Not for sure if this is in the right section but is good engine knowledge. I was in a conversation about using longer connecting rods and wanting some opinions on your part. A longer rod will sit by the top longer than a short rod engine, everything else being equal. So to make up the difference does the piston move faster at it's peak speed or is the peak speed the same for both but the long rod will ramp up to it faster ? Also we discussed rod length affecting detonation and was told it had no affect, I disaggree. And the longer rod should broaden the torque curve which should make it more street friendly, yes/no ?

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85 GT 3.4
14.9 @ 90 1.9 60' Old TH125/3.06
Unknown New 4T60/3.42

The results in this link will answer some of your questions.

http://www.rustpuppy.org/rodstudy.htm

I didn't read Joseph's link, but the length of a rod by itself doesn't change the speed of a piston significantly... it's the stroke (or the distance from the centerline of the rod journal to the centerline of the crank journal) that affects the speed of the piston. The only thing a longer rod does is improve the angularity of the rod throughout it's entire stroke. By improving angularity, you improve the efficiency of the power stroke since the forces on the piston act more in-line with the crank... so it will improve torque.

[This message has been edited by Bloozberry (edited 09-07-2010).]

Here is another interesting read on Rod Length by some HP Industry Professionals over at SpeedTalk forum: http://www.speedtalk.com/forum/viewtopic.php?t=406

quote Originally posted by Lilchief: ...the longer rod should broaden the torque curve which should make it more street friendly, yes/no ?

The last two paragraphs of the article which Joseph Upson's link references suggests any gains would be minimal:

"The data in this report seems to indicate that the differences between the rod lengths are exaggerated in the literature. In many (most) cases claims are anecdotal and represent the vested interests of the suppliers. I have seen no objective dyno testing of rod lengths but keep hoping for one.

There are real gains to be had by going to longer rods but they are small, usually a lot less than 2 percent. However, the hard-core racers are grasping at every tiny bit of performance and can justify the expense. For the more average rodder I would suggest staying with the rod length specified by the factory. Money would be far better spent on improving the heads, cam, and induction and exhaust systems. (and perhaps a supercharger..)"***
A separate source (not surprisingly also using SBC data, as opposed to data for our little V6s) suggests much the same thing re switching from shorter to longer connecting rods, torque gains which are minimal:

code:

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          Short       Long
           Rod         Rod
         (5.56")     (5.70") 
RPM       Torque      Torque
2500       341         341
3000       356         356
3500       359         363
4000       338         338
4500       309         310
4750       289         292
5000       253         257

---

Source: Page 80 of How To Build & Modify Chevrolet Small-Block V-8 Pistons, Rods & Crankshafts, David Vizard, 1992

The *average* piston speed is determined by the stroke. The *peak* piston speed is significantly different than this.
Shorter rods will give a higher peak piston speed (and acceleration) than longer rods.
As the piston moves up in the bore, it moves less than half way when the crank is 90 degrees ABDC/BTDC. This means the piston is moving faster in the "upper" half of the crank revolution and slower in the lower half. This effect is reduced by lengthening the rod. Lengthening the rod takes piston motion closer to being the ideal sinusoid.

Also, since shorter pistons are lighter, longer rod engines have lower reciprocating mass than shorter rod engines. The crankshaft can thus be balanced lighter, which may lead to a measurable loss of MOI in the crankshaft.

However, because of the higher piston speed in the upper have of the stroke, a short rod engine "draws" harder on a carbeurettor than a long rod engine and may produce better mixture quality through superior signal at the venturies.

[This message has been edited by Will (edited 09-06-2010).]

Long rod length = less wear on the cylinder and more tdc time

[This message has been edited by Erik (edited 09-07-2010).]

quote Originally posted by Erik: Long rod length = less wear on the cylinder and more tdc time

The link I posted gives some examples of percentage point differences involving those aspects also and they are minimal in magnitude. I agree that except for racing where such small benefits can be the difference between winning and loosing a race, it's more of a marketing enhancement than significant gain for the average engine build. More and more engines are being produced with oil squirters that hit the piston and cylinder wall now.

Todays metals and piston treatments probably makes the side thrust loads a non issue not that they ever were a problem. I recall it being a concern in the old Sbc 400 with a 3.75" stroke and 5.40" connecting rod and dwell time was a big topic years ago also but fast burn cylinderheads have probably had there effect on the significance of that also. A lot of todays theory is still based on old experience untested on new technology and sometimes we limit the potential of a performance build by adhering to it.

quote Originally posted by Lilchief: ...Also we discussed rod length affecting detonation and was told it had no affect, I disaggree.

Yours seems a valid point to me, but perhaps for reasons less complicated than your discussion may have focused on.

More specifically, if one switches from shorter rods to longer rods and does nothing else, those rods' greater length, aside from potentially increasing the risk of valve-to-piston interference issues, will increase the engine's compression ratio, and presumably the engine's risk of detonation as well. Because of that, doing nothing else when switching to longer rods from shorter ones often may not be a practical option.

Accordingly, to reduce the increased compression ratio that otherwise would be created by the installation of longer rods, some may employ, for example, new pistons whose wrist pins are located higher within the piston, similar to that which one might do to reduce the increased compression ratio that otherwise would be created by the use of the longer "throw" of a stroker crankshaft to transform a 2.8L V6 Fiero engine into a 3.1L V6 Fiero engine.

Learning of that "new-pistons requirement" actually was one of the reasons I ended up opting instead for a pushrod 60^o 3.4L engine transplant, and abandoned my earlier plans to transform my pushrod 60^o 2.8L Fiero engine into a pushrod 60^o 3.1L engine via a stroker crank.