PISTON SPEED/ROD LENGHTH

[Anonymous]

OK guys,here's one for debate,on another stite some guy is saying that by putting longer rods in an engine,you can reduce piston speed.
He even included this link.
http://hotrod.com/techarticles/pit/index14.html

I checked it out,and although the guy who wrote it makes it sound feasible,mathematically,it just doesn't add up.

A motor with a 3.5" stroke,the piston must move a total of 3.5" per stroke,no matter what the lenght of the rod,this equals 7" of movement per revolution.(one stroke up,one stroke down.)

So at say,1000 rpm,the piston speed will be 7000 inches per minute,short or long rods.
Different rod lengths will give different angles in between,and maybe different dwell times at tdc or bdc,but I don't see how it can change piston speed.
In fact,if the piston dwells longer at either,I would think it would have to move faster somwhere along they way,to still be 7000 inches per minute.

Anyone else have any ideas on this?

 

[80broncoman]

Unless the piston moves in only one direction, Its speed changes.

 

[StrangeRanger]

Rod length has NO effect on maximum piston speed; that occurs when the crank throw is horizontal and piston speed equals the instantaneous crank speed in the vertical direction. Nor does it affect average piston speed, that is as you described. Rod length does affect piston acceleration at top and bottom of stroke and, more importantly, piston side loading throughout the stroke. Both are decreased by increases in rod length. Longer rods also produce longer dwells at near TDC so they have the potential to produce more BMEP, therefore more torque. A ratio of 1.75: is generally considered optimum; the gains diminish rapidly beyond this point.

None of this matters a rat's ass on any real-world engine. The gains are infantesimal. If you're going to Bonneville, by all means, worry about rod length. If you're going to take an occasional trip to the local dragstrip, spend your money elsewhere.

If you are talking about the 300, which has an admittedly poor 1.56:1 rod length:stroke ratio there is a fairly cheap solution. A set of 240 rods and a set of trick pistons with a 1.71 compression height will get you to a 1.707 ratio. It's not worth rebuilding an engine for the sake of doing this, but if you have to rebuild one anyway, it might be worth considering.

 

[Anonymous]

Rod length has NO effect on maximum piston speed; that occurs when the crank throw is horizontal and piston speed equals the instantaneous crank speed in the vertical direction. Nor does it affect average piston speed, that is as you described.

That's exactly what I think,the average or overall piston speed doesn't change at all,I think the guy that wrote the info in that link is wrong,on that point anyway.

 

[rickwrench]

Rod length -definitely- affects piston "speed".
Mean, or average, piston speed is the same for any given rod length for a specific stroke, and is an essentially useless factoid.
Piston acceleration is a whole different subject.
Longer rods have lower piston acceleration speeds. Lower acceleration speeds = lower stress on the rods and crank.
An engine with a 3.5 inch stroke and, say, 5.7" rods is not going to be able to safely spin up as fast as the same stroke with a 6.125" rod. The longer rod reduces the maximum piston acceleration. Piston acceleration is what tears rods in half.
Decent articles on acceleration:
http://e30m3performance.com/tech_articl ... rod-ratio/
Rick(wrench)

 

[StrangeRanger]

The term "acceleration speed" technically has no meaning. Acceleration is the rate of change of velocity; Velocity is speed and direction.

 

[Lazy JW]

...If you are talking about the 300, which has an admittedly poor 1.56:1 rod length:stroke ratio there is a fairly cheap solution. A set of 240 rods and a set of trick pistons with a 1.71 compression height will get you to a 1.707 ratio. It's not worth rebuilding an engine for the sake of doing this, but if you have to rebuild one anyway, it might be worth considering.

This may well help you at Bonneville but a longer rod will only hurt low end torque, which is what I want and am unwilling to compromise on.
Joe

 

[rbohm]

...If you are talking about the 300, which has an admittedly poor 1.56:1 rod length:stroke ratio there is a fairly cheap solution. A set of 240 rods and a set of trick pistons with a 1.71 compression height will get you to a 1.707 ratio. It's not worth rebuilding an engine for the sake of doing this, but if you have to rebuild one anyway, it might be worth considering.

