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?

[technicalninja]

I am an old school engine builder as well.

I've always been envious of the bike engine builders as their parts are so much smaller/lighter than automotive engines.

The issue of time for flame front travel across the cylinder bore is usually not an issue for a small diameter bore.

My suggestion is to search on "Jake Raby" posts on all of the forums you can find that he has frequented.

His tips and suggestions changed my outlook on the type 4 engine.

A single tip that helps you attain your goals will be worth the time spent.

One tidbit that he shared was bores above 96mm really required dual plug heads to make more power.

Another is "more money spent on the heads than the lower end."

He stated that he takes the new castings (AA or AMC) and removes EVERYTHING including the seats and guides. ON a single plug head, he welds the plug holes up, recuts to 12mm and changes the plug orientations.
This is a BUTTLOAD of work to make his power figures.

I did a bunch of research, and it altered my outlook.

Controlling heat is the single most important factor in making one of these work properly...

Over 400 degrees is a head killer, and that number is damn easy to hit. Just miss-tuning one of these (a STOCK one) and you're there...