Can anyone explain why a longer rod is better?

What is the point of diminishing returns when it comes to rod length?

 

John,

There are some arguements for this. Some people like shorter rods, some like longer.

On a NA engine I like a longer rod. You can have a lighter piston because of less compression height which means lower piston G's on the rod and crank. The longer rod also has more dwell around TDC which can reduce timing some. A 6.00" rod to a 6.25" rod can reduce timing 2 degrees in some cases.

Basically the limit is the stroke/deck height and that leads to a low compression height. Bascially a 1" compression height is the limit.

Bret

 

And, the shorter the rod, the more side loading of the piston you have.

I don't think you can accurately say that any rod/stroke ratio is perfect.

As to whether longer is better for power...

It is assuming the same stroke. But, in many cases a longer stroke at the expense of rod length is better...otherwise we'd all be running stock strokes with super long rods. The extra CIs win out over the efficiencies of the longer rod.

The higher the RPM you turn, the more important it is to have a good rod/stroke ratio. This is due in part to friction, side loading, and time spent at TDC. This is the reason my new C5R engine has a 4" stroke rather than a 4.1 or larger. Had we been planning for 7000 rather than 9000 RPM I would have gone with the shorter rod/longer stroke setup.

 

^ Hes got it!^ <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />

 

Less Piston rock and more TDC. Longer is better.

 

Mainly rod length cuts down piston weight. Your rod ratio is stuck in a fairly narrow range as far as what it can really change in geometry due to either the counter weights hitting the piston pins and skirt and the deck height you have already built into the block you have. Any rod ratio you lose in getting stroke is usually beneficial in reducing rpm and the associated loads it generates. Whatever stroke you do end up with though it's nice to have the longest rod possible. 10 grams of weight off a piston is more inertial piston and side loading than going from a rod ratio of 1.50 to one to 2 to one on a 4 inch stroke engine going 7500 rpm for instance so rod ratio is not going to be any huge gain ever. Of course you get the benefit of a slightly better rod ratio AND less weight by running longer rods!

 

Sorry to be a nuisance, and to be off subject.. But you guys should really read some of these posts out of context. Maybe I'm immature, but I was laughing my *** off. Some people like longer rods, some people like shorter rods. In any case, I was always curious about this subject and thanks for the informative info.

 

Maybe you're just immature. There's nothing mysterious or new about rod length, and the posts in this thread are right on the money. One of the big advantages that LS1's have over old-school SBC's is the longer OEM rod length.

If you have ever built a stroker motor, rod length is just one of the design considerations to consider, and it's not a trivial issue.

 

Threads like this are exactly the reason why this board is a great asset to the shadetree LS1 modder like myself. I always welcome gaining a better understanding of the physics of what's going on inside my motor. It's funny, I'd say in the last year 90% of the in depth technical information I've learned about the internal combustion engine has come from this board <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />

If I have an urge to laugh about rods and strokes, I'll listent to Andrew Dice Clay <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />

 

So, going the other way, what is the downside of running a big stroke motor, like 4.125 with a 6.000 rod?

 

A little more cylinder wear due to side loading.

The steep rod angle is not as good if you want to spin 9000 RPM. At some point of increasing RPM and decreasing rod/stroke, inefficiency (less time spent at TDC and frictional losses) and block integrity (side loading) become more of a deficit than the increased stroke can afford.

If running semi-sane RPMs I'd go with the longer stroke over the longer rods any day. If NASCARland is your game, you need to study deeper.

BTW, I think it is possible to run a longer rod than that with that stroke.

 

Cool info guys!!!

Col, we can't run a ~9:1 piston with a 4.125 stroke and a 6.125 rod that would be the exception.

 

You should always try to get the biggest bore possible instead of adding stroke. Only add stroke when you are at the limits of your bore size. This will give you a more efficent engine over one that has a lot of stroke in it. The 1.8 rod ratio is what is considered desirable in the racing world.

Chris

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by SStrokerAce:
<strong> John,

There are some arguements for this. Some people like shorter rods, some like longer.

