Why do shorter rods increase piston speed and vice versa? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 03-08-2005, 04:46 PM
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Why do shorter rods increase piston speed and vice versa?

Why do shorter rods increase piston speeds and longer rods produce faster piston speeds? I think I understand the differrence between piston speed and rpms but im not sure on it. could someone fill me in? thanks, bf

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Old 03-08-2005, 05:30 PM
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Geometry.
No matter what rod youre running it still takes the same amount of time for piston to go from top of stroke to bottom and halfway point is also still the same. But different rod lengths for same stroke change the “curve”.
Longer rod causes piston to move less distance for a given number of degrees at TDC and BDC and faster approaching and leaving halfway point.

The whole benefit of a longer rod is that piston wont start moving down in cylinder as much at TDC while cam is continuing to open the intake valve this aids engine breathing.

I suppose it may help reduce exhaust dilution in cylinder too since the exhaust valve will be closer to being completely shut before piston takes off for the bottom. And maybe combustion efficiency but that’s over my head.
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Old 03-08-2005, 06:06 PM
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Average speed will be the same. You're still moving the same amount of distance over the same amount of time. There is a slight difference in the speed leaving TDC, but more importantly, the acceleration is lower with a longer rod. Of course, what makes a VASTLY bigger difference, is how many RPM you're willing to spin the motor in the first place.

This is an over thought concept and one that pays zero dividends on the street.

K
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Old 03-08-2005, 07:02 PM
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Thats a good point.
On average street engine theres so many other things to make better first this would not pay off untill all other restrictions are eliminated.
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Old 03-09-2005, 07:14 AM
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It increases the piston's leverage (torque and HP) and reduces piston rock in the bore (wear). Those are two benefits regardless of where you're at.

In terms of doing other mods first, I think this (longer rod) would impact (increase)cylinder pressure thereby making it a consideration when the cam grinder makes your cam. The cam would then influence intake and exhaust requirements. The point is, one grind wont be perfect for both conditions.

Another consideration is the reduced piston weight due to the raised pin location. This would require a second balance job when changing pistons.

I believe that rod ratio is an important consideration when setting up the "foundation" of a motor and will impact choices made with regard to other components.
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Old 03-09-2005, 08:12 AM
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I agree to an extent, but I certainly don't think you're going to see noticable differences on the dyno (like you mentioned, in terms of torque and HP) even by a noticable change in rod length. At least for street motors, 500 HP and lower.

I think the money for a rod upgrade and special pistons could be vastly better spent on the next better set of heads-

K
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Old 03-09-2005, 08:41 AM
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The amount of stroke increase piston speed. Rod length affects dwell time at the TDC.

Chris
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Old 03-09-2005, 09:41 AM
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56Maynard is correct, but so is Killer. I saw a HP variance of up to 3.5% on my engine just by changing rod lengths, but that was in the 7,000 - 8,000 rpm range. For 99% of the street rodders out there it isn't worth worrying about.
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