engine and drivetrain Rod Size?

[tim385]

What advantage or disadvantages to using 135mm or 144mm rods.Engine to be built for track use.

[Chamonix72]

144mm give you a longer stroke, so a bit more displacement. That should lead to more torque I think. The downside is that you will need custom pistons to relocate the wrist pin mount. Of you are already going with custom pistons, then not too big a deal.

The stock length of course is more plug and play.

[allbim]

144mm give you a longer stroke, so a bit more displacement. That should lead to more tongue I think. The downside is that you will need custom pistons to relocate the wrist pin mount. Of you are already going with custom pistons, then not too big a deal.

The stock length of course is more plug and play.

OMG.  Did you mean plug and pray?  

[FB73tii]

144mm give you a longer stroke, so a bit more displacement.

 

No, stroke is from crank throw, not rod length. Displacement comes from the particular combination of stroke, rods, pistons, and combustion chamber volume being used.

 

Using a longer rod for a given displacement (which requires moving the wrist pin higher in the piston) increases the rod/stroke ratio. This has a number of effects but I am not an expert here so I will post a few relevant links below and let others chime in. The guy who built my slide throttle (and who works on engines for Daytona Prototypes and land speed cars) said that while many people think the longer dwell time at TDC is a benefit, it is really the piston speed that affects things. Something about the longer rod helping scavanging or packing, but I don't have my notes from that conversation. Doing 144mm with raised pin on a 2.0 L is fairly standard. Others have gone with even longer rods, getting the wrist pin up into the ring area and requiring custom pistons to accommodate this. John Forte was using 162mm rods in his GT3 tube frame 2002, I believe that was a 2.2 or 2.3 liter engine.

 

Here's a few articles to get the conversation started:

http://www.stahlheaders.com/Lit_Rod%20Length.htm

http://www.mayfco.com/rods.htm

 

Fred '74tii (2.2L W/144mm rods, in process) & '69 (2.0L w/144mm rods, in process)

[TobyB]

Smokey Yunik had a good set of explanations in "Power Secrets"-

reduced piston skirt loading and thus friction, improved torque,

and while the dwell's a bonus (lets the explosion build higher cylinder pressure

at higher revs) it's the shape of the piston travel curve (velocity vs. position)

that really gains ft.- lbs.

 

Not legal in SCCA Production, though.  Also means you have a set

of rods that can't play nicely except with their own pistons.

And Smokey said that was a bad idea, too...

 

hee

 

t

[tim385]

Thanks for all the good information, Ahhh decisions decisions.

[Chamonix72]

Thank you for that correction to my faulty information.

[Rocan]

Toby b has it exactly right. Theoretically if you have an infinitely long conrod the piston will travel perfectly vertically in its bore and will see no side loading whatsoever.

Since the m10s see so much skirt wear, long rods are a beneficial upgrade to these motors and even just a few more mms makes a big difference.

[allbim]

One of the effects of a long rod is that it moves more of the weight of the piston below the wrist pin, thereby preventing the piston from being top-heavy and wanting to tilt with every stop and go.

[Tommy]

I don't see any disadvantage in using longer rods. Advantages have been mentioned. I have 146mm custom rods with stock 80mm crank.

 

  Tommy

[TobyB]

The ONLY disadvantage, Tommy, is that one word- "custom".

 

But since stock pistons cost so much (when you can find them) it's a pretty

good incentive to modernize the engine a bit.

 

Plus, ring sealing technology's come a long way, so you don't need nearly

the room for the rings that they did in 1958.

 

Topheavy?  take a look at a late E21/early E30 M10- they moved the pin DOWN to destroke it!

I always thought that a 180mm rod on that crank might be a real treat... if you could get the valve train

to hold together at 10k.

 

t