916/996/748 Crank Swapping for Stroke Increase

[JEC]

I'm studying up on 4V tuning and I came across an interesting possibility that I don't often hear about...

900SS/ST2 crankshafts have a stroke of 68mm
916/996 = 66mm
748 = 61.5mm

For the 748: You can install a 916/996 crank in a 748 to get 803cc, while installing the SS crank will give you 827cc.

916: SS crank = 944cc
996: SS crank = 1026cc

The benefit of stroking a motor vs boring it is that stroking gives you more torque and midrange instead of top end power, and doesn't weaken the barrels by making them thinner than originally intended. You also don't mess with the weight of the pistons, which can increase vibration and force you to rebalance the cranks.

You keep the stock conrods out of your SBK (which are 124mm vs the SS's 130mm) and install them on the SS crank. In the case of the 916-748 swap the conrods would be the same length anyway (but possibly a different design depending on the model, variations abound).

This is particularly impressive in that you can pick up a common spare part (the SS crank) and install it for a significant stroke increase, without resorting to ridiculously expensive aftermarket items. A complete spare motor is often cheaper than a new stroker crank.

For those that are doubtful, just remember this - the 916 is basically just an 888 with an extra 2mm of stroke and a different airbox design

[853MACHINE]

Ok but if you increase the stroke without exchanging the pistons with ones that have less distance from the pin to the piston top or get longer cylinders, won't your piston sort of hit your head?

I thought of this when doing my 853 kit untill I realized, if I put a 916 crank in it with the 94mm pistons it would be a ...... 916!
The 68mm crank would be neat though !

[JEC]

Originally Posted by 853MACHINE

Ok but if you increase the stroke without exchanging the pistons with ones that have less distance from the pin to the piston top or get longer cylinders, won't your piston sort of hit your head?

I thought of this when doing my 853 kit untill I realized, if I put a 916 crank in it with the 94mm pistons it would be a ...... 916!
The 68mm crank would be neat though !

Strangely, the manual doesn't address this at all - but if you can buy stroker cranks, then you can buy pistons with increased crown heights to match. This way, you save some cash by buying a standard crank with a different stroke.

[JEC]

Also, if you had a newer style motor with the copper head gaskets, you could theoretically alter your squish with some thicker, earlier fibre type gaskets that are about 1mm thicker - but that wouldn't solve the problem entirely.

Someone should post the piston-to-head clearances of each combination if they have the means to calculate it, then it would just be a matter of getting some pistons machined to match your crown height needs.

[Synergy]

Warning about changing cranks!!!

The single phase and 3 Phase charging systems have different crankshaft spline configurations for the stator assembly. You can put a crank from a 95' 916 into a 01' 748 but you will have to change the whole charging system with it.

[Synergy]

This post got me thinking more and I am not trying to take the wind out of your sails.

My prior post was talking about superbike charging systems. I have no Idea about SS charging systems. I doubt they would be interchangeable. Different computers, different wiring. It would take some serious work measuring and checking crank shaft dimensions and splines before dropping a SS crank into a SBK. What about big end dimensions? Are the mains the same?

900SS crank 68mm
748 crank 61.5mm

A difference of 6.5mm. Where is that 6.5mm going to go? Does the dome of the heads have enough volume for the piston to clear the valves with that added 6.5mm stroke. What about your squish?

I am far from a engine builder or a mathematician. I am just trying to throw all of the variables out there. Food for thought.

http://www.circletrack.com/techartic...e_compression/

http://www.circletrack.com/techartic..._head_volumes/

Originally Posted by JEC

The benefit of stroking a motor vs boring it is that stroking gives you more torque and midrange instead of top end power, and doesn't weaken the barrels by making them thinner than originally intended. You also don't mess with the weight of the pistons, which can increase vibration and force you to rebalance the cranks.

If you have the crank out of the motor its very inexpensive to balance its 130 bucks. If your building a hot rod motor another 130 bucks shouldnt break the bank.

Originally Posted by JEC

This is particularly impressive in that you can pick up a common spare part (the SS crank) and install it for a significant stroke increase, without resorting to ridiculously expensive aftermarket items.

How about the electrical charging system? Thats another cost and usually need a donor bike to get a complete system.

Originally Posted by JEC

Someone should post the piston-to-head clearances of each combination if they have the means to calculate it, then it would just be a matter of getting some pistons machined to match your crown height needs.

Do you know what machine labor runs? Upwards of 80 bucks an hour. To machine two, one off pistons would be incredibly expensive, unless you have a mill at your disposal. Machining pistons your back to different weights, back to balancing.

For cost effectiveness, from your initial post I gather that is what the goal was. This process is more expensive and more of a headache than a big bore kit. A big bore kit is plug and play, just swap the top end. You dont have to crack cases and swap cranks, electrical systems, rods, etc...