Does connecting rod length matter?
Thread Starter
TECH Addict
Joined: Nov 2001
Posts: 2,556
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From: Nawf Carolina
Does connecting rod length matter? In a naturally aspirated motor, is there really that much to be gained. I've heard that sticking the longest rod possible in the motor is great, more dwell at TDC, more torque, better cylinder filling, etc. But then I've heard that the difference in a 5.7" rod and a 6.2" in a SBC is barely enough to tell. While it is a gain its insignificant except maybe at a Winston Cup/Pro Stock level.
I'm not talking about power adder situations, when you could possibly be trading durability by moving the wrist pin up too far.
I just read PSJ is using a 6.2" rod, and has said something about spraying also. Does this mean a LS1 built to run on motor would use a 6.325 possibly? How much would be gained? What does the SAM SS run?
I'm not talking about power adder situations, when you could possibly be trading durability by moving the wrist pin up too far.
I just read PSJ is using a 6.2" rod, and has said something about spraying also. Does this mean a LS1 built to run on motor would use a 6.325 possibly? How much would be gained? What does the SAM SS run?
FormerVendor
Joined: Nov 2001
Posts: 3,065
Likes: 6
From: Houston, Tx.
Re: Does connecting rod length matter? ""What does the SAM SS run?""
We had a deal where we had some custom 6.175 rods which went along with our 1.045 CH pistons and 4 inch stroke. We then decked it down to what we wanted.
The rod length is not very important and I'd go with a common size to save money. We were stuck because we had an even bigger crank and it didn't arrive in time for the BG event and it was circus getting that engine together and on the road. We ended up just using the old 4 inch crank like everyone else. With that you could run a 6-6.250 rod depending on what you wanted to do with the motor.
Obviously the crank and rod set the CH of the piston which needs to be sized for the usage it will get. So anotherwords you'd run more CH usually for like NOS etc.
We had a deal where we had some custom 6.175 rods which went along with our 1.045 CH pistons and 4 inch stroke. We then decked it down to what we wanted.
The rod length is not very important and I'd go with a common size to save money. We were stuck because we had an even bigger crank and it didn't arrive in time for the BG event and it was circus getting that engine together and on the road. We ended up just using the old 4 inch crank like everyone else. With that you could run a 6-6.250 rod depending on what you wanted to do with the motor.
Obviously the crank and rod set the CH of the piston which needs to be sized for the usage it will get. So anotherwords you'd run more CH usually for like NOS etc.
Teching In
Joined: Nov 2001
Posts: 39
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From: Here, Duh
Re: Does connecting rod length matter? The way it was explained to me was like this:
Longer the rod, the more hp that could be produced high in the power range, but less torque would be made, at a higher RPM as well.
Shorter the rod, the more power made in the lower RPM range. When it was explained to me, the 400sbc was given as an example. I was told that a reason they make so much power out of the hole, was because they have the short rods.
How true is this? I dont know, Im no engineer, who am i to dispute. I dont have any reason to dispute because this was coming from the guy who rebuilt my cousins Dirt track motor, and you know which one im talking about <img src="images/icons/wink.gif" border="0"> Oh yeah by the way, after hitting the auctions this year he has enough stuff to build a SB2... but for some reason he's not? He's going to start with a DART aluminum block, and throw on some Chapman heads(edelbrock i beleive).
Longer the rod, the more hp that could be produced high in the power range, but less torque would be made, at a higher RPM as well.
Shorter the rod, the more power made in the lower RPM range. When it was explained to me, the 400sbc was given as an example. I was told that a reason they make so much power out of the hole, was because they have the short rods.
How true is this? I dont know, Im no engineer, who am i to dispute. I dont have any reason to dispute because this was coming from the guy who rebuilt my cousins Dirt track motor, and you know which one im talking about <img src="images/icons/wink.gif" border="0"> Oh yeah by the way, after hitting the auctions this year he has enough stuff to build a SB2... but for some reason he's not? He's going to start with a DART aluminum block, and throw on some Chapman heads(edelbrock i beleive).
FormerVendor
Joined: Nov 2001
Posts: 3,065
Likes: 6
From: Houston, Tx.
Re: Does connecting rod length matter? ""How true is this?""
Not very true. The more you stroke an engine the shorter the rod will necesarily get to keep the same CH piston. Bigger engines have more tq than smaller ones. The longer a rod is though the less sever the acceleration off of TDC so the highest instantaneous piston speed is lower with longer rods. This will push power a little higher but this is a tiny effect since the rod ratios attainable are so close anyway no matter what rods you use. The longer a rod you use the less CH you can use which lightens up the piston too.
Not very true. The more you stroke an engine the shorter the rod will necesarily get to keep the same CH piston. Bigger engines have more tq than smaller ones. The longer a rod is though the less sever the acceleration off of TDC so the highest instantaneous piston speed is lower with longer rods. This will push power a little higher but this is a tiny effect since the rod ratios attainable are so close anyway no matter what rods you use. The longer a rod you use the less CH you can use which lightens up the piston too.
TECH Addict
Joined: Nov 2001
Posts: 2,207
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From: College Station, Tx
Re: Does connecting rod length matter? Going along with what Racer7078 said here is a Excel spreadsheet I made awhile ago that lets you play with instantaneous position, velocity, acceleration, and jerk with different rod and stroke lengths (you just need to change the top entry and the rest are cloned).
http://shared.slowcar.net/pistonmotion.xls
Chris
http://shared.slowcar.net/pistonmotion.xls
Chris
