Balancing Problems???

[V-8]

Hi guys;
Im going to buy these parts separately any balancing problems can be occur?
http://www.cnc-motorsports.com/produ...545&CtgID=7500
http://www.cnc-motorsports.com/produ...147&CtgID=8183
And custom made Ross Pistons.

[blykins]

I've heard stories about trying to balance Scat cranks...but I guess there's probably a story floating around about every other brand. I doubt you would have any trouble balancing the assembly. Make sure you take the rings and bearings with you too....and of course your flywheel/flexplate.

I thought you were going with a 393? Those rods are way too long for that stroker combo. Also, you can do better than $800 for a Scat crank....I see them all the time for a little over half that price.

[V-8]

http://www.cnc-motorsports.com/produ...300&CtgID=9059
I think im going to buy this kit.

[V-8]

I will run with a later stock block with Probe 1/2 main girdle and ARP studs.
Now im using TFS tw heads and 750hp series Holley carb, they can feed a 408w to the 6500rpm?

[scottj]

V-8,

If the crank pins are gun drilled it will balance easier (less heavy metal). The cranks cost more this way but it can usually be offset by the savings in heavy metal. Also it will be lighter.

Cheap parts don't always equate to cheaper to build.

Scott

[blykins]

Is there any reason you're set on the 6.25" rods? It will not only slow down the reciprocating mass that you're trying to spin to 6500 rpms, but with that long of a rod, the compression height on the piston is probably gonna be less than 1.25"....which may cause you to have to put an oil ring in the wrist pin hole. While this isn't a terrible thing to do, you'd be a lot better off without it.

[scottj]

I wouldn't go any shorter than a 6.25 rod with a 4" stroke and rpm's are why. The 6.25 rod allows for a lighter piston with very little increase in rod weight. Lighter pistons mean less stress at the high piston speeds you see with a 4" stroke. You also reduce friction somewhat. Also, if it is a high-HP motor, the longer rod makes for a smoother power delivery than say a 6" rod which tends to have "snappy" power.
Scott

Edit: In my motor we cut the deck down from 9.5 to 9.3 and used a 1.063 CH piston. We did this because I turn it 8000 rpm (endurance not drag) and light pistons are manditory with a 4" stroke. The pins are in the rings but it causes no problems at all.

[V-8]

I know that longer rods are better?

[V-8]

The longer the rod, the more 'leverage' on the crankshaft. Piston dwell increases, piston speed decreases, sideloading decreases, it's a win-win deal

[blykins]

I may be showing my ignorance....but....

The length of the rod to me doesn't increase any leverage. You get leverage on the crankshaft from its stroke. Think of leverage with a wrench situation....the longer the wrench the more torque on the bolt....it doesn't matter how long your arm is that's pushing down on the wrench....just how long the wrench is.

What do you mean by piston speed? The speed of the piston is determined by how fast the crank is turning...

Do you mean piston travel distance? If so, the distance is gonna be the same with any length rod...The piston travel is determined by the stroke....

I also disagree with the sideloading. A longer rod is gonna cause more of a sideload. Think of the piston at the bottom of the stroke. A longer rod is gonna be at more of a greater angle to the cylinder skirt...that greater angle is gonna put a greater sideload on the piston.


Scottj....I've never thought of it that way. I've seen many stroker kits....and most of them use the shorter rods...I think of it as a reciprocating mass sort of deal. The less mass you're spinning, the less inertia to overcome with a crank rotation. Since rods are made of steel (generally in street cars), it would seem to me that a shorter rod would decrease mass more than a little aluminum piston.

I've got some pondering to do.....

[scottj]

You think you're pondering.

While my application is extreme, imagine my thoughts the first time I took my new 9.5" aluminum block, 6" Carrillos, and 1.5 CH pistons, and 4" crank to my engine builder and I learned this:

The brand new crank was to heavy, the block was too tall, the piston at 1.5CH were too heavy, and the rods were too short. They said if they built it with my parts it would probably tear the pins out of the pistons in short time.

Realize that the combination I described may not be necassary for a street build but, as the envelope is pushed with these stroker combinations, especially in dirt cars and offroad truck racing, this is the combination you will find...and they're bullet proof.
Scott

[V-8]

I'm using TFS tw heads and 750hp series Holley carb, they can feed a 408w to the 6500rpm?

[blykins]

I would think so.

