Turbo/Supercharged Engine Rod Legnth
For all else being equal - assuming identical throw of the crank a shorter rod will create a little more torque down low, and a little less up top. You will increase side piston loading, but the rod will be stronger, thus you sacrifice some long-term longevity of the motor in exchange for short term survivability of the rotating assembly. Vice versa with a longer rod
Practically, the above only influences the numbers a couple % either way and it is more of an academic discussion than a practical one.
The more practical decision is what you are going to be doing with the motor and then just selecting the cubes accordingly - that larger decision will tightly confine what rod ratio you can actually use. For example - say you are planning on a Forced Induction pump gas motor. For this you will want as much cube as you can get. The larger cubes will steer you towards a 4.125 bore and 4"-ish stroke. Additionally, you will need to dish the piston and run a FI piston which will demand thicker crown and move the pin down. Pretty quickly you get to a spot, where you're deciding between a 6.1 rod or a 6.135 rod......not much room to maneuver.
Practically, the above only influences the numbers a couple % either way and it is more of an academic discussion than a practical one.
The more practical decision is what you are going to be doing with the motor and then just selecting the cubes accordingly - that larger decision will tightly confine what rod ratio you can actually use. For example - say you are planning on a Forced Induction pump gas motor. For this you will want as much cube as you can get. The larger cubes will steer you towards a 4.125 bore and 4"-ish stroke. Additionally, you will need to dish the piston and run a FI piston which will demand thicker crown and move the pin down. Pretty quickly you get to a spot, where you're deciding between a 6.1 rod or a 6.135 rod......not much room to maneuver.
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Crankshaft Rotalion v/s Piston Position Hi ALL, those here who do not understand how Pre-Ignition is caused will benefit when a CORRECT R/L is chosen.
The crankshaft rotates AND the position of the piston changes.
A HIGH R/L, a "long rod engine", will keep the piston position HIGHER in the cylinder during combustion as the engine's crankshaft moves. This causes the NEED for higher Octane fuel which is more resistant to Detonation.
A LOWER R/L, in the engine, will "move" the piston away from the combustion chamber faster as the pressure increases.
Lance
The crankshaft rotates AND the position of the piston changes.
A HIGH R/L, a "long rod engine", will keep the piston position HIGHER in the cylinder during combustion as the engine's crankshaft moves. This causes the NEED for higher Octane fuel which is more resistant to Detonation.
A LOWER R/L, in the engine, will "move" the piston away from the combustion chamber faster as the pressure increases.
Lance
Last edited by IGN-1A; Apr 20, 2024 at 01:49 PM.
True, but like everything in a motor, it is about compromises. A small increase in residence time is usually not worth the headaches a much longer rod arrangement causes. Extending the rod to get a better rod ratio will usually sacrifice much-desired strength at the piston crown and push the top ring too close to top of the piston for a FI setup. Conversely, if you try to get a better rod ratio by shortening the throw of the crank, you will end up with a smaller cube and will need to run several psi more of boost to make the same HP. Not a problem with a methanol or E85 setup, but I can guarantee the additional couple pounds of boost you have to run to make the same HP will cause more detonation issues than the small improvement in residence time you get from playing with the rod ratio.
As such, you are usually better off sticking with an off the shelf rod in the 6.1-ish range.
As such, you are usually better off sticking with an off the shelf rod in the 6.1-ish range.
Last edited by NoGo; Apr 16, 2024 at 11:42 AM.
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Piston Distance from Head at Peak Cylinder Pressure SURE, almost always the same.
The difference can also be measured in Crankshaft Rotation Degrees.
Thus the "leverage" is different with a low R/L and a high R/L engine.
I also state a stronger piston and stronger pin (.990") is more easy to fit.
Thus, LOWER the R/L with higher cylinder pressure.
The difference can also be measured in Crankshaft Rotation Degrees.
Thus the "leverage" is different with a low R/L and a high R/L engine.
I also state a stronger piston and stronger pin (.990") is more easy to fit.
Thus, LOWER the R/L with higher cylinder pressure.
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Hi ALL, those here who do not understand how Pre-Ignition is caused will benefit when a CORRECT R/L is chosen.
The crankshaft rotates AND the position of the piston changes.
A HIGH R/L, a "long rod engine", will keep the piston position HIGHER in the cylinder during combustion as the engine's crankshaft moves. This causes the NEED for higher Octane fuel which is less resistant to Detonation.
A LOWER R/L, in the engine, will "move" the piston away from the combustion chamber faster as the pressure increases.
Lance
The crankshaft rotates AND the position of the piston changes.
A HIGH R/L, a "long rod engine", will keep the piston position HIGHER in the cylinder during combustion as the engine's crankshaft moves. This causes the NEED for higher Octane fuel which is less resistant to Detonation.
A LOWER R/L, in the engine, will "move" the piston away from the combustion chamber faster as the pressure increases.
Lance
Their theory was the longer rod increased the piston dwell time at TDC providing a more efficient combustion cycle enabling them to run a higher compression ratio on lesser grade fuels.
