What are people opinions on long v's short rods?

Damo · Post by **Damo** » Thu Mar 03, 2011 9:46 pm

Have been doing a little reading reicently, and the latest thing that has caught my attention is rod/stroke ratio. The school of thought is that a longer rod will have the piston dwell around TDC for longer thereby imparting greater force onto the crank as it is leaving the combustion chaimber smaller for longer. Also, it has the advantage of keeping the rod more vertical to the crank imparting greater force.

I have a whole bunch of problems with the things I am reading as the results spoken of seem to contradict reality.

If you have a theory, let it rip.

I'll add my 2 cents later.

Damo

Kimmo · Post by **Kimmo** » Mon Mar 07, 2011 10:17 pm

Intuitively, a longer rod seems better; seems to me minimising the rod's angle is a good thing, for some reason... I imagine it'd rev better.

Up to the point your rod starts to get a bit heavy, I guess

Post by **tassuperkart** » Tue Mar 08, 2011 7:07 pm

Ive not experiment myself but by all accounts its a winner on all fronts.
L8r
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pauly72 · Post by **pauly72** » Tue Mar 08, 2011 8:09 pm

the longer rods minimise the the size of the chamber there by giving the piston more velocity on the way down, making it go faster with less distants it travels before its at TDC again
the rod impacts with the crank higher in its revolution being longer,so lessoning the angle right ,so rev easier
but as kimmo said good till your rods get to heavy and you've lightened the flywheel and crank as much as you can
throttle responds would be wild

Pauly

Damo · Post by **Damo** » Tue Mar 15, 2011 10:58 pm

pauly72 wrote:the longer rods minimise the the size of the chamber there by giving the piston more velocity on the way down, making it go faster with less distants it travels before its at TDC again
the rod impacts with the crank higher in its revolution being longer,so lessoning the angle right ,so rev easier
but as kimmo said good till your rods get to heavy and you've lightened the flywheel and crank as much as you can
throttle responds would be wild

Pauly

Pauly, I have no idea what you are trying to say?

The idea of long rods has been around for a very long time. The whole keeping the piston dwelling at TDC for more crank degrees allows the air/fuel mix to burn for a greater time at a higher pressure. This in effect is supposed to generate more torque at higher revs.
An average 'burn' time for normal (non oxygenated) pump fuel is around 2m/s, so it only make sense that at elevated revs using the 2m/s fixed guide, you will end up loosing pressure in the cylinder as the piston moves futher away from TDC (effectivly increasing the combustion chaimber size) before the charge has fully burned.
So the answer (supposedly) is to go for longer rods to increase dwell at TDC.
The problem with this is there is always a trade off in engineering any method. If the rod ratio is 'too' long you end up dropping torque low down, as the fixed burn time is still 2m/s and combustion is completed well before the crank is at a great enough angle to the rod to take advantage of all that combustion pressure which is already cooling, at lower rpm.
There is also a greater risk of detonation which requires either high octane fuel or reduced ignition timing to keep in check. And for what gain?, possibly a couple of hp at the top end! Which I have seen no dyno evidence.

Then there is the matter of what exactly a long ratio is? From what I have read, greater than 1.75 is considered to be long, And less than 1.6, short.
Why is a long rod ratio needed for 'high' rpm anyway. All the hipo Honda 8000rpm uber donks run LESS than 1.6! So what revs does a long ratio rod support?
F1, the ultimate big engine, high reving donks (18,000-20,000rpm) use around 2.2-2.4:1 ratios.
So it seems that every point of ratio in rod gives around 2,000rpm higher effective peak power point if you use a couple of loose examples.

Being able to control combustion via having a very well designed squish area and equaly high comp ratio frees up revs far more effectivly than a change in rod ratio ever will.
Old ohv Commo's from the 90's ran 8.5:1, these days 10.5:1 is well achivable on even bargain basement cooking model engines on 91 octane.

I have even heard/read stuff about "high octane fuels burning slower which supposedly releases pressure more effectivly at higher revs." But if you look at the process behind it, a 'lower' octane fuel with faster burning qualities would be better suited at high rpm rather than high octane high rpm. Either way, that is mis-imformation.
Low or high octane, pump fuels burn at the same rate.
High octane fuel resists detonation from higher peak cylider pressure which occure at LOWER rpm.

Rod ratio, Over rated! IMHO!

Damo

Post by **tassuperkart** » Wed Mar 16, 2011 6:13 pm

pauly72 wrote:the longer rods minimise the the size of the chamber there by giving the piston more velocity on the way down, making it go faster with less distants it travels before its at TDC again
the rod impacts with the crank higher in its revolution being longer,so lessoning the angle right ,so rev easier
but as kimmo said good till your rods get to heavy and you've lightened the flywheel and crank as much as you can
throttle responds would be wild

Pauly

Firstly, the length of the rod should have NO impact on the size of the combustion chamber. You must lower the piston deck height in proportion to the increase in crank length to maintain static CR.

