Rod/piston length questions
Thread Starter
12 Second Club
Joined: Dec 2017
Posts: 1,030
Likes: 220
From: San Antonio
Rod/piston length questions If the info I found is correct, then the stock rods are 6.098". A lot of aftermarket rods are 6.125". If you replace the stock rods with longer rods, will that increase compression? The displacement doesn't change, nor does the stroke. The longer rod will change where the stroke occurs within the cylinder, though. It would seem that the compression would increase, due to the piston coming up higher, and possibly going above the deck. I'm guessing that is why they make pistons specifically for 6.125" rods.
Another scenario to discuss:
Let's say you have a 6.098" rod, and a piston made for a 6.125" rod. Would that lower compression? Would this pull the piston close enough to the bottom to cause it to rock at BDC?
These are all things I think that I need to understand before contemplating any future builds. I have read threads about the advantages of the longer rods. I just didn't see anything about how the longer rods affect things like compression, PTV, and other clearances. I have seen pistons advertised as 6.125" rod pistons. I just don't know what is different about them. Any insight is appreciated.
Another scenario to discuss:
Let's say you have a 6.098" rod, and a piston made for a 6.125" rod. Would that lower compression? Would this pull the piston close enough to the bottom to cause it to rock at BDC?
These are all things I think that I need to understand before contemplating any future builds. I have read threads about the advantages of the longer rods. I just didn't see anything about how the longer rods affect things like compression, PTV, and other clearances. I have seen pistons advertised as 6.125" rod pistons. I just don't know what is different about them. Any insight is appreciated.
Deck height of an LS block is 9.24 inches and when setting up a rod piston combination for a given crank you apply a formula
Stroke divided by two plus the rod length and compression height of the piston. Example: 3.622/2=1.811+6.098+1.33=9.240 (Stock)
Now throw in the 4.00 stroke crank and now you have 4.000/2=2.000+6.125+1.115=9.240 (Stroker)
Stroke divided by two plus the rod length and compression height of the piston. Example: 3.622/2=1.811+6.098+1.33=9.240 (Stock)
Now throw in the 4.00 stroke crank and now you have 4.000/2=2.000+6.125+1.115=9.240 (Stroker)
If the info I found is correct, then the stock rods are 6.098". A lot of aftermarket rods are 6.125". If you replace the stock rods with longer rods, will that increase compression? The displacement doesn't change, nor does the stroke. The longer rod will change where the stroke occurs within the cylinder, though. It would seem that the compression would increase, due to the piston coming up higher, and possibly going above the deck. I'm guessing that is why they make pistons specifically for 6.125" rods.
Another scenario to discuss:
Let's say you have a 6.098" rod, and a piston made for a 6.125" rod. Would that lower compression? Would this pull the piston close enough to the bottom to cause it to rock at BDC?
These are all things I think that I need to understand before contemplating any future builds. I have read threads about the advantages of the longer rods. I just didn't see anything about how the longer rods affect things like compression, PTV, and other clearances. I have seen pistons advertised as 6.125" rod pistons. I just don't know what is different about them. Any insight is appreciated.
Another scenario to discuss:
Let's say you have a 6.098" rod, and a piston made for a 6.125" rod. Would that lower compression? Would this pull the piston close enough to the bottom to cause it to rock at BDC?
These are all things I think that I need to understand before contemplating any future builds. I have read threads about the advantages of the longer rods. I just didn't see anything about how the longer rods affect things like compression, PTV, and other clearances. I have seen pistons advertised as 6.125" rod pistons. I just don't know what is different about them. Any insight is appreciated.
Thread Starter
12 Second Club
Joined: Dec 2017
Posts: 1,030
Likes: 220
From: San Antonio
I've been playing with this calculator: https://www.rbracing-rsr.com/compstaticcalc.html
The only way I see to account for a rod length change, would be in the "Deck Height" box. Assuming you go from a stock (6.098") to an aftermarket rod (6.125"), it should increase the deck height by .027". If it works this way, then a longer rod would increase compression (all else equal). Does this sound correct, or am I way off base here?
Here's my rational for doing this:
If I want to swap the stock rods and pistons with a forged set, how do I maintain the same or close to the same compression? There are no forged, 0cc, Flat-top pistons on the market for an LS1. The closest I could find are the Summit +3cc mini domes, and some -5cc flat tops with valve reliefs. I'm currently at 10.23:1 compression with stock rods/pistons (0cc flat tops).
Option 1: -5cc pistons (made for 6.098" rod)
These 5cc valve reliefs would decimate my compression (9.67:1) with a stock-length rod. A longer rod would help compression (10.27:1) with these -5cc pistons (made for 6.098" rod). The longer rod should increase compression with these pistons. I am hoping that it works this way.
Option 2: +3 cc mini dome (made for a 6.125" rod)
These +3cc mini domes might be ok with a 6.125" rod (10.61:1 compression). If my end goal is to add boost, then I'd rather keep the compression down. These pistons (made for a 6.125" rod) with a stock length rod should lower the compression to 9.97:1.
So, which option would be better? Are either of these options actually practical?
The only way I see to account for a rod length change, would be in the "Deck Height" box. Assuming you go from a stock (6.098") to an aftermarket rod (6.125"), it should increase the deck height by .027". If it works this way, then a longer rod would increase compression (all else equal). Does this sound correct, or am I way off base here?
Here's my rational for doing this:
If I want to swap the stock rods and pistons with a forged set, how do I maintain the same or close to the same compression? There are no forged, 0cc, Flat-top pistons on the market for an LS1. The closest I could find are the Summit +3cc mini domes, and some -5cc flat tops with valve reliefs. I'm currently at 10.23:1 compression with stock rods/pistons (0cc flat tops).
Option 1: -5cc pistons (made for 6.098" rod)
These 5cc valve reliefs would decimate my compression (9.67:1) with a stock-length rod. A longer rod would help compression (10.27:1) with these -5cc pistons (made for 6.098" rod). The longer rod should increase compression with these pistons. I am hoping that it works this way.
Option 2: +3 cc mini dome (made for a 6.125" rod)
These +3cc mini domes might be ok with a 6.125" rod (10.61:1 compression). If my end goal is to add boost, then I'd rather keep the compression down. These pistons (made for a 6.125" rod) with a stock length rod should lower the compression to 9.97:1.
So, which option would be better? Are either of these options actually practical?
Thread Starter
12 Second Club
Joined: Dec 2017
Posts: 1,030
Likes: 220
From: San Antonio
So you can follow along with the calculator, here's my current set-up:
Bore: 3.898
Stroke: 3.622
Head Gasket: .052
Deck Height: 0
Piston volume: 0
Chambers: 66.5
NOTE: My 317 heads have been milled about .030" to achieve the 66.5cc. I don't know the current PTV clearance, but it is something to consider.
Bore: 3.898
Stroke: 3.622
Head Gasket: .052
Deck Height: 0
Piston volume: 0
Chambers: 66.5
NOTE: My 317 heads have been milled about .030" to achieve the 66.5cc. I don't know the current PTV clearance, but it is something to consider.
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