This may well help you at Bonneville but a longer rod will only hurt low end torque, which is what I want and am unwilling to compromise on.
Joe

8) not always. it depends on teh rest of the combination as well. you have to remember that an engines output depends on more than just one item. yes you might lose a few ft/lbs of torque at low rpm, but that can made up rather easily by changing the cam profile slightly. something else to consider, when you have a better rod length/stroke ratio, you dont need as much cylinder port size, and that will help improve low end power as well by improving mixture velocity. a side benefit of this is that you also get an improvement in fuel economy. remember though that the difference will be small.

 

[Lazy JW]

The late Gene Berg did extensive dyno testing on air-cooled VW's to see what different combinations did as for performance. Of course, Fords may obey different laws of physics than Volkwagens, but he was able to demonstrate consistant results showing better low speed torque with a short rod using a wide range of head/cam combinations. He concluded that for low end torque you want to yank that piston down from TDC as quickly as possible to get a good vacuum pulse in combination with early valve timing to pull in the air/fuel charge. Makes sense to me.
Joe

 

[rickwrench]

The term "acceleration speed" technically has no meaning. Acceleration is the rate of change of velocity; Velocity is speed and direction.

Correct.
I should have used the term Gmax, and maybe math more complicated than 2+2=4.

Rick(plays to the audience)

 

[Anonymous]

Here's a good article on the effects of changing rod ratio.

http://www.e30m3performance.com/tech_ar ... rod-ratio/

Longer rods do slow the piston speed down close to TDC. Make note of the acceleration charts, and you'll see that the piston with a shorter rod has higher acceleration rates close to TDC and BDC. This means that it comes to a screeching hault closer to TDC, and likewise accelerates away from TDC faster than a longer rod.

 

[Anonymous]

The late Gene Berg did extensive dyno testing on air-cooled VW's to see what different combinations did as for performance. Of course, Fords may obey different laws of physics than Volkwagens, but he was able to demonstrate consistant results showing better low speed torque with a short rod using a wide range of head/cam combinations. He concluded that for low end torque you want to yank that piston down from TDC as quickly as possible to get a good vacuum pulse in combination with early valve timing to pull in the air/fuel charge. Makes sense to me.
Joe

Joe, it's important to note that AC VW engines have rod ratio's of about 2:1 stock. So, without getting into a long sermon, lowering the rod ratio significantly on a VW engine still nets a ratio of around 1.75:1. It's all relative. But based on my experiences of playing with rod ratios on the dyno (especially seeing large stroker cranks with stock length rods), In an ideal world, my 300 would have slightly longer rods mainly to reduce piston side-loading.

 

[StrangeRanger]

...In an ideal world, my 300 would have slightly longer rods mainly to reduce piston side-loading.

Bingo! Forget about acceleration at TDC/BDC, forget about BMEP, forget about dwell. They all offer at best miniscule changes. reduced side loading means reduced skirt scuffing, longer bore life, etc. Those are real-world benefits.

 

[Lazy JW]

...In an ideal world, my 300 would have slightly longer rods mainly to reduce piston side-loading.

Bingo! Forget about acceleration at TDC/BDC, forget about BMEP, forget about dwell. They all offer at best miniscule changes. reduced side loading means reduced skirt scuffing, longer bore life, etc. Those are real-world benefits.

Agreed. I used Dr. Gene's example to point out the futility of spending large sums of money for tiny gains. And considering the reputation the 300 enjoys regarding longevity it would appear that Ford didn't compromise excessively in their quest to make a torquey engine that fit under the hood line.
Joe

 

[300nut]

I think the original term "piston speed" was used only to determine
the distance a piston traveled at a given RPM and a given length of time.
The longer the stroke the more piston speed.

For what it's worth--If you order a custom ground cam for a short rod
engine and a long rod engine,the rest of the engine being the same
there will be slight differences in the intake center line and the lobe
seperation angle. So the dwell time at TDC and how fast the piston
moves away from TDC do change when changing rod lengths.

300 nut

 

[Lazy JW]

...In an ideal world, my 300 would have slightly longer rods mainly to reduce piston side-loading.

Bingo! Forget about acceleration at TDC/BDC, forget about BMEP, forget about dwell. They all offer at best miniscule changes. reduced side loading means reduced skirt scuffing, longer bore life, etc. Those are real-world benefits.

