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There is a lot of speculation as to which should be used in which application. The common phrases that are thrown around fall into one of the following: Long rod = more top end reliability and short rod = faster revs, long rod = more torque and short rod = more hp, etc.

The truth of the matter is that there are benefits and downsides to each. Before the technical side of things is covered, the objectives for the build in question should be determined.

Where do you hope to make your power? Is this bike a high revving road racer or a woods dwelling torque monster? Is this bike to be used for a single season or 5?

There are other concerns, but I'm sure the inclusion of those would bore those of you who could already care less.

Long rod: With a long rod, the closer your piston is to TDC or BDC, the more your piston will dwell. This is great for cylinder filling as your ports are open for longer. However, the closer your piston is to 90 before and after TDC and BDC, the faster it is moving. This is so with all engines, short and long rod alike, BUT your piston accelerates more with a long rod than with a short rod at the same RPM. The reason for this is that, due to gemetrical math stuff, long rods translate less vertical change as it moves left and right (as viewed from the side)with the crank pin, leaving all of the vertical movement to be made as the crank approaches 90 and 270 degrees.

Now, with that in mind, since your piston dwells more when it's closer to TDC and BDC, your piston ultimately makes it's way closer to the head faster, but maintains a sqiush for longer. Following suit with the previous, it takes it longer to make it's way from TDC (once it does hit a certain threshold, it accelerates above the normal rate). With Ignition timing in mind, where does this leave us?

There is much speculation amongst engine builders as to whether it affects timing. The fact of the matter is that it does for most of us. The objective of ignition advance is to ensure that the mixture reaches maximum pressure immediately after TDC so that more work can be derived from the burn. When the piston dwells at TDC for longer, the innacuracy of timing is less pronounced. For those who have the ability to precisely calculate this exact point, long vs. short isn't a concern of time.

Heat exchange is also another issue. Since the burning/burnt mixture is dwelling at TDC for longer, more heat is being absorbed into the head and piston than a short rod, even if the exhaust duration is 180 for both. Imagine a stove burner and how hot it is at 3" and how hot is at that 3" for twice the amount of time. Saturation of anything makes heat transfer more. When the mixture is compacted more, more heat is absorbed into the piston and head. (keep in mind that in most cases, this is negligible)

Wear is the other concern for most builders (or should be). Since the piston sees more of a vertical movement as opposed to the side-side pull of a short rod, sleeve wear is reduced. However, there are some who say this is non existent. For those of you, I ask you to go ask any mustang 5.0 owner who has had their engine stroked to a 331ci. Ask them how much oil they go through. Now, wear on the big end of the rod and bearings is increased. Remember how the piston dwells at TDC for longer? During that time, the piston is pushing nearly vertically on the crank, making no power in that time AND wearing more on the bearing.

Lastly, consider that a long rod weighs more and can hinder low end torque (which is also gained due to dwell at BDC) and can reduce top end reliability. With the piston accelerating more, the rings are more prone to flutter as the piston also has to decelerate faster.

Short Rod: Unlike a long rod, the short rod setup allows for a more even acceleration of the piston. Since the side-side movement (again, as viewed from the side) has a more direct effect on vertical movement, the piston begins to accelerate sooner and doesn't see as high of a piston speed as a long rod would at the same RPM. However, since there is less dwell, cylinder filling is harder to accomplish. For this side of things, I leave for you porting experts of which I am not.

Now, with ignition, a short rod is more finicky as the piston builds an angle on the crank and can generate torque sooner in the stroke than a long rod. This means that the accuracy of ignition timing is more important than in a long rod (both are very important). If you build pressure too soon in a short rod, there is more push back than with a long rod, again because of the angle on the crank at positions closer to TDC and BDC. I've never actually seen it, but I've been thinking about how much power can be gained from advancing the ignition in a short rod, vs. the amount to be made with a long rod... hmm.

In addition to this, the purpose of a chamber that designed with a squish band must also be accounted for. The original idea (I'd like to reference the Two Stroke Tuner's Handbook) was to compact the majority of the AF mixture into an area so as to allow for a quicker burn while allowing the outlying ares (in the squish band) to be cooled by the proximity to the head, allowing for better cooling of the mixture to fight detonation. With the short rod, the piston approaches the head at a slower rate and can detract from the ability of the squish band to properly cool the AF mixture in the squish band. This can lead to detonation. Again, making timing more critical.

With the short rod, wear is more pronounced on the sleeve as the piston travels in a less vertical manner.

Lastly, a short rod weighs less and can aid engine acceleration. Since the piston doesn't have a maximim acceleration tht is as close to that of a long rod, piston speed is reduced for a given RPM as opposed to a long rod. This means higher safe RPM with the same type of ring.


I hope this was beneficial to at least someone. Though you may have learned something, please don't tell any professional engine builder how they should build your engine. Their reputation lies on their ability to produce a quality product. Allow them to decide if you should run a short or long rod.

-Faith
 
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