UnBalanced SBM Engine build

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dartndodge

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OK, a search of FABO shows that everyone recommends a crank balance.
But who has just "thrown together" a 360 Mopar over the past 30 years and never had it balanced?

Well I have. In the early/mid 80s I was young and built my "Good" 360. I used aftermarket pistons (cast) stock rods, std crank/balancer/flexplate/2bbl converter.
I never weighed anything, or balanced anything etc, in fact as I didnt trust "Chevy Shops" I even pressed on the pistons myself...breaking a few ring lands in the process, but...oh well.

Long story short...I put it in a 4 door 68 Valiant, popped in an ST-21, a rubbish Offy 360deg equaflow, 750 vac, 4.1s, 727, stock converter, and use to click of 12.6-12.8 IN FULL street trim...pipes on, retread tyres etc. Fastest mph was 111mph with a borrowed 850 mech.

The engine was a bit buzzy, but never overly, I use to rev to 6500 occasionally, when I put the same engine in my Dart in the mid 90s and installed a worn out 2bbl 360 cam, it was relatively smooth. When I rode in my friends Hot 360 70 Cuda, his engine, to me, vibrated more than my old smogger, and his was balanced blueprinted etc etc.
Now, Ive decided to freshen the old girl up (its only been 30 years:-\"), Im a bit worried about this "Balancing stuff".
Im using some nice polished factory rods, KB190 pistons (both second hand of course), which on my cheap chinese scales, weighs 1490g all up (bearing, pin)
Ive weight some stock 75 360 pistons/rod same, @ 1559g, my old cast combo weighs @ 1565g. I notice a thread on factory weights and a stock 360 piston/rod/pin = 1548g supposedly.

Now as my new piston rod combo is around 70g less than my old 'Overweight" combo, will my engine be seriously underbalanced, or will the lighter new combo not affect much.
As my poor old block at +40 spewed out so much flaky rust that im surprised it has a bore left, I dont want to spend a fortune on balancing, as I may go to a different combo in the future ....maybe.
Im going to use the 272 mopar cam, maybe rev to 6K.

So how many other people have built an unbalanced combo b4, and have you had any problems or not?. thx
 
we threw Ford motors together from 8 different sources when we were blowing up 351Cs in high school. Nothing over stock on the reciprocating parts though. There is a formula for known bobweights and how much you need to take off the crank throws (youll need to drill a few holes on yours to make it "blueprint" but I dont know it offhand. I read a lambo motor was 80G out from the factory from a blueprinter that balanced it. Lumpy cam motors are never real smooth anyway. You can also add weight to a flexplate if you know the centripital weight formula, like farther away from the axis compounds the weight...? Check this example out and maybe you can determine your bobweight amount you need to drill out:
Piston 250
Pin 72
Retainers 4
Ring Set 95
Reciprocating Rod 130
___________________________
551

Rotating Rod 295
Bearing Pair 55
___________________________
350 X 2 700

Crank pin plug 5
Est. oil in crank pin 4
___________________________
Weight of bobweight 1260
 
I can't quote you any specific numbers, that's what I paid the shops for....lol, but since you're light on the top now, you shouldn't have to buy any heavy metal for the crank. I'm not sure i'd even try it on a driver that saw only 3k, but if you are going into the 6k range, I think it's a must. Not sure what it cost's in your neck of the woods, but a few hundred would be money well spent in my opinion.
 
1 gram at 5K rpm is something like 500 lbs of rotational force... have it balanced, it reduces friction and will "free" up hp. that being said i didn't balance my slant that is low 14's, 13's when i have some time again to get to the track...
 
OldmanRick said:
I can't quote you any specific numbers, that's what I paid the shops for....lol, but since you're light on the top now, you shouldn't have to buy any heavy metal for the crank. I'm not sure i'd even try it on a driver that saw only 3k, but if you are going into the 6k range, I think it's a must. Not sure what it cost's in your neck of the woods, but a few hundred would be money well spent in my opinion.
Click to expand...
its about two bills here without heavy metal. i'm with you rick get it balanced...
 
X3.The amount of time in assembly,plus gaskets,and bearings to do it a second time.Stock parts,ok. Knocking that much off the bobweight over stock. Simply not worth it,unless you want to kill it.
 
The one you threw together had cast stock replacement pistons (I don'y know of any cast performance ones...). They would work fine without balancing. "Fine" being defined as "as good as the factory did" which in terms of real performance which was terrible but runs ok in stock form.
The new engine needs to be balanced. If this is a short term fix - maybe find another place-holder or just anticipate holding the paperwork and when you sell it - it's a selling point.
 
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