Oversquare Motors Build, (Shorter Stroke, Bigger Piston Build) Options

[tjtryon]

Good Morning!

I'm looking at a rebuild of my 1973 2.0 Motor. When I raced motorcycles, I was constantly rebuilding my motors, chasing the speeds I want, and putting the power where I wanted it in my motor RPM. One thing I (and most that I raced with) did was rebuilding with an over-square motor build. The 2.0 914 Type 4 motor has a stock stroke & bore of 94mm (bore-cylinder width) and 71mm (stroke - piston travel). By moving to a shorter stroke, such as 68mm, the piston would move less in the cylinders, which would allow me to run a bigger piston with less stress on the piston (rings move less causing less friction and less heat) allowing me to run a bigger piston, and would move max power higher up in the RPM range, and allow for a higher max RPM. Less movement for the piston would cause less movement on the rods and crank as well.


My thoughts are to run 103mm pistons, and a crank of 68mm, allowing for a displacement of a 2.3l (2266 cc). I've been looking into a Falicon forged crank, Carrillo Titanium Rods and JE forged pistons. I think this could allow for a much stronger engine than any other I would have.

Does anyone here have any experience with a short stroke, big bore motor? How about those engine builders reading through this? Any thoughts or experiences?

[914werke]

what you are leaving on the table is torque. A longer rod will make more torque with the same piston force, & since it's less angular than a shorter rod, will reduces sidewall loading and decrease component friction. All of this adds up to more power, the reason most popular T4 eng. build combos are strokers. Though if engineering a motor from scratch yes all the concepts you note would aid in the "combo"

[tjtryon]

what you are leaving on the table is torque. A longer rod will make more torque with the same piston force, & since it's less angular than a shorter rod, will reduces sidewall loading and decrease component friction. All of this adds up to more power, the reason most popular T4 eng. build combos are strokers. Though if engineering a motor from scratch yes all the concepts you note would aid in the "combo"

Yes, the Carillo rods will be based on the stroke length. With a 68mm stroke, I could lengthen the rods by about 2mm to increase the compression ratio to about 9:1. Since the stroke is close to the original length (-3mm), I could still run the stock length which lowers the compression ratio only marginally. The 103mm pistons have valve pockets cut, which is more than enough to add +2mm to the rod length. I think I could go as high as +4mm to the rod length, but the compression ratio would be about 11:1, which would cause things to run MUCH hotter. I don't want that much extra heat, as that goes against my plans to remove as much extra heat as possible.