My 350 shortblock is awaiting parts to be built up by my local engine builder but i have a bit of a dilema. The guy building the engine said that because the engine will only produce around 370 bhp and will not rev past 6000rpm it will not need balancing yet everyone i speak to said that it should be balanced.

The shortblock is having all new parts apart from crankshaft , which is being reground , including forged pistons . Do you think balancing is a must or not ? could you give your reasons please so that i understand the process better. Thanks. Craig

 

Balancing matches each of the moving parts to its brothers (all rods the same, all pistons the same); and matches the crankshaft to those weights.

Any time you change the weight of the moving parts, the crank needs to change with them. In your case, going from stock pistons to any kind of forged ones, is guaranteed to result in a change of weight.

I'd recommend having it done. I myself would not build that motor without it.

 

As said in the above post you are changing the components and weight. Rebalance.

 

Yeah i thought that may be the case. Its just the guy who is building the engine said that he has not got the means to rebalance V8. Is it an involved job or is it that specific equipment is needed ?

 

It just means that his equipment is sized for something like an A series engines, not a big American V8.

You could find another shop to do the balancing.

 

I'm based in the U.K and the guy i have been using has come highly recommended. The problem i have is that i know that he is an excellent builder as many of my friends have had engines built by him so i know he will do a top job.

There is a local guy who has got the capabilities to do it but i have been warned not to go with him. I know you guys recommended balancing, and i do respect your opinion , but what problems might i run across by not balancing.

 

The engine might vibrate objectionably while running and it can beat the crap out of the bearings. It might be worth looking outside your immediate area for a shop that can do it.

 

What Apeiron is saying is a possibility, but not a definite one. Remeber that motors were built a long time ago, and still are from the factory for the most part, unbalanced. Whether the rotating assembly is balanced or not, the bearings are still gonna see abuse. Balancing will get you a lot more longevity out of your parts though. In short, it doesn't have to be done, but it's a nice insurance policy against engine failure. One possiibility is to see if the guy you trust has the equipment to match the pistons and rods to within half a gram of each other. If he can do that, but he can't balance the crank, either buy a replacement crank with the proper counterweight balance or farm out the work as Apeiron suggested. Another point worth mentioning, is that pre 87 engines are internally balanced. 87 up engines are a form of externally balanced engines. 87 up engines have a 1 piece rear main seal. The 86 and earlier have a 2 piece rear main seal. If you have an 87 up block, that might explain why your machinist doesn't feel it's necessary to balance. He could order a correctly balanced flywheel and balancer, solving the problem. Wow! This is a lot more writing than I intended to do. Let me just finish by saying that neither Apeiron or myself are wrong. I tend to agree with him on balancing for insurance, but I also know that you can build it without balancing. Okay, I'm gonna shut up now before I wind up writing a book.

 

I would take the motor to someone that knows what there doing

 

"If the budget exists, balancing the rotating assembly is recommended. But be aware that your motor will be just as reliable and run as well without balancing”

"Foregoing balancing does not mean that there will be more destructive internal engine loads present. What it does mean is that whatever forces are generated at one journal may not be fully countered by forces generated at another. The result is an out-of-balance engine. But the engine is no more likely to fail than if it were perfectly balanced. If the budget is available, get the crank balanced. If you buy a rod/piston balancer you will save about 40% of the cost of a normal balance job because you rods and pistons will already be done”.

...That is according to David Vizard. Probably the only guy whos writings I would completely trust. My engine isn't balanced and it runs perfectly fine, runs faster than most said it would, and I checked the bearings a few months ago after 2 years of abuse and they looked brand new. I wouldnt waste my money on balancing unless I had extra money laying around...

 

None of that has anything to do with the decision to balance or not.

Factory engines aren't individually balanced because entire runs of engines are mass produced from similar parts. Production tolerances are close enough that the end result is a reasonably (but not perfectly) balanced engine. If you're rebuilding with stock or close to stock parts, the balance may be acceptable when you're done. When you start changing things you can't count on it anymore, especially when you start using heavier forged pistons.

Balancing is cheap insurance. I wouldn't waste money on not balancing, unless I had extra money laying around. It shouldn't be too hard to find someone nearby who can handle the job, especially considering you can drive from one end of the country to the other in a day.

If you want, you could get your machinist to calculate the bob weights of the old and new engines. He doesn't need to spin it up for that. If there's a significant difference (I'd be surprised if there wasn't), then you'll probably want to balance.

 

I have to disagree with this statement. If balancing is the way he goes, then knowing the difference between the two is very important. If the machinist says "No problem. I'll have the crank balanced in no time." And you're dealing with the external balance of the flywheel and the balancer of a 1 piece rear main seal, this would be a good time to leave. Unless of course you don't mind paying for more problems. And the fact that the assembly line can't afford to balance each motor proves that he can rebuild without balancing if he so chooses.

 

A 1 piece RMS setup doesn't take any additional time or effort to balance compared to a 2 piece RMS, so I don't know what you're trying to say here.

