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Help with a long-rod project, please.

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Hey guys. I have a Honda CR125 motor bored and stroked to 167. 60mm bore, unsure of exact stroke(think it's actually a 166.82cc?). Original built by Rick Peterson and based around the old powervalve style 1991-1999. It has the stock-length rod (104mm) and I've always wanted to put a longer rod in it. I feel the stock length is contibuting to the short engine life. It has also cracked the exhaust port, twice. I consider it a 10 hr motor, tops. Perhaps a longer rod would free up a bit of power too but i'm really looking for better longevity and freer rev through less sideloading. It's used for iceracing and is like 10-time champion in the Steel shoe fund 3 hour endurance race.

Any input, guys? Anyone have direct experience??! At very least, I know there is an optimal stroke-to-rod length range but I'm unfamiliar with the exact figures, and know one of you can suggest an optimal length.

Do any companies make custom rods that aren't rediculously expensive?

Thanks!

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i have alittle experience swaping rods. how are you sure the total cc if you dont know the stroke ? if your guessing its 166cc with 60mm bore and 104 rod then you would have approx 1.79:1 rod ratio. stock motor would of been around 1.9:1. to get it back to 1.9 with your stroke (assuming its stroked to 58mm) you would need about a 110 rod. what i would do is go to the prox site and they have a catalog that tells you all the dimensions of most rods from nearly all manufacturers of bikes, qauds, snomobiles, watercraft etc. take your stock rods dimensions and find a rod from another machine that will get you the desired length and fit with the least amount of machine work. be sure to find a candidate thats plenty strong also. its highly unlikey youll find something that just drops right in but with some machine work you should be able to make something do the job. had to modify the piston and machine the crank on mine plus do alittle case work. honda rod in a ktm is what i did. dont forget youll need to make a spacer plate under the cylinder

im not sure theres a exact rod ratio thats perfect for every bike. most you big 500cc are likely in the 1.7-1.8 range. cr500 in 1.82 .kx500 is 1.68. smaller motors like yz80 and 125 will be more around 1.9-2.0

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Chevrolet engineering long ago postulated the idea that the ideal range for rod to stroke length ratio was from 1.6:1 to 1.9:1, with 1.75-1.8 being most favorable.

v8 formula one engines dont seem to fit in that ideal range :bonk:.

imo i wouldnt go any lower than what the factory has done. stock is 1.9 on the bike in question if im not mistaken. i like the idea of using the longest rod you can find but theres 1000 opinions on this subject

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v8 formula one engines dont seem to fit in that ideal range :bonk:.

I'd imagine they'd be longer. You have to bear in mind that the higher the rev range the engine is likely to run in, the longer the ideal rod will be. The small block V8 was designed to develop peak power at between 6000 and 7000 RPM. The standard rods were 5.7" for a number of years until the 350 cid engines came along with their 3.48" stroke. The racing division began making 6" rods shortly after that, and longer ones are available now.

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since the longer rod increases dwell time, i wonder if your typical 18K rpm formula one engine piston would outrun the combustion process with shorter rods ? thought i read something about that

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MotoGP 125's look to be 2.25-2.50 rod ratio.

I've also read theory about the piston outrunning combustion with too short of a rod ratio.

Keep in mind that changing the rod ratio will also change the way the porting works, since the piston is at a different position in the cylinder at the same crank position.

Also, I'm not sure a longer rod will buy you much in engine life on a built to the max small engine being used in an environment like ice racing.

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since the longer rod increases dwell time, i wonder if your typical 18K rpm formula one engine piston would outrun the combustion process with shorter rods ? thought i read something about that
Yup, that's the basic problem. The speed of the flame is not entirely constant and fixed, but for practical purposes, it can be said to be. Consider the simple issue of why spark advance is needed. There has to be time enough allowed for the burning fuel to create a useful amount of pressure against the piston when the rod reaches a mechanically advantageous relationship with the crank.

Lengthening the rod places the crank and rod at a favorable angle to each other both later in the stroke, and for a longer period of time in the stroke. This will also reduce thrust against the cylinder walls and piston skirt on the power stroke. The same thing can be accomplished with a shorter rod by offsetting the bore toward the down stroke side of the crank (i.e., forward on a forward rotating crank).

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On four stroke engines such as F1 the rod length is designed to be as short as possible until the G force at tdc begins to lift the ring and lose its seal. The reason that engineers want the rod as short as possible is that the block deck height can be as short as possible for weight concerns (longer rod = heavier block). Thus rod length and ring mass go hand in hand. When ring manufactures can develop thinner rings the rod lengths begin to shorten. My first career was developing engine simulations for race teams (two F1 teams) so I feel reasonably qualified to discuss this subject. Also, there are not any combustion or gas dynamic advantages/disadvantages with a shorter rod. If you plot pistons position versus crank angle (long or short rod) it is has negligible influence on combustion

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im no formula one expert but the info ive found shows one engine to be 98x39.77 with a 102mm rod. puts the ratio at 2.56:1. if im understanding you correct, the only reason the rod isnt shorter than 102 is because the rings lose their seal at that point ?

