Will a TRX cylinder fit on an ATC?

[Ryah]

I've got a short rod 85 250r. Will a TRX cylinder from an 87-89 fit on it? If so, what year piston do I need. What are the differences between the 85-87 cylinders and the 88-89 cylinders? I'm going to need a cylinder any way I look at it so if I can use a TRX it just opens up my chances of getting one.

[X2guy]

An 87-89 cylinder will fit on your short rod bottom end with an 85-86 piston if I’m not mistaken. I have an 86 that I converted to a long rod crank. To compensate I had to use a piston for an 87-89 as the piston pin is set 5mm higher.
There are 3 combinations that I am aware of…
1. Short rod w/85-86 piston
2. Long rod w/87-89 piston
3. Long rod w/85-86 piston & a 5mm spacer under the cylinder.

[fyi]

Why did Honda increse rode length? Additional torque?

I would think the longer rod would increase TDC dwell time and therefore increase torque production. Hmmmmm.....

[X2guy]

More torque by reducing the rod angle.
What do you mean by dwell time? The time the piston spends at TDC would not change.

[fyi]

Increased torque output is derived directly from piston dwell time, amoung a few other details of the build. The longer the piston remains at TDC (usually the swing from 15* before and 18* after actual TDC) the longer the expanding combustion event has the ability to propogate across the cylinder/piston crown to increase cylinder pressure from a more complete burn. In a nutshell, more PDT, the greater the torque output to some extent.

As another example using a SBC~ An engine built w/ a 4" stroke and a 5.7" rod will produce far less torque than would an engine w/ the same stroke, but w/ a 5.85" or 6.0" rod all else being equal. Usually the limiting factor is pin height and how far you're willing to move the ring pack across the piston pin axis. (the greater the pin axis intersection, the less stable the ring pack is at high engine RPM)

We build many of our race engines for automotive use in that fashion and it does make a difference.

[X2guy]

I understand the theroy behind giving the mixture more time to burn but how does the piston on a long rod engine gain more dwell time? Just thinking about it...if your engine is turning 6000 rpm even with a rod twice as long the piston will spend the same amount of time between two given points.
What I understan is that when the crank is at say 90 ATDC the longrod is at less of of and angle than a short rod thus transfering more of the force into the rotation of the crank. A simple example would be when pushing a trike stuck in the mud you push straight from behind and not at a 45 deg angle where only 1/2 of your pushing force would be effective for forward movement.
Anyway...I'm not an expert. I just like to learn.

[fyi]

The one key factor that is most easily forgotton by some, but is VERY important to factor in when building an engine, is the higher piston pin height that's present w/ a longer connecting rod engine combination.

Oversimplified: The longer the rod is, the higher the pistin pin height becomes, which ultimately alters the fulcrum point where the small end of the rod pivots.

A short-rod engine will produce piston speeds far greater than will a long-rod engine. I.E.~ the de-acceleration/acceleration rates from TDC with a short-rod is much faster than would be w/ the latter.

Typically, high-RPM forced induction engine combinations like short-stroke, short-rod, big-bore, short-PDT engine combinations because it's not as important for the piston to remain at TDC as long due to the positive intake pressure that's present from using a supercharger/turbocharger. Additionally, F/I engine's like the big-bore also, because the greater surface area of the piston crown is exposed to the combustion event- thus providing more power. This is why you typically see turbo or blower cars having small C.I. displacements and strokes, but produce HUGE power with big bores for their small cubic inch size.

Long rod, long stroke, small bore, long PDT engine's are just the converse of the latter combination and usually spin far less RPM to make power, but produce far greater torque for any given RPM because of that combination.

Piston TDC dwell time is a very important dynamic engine factor to consider when building an engine for a given application, but unfortunetaly, it's the most often overlooked.

That's why I was asking about the long-rod vs. short-rod approach for these Jap engines, since I don't have as much experience with them as I do w/ automotive racing engines. I do see now that increasing PDT w/ a longer rod combination would be a cheap and easy way for Honda to increase the torque of their 250cc 2-stroke engine, without having to alter much else to achieve their goal.

[X2guy]

OK....I understand now how a short rod engine has less PDT. The shorter rod causes the piston to accelerate much quicker away from TDC. Thanks.
Do you think that as a long rod will always be at less of an angle than a short rod engine this could also contribute to a torque gain?

[max]

just answer the dudes question.