I would like to get a second opinion from some of y’all expert engine builders.
I’m putting a new rotating assembly in this winter. All forged.
the kit says it is internally balanced, but doesn’t include a flexplate or balancer.
Will my stock units suffice?
I called the company and asked, they said they balance it with stock components, so my stock components will maintain its balance.
Seems a little odd to me, I imagine my decades old balancer probably has a different weight than theirs.
Thoughts or experiences?
usage is mainly track/hard driving

 

What do you have now? If you have a 2 piece rear main 350 and the kit you buy is neutral balance it should work with the stock balancer and stock flexplate/flywheel. If you buy a kit for a 1 piece rear main you will need a different flexplate/flywheel. Now if you have a big block, the 427 is neutral balance and a 454 is external balance.

 

Currently a 355 sbc 2 piece rear main. Want to make sure before spending the few thousand.

 

The stock balancer and flex plate should be neutral balanced and the weight really shouldn't matter. SB were internally balanced up until 86 or 87 somewhere
The externally balanced motors (ie the 454) need (should) have the flexplate and balancer installed when the balance the crank
M

 

Thanks guys

 

Dumb question aside from the balancing of the thing. Sounds like you’re building a fairly serious motor. Any idea of the HP and RPM you’re looking at?

 

I’ve only built a handful of sbc’s, and this will be the first higher hp one. I’m used to having the machine shop balance the assembly. Never tried a “pre-balanced” unit before. I think the suspicion is understandable.
The current motor runs great, but I would like to be faster. Easiest way for me is to just buy bigger NOS jets. But I don’t feel comfortable with my current cast rods and hyper pistons running more than a 125 shot.
Which is why I’m putting a forged assembly in
but 6k rpm and the jets I’m buying are 225 250 and 300

 

Kinda long winded but hear me out on this.

I was doing some snooping around on the forum and it looks like you drag race this car quite a bit. Nothing wrong with that. I just wanted to understand what you do with the car alittle better. I can't find the thread now but if I recall correctly you're running 8.0's at 90 mph. That's mid 12's in the 1/4. That works out to roughly 370ish at the wheels. I assume these times are on the bottle, albeit you aren't spraying the **** out of it due to the current pistons. Being you're building a forged stroker and planning to hit it with a pretty healthy dose of crack (nitrous) I would not be surprised if your rwhp jumps to around 500ish. Doing some rough figuring that will put you to a roughly close to a 7 flat @ 98 mph in the 1/8th or 11.1 @ 130 mph. This is pretty damned big of a jump in the speed of the car.

If break out of the 11/7 second brackets. You're gonna need an SFI approved balancer and flywheel. The certifications you'll need to look for is a balancer certified to SFI 18.1 and flexplate to SFI 29.1 respectively. Given your building this thing new I would personally just include a new balancer and flexplate certified appropriately that way if something were to happen you don't get your legs cut off at the knees. If I recall correctly your generation of car has a fiberglass firewall and floor from the pedal box forward, and neither are going to do alot to stop a flywheel if it comes apart. While you may not need it, it won't hurt anything either than cost you a couple of hundred more dollars to put it together.

Granted there are alot of other things here that if the car is going to be as quick as I think it could be then you're gonna need ALOT more than just a balancer and flexplate. IE 6 point, belts, seat, not sure if the rear IRS is permitted etc etc.

 

And a scatter shield!

 

Good info Kossuth!
but yes I would like to get into the mid 7’s.
I’ve only done grudge racing, so it’s not a sanctioned event. But I would like to compete one day in some actual events.
I feel good about the rear end and trans holding up because I don’t spray off the line.
I do think I’ll get a sfi balancer, that would give me a better piece of mind about the balancing too. I replaced the flexplate some years back, need to see if it was just a stock or sfi replacement

 

[QUOTE=randallsteel;1606931684
I called the company and asked, they said they balance it with stock components,...........
Never tried a “pre-balanced” unit before. I think the suspicion is understandable.
Thoughts or experiences?
[/QUOTE]

When you receive the "kit", look on the rods and pistons, I'll bet you won't find any weight corrections made beyond that executed by the component manufacturer!

"Balanced" surrrrre!

Scott.

 

Thats what I’m saying!
I was hoping someone would chime in and validate the “pre-balanced” idea.
Ima go for it anyways. But I am going to get a new balancer.
The kit is from SCAT, it is made to order so maybe they actually do balance it. I’ll post some pics when it gets delivered in about 8 weeks.

