Rotating weight.....crankshaft....bobweight

[6bangerbill]

I am reading alot of reasons I should not attempt to lighten my 300 ford crank, and very few reasons to attempt it. A lightened 300 crank (3.98" stroke) would likely be much more prone to self destruct than a smaller stroke 250 six crank(3.5" stroke). The forged Cranks for the 300 are becoming very rare and expensive; and a mistake potentually very destructive from this discussion. I plan to proceed with a set of 6.25 center chevy 2.1 journal Eagle H beam rods and Manley forged 351 pistons for a limit of 7500 rpm. I have bought (4) NASCAR P7 Dodge heads for this engine to build a hybrid head and a solid roller 256/260 cam and lifters, but this is the wrong post for that discussion. Some of us 6 banger guys are a little soft in the head when it comes to our projects, spending years of fabrication for a smaller hp result. Thank you all for your help.

bill

[BrazilianZ28Camaro]

Some of us 6 banger guys are a little soft in the head when it comes to our projects, spending years of fabrication for a smaller hp result. Thank you all for your help.

bill

Maybe Kirby Sissel can help you in the head department?

Maybe you'd like to see our stroked 312" L6 with billet crank here:

http://www.youtube.com/watch?v=GuXVetGVTDc

This car ran high eights on the quarter with the STOCK ported head. N/A engine, EFI, burn some nitro.

Sorry OP, a bit off topic here

[af2]

A 39 Chevy at 3200# up hear ran 10.70's consistently before the rod hit the pump drive after every 4th or so run...
To say the crank don't move around is like saying the world is flat. This engine was shifted at 8500 with a Doug Nash 5 speed.

[6bangerbill]

Was this a ford or chevy engine In the 39 chevy? Maybe I shoud limit my rpms to 7k intead of 7.5k!

[ICEMAN]

As a rule, speaking L6, what does the following do to harmonic (raise or lower frequency) and torsional vibration (more or less amplitude)?:

-Lengthening and shortening stroke
-15-20% reduction in reciprocationg mass with unchanged counterweights
-Change in rotating mass+- (rod big end)
-Reducing number of counterweights as opposed to fully counterweighted (12 for L6)
-Increasing output of engine through improvements in top-end

[fishman]

Let me ask this lets say i have a crank with 1850 grams bobweight and my piston..rings...bearings...pins...rods...bobweight out at 1720 grams and need to lighten crank up by 130 grams to balance the crank...will create more horsepower because u have less moving mass now.........or am i understanding wrong

[MadBill]

For reasons I don't understand, this is likely to flare into a heated discussion, but simply put: The mass of an engine's moving parts do no affect it's steady speed power. E.g. If you run it at WOT at a steady 5,000 RPM on the dyno and it produces 400 HP, then you substitute pistons that weigh twice as much and rebalance it accordingly, it will still produce 400 HP, as measured to any typical degree of accuracy. Similarly, if you substitute a 40 lb. flywheel for a 3 lb. flexplate, no effect.

Actually, the arguments start when the discussion turns to the effect of engine mass on power as the engine accelerates and I won't go there...

[Baprace]

For reasons I don't understand, this is likely to flare into a heated discussion, but simply put: The mass of an engine's moving parts do no affect it's steady speed power. E.g. If you run it at WOT at a steady 5,000 RPM on the dyno and it produces 400 HP, then you substitute pistons that weigh twice as much and rebalance it accordingly, it will still produce 400 HP, as measured to any typical degree of accuracy. Similarly, if you substitute a 40 lb. flywheel for a 3 lb. flexplate, no effect.

Actually, the arguments start when the discussion turns to the effect of engine mass on power as the engine accelerates and I won't go there...

well MadBill I might as well be the first one to slightly disagree, I know what you are saying about steady rpm horsepower BUT it takes more horsepower to throw a 5 lb piston in different directions that it does to throw a 1 lb piston in different directions on a maximum WOT application, thats the best way I know how to discuss my point of view, not wanting to argue but I believe my point is correct , flywheel weight you are correct , it can be a flex plate or a 100 lbs and no difference in horsepower but both engines will drive totally different , agree ?

[MadBill]

That's the problem: It seems like it must take more power to yank the heavy piston up and down the bore, but apart from the infinitesimally greater energy lost in stretching/compressing the rod (e.g., if you bend a paper clip back and forth, it gets noticably warmer before it breaks), all the energy lost in accelerating it away from TDC/BDC is transferred back to the crank when decelerating it at the other end of the stroke.

[David Redszus]

Assuming that the stroke, rod length, rod weight and rpm are the same, there will be no change in piston acceleration G forces. But if the piston mass were to increase, the loading on the rod big end would increase proportionally to the increased weight. The increased frictional loading will reduce torque but since most weight differences are incremental, so will the performance differences be as well. The unanswered question remains: if the friction is raised by 2%, how does that relate to actual changes in torque?

[BrazilianZ28Camaro]

Assuming that the stroke, rod length, rod weight and rpm are the same, there will be no change in piston acceleration G forces. But if the piston mass were to increase, the loading on the rod big end would increase proportionally to the increased weight. The increased frictional loading will reduce torque but since most weight differences are incremental, so will the performance differences be as well. The unanswered question remains: if the friction is raised by 2%, how does that relate to actual changes in torque?

David, sorry if its a stupid question, but wouldn't the forces increase exponentially with RPM and the increased piston weight??

[kassemd]

All 250 ci Chevy counterweights were cut on what I would call a 60* angle ( this may be incorrect as I don't remember the angle right now ) BUT it vibrated as the rpm's went past 6500 , I think I had under counter weighted the crank But It did balance force & couple with no problems, no bobweights are installed on an inline 6 cyl crank when you balance it, if I had some time I think I would like to cut up a crank to make it a single cyl engine and maybe balance it with a bobweight of 54% +/- like I do some single cyl engines that seem pretty smooth at race rpm"s, JMHO

I'm working with an inline 6 tractor engine that has 4 mains & took out the 2 center mains while the end ones looked fine. What I think is happening is that since the 2 center crank throws are on the same side, the centripetal force of the throws & con rod load are causing the crank to flex kind of like a jump rope. It has some counterweight opposite of the throws which is probably adequate for the 2500 RPM it ran from the factory, but isn't for the 5000+ that I'm trying to run. I'm planning to cut a scrap crank at the # 2&3 mains so I can see how much counterbalance it has & plan to weld more on. How much I need is the million dollar question. I can definitely justify 100% of the rotating rod weight; if I'm reading you correctly you would suggest 100% rotating + 54% reciprocating? Is there value in calculating acceleration forces at TDC & BDC and somehow get from there to the % of reciprocating? If so, how...

Thanks,
Mark