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N/A Engine R/L Hi, OK fair is fair.
The worst N/A engine I assembled using a 4" crankshaft in a 351-C Crankcase required a "short" rod, a low R/L.
The ignition advance required was high for the engine to make power.
YES, there was Pre-ignition at LOW RPM.
MY "post" concerns "supercharged" engines.
The worst N/A engine I assembled using a 4" crankshaft in a 351-C Crankcase required a "short" rod, a low R/L.
The ignition advance required was high for the engine to make power.
YES, there was Pre-ignition at LOW RPM.
MY "post" concerns "supercharged" engines.
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I...did a straight six build years ago where they built the engine with a short 5.7" rod/piston combo with 11:1 compression and had to use a high-octane fuel (101?) to avoid detonation then installed a longer 6.125" rod/piston combo, still 11:1 and was able to run it effectively on pump gas fuel.s.
.
. So many variables to control for:
Do you think they had the same piston crown to cylinder "quench", same camshaft timing, REALLY the same dynamic compression ratio between the two "Rod Lengths"
SO many minor variables and tolerance stacking to enter the equation here, it's really hard to believe the octane rerquirement could change by 8 or 10 whole units on the SAME compression and stroke with JUST a 7% rod length change.
Juggling piston pin heights, compressed gasket thickness, and potential timing changes with each head to block assembly leaves a strong possibility of OTHER parameters introduced into the compression vs spark advance relationship.
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IGNA1 - " higher octane fuel is less resistant to detonation " this must be a typo or did you mean to say that ?
I agree with the many variables in comparisons , first off ign timing must be different (less advance for the long rod ratio) if your shooting for the same peak cylinder pressure, thinking that the rpm vs advance curve probably has to be tweeked for perfection. Even valve events should be changed as the dwell time at TDC alters piston movement on the intake stroke also . For forced induction that's not a big factor but the dwell time is probably going to increase cyl pressure enough to require less ignition timing and you're back to the same power ( assuming you're on the edge already) Unless you're all out racing and searching for every last hp it's not worth it . Certainly not if your power goals can be reached with a SBE . The stock Ls rod is considerably longer than gen 1 stuff already, and guys have made good power with sbc 400 rods @ 5.565" on a 3.75" stroke - terrible rod/stroke ratio . Lots of variables as said , quech and ignition advance being the biggest IMO
I agree with the many variables in comparisons , first off ign timing must be different (less advance for the long rod ratio) if your shooting for the same peak cylinder pressure, thinking that the rpm vs advance curve probably has to be tweeked for perfection. Even valve events should be changed as the dwell time at TDC alters piston movement on the intake stroke also . For forced induction that's not a big factor but the dwell time is probably going to increase cyl pressure enough to require less ignition timing and you're back to the same power ( assuming you're on the edge already) Unless you're all out racing and searching for every last hp it's not worth it . Certainly not if your power goals can be reached with a SBE . The stock Ls rod is considerably longer than gen 1 stuff already, and guys have made good power with sbc 400 rods @ 5.565" on a 3.75" stroke - terrible rod/stroke ratio . Lots of variables as said , quech and ignition advance being the biggest IMO
.
.
. So many variables to control for:
Do you think they had the same piston crown to cylinder "quench", same camshaft timing, REALLY the same dynamic compression ratio between the two "Rod Lengths"
SO many minor variables and tolerance stacking to enter the equation here, it's really hard to believe the octane rerquirement could change by 8 or 10 whole units on the SAME compression and stroke with JUST a 7% rod length change.
Juggling piston pin heights, compressed gasket thickness, and potential timing changes with each head to block assembly leaves a strong possibility of OTHER parameters introduced into the compression vs spark advance relationship.
.
. So many variables to control for:
Do you think they had the same piston crown to cylinder "quench", same camshaft timing, REALLY the same dynamic compression ratio between the two "Rod Lengths"
SO many minor variables and tolerance stacking to enter the equation here, it's really hard to believe the octane rerquirement could change by 8 or 10 whole units on the SAME compression and stroke with JUST a 7% rod length change.
Juggling piston pin heights, compressed gasket thickness, and potential timing changes with each head to block assembly leaves a strong possibility of OTHER parameters introduced into the compression vs spark advance relationship.
IIRC the piston dwell time provided more time for the flame front to travel increasing its efficiency so less ignition timing was required and more compression with pump gas could be used.
A guy I knew around this time built a 355 SBC with a 6.0" rod, 11.5:1 compression and heavily ported factory iron heads that he ran on pump gas that worked really well.
I don't recall him having to run abnormally low timing or anything.
@IGN-1A My assumption is boost would benefit from this either way.
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design your piston and choose your stroke first. those will determine your rod length.
im doing a fairly fancy ecotec and worked with diamond for pistons. we chose to use the shortest shelf rod that molnar makes to get the piston height ideal.
im doing a fairly fancy ecotec and worked with diamond for pistons. we chose to use the shortest shelf rod that molnar makes to get the piston height ideal.