And carrying this thought into the realm of the ultimate torque engines, aka farm tractors, I can't recall ever seeing a tractor engine with short rods. I don't have any numbers to compare and I never really gave it any thought when I had them opened up, but I would guess that none of the tractor engines I have worked on had less than a 2:1 rod/stroke ratio. This on engines that were rated at 2000 rpm or less. Reckon the designers were more interested in longevity than any negligible torque/horsepower numbers. But I'm still sticking with my stock rods
Joe

 

[Anonymous]

I think the original term "piston speed" was used only to determine
the distance a piston traveled at a given RPM and a given length of time.
The longer the stroke the more piston speed.

A given rpm is a lenghth of time,a revolution per minute,more rpm=more piston speed,regardless of lenghth of rod.

 

[83F150]

A good example of how Stroke isnt all that equates low end torque is the 2.9L V6 in the Ranger pickups.....even with a short 2.83 inch stroke, and a 3.66 bore it still churns out a ton of low end (for what it is), because of its intake design, and cam, with a peak torque of 2600RPM.

The 3.0L Engine that replaced it has a longer stroke but a different cam and couldnt pull a greasy string from a cats rear end....making 5ftlbs less 1000RPM higher.

Then some Engines (like the 300) with a long ass stroke and a pretty flat cam make nothing but low end torque.

 

[xctasy]

In the old days, Increased rod length greatly increased expense and weight.

You look at the Hemi 426. Great rod ratio, great power, but my goodness, what a heavy, heavy engine.

Mopar found they could save many dollars per engine by going to a 440 engine, with a worse L/R ratio and a head which wasn't as efficeint.

Ford must be THE most cost conscious auto marker around, because they don't mind having there little engines having awesome rod ratios, but the versions of the engines that sell in volume are always the big inch, long stroke, short rod engines.

Examples are the 240 with a great rod ratio, but the volume was the 300 with a so-called bad rod ratio.

221 with a great rod ratio, 302W and last of all, the 342W (Aussie 347 with stock bore) had long stokes and worse rod ratios. The 144/170 with a reasonable rod ratio, the 200 and 250 with a bad one.

The 375 000 of the garden variety Aussie 302Cleveland with a brilliant rod ratio, lovely reving engine which was smooth and quiet, or the million or so American made 351C with a baratone vibe and a very grumpy stroke to rod length ratio.

Things have changed since 1990, when Zoliner made the first shallow deck 1.22" pistons for the Romeo 4.6. Since then, the US OE and aftermarket have allowed us to piuck up cheaper low deck psitons than ever before.

Look at the 5.7 liter 346 LS-1 and the later 6.0 alloy version, with a tall 6.1" rod, shallow 1.08" pistons , tall 9.5" 351w stlye deck. The smaller and dynamically better 4.8 and 5.3 versions, which are sweeter and smoother, are now more expensive to produce than the shallow pistonsdon't even rate.

Generally, production engineers are the ones to follow. If you've only got a few hundred dollars markup form each engine, and 400 pounds of grey iron or aluminim to build it with, then you used to be freeking mad to make shallow deck pistons.

Now, there are 1 and even 0.875" deck pistons availabe for Scat and World 351 blocks.

So 300 I6's can run 7 and even 7.135" rods.

1.76 and 1.79:1 rod to stoke ratio.

The only issue is that the cost (of pistons and rods) is disproportionate to the result. A potential 14% rod ratio improvement might trigger an engine rebuit, and a cost of perhaps 4 grand extra to fit just 3 grands worth of parts, and then only give another 6 or 7% more power. A Desktop Dyno simulation will show about that much on a given optimised combo. On a race engine, that's the difference between grinning winner or looser. On the street, that's 4 grand worth of whatever you fancy.

You do the math.

 

[Twinscrew]

The magic in the rod ratio becomes apparent at higher RPMs. The higher the rod ratio the longer the dwell at TDC. In lower RPM applications there is sufficient dwell time to build Brake Mean Effective Pressure which, when applied to the crankshaft, is what creates rotational torque. As RPMs increase more time is required to create BMEP because in the grand scheme of things combustion is a slow process. Therefore the longer the piston can dwell at TDC at high RPM the more BMEP can be applied to the crankshaft before the exhaust valve opens. In our relatively low RPM six cylinders, more power from an increase in rod ratio would be nearly impossible to measure on the dyno.