you are using expensive parts.
If you are using stock induction which won't breathe or make power above 5000 rpm, (like TPI) it isn't necessary.
Now be honest, if you have a good cam, intake, and heads, do you think you will always shift when that engine is really starting to pull at 5000 rpm. Even when the guy in the other lane is level to your back bumper?
Engines that aren't balanced and are pushed above 6000 rpm regularly, will "throw a rod"

 

Well this is a daily summer car, but I have a leadfoot. And my powerband will peak around 5500-6000.

I'm so confused

 

If you're using a used crank, a set of weight-matched pistons, and a set of weight matched rods, you might end up with something that will work reasonably well. Your H beam rods and forged pistons are probably considerably heavier than what the crank was originally balanced with though, which may or may not be a problem.

Of course, if you get it balanced, you won't have to wonder or worry about it.

 

Balancing is an extra $300 or so, isn't it? Money is an issue!

 

Nothing is "necessary". You don't "have to" do anything.

Well, all except one thing, anyway; everything else, even taxes, is just a matter of options and consequences.

If you had a stock crank with stock rods and stock pistons, you could get away with not doing it. Or a crank that had already been balanced for a combo of parts very similar to what you're building. But with the collection of parts you're talking about, you're going to end up SO FAR away from the crank's stock balance, you'd be committing economic suicide by NOT doing it.

You'll probably end up with a motor that vibrates so bad that it will be unuseable. It will tear itself to pieces, wipe out main bearings, and so forth.

It doesn't cost $300 to balance a motor; around $175 is more typical.

If you think money is an issue now, how much of an issue will it be, when you've already sunk all you have into something that turns out doesn't work, and now you have to go back and fix it?

 

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I called the shop my teacher recommended and it's $200 plus the price of heavy metals.

 

$200 is within reason.

I doubt you'll need any Mallory metal, but you never know.... that stuff is $$$HIGH$$$$. Get them to spin it up and tell you what it's going to take, before they just plow in there and do it. It's SO MUCH money, it's a total waste to do tha tto a stock crank, unless your class rules require a specific casting or forging. You can possibly buy a whole other aftermarket crank for less money than the Mallory you might end up using. Of course, they can also tell you how far off it is in the first place, and then you can decide if you really need them to do anything at all, or not; and just pay them for checking.

Instinct tells me though, you're going to find out you need it. I'd guess you'll be 25 to 30 grams off on the bob weight, maybe more.

 

If the crank already has sizeable balancing holes in it, they might get away with using steel plugs instead of mallory. Pretty much no question that they'll need to add some kind of weight to it, though.

 

My metal costed 300 bucks PLUS 250 for the balancing. if I remember correctly it was 60 bucks a plug for the mallory metal and I went through 5 of them. 3 in the front counter weight and 2 in the back. I had a stroker crank internally balanced. Thats why.

 

That's a bit of an extreme example, you're needing to add a substantial amount of mass to the crank there. Likely considerably more weight than the counterweights had as-cast. In his case, he may only need to add back some of the material that was removed during the original factory balancing, which can be done with inexpensive steel plugs.

 

Holy crap.


Anyway my crank is a stock ZZ4 forged that was turned 10 and 20 on the rods and mains.
There are pretty big holes already drilled into the counterweights

 

They should be able to just fill those holes back up again, then.

 

I hope this won't cost a fortune
the crank was only $150 and was freshly turned before I bought it