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Discussion Starter · #1 ·
I'm planning out a build of a 532 cubic inch big block for a project, gonna be twin turbo, sequential fuel injected, coil near plug ignition, somewhat normal stuff for serious performance. The weird stuff starts here, I decided to go with a flat plane crankshaft, long aluminum rods, and short pistons so I can spin this thing higher than I could otherwise, maybe even as high as 9,000 rpm, not sure yet. For the proper quench I want, the static compression I'm eyeing, and the RPM I'm looking for I'm gonna use 6.7" aluminum rods and 1.1" tall forged aluminum pistons, netting me 0.021" quench due to the head gasket thickness. What issues will I run into? First thing that comes to mind is with that piston height the wrist pin is going to start getting into the oil control ring, would I see issues with oil consumption? If the pin is moved down towards the bottom would I have stress issues that could/would lead to cracking and breaking of pistons over time? On the crank I'm thinking an Up-Down-Down-Up rod journal arrangement so as to have a firing order closer to that of an LS, however I have no clue what the bob weight on the crank will be and in the response email I got that was one of the things they'll need, is this just the weight of the crankshaft?

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Bob weight is a term used in balancing....it is partly a measured amount and partly a calculated amount, the math formula is this...
100% rotating weight + 50% reciprocating weight + a small amount for oil clinging to parts(usually just 4 to 8 grams is commonly used).
Rotating is everything bolted to a rod journal, rotating with the crank...big end of both rods, 2 sets of rod bearings
Reciprocating is everything moving up the bore....small end of rod, wrist pin, piston, rings set, pin locks.
Rods are weighed by hanging one end while weighing the other end on a scale.

so, to get the bob weight that is bolted around each rod journal on the crank during the crank balancing process, we have to weigh all our parts in the rotating assembly and calculate the final result.
So..."100% rotating weight + 50% reciprocating weight + a small amount for oil "....
Big end of both rods, 2 sets of bearings + 1 small end of rod, 1 piston, pin, locks, ring set + oil = bobweight

As an example, small block-ish...
625 gram connecting rod(450 gm big end, 175 gm small end) 680 gm piston/pin/rings/locks assembly, 25 gm rod bearing.
450 big end x2, plus 25 rod bearing x2 = 950 gms Rotating
175 small end x1, plus 680 piston x1, plus 5 gms for oil = 860 gms reciprocation.
Total bobweight would be 950 + 860 = 1810 grams.

BBC stuff s a bunch heavier.
They need to know approximate bobweight so they know how big to make the crank counterweights to offset that bobweight. So you need to know what your rod and what your piston are going to weigh.
And that's just the beginning of all the things you are going to need to know to pull this off, or if it is even physically possible given the space constraints of the BBC block design, size, and shape.

So what is going to be the bore and stroke of this wonder engine?

You want the LS firing order, just get a 4-7/2-3 firing order swap camshaft for a BBC.

Flat plane crank has been tried before....it never works out well retrofitting into an engine design that never included it.
Bad idea.
 

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Discussion Starter · #3 ·
Bob weight is a term used in balancing....it is partly a measured amount and partly a calculated amount, the math formula is this...
100% rotating weight + 50% reciprocating weight + a small amount for oil clinging to parts(usually just 4 to 8 grams is commonly used).
Rotating is everything bolted to a rod journal, rotating with the crank...big end of both rods, 2 sets of rod bearings
Reciprocating is everything moving up the bore....small end of rod, wrist pin, piston, rings set, pin locks.
Rods are weighed by hanging one end while weighing the other end on a scale.

so, to get the bob weight that is bolted around each rod journal on the crank during the crank balancing process, we have to weigh all our parts in the rotating assembly and calculate the final result.
So..."100% rotating weight + 50% reciprocating weight + a small amount for oil "....
Big end of both rods, 2 sets of bearings + 1 small end of rod, 1 piston, pin, locks, ring set + oil = bobweight

As an example, small block-ish...
625 gram connecting rod(450 gm big end, 175 gm small end) 680 gm piston/pin/rings/locks assembly, 25 gm rod bearing.
450 big end x2, plus 25 rod bearing x2 = 950 gms Rotating
175 small end x1, plus 680 piston x1, plus 5 gms for oil = 860 gms reciprocation.
Total bobweight would be 950 + 860 = 1810 grams.

BBC stuff s a bunch heavier.
They need to know approximate bobweight so they know how big to make the crank counterweights to offset that bobweight. So you need to know what you rod and what you piston are going to weigh.
And that's just the beginning of all the things you are going to need to know to pull this off, or if it is even physically possible given the space constraints of the BBC block design, size, and shape.

So what is going to be the bore and stroke of this wonder engine?

You want the LS firing order, just get a 4-7/2-3 firing order swap camshaft for a BBC.

Flat plane crank has been tried before....it never works out well retrofitting into an engine design that never included it.
Bad idea.
Bore and stroke will be 4.6" and 4" respectively

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Discussion Starter · #4 ·
Okay, so what would the quench height I'd want to avoid piston contact with the valves?

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Quench height has almost nothing to do with piston-to-valve clearance.....you don't decide on a quench clearance using valve clearance as a criteria.

Seems from your questions so far that this level of build is well beyond your abilities and understanding of basic relationships between parts??
 

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Discussion Starter · #6 ·
Quench height has almost nothing to do with piston-to-valve clearance.....you don't decide on a quench clearance using valve clearance as a criteria.

Seems from your questions so far that this level of build is well beyond your abilities and understanding of basic relationships between parts??
I made a mistake on which thread of this I posted that comment on. I understand that valve to piston clearance is separate from quench distance.

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