The assembly line doesn't have to balance each motor because they specify all the parts to produce a reasonably balanced assembly when it's all put together. Essentially they're building "pre-balanced" engines. If you're rebuilding an engine with stock parts, then you might decide to not balance it knowing that the combination you're using is reasonably balanced for a stock application. He's not doing a stock rebuild though, he's using forged pistons which will have a significant weight difference compared to stock pistons.

 

I'm by no means an expert when it comes to balancing engines, whether or not it should be done, nor how to do it. But it would seem to me that it all boils down to how much modification is being done to the engine.

If you have a complete original engine and are doing nothing more than having a basic rebuild, even with forged pistons added (crank ground down, block bored and honed), it isn't "necessary" to have the rotating assembly balanced AS LONG AS the pistons are all matched to each other.

If the rods are being resized then they need to be matched to each other again.

If you're buying new rods and new pistons you may as well have the entire rotating assembly balanced.

If you're buying a new crank, new rods and new pistons then you absolutely must have the rotating assembly balanced, IMO.

The reason I don't feel a basic rebuild needs to be balanced again is simple.... Everything is already matched enough from the factory. Adding another 10 oz or even 40 oz. to each piston isn't going to be enough to mess up the counter balance of the throws. Generally, a crank is balanced heavier than the piston/rod combo anyway. As long as each piston is equal, that's what really matters.

Another thing......
Whenever a rotating assembly is balanced, it's done at "X" RPM (the norm is 500 RPM). How much time does your engine spend at that RPM? If you drive 100 miles per day (like I do) and 80% of the time the engine is at 2,500 RPM, then wouldn't it make sense to have it balanced at 2,500 RPM? That engine should last a very long time if it was. Or at least it would seem to me.

But what about continuous stop-and-go driving? What RPM do you use then? The engine is only balanced at one RPM, yet spends 99% of it's time at other RPMs.

Kind of like tires. They're balanced at 65 MPH. Any speed below that and they're out of balance. Any speed above that and they're out of balance.

Granted, they're not as out of balance as not having them balanced at all, but let's say you have a tire that you only add a .5 oz weight to get it to balance out. Then you have another that takes 4 oz. Which tire do you think would wear faster if it hadn't been balanced? The 4 oz tire, right?

Well... not if the car never went over 10 mph, or maybe went 90 mph 90% of the time. There will be a speed (or "RPM") where even the tire that's off by 4 oz. will balance out even if a weight is never added.

Anyway, it all depends on your budget, your intended use, how many miles the engine will see, and what parts are being replaced.

 

sorry i didn't make it clear what i was actually doing with the build up.

My current engine is a real smogger so preaty much everything is being renewed including , as already previously stated forged pistons,. The bores are being done +.030 " and the crank .010". The only component i'm not renewing is the crank and this mainly down to cost . The cam will be something like a Comp cam 280 hydraulic camshaft , should be preaty good for street and strip use and my budget for heads is something like £800 ( i think thats about $ 1300 ) assembled but i'm not to sure what to run but my shortblock is the main concern at the mo.

What i'm getting at is that most of the rotating assembly will be new and the build has to be right first time as i'm not overly keen on dropping the engine in a hurry again.

After reading all your comments and other sources i think it would be stupid of me to pay over £2000 on components and labour without paying the extra £150 or so pounds to get the bottom end balanced

 

THAT's not true. IF they're truely balanced at any speed they're balanced at every speed. The thing is, due to forces increasing by the square of the RPM, it's easier to "see" a slight imbalance, the faster you spin something- any slight or unnoticable imbalance at a low speed, becomes apparent at a higher speed due to the increased centripital force.

The "balanace" of an engine has ZERO effect on rod bearing life. The load on the rod bearing is completely unbalanced -or rotating around the journal- no matter how much balancing you do! The piston and rod are reciprocating, and no amount of balancing will take that "imbalanced", or uneven load out of the picture.

On the other hand, a properly balanced rotating assy, will lessen "loads" on the crank bearings and block. If the rotating assy is balanced perfectly, in theory, the only load the crank bearings would see are the force of combustion, and gravity.

The forces applied to the block through the crank bearings of a non balanced, or out of balance engine won't have any effect on reliability, IMO...especially at the power level the original poster is looking at. A balanced engine would most definitely feel smoother to the passengers however.

Most of the time I think Apeiron thinks there is only ONE way to do things; his way, which frequently happens to also be the most expensive way. He often backs his opinions with prophesies of doom if, you don't follow his advice. Frequntly he's right, but in this case he's not. Other posters in this thread have proven, running examples of engines that are performing, and lasting, "unbalanced". I myself have more of the same- engine's I've build, changed parts, not balanced, and they've performed well and continue to do so. And then there is Vizard's opinions, quoted above. He's got some experience building engines. I'd say more than the original poster, me and everyone else in this thread put together, and look at what he has said. It's pretty clear.

It's all about being able to apply common sense to each scenario, and do what makes the most sense. What are you going to get out of it, in your situation, for your money? Unless the original poster has "extra" money left over from his re-build plans, I'd say it's probably not worth it.

 

What can I say? I like to spend money.

 

LOL, I'll have to admit; I do too...when I've got it to spend.

-Tom