Stuffing the crank

[MTrat]

Does anyone have experience or theories on 'stuffing the crank' to enhance performance?

[Bullfrog]

Well, the general theory was something like . . . filling depressions/cavities in the volume below the piston tended to increase the "compression ratio" for the area under the piston. This was predicted to "pump" the air/fuel mix in the crankcase(below the piston) up through the transfer ports better.

More modern thinking is to greatly enlarge the transfer passages to allow the air/fuel mix to flow more easily. This concept actually reduces the "compression ratio" for the area under the piston (which includes the transfer passages) - but results in much improved transfer of the air/fuel mix from the crankcase to the combustion area (the same result which was originally envisioned for stuffing the crank).
Ed

[racerclam]

I have stuffed many cranks for my customers as well as my self with great results with response as well as over all power , I have just compleated one for mu yamaha 175 build. First by weighing the halves and drilling to remove weight to compensate for the added material . An added bonus to me is that the new con rod from Yamaha was 15grams lighter than the old one plus I polished the beam and took off another 2 grams.

Rich

[racerclam]

Here is the crank from my super combat that i did several years ago . I had welded sheet metal around the cutouts to contain the material since once I had material fly out from centrifugal force . This stuffing really made a huge difference . Also since it was in the same group of pictures I included my bridged exaust port in the super combat , it enabled me to widen the port by 12mm another big performance gain. bot were very time consuming. And back to stuffing according to Harry the only possible negative to stuffing (MIGHT) be is the additional transfer air velocity from the boost port (might ) blow too much out the exaust port at low speeds due to the slow piston speed , But I have never experienced that. I have always had a gain on the bottom ens as well. Afetr all crankcase signal to intake is improved by the lesser crank case volume.

Rich

[viclioce]

What, exactly, is "stuffing the crank?" I've never heard this term before. Victor

[taber hodaka]

When the piston starts down it is building up pressure in the crankcase so with a smaller area you have more pressure. When the transfer ports open the added pressure would result in more air fuel rising to the compression chamber.-----Clarence

[MTrat]

Racerclam, what material do you use to fill the holes in the crank?

[racerclam]

Besides building crankcase pressure it also develops a stronger vacuum signal on the intake stroke resulting in better intake response. The material I use i LPS strong steel stick but over the years I have used other stuff , Devcon , marine tex . Many things wil work. Harry told me he used cork on the under side of the piston once and it ran really good but it didnt last long. Oh I also often tig weld the ends of the piston pin shut , any area below the ring is crankcase volume.

Rich

[Hydraulic Jack]

I wouldn't weld the pin for metallurgical reasons, but stuffing the pin cavity is used a lot on race engines. Even wood plugs will work if they stay put.

[ossa95d]

Back in the day a lot of the two stroke engine builders around here would mill holes in the crankshaft halves and stuff them with cork to lighten the whole crank/flywheel assembly as well as reducing crankcase volume.

[racerclam]

Welding the pin isnt an issue if done properly. I clamp the pin in a vise with aluminum jaws that absorb the heat rather well and let it cool before doing the other side. I use a cut off screw head to initially plug the ends then with a tig i melt it into the pin which doesnt expose the pin to a long duration of heat . I have not had a problem .

Rich

[Hydraulic Jack]

Rich,

You, sir, are an artist and a pioneer.

[racerclam]

Thank you 8)