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As the rod gets shorter the g force at tdc gets higher. This g force is responsible for unseating the ring. Once this happens the combustion pressure escapes around the piston. This is also the purpose of a vacuum pump to help keep the ring seated at high rpm. Unfortunately too much vacuum will make the oil boil so one can only go so far with this technique.

Rod angle ratios are really meaningless. Engineers are simply concerned with maximizing g force at tdc but not exceeding ring float g levels.

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Ring float under power is only a factor at the extreme speeds at which F1 engines operate, and now that they've been limited to 18K, it's not as big a factor as when they were running at 20K. Piston speeds at 18K with a 2.1" stroke and a 4.4" rod exceed 10,000 FPM, compared with less than 8000 in a YZ450 at 11500. Makes a huge difference in ring inertia.

The original purpose of crankcase evacuation was to reduce the pumping losses resulting from pushing air back and forth under the piston.

Although there are other considerations in such high speed engines, he mechanical advantages of reduced rod angularity are primary.

One of the other things you run into sooner with small engines (F1's are 300cc per cylinder) is that there's a practical limit to how short the rod can be made before the crank counterweights hit the bottom of the wrist pin bosses.

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this is a bit of info from engine builder mag.com. i have no idea about their knowledge or credentials but i would guess they know alittle about the subject im sure. im thinking it mainly deals with your v8 street car engine, atleast in this particular article. but none the less it says to me that theres atleast some importance of having a particular length of rod.

Top engine builders are always experimenting to find the ultimate combination that produces the most power and torque in the rpm range where they want it. There is no pat formula for rod ratios that work in every engine or every application. Many of today’s aftermarket performance heads flow tremendous amounts of air, so finding the right rod ratio for a given engine/head/cam combination is a trial-and-error process that separates the winners from the also-rans.

http://www.enginebuildermag.com/Article/16280/connecting_rods_so_many_choices.aspx

but just as there are many opinions on premix ratios and whats best, there most likely be endless debates on rod ratios as well

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alot of info out there says other wise

Rod ratios are simply a mathematical outcome of a design. Nobody starts an engine design with a rod ratio in mind. It is the same as cam centerlines. I can’t believe how many people get hung up on centerlines. Centerlines are simply the outcome of where you open and close a valve – nothing else. But believe me you can try and explain that to an “engine builder” until you are blue in the face and it won’t sink in. Like a dog that stares at your hand instead of seeing where you are pointing. Centerlines and rod ratios are just outcomes of other more important parameters.

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the OP's bike would probably get better piston life with a long rod. But by how much, 1 ride, 2 rides, 10 rides? will it cost him power from the extra case volume from the cyl shim? who knows? surely not hot rod magazine SBC tech.

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Rod ratios are simply a mathematical outcome of a design. Nobody starts an engine design with a rod ratio in mind. It is the same as cam centerlines. I can’t believe how many people get hung up on centerlines. Centerlines are simply the outcome of where you open and close a valve – nothing else. But believe me you can try and explain that to an “engine builder” until you are blue in the face and it won’t sink in. Like a dog that stares at your hand instead of seeing where you are pointing. Centerlines and rod ratios are just outcomes of other more important parameters.

not sure if your familiar with the honda cr500 back in the mid '80s when they were using a 139mm rod with 79mm stroke. then in '87 they lengthened the rod to 144mm and stayed with that combination for the following 14yrs. probly 100 theories out there why. ive got my own theory as well. be cool to hear from the engineer and know the real reason.

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the OP's bike would probably get better piston life with a long rod. But by how much, 1 ride, 2 rides, 10 rides? will it cost him power from the extra case volume from the cyl shim? who knows? surely not hot rod magazine SBC tech.

i suggested at the start of this thread he should try and get back to somewhere around 1.9:1 like the motor originally had. if for no other reason than to help longevity of the internal parts since 125cc is typically spinning to the moon constantly. hard to say what the extra volume from the spacer plate would do. i highly doubt there would be a significant decrease in power. it may even help his power. im pretty sure not every dirtbike engine is designed with the perfect amount of case volume. you start doing porting, custom made pipes, different head designs, and the factory case volume just might not be ideal any more anyways.

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Crankcase compression ratio is critical in a two stroke. Again as I said before rod ratio is an outcome of other more important parameters. You cant say I want a particular rod ratio for my engine as there are other far more critical parameters that need met. Such as crankcrase comp ratio or blow down timing or transfer port timing or g load etc.........

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Greater crankcase volume can work good if you can build enough suction in the pipe. The divergent convergent cones usually result in a fat pipe that's hard to package on a bike. Sometimes hard to squeeze in on a sled too :bonk:

CRANKSHOP-1180-REV-3.jpg

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