 

I picked up a Scat balanced rotating assembly for my engine build and had it checked by the machinist doing my block work.
He said it needed work, sorry I forget the specs he found and what he went to but it wasn't close enough.

 

When I built my 383 My Machinist told me to not bother getting a balanced kit and have somebody local do it which i did. The Scat 9000 cast crank is very popular but it takes some heavy metal to balance so its another $80. I thought this was odd but others have confirmed the same thing. They sell a lot of them.

 

The 5.7 rod SCAT crank is set up for external balance (or heavy metal).

If you choose the same SCAT crank for the 6" rod, it will internal balance without the need for heavy metal. 6" rod setup leaves enough room for the larger counterweight required for internal balance. Just need to be sure to get the 6" rod crank and the 6" rods/pistons.

 

I have personally balanced crankshafts for two different companies when I was younger (one was World products)........all neutral balanced crankshafts are balanced without a balancer or flexplate. No need for it unless you are building a Comp. Eliminator engine or something of that caliber. I always tried to get them as close as possible to the bobweight within 1-2 grams but the accepted balance for both shops was 50 grams heavy and 25 grams light. There is absolutely no problem running a street engine to 6000+ RPM using these parameters.
During the pandemic I built three engines with rotating assemblies balanced by Eagle because literally no one would balance a crank anywhere at any price and the one guy who said he would was 4 months out....screw that. I took a gamble and all three engines are fine.
I built three 408 Windsor Ford's over the last 8 months and one 385ci small block using pre-balanced Scat assemblies......and all four of these engines are fine......the 385ci revs to 5800 RPM like a watch.
This 50 heavy 25 light will not be a close enough spec for the average performance engine builder as they will get it much closer........but it doesn't mean it's no good. Factory stuff was much more than 50 grams.......
Now...this 50/25 is the bobweight overall........a small block crank will come in around 1800 grams bobweight......so at 50 grams it is less than 3%......

The 385ci had a Scat 9000 crank in it for 6" rod and it was externally balanced and came with balancer and flywheel. These items on external have to be installed during balancing or it can't be done.....

I am all for having **** on the money......but I will install a balanced assembly any day of the week knowing what I know now......

Jebby

 

Thanks Jebby.
I understand the concept of balancing, and why it can be important but I know nothing of tolerances and what is acceptable.
It's good to read what works for experienced builders.

 

Thanks for the feedback everyone.
really good to know about the tolerances and you’re own experience Jebby.
the price is really hard to beat for a full forged assembly. I’m going with a +3.7cc piston instead of my current +5cc. Should add another .2 to CR.

Got the motor pulled yesterday, only involuntarily separated one wire lol! Dang starter ground. I was cranking from the driver side and didn’t even notice. Easy fix though.

 

I don't really want to know where they came up with this "50/25" specification, but I fully understand the logic in the 'why'! But when known reasonably proper processes have been established and apparatuses have been made available for quite a number of decades to acquire such, and this specifically in conjunction with the automotive industry, this is nothing more than a practice to take advantage of the consumer as they most often are not going to have a balancing machine in their garage to catch them cheating.

And Cheating is exactly what this practice constitutes, as there are established standards of "balancing", and if one is not of the intention of providing a product within such definition, then one does not have the right to express that such a product is being supplied, unless just practicing dubious behavior.

And yeah, thank goodness for the rubber insulator engine mounting systems created for the purpose of dampening out much of the "excessive" vibratory result.

B.T.W. I'm not sure in the definition of the the statement of "the factory stuff was much more than 50 grams [off]", perhaps this implies a total sum of both ends of the crankshaft added together (the couple imbalance of a V8)? Which at that point I would probably agree with, but as in my experience normally when spoken of, we would say that the crank was out say: 25 grams "per-end" (+/-), as the values are most often offsetting in positioning, and this is more in line with what I have generally found as that of the "factory stuff" more closely presents (but then in the last 40yrs. I've done more O.E.M. componentry Fords than Chevy's).

Scott.

 

My 383 package was ordered threw CNC Motorsports. Scat 9000 Crank, Scat 6 inch H Beam Rods, ICON Forged Pistons. Since I thought I was building about as standard of a performance SBC as can be doing little or nothing out of the ordinary of which 1000's before me have done this was one of the few things I found that was odd. I'm real happy with the end results though and that's what matters most !