Anyone know their bobweights??
 

I just calculated mine and curious how it compares. Please state what rod/piston combo along with rod length and bore size. I think mine is very light and can't wait to get it together and see how it revs.

1940

I've got BME 6.8 rods/ 4.211 Ross pistons = 1875.9 grams

That's light

Most of the 6.800" steel rod, shelf style pistons with big rings are around 2150g.

My little 400ci came in right at 1700g.

The 7.100" Carrillo rods weighed only 614g, had a short skirt flat top piston and .043" rings.

1875g is pretty light, what rod journal?

Calvin Hill
Hill Performance
708-250-7420

Scat 6.80 rods, RaceTec 4.31 pistons, bob weight 2185
Eagle 6.80 rods, RaceTec 4.185 pistons, bob weight 2149
Eagle 6.80 rods, Ross 4.180 pistons, bob weight 2110
RPM 6.80 rods, SRP 4.180 pistons, bob weight 2036.4

2.20

I can't believe those steel rods are that light...wow.

Nitro Pontiac= 2420 grams 4.250 bore. BME 426 forgings, 1.094 pins, Big, thick BME pistons. Strength over weight. 8500 RPM. .Rods 6.7" Piston weight 630 Grams We overbalance 1%

When forcing nitro in, you definitely consider strength over weight. Your pistons are almost 200 grams heavier than mine, but mine wouldn't last for a burnout in your application.

Yea, you do need the extra weight in some power adder applications. When we were first working the tune-up a couple years ago we bent one of those 1.094 VascoJet wrist pins. I couldn't believe it!. It was bent .003" The flat top pistons are almost .400 thick. As far as pistons for NA and high RPM Pontiacs, Because of the short cylinders, I would try to keep as much structure and support in the skirt area as possible. Certainly around the pin boss and the underside of the piston, they can be light.

eagle 6.8 rods, probe 4.155 pistons, 2160 Bob weight

Do you mean 6.100 rod ?

Those are dinky.

Not being a smart azz but you did double real big end and the rod bearing weight as well?

need 1 BME 426 6.625 with 927/2.200 ends to complete a spare Street motor.

In my 461 stroker, 4.155 ROSS pistons and 6.8 RPM Ultra Lite A beam rods came in at 1957g.

Yep. Don't have the numbers in front of me but the rods weighed 660 total (about 450 big end/210 small end), bearing about 50-ish, 444 for pistons, 145-ish on the pins, 7.5 support rails, 57 rings, 4.8 locks, added 5 for oil. Again, those numbers are off the top of my head but I know I'm really close on the numbers for the rods and pistons.

Bob weight 1972.7 gms

No, these were used in my stock block 400ci.

SBC rod journal, .866" piston pin.

Calvin Hill
Hill Performance
708-250-7420

Ross 4.375"
GRP 6.735"

http://rexracing.com/bobweight.jpg

I'm sitting here thinking bout this new build and lightening it up from what i had, I'm surprised how light some of these are, compared to mine which i thought ran pretty damn good!

You're using a heavy rod and ring pack.

Crower Lightweight 4.125 billet crank
Crower Midweight billet 6.8 rods
JE 4.310 pistons

2303.4
1/2 = 1151.7


.

If he were talking about a Carrillo 366 cid engine rod or 303 cid engine rod they would be 7.080" inches long John M.
He did not say 366 or 303 engine so assume your rod length question is valid (he had a typo error).

Tom V.

Must be why my car is a turd, got heavy rods in, and a crank that weighs a ton

Yeap, Going to be changing that Paul!

Yea, turd , right lol

2278.6.

what's the combo?

455 + .030. Eagle rods, Sealed power forged pistons. factory crank. neutral balance

Says the fairweather racer. :D

My rods weigh around 850g, crank weighs a ton, don't worry to much about weight. Use the K.I.S.S method! Imo there is next to nothing in reducing crank, rod, piston weights as far as performance goes.

Have u tried lighter stuff Brian? To me anything that helps lighten the assembly the easier to turn n less wear n stress?

Generally that's how it starts out, the lighter the better. As you make more power you need heavier parts to stand up to the additional load. Next you have to pay the big bucks for stronger lighter parts, like ones made out of titanium.

Ive had lighter rods and crank, no performance difference, in another engine we went from alum rods to steel, no performance difference. The lighter rotating assembly seem to rev quicker in neutral but that's it.

When you switched to the aluminum rods and saw no difference, were the counterweights turned down or drilled?

We've never had counter weights turned.
FWIW Chris U on yellowbullet has a 9000+rpm record setting bbc with 800+g rods

When switching from a steel rod to aluminum rods you want to turn the counterweights down rather than drilling holes when balancing the crankshaft. Any gain you might of seen from the lighter rods may have been absorbed in windage losses due to the holes machined to balance the crank. You'll also benefit from the shorter counterweight.

If that balance hole offsets the gains from the alum rods, then the gain was so small it wasnt going to be noticable.
Besides that how does lighter rods increase performance?

The gains or losses from different reciprocating weights are usually small but real. Manual trans guys have played with flywheel weights for decades to increase their cars performance. It's kinda like the difference between power producing atmosphere and warm temperatures with lots of humidity and low barometric pressures.;)

So the lightest possible flywheel is always the quickest for stick cars?

Light weight has its benefits but 99.9% of the people won't realize those benefits. It just costs more money and increases maintenance.

Not necessarily

Why does it cost more money and increase maintenance?

Just picked up block etc for my brother's motor Friday. Eagle 6.625" rods, Icon forged pistons bob weight is 2190.6 g.

But lighter is better isn't? :D

Depends :D

There used to be a 56 chevy stick car that set a record years ago at our track. He had a very heavy flywheel, and a 265 small block.
Dan Whittmore about a couple of builds where the customer insisted on changing from steel to aluminum rods. Seen no difference. He generally did not like aluminum rods. Worrying about cycles and warming up your engine. Block heaters and all that. Had a customer who lived in cold weather and did not pay a lot of attention to warm ups, windowed the block.

So My New Combo is balanced and with the bigger bore, but lighter rings etc, its lighter then last combo just a tad at 2238 now..