On a NA engine I like a longer rod. You can have a lighter piston because of less compression height which means lower piston G's on the rod and crank. The longer rod also has more dwell around TDC which can reduce timing some. A 6.00" rod to a 6.25" rod can reduce timing 2 degrees in some cases.

Basically the limit is the stroke/deck height and that leads to a low compression height. Bascially a 1" compression height is the limit.

Bret </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">In addition to increased dwell at TDC, which incidentally helps build cylinder pressure early, you also have the benefit of a slower moving piston coming off of TDC, which helps give you a more complete burn. Hence, the effect of needing less timing as SStrokerAce mentioned above. Beyond 15 degrees of crank rotation, the longer rod actually accelerates the crank faster than the short rod, helping you get a better cylinder fill. Generally, the smaller and less efficient the intake port, the better the engine responds to a long rod. That is why BB Chryslers were such torque monsters. I think stock 440's had a R to S ration of over 1.7. The stock cylinder heads didn't flow worth a damn, but if a BB Chevy or Hemi guy wasn't on his game, he was going to be looking at taillights.

On the other end, as the crank approaches TDC, the long rod engine also slows down more than a short rod engine, once it gets to within 15 degrees of TDC. The overall effect of this on camshaft selection would mean that you can get away with a cam that has gentler initial opening and closing ramps, which are not only easier on the valve train in general, but help to prevent valve bounce.

 

So show much more piston velocity and side wear will there be with a 6.125" rod and 4.125" stroke compaired to a 4.0" stroke? Is it an 'issue' when spining the motor to about 7300 or 7400 rpms? If the rod ratio is import, why do the 383 Stroker kits use 6.125" rods rather than 6.2" rods? There is enough deck height for a 6.2" rod with a 4.125" stroke.

 

Of course the old school example has an easier time of getting to 1.8 with it's bigger BB bore, what is is like a 4.500 bore?

In some respects (tangent alert) this validates the thinking behind a 4.150 SBC with a a stock 3.48 stroke (377ci).

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Pro Stock John:
<strong> Cool info guys!!!

Col, we can't run a ~9:1 piston with a 4.125 stroke and a 6.125 rod that would be the exception. </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">I thought that ARE built up a motor with a 4.125" stroke and a 6.225" rod?

What would the rod limit be on a motor with a 4.25" crank? What about rpm potential? Can a 4.25" crank motor rev to 7,500? <img border="0" alt="[cheers]" title="" src="graemlins/gr_cheers.gif" />

 

1camwinder, it depends. It depends on the desired compression of the motor.

 

Col, we can't run a ~9:1 piston with a 4.125 stroke and a 6.125 rod that would be the exception

no ya cant but ya can enlarge the combustion chambers to offset that.

good info guys. i dont think i will have to bring up the "mr smartie pants" comment again. (brett;D)

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by ls1290:
<strong> So show much more piston velocity and side wear will there be with a 6.125" rod and 4.125" stroke compaired to a 4.0" stroke? Is it an 'issue' when spining the motor to about 7300 or 7400 rpms? If the rod ratio is import, why do the 383 Stroker kits use 6.125" rods rather than 6.2" rods? There is enough deck height for a 6.2" rod with a 4.125" stroke. </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Anything with a stroke over 4.000" has a issue with piston speed over 7000rpm. Now yes there are big blocks that do it, but they are drag racing units that need to have parts replaced more often than you want them too. Piston speed is really the limiting factor to stroke lenght sometimes.

Right now I have a stroker assembly for a 383 LS1 that has a custom Cola Crank, Oliver Rods, and Diamond pistons. It's a 4.000" Stroke, 6.200" Rod and a 371g piston. It comes with H13 N2O Pins, support rails, and is balanced already. It's designed for a 900hp N2O engine. If anyone is interested in it, shoot me an e-mail.

You don't have enough deck height and compression height in a LS1 to run a 4.125/6.200 combo but you could run a 4.125/6.125 combo.

Bret