The longer rods also allow the piston to dwell longer over TDC and allows far reduced ignition timings to achieve maximum combustion chamber pressure at the ideal crank/rod angle so less energy is lost trying to shove the crank straight down but actually convert to torque.

Side thrust on the piston is also reduced due to the lessened rod angle off vertical.

This is what I understand are some of the plusses of increasing rod to stroke ratio.

L8r
E

As far as making the engine "revvier"....bollocks

Damo · Post by **Damo** » Wed Mar 16, 2011 7:40 pm

The longer rods also allow the piston to dwell longer over TDC and allows far reduced ignition timings to achieve maximum combustion chamber pressure at the ideal crank/rod angle so less energy is lost trying to shove the crank straight down but actually convert to torque.

Side thrust on the piston is also reduced due to the lessened rod angle off vertical.

This is what I understand are some of the plusses of increasing rod to stroke ratio.

L8r
E

As far as making the engine "revvier"....bollocks[/quote]

Side loading would be a non issue due to offset piston pin design of even the humble E15, but yes, a longer rod will reduce loading in a non off set combo.
It seems there are far more compramizes in the humble engine than at first glance.

Damo

Post by **tassuperkart** » Thu Mar 17, 2011 6:46 pm

Ummm nah Damo, I think offset pins are an effort to reduce noise from piston "rocking" no???

Either way ANY reduction in side loading is a bonus.

L8r
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Damo · Post by **Damo** » Thu Mar 17, 2011 9:22 pm

tassuperkart wrote:Ummm nah Damo, I think offset pins are an effort to reduce noise from piston "rocking" no???

Either way ANY reduction in side loading is a bonus.

L8r
E

hmmmm.... I may have to look into that futher.

I can remember back in Auto tech days (y11/12) seeing exagerated drawing of a piston on the power stroke. The pic was two images showing the difference in how force was being loaded with a central pin 'falling' in the direction of crank rotation, compared with a more vertical drop with the offset.

I wonder if I can find it on the net somewhere.

Definitly the less side loading there is the better.

Damo

Damo · Post by **Damo** » Thu Mar 17, 2011 9:31 pm

Not what I was after, but found this:
http://www.ftlracing.com/tech/engine/rsratio.html

and then this:
http://www.motorcycleproject.com/motorc ... ffset.html

Your part way there old buzzard!

Damo

Damo · Post by **Damo** » Thu Mar 17, 2011 9:57 pm

Also found this:
http://www.epi-eng.com/piston_engine_te ... basics.htm

This is a pretty good summary of everything. It also throws in another interesting thing, the shorter the rod/stroke ratio, the greater the out of balance the engine is, the harder it revs.
So the longer the rods you can get into the engine, the smoother it will be at full power revs.

Another interesting thing, are forged pistons machined with offset pins? If not, is this where forged pistons get their "slap" from?

Damo

Kimmo · Post by **Kimmo** » Fri Mar 18, 2011 7:40 am

Damo wrote:Another interesting thing, are forged pistons machined with offset pins? If not, is this where forged pistons get their "slap" from?

Wow, given the almost scary degree of insight into this stuff you've displayed, that seems like a dumb question...

Why would the fact a piston's forged mean it couldn't be made with an offset pin? And I've seen it mentioned often enough around here that forged pistons slap cause they expand more thus have greater clearance when cold.

Ah well, all this reciprocating malarkey can't stay relevant for too long anyway... in fifteen years we'll prolly be talking about which transistors are best to use to control our brushless motors.

Post by **tassuperkart** » Fri Mar 18, 2011 7:34 pm

Kimmo wrote: Why would the fact a piston's forged mean it couldn't be made with an offset pin? And I've seen it mentioned often enough around here that forged pistons slap cause they expand more thus have greater clearance when cold.

Proverbial nail on the head Kimmo.!!
Correct! Go straight to the head of the que!
L8r
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Damo · Post by **Damo** » Sat Mar 19, 2011 2:08 am

That probably should have read, "Are ALL forged pistons machined with centred pins unless requested?"

I was thinking some people may have been mistaking piston slap from centre machined piston pins, with cold forged piston clatter, and these people just assumed all the noise was from the fact that they are forged rather than pin location.

Damo

Post by **tassuperkart** » Sat Mar 19, 2011 4:48 pm

If you have cast piston clattering, the position of the pin will have little effect.
The issue is they are ROOTED. Pure and simple!!!

Forged type pistons will rattle when cold and be more noisy hot regardless of where the pin is located. Way it is with them. One of the reasons why I dont use the bloody things.

L8r
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