Longer Con Rod

[St George]

I know my 200 has been fitted with a longer Japanese con rod and a 3 mm Packer to compensate. Obviously this makes the stroke longer. Is this purely for touring engines. What other advantages are there etc. May seem a silly question to the experts on here, but we all learn every day. Thanks lads.

[Adam_Winstone]

A longer rod does not change stroke as the stroke is controlled by how far out the centre of big end pin is from the centre of the crank. If the rod is longer the piston starts higher up the bore but equally remains that amount higher up the bore at BDC, meaning that stroke is unchanged. A 58mm stroke crank remains 58mm stroke, regardless of con rod length. However, move the crank pin 1mm towards the outer edge of the web and the piston then travels 1mm further up the barrel and 1mm further down the barrel as the crank rotates, equalling a 2mm increase in stroke (60mm stroke).

There are a number of reasons why you can benefit from changing to a longer rod, even when keeping the stroke unchanged. These reasons may include; allowing for a piston with a different compression height (many Jap conversions), reducing primary compression / increasing volume for transfer, allowing a reconfiguration of port heights (you choose whether packer goes at bottom or top) and performance, etc. All of these reasons are valid but it sounds as though yours may have been done to move piston crown up in relation to port height, therefore reducing port timings and providing a broader spread of power (e.g. Taffspeeds fitting of 110mm rods to standard TS1 kits).

^... all basic stuff but an introduction to some of the issues.

Adam

[St George]

Thanks Adam. I know that it has an AF Crank and apparently the con rod is upgraded ? Does this affect anything in your otherwise comprehensive reply. Just trying to understand what state of tune I have. Regards.
PS Can you elaborate on the volume of transfer you mentioned.

[Adam_Winstone]

TBH the volume of charge to transfer is probably the least impact of any of the elements mentioned, with people having differences of opinion over any benefit it provides. What it relates to is that a longer conrod places the piston further up the bore (unless the compression height of the piston is reduced) so there is more physical space in the crankcase to fill with fuel / air charge, which in turn means more mix to push up through the transfers as the piston comes back down again, hopefully resulting in a bigger bang when combustion happens. Also, increasing this space reduces pressure, which in turn effects the pressure/speed that the mix is transferred from crankcase to combustion phase.

Unfortunately, without knowing the length of conrod, confirmation of the stroke, confirmation of piston type fitted (compression height), and any tuning that may have been done to the barrel, there really isn't much that we can tell you about the state of tuning. Welcome to the 'thin edge of the wedge' that is understanding tuning.

Adam

[St George]

Thanks again. Sometimes I wish I had a standard 200 engine. No wonder the forum is so busy. I will plod on with my jetting tests.

[Fast n Furious]

Back in the mid 80's when reed valve cylinders started to make an appearance, it became apparent that the your part worn out standard crank wasn't going to be reliable for much longer. SIL had the lions share for replacements but these were not up to it either.
107mm conrods are pretty much a Lambretta size thing, much like their 22.9mm bigend pins. I can't think of any other key manufacturer who used this length at the time and certainly no-one else used 22.9mm bigends.
Yamaha parts, strangely, seem to have an affinity with our lammy's and the quality Japanese made RD350 110mm conrod became a natural choice for a number of reasons. It just needed someone to make decent webs with 22mm diameter bigends and then it became something capable of handling the extra power.
It was more a reliability development than a technical /performance one.