Strokin' It

Im building a backup motor. Its a BBC 9.8 deck. If you had an option for stroke would you use a 4.0, 4.25 or 4.375? If so why? Ive been running a 4.25 for quite some time. Was thinking about changing to the bigger 4.375. It will have a 14/71HH SSI around 20 to 30% OD, Big Chief Heads, blah blah blah, Car weighs around 2500.

 

stroke

That getting to a pretty short Rod. Don

 

shorter stroke will give more reliability.

 

Would you gain any power from a longer stroke or does it matter with a blower?

 

You're going to lose RPM. Depends on gear ratio. With long gears, the longer arm will pull better, with short gears, you'll kill the mph

 

Stroke

We run a Dart big M 9.8 block and 4.25 stroke. At BDC the piston skirts come approx 1/2 way out the bottom of the cylinder as they are rocking from one side of the bore to the other. There is a definite marking on the skirt where the bottom of the (gently relieved) bore is and though it has not caused us any problems that I am aware of, I get the feeling that we are running a fine line between good and bad More stroke will only make this issue worse and I dont know how far it can be taken before it becomes a reliability problem. In our own case if I was building again I would definately have a taller block, or no more stroke- preferrably a little less.
As a side note- something that has become apparent to me as a reader of this highly educational forum is that the size of the engine does not seem to have a huge effect on the power output if ALL other factors are the same- eg same cam, heads, manifold, blower, OD. I seem to remember a professional engine builder/tuner posting that the difference between a approx 460 ci (dragster) and 526 ci(funny) engine was 20? ftlb of torque when all the other parts were the same. Someone correct me here if I`m wrong.
Jim

 

On the pulling side of things,,,
There is getting to be a bigger and bigger group of us that run 550-650 cube blown alky motors. Some hemi, some chevy.
Ken Veney's personal tractor pulling hemis that he has for sale right now are 541cid so 4.5" stroke and i beleive he runs about 8200 RPM.
The harder running chevys I run against are 588's and 572's with 8-71's (so these are 4.5" strokes).
If the rules would let me I would run 632's, but that's my philosophy and its not dyno tested.

 

I run a 572" motor 4.5 x 4.5 spin it 8200. 7.000 rod. It makes power with not much boost.