[brainsboy]

I have to agree with BLYKINS,

Some years ago some tech at MIT probably came up with longer rods are better and gave a little bit of technical reasons behind it, ever since your average back yard mechanic and average machine shop will say longer is better. They threw out the old saying more dwell time at TDC, and ever since every one building motors seems to know it like a bible. The truth is its complete bull%$&t. It all depends on what your trying to do. You always hear the famous lines, longer rods have more dwell time at tdc bla bla bla.. I hate hereing this, because people probaly dont even understand what it means, or the physics behind it. For every reason to use a longer rod there are reasons to not use it Longer rods are not better, as shorter rods are not better. I can gaurentee two motors 100% the same one has 6.2" rods and one has 6.0" rods you wont see a difference in HP due to rod length. The differences come into play with matching your products, which is something people seem to have forgotten. Rod length has more effect on things like intake port size and exhaust port size. Nitrous tends to work better with shorter rod ratio's.
A 6.250 vs. 6.00 rod in a 4" stroke is only .06 difference in rod raito. Not to mention no one ever explains that a 6" rod works better with nitrous and supercharging. Or that 6" rods allow you to run a .200" lower top ring on the piston for heat purposes, or the pins being in the oil ring. There have been stock engines with as low as 1.45 rod ratio's. I am not saying here that shorter is better, but it certainly is not longer is better. There are so many things that rod length has an effect on that need to be accounted for.. There is 100% nothing wrong with a 6" rod with a 4" stroke. I have done it for years, with no ill effects. Like I said im not saying longer is not the way to go. Im saying it onlyworks to a point and more then often people run too long and sacrifice other area's.

[brainsboy]

For what its worth Here are some advantages of a shorter rod.


6" rods provide very good intake and exhaust velocities at low to moderate engine speeds causing the engine to produce good low end torque, mostly due to the higher vacuum at the beginning of the intake cycle. The faster piston movement away from TDC of the intake stroke provides more displacement under the valve at every point of crank rotation, increasing vacuum. High intake velocities also create a more homogenous (uniform) air/fuel mixture within the combustion chamber. This will produce greater power output due to this effect.
The increase in piston speed away from TDC on the power stroke causes the chamber volume to increase more rapidly than in a long-rod motor - this delays the point of maximum cylinder pressure for best effect with supercharger or turbo boost and/or nitrous oxide.
Cam timing (especially intake valve closing) can be more radical than in a long-rod motor

[Mr.Fixit]

brainsboy,

how does a short-rod motor get the piston to move away from TDC faster than a long rod motor? The piston will dwell longer at TDC with the long rod, but it will move faster as it hurries to get to BDC, where it will again dwell longer than the short rod motor's piston.

If the piston dwells longer at TDC and BDC with a longer rod, there is less time for it to go up and down, so it's piston speed must be greater than the short rod motor.

[blykins]

Make sure and don't confuse piston speed with piston acceleration. Piston acceleration is what Mr. Fixit was explaining....

[brainsboy]

Well let me explain why

Ratio "n" = Rod Length ÷ Stroke

The rod’s length is measured (for this purpose) from the center of the piston-pin opening to the center of the big-end bore, not overall. There is a small range of ratios for most conventional piston engines: the rod is between roughly 1.4 and 2.2 times the stroke length. It’s not possible for the rod to be the same length as the stroke, and rods much longer than twice the stroke make the motor very tall, and are not practical for most purposes (although used for racing).
The rod angle must not encourage excessive friction at the cylinder wall and piston skirt. A greater angle (smaller value of "n") will occur by installing a shorter rod or by increasing the stroke. A reduced angle (larger value of "n") will occur with a longer rod or a shorter stroke.
If the rod length is decreased, or the stroke is increased, the "n" ratio value becomes smaller. This has several effects. The most obvious is the mechanical effect. Motors with low values of "n" (proportionately short rods or long strokes) typically exhibit the following characteristics (compared to high "n" motors): physically shorter top-to-bottom & left-to-right (more oil pan, header, and air cleaner clearance) lower block weight (400 vs. 440, for example) higher level of vibration, shorter pistons, measured from the pin center to the bottom of the skirt. Greater wear on piston skirts and cylinder walls. Slightly higher operating temperature & oil temperature due to friction
There are also differences in how the motor breathes: intake vacuum rises sooner ATDC, allowing bigger carburetors or intake port runner & plenum volumes to be used without loss of response
on the negative side, a small or badly designed port will "run out of breath" sooner. Piston motion away from BDC is slower, trapping a higher percentage of cylinder volume, making the motor less sensitive to late intake valve closing (hot cams)
Spark advance is also affected: earlier timing (more advance) is required, as the chamber volume is larger (piston is farther from TDC) at the same point of rotation. The motor may also be less knock-sensitive, as the chamber volume increases more rapidly ATDC, lowering combustion pressure (this is useful for nitrous & supercharged motors)

[Mr.Fixit]

If you are going to cut n' paste somebody else's work, at least have the common courtesy to cite the source.

http://victorylibrary.com/tech/crod-c.htm

[brainsboy]

My father neibour "Jeff Diamond" , satisfied now?

we all learn from somewhere, I suppose you learned everything on your own, with out reading? it sure is easier cut and paste then try and sit and type everything I learned from him.