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?

[burton73]

Mark and I just rebuilt a 2270 built by Jake Raby in 2005. HAM RAT heads, Keith Black Pistons, CR rods special cam and well on and on. $3,000 plus in machine work and new parts from George at European Motorworks. 78 Stroke, 96 MM Pistons

Best Bob B


Old posting from March 2009


Jake Raby
Mar 11 2009, 03:24 PM
It's been so long since I started a new post here that I almost forgot how to do it... I have refrained from "Chest beating" on the 914 forums for a good while, but I figured the results we experienced with this engine was worth the BS that the post would probably create, so I decided to post it and take my chances.

I cross posted this from my forums.aircooledtechnology.com site
------------------

We built a 2270 for one of our race customers 7 years ago. He wanted huge torque and not a ton of revs, so I created him a combo based from these desires.

The engine was, at the time one of our higher HP models of the 2270, running around 10.5:1 CR, Web split duration cam based on the 119/163 base grinds and 2.0 heads with 48X38 valves coupled to 48 IDF carbs.

The engine ran for 7 race seasons flawlessly with only routine maintenance, it still had the same carb jetting that it left our dyno with! I don't think it was adjusted much at all as it looked the same as when it left!

So the graphs below tell the story.. 64HP came from the same exact engine with only a head, CR and camshaft change made!! Look at the torque, the 09 design made more at the lowest test RPM than the 02 model did at peak!!!

This illustrates how performance really evolves and I don't think many people realize the difference even one year can make here!

The 09 design has throttle response like a rocket and the power is very crisp to say the least- the owner is already buying new tires!

I won't go into detail here about what was done since this is a competition engine, but I will say the engine still runs on the same fuel that it did before, has a RAT 9700 cam (non roller) and a set of cylinder heads based off the LE 200 port profile that are not yet released to the public. (they still flow 200 CFM @ .500!)
The engine still uses the same dizzy, same set of carbs and same venturi size as in 2002, but it wants 8 degrees less total advance and has lower BSFC numbers!

There is only one place to buy our level of development and this proves what we have accomplished. I'll bet it lasts another 7 seasons.

Here ya go, the difference in yesterday and today...
The graph from 2002 was still in the customer's file.. There is a huge benefit to keeping records!



http://www.914world.com/bbs2/lofiversion/i...php?t93548.html


Mark and I just rebuilt a 2270 built by Jake Raby in 2005. HAM RAT heads, Keith Black Pistons, CR rods special cam and well on and on. $3,000 plus in machine work and new parts from George at European Motorworks. 78 Stroke, 96 MM Pistons

Best Bob B
THESE ARE JAKES DYNO NUMBERS