Rod ratio and valve events...
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From: Portland, Oregon
Rod ratio and valve events... What changes to the camshaft are necessary for different rod ratios?
Rod ratio dictates piston dwell at TDC, and the speed at which the piston moves away from TDC/how fast pressure drops in the chamber. These both seem relatively important, and I want to know how it changes the necessary valve events.
For the sake of this discussion, we will use a pair of 427ci LSX motors... one tall deck, one standard deck. Both have 4" stroke, and 4.125" bores. Same heads/intake/exhaust on both. Same vehicle. Same purpose for both.
Tall deck: 6.560" rod = 1.64 rod ratio
Standard deck: 6.125" rod = 1.53 rod ratio
What would change as far as valve events between the two?
From what I have read, it appears that low rod ratios promote high air speed, and helps low end torque production. However, the quicker drop in cylinder pressure hurts midrange and top end performance.
Higher rod ratios mechanically lock the piston at TDC and build/hold cylinder pressure really well, but provide less pull on the intake. This hurts low end torque production, but facilitates better midrange and top end performance.
Now, do you cam it to compliment the qualities of its rod ratio, or do you cam it to make up for the drawbacks of its rod ratio?
Thoughts?
Rod ratio dictates piston dwell at TDC, and the speed at which the piston moves away from TDC/how fast pressure drops in the chamber. These both seem relatively important, and I want to know how it changes the necessary valve events.
For the sake of this discussion, we will use a pair of 427ci LSX motors... one tall deck, one standard deck. Both have 4" stroke, and 4.125" bores. Same heads/intake/exhaust on both. Same vehicle. Same purpose for both.
Tall deck: 6.560" rod = 1.64 rod ratio
Standard deck: 6.125" rod = 1.53 rod ratio
What would change as far as valve events between the two?
From what I have read, it appears that low rod ratios promote high air speed, and helps low end torque production. However, the quicker drop in cylinder pressure hurts midrange and top end performance.
Higher rod ratios mechanically lock the piston at TDC and build/hold cylinder pressure really well, but provide less pull on the intake. This hurts low end torque production, but facilitates better midrange and top end performance.
Now, do you cam it to compliment the qualities of its rod ratio, or do you cam it to make up for the drawbacks of its rod ratio?
Thoughts?
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I would say first determine what type of engine you want. High rpm street/strip? Drag race only? Road race? Street? Then build the engine and determine what deck height.
Then I would cam based on the strengths but keep in mind the weaknesses. I’m no cam guru, so I have leaned on Mamo and had his help with my cam... especially since I’m using his heads and intake.
Then I would cam based on the strengths but keep in mind the weaknesses. I’m no cam guru, so I have leaned on Mamo and had his help with my cam... especially since I’m using his heads and intake.
Launching!
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You are on the right track. The "dwell" time is less of a consideration. The piston speeds are quicker in the lower half of the bore with a longer rod. This effects the ramming phase as the piston will be at a higher point in the bore for the same degree of crank rotation on the compression stroke with a longer rod.
That said... the difference between a 1.64 and 1.53 rod ratio, in the real world, is negligible. The difference between a 6.56" rod and 6.125" rod, in your example, is just .02" at 70* ATDC (near peak piston speed). For me, I would see no change in cam choice. This is why I tend to not prioritize rod ratio in a build because there's more important factors.
Let's say both of those engines are spinning 8,000 rpm with the same crank, same pistons, and different rod lengths and deck heights. The longer rod / taller block would have more rod weight which would need more counterbalance, which IMO would cancel out any benefit from the longer rods. You also have to consider with a taller deck comes longer pushrods which means more valvetrain weight/inertia. I'd want the shorter deck on that aspect alone as that would have more effect on the lobe intensity and jerk in the valvetrain than any changes needed for the rod ratio.
That said... the difference between a 1.64 and 1.53 rod ratio, in the real world, is negligible. The difference between a 6.56" rod and 6.125" rod, in your example, is just .02" at 70* ATDC (near peak piston speed). For me, I would see no change in cam choice. This is why I tend to not prioritize rod ratio in a build because there's more important factors.
Let's say both of those engines are spinning 8,000 rpm with the same crank, same pistons, and different rod lengths and deck heights. The longer rod / taller block would have more rod weight which would need more counterbalance, which IMO would cancel out any benefit from the longer rods. You also have to consider with a taller deck comes longer pushrods which means more valvetrain weight/inertia. I'd want the shorter deck on that aspect alone as that would have more effect on the lobe intensity and jerk in the valvetrain than any changes needed for the rod ratio.
In a nutshell, without typing 3 pages of technical jargon that few will understand and even fewer will actually read, lower rod ratio engines like more duration or at least can tolerate more duration than higher rod ratio engines, but the difference in camshaft design is not going to happen to anyone except perhaps NHRA Pro-Stock guys and nascar engines built for Talladega or Daytona. We’re talking a few horsepower here, and not worth the cubic dollars needed in R&D to find vs. Reward. You’ll find more reward easier messing with your ignition dwell and advance/retard settings for this scenario than actual camshaft design. Cool topic to discuss on Tech however.
Thread Starter
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From: Portland, Oregon
From my understanding, the lower rod ratios actually allow more ignition timing advance than higher rod ratios, due to how fast the piston accelerates away from TDC. The lower rod ratios are said to be more forgiving to tune.
I have also seen a trend of relatively tight LSA's in the short rod ratios. That may be just a trend of the times, overall, and strokers with low rod ratios happen to be popular right meow, but it is a pattern that I have noticed nonetheless.
Shorter rod ratios help lazy heads, due to the piston speed... and everyone is super dickhigh on LS3/7 heads these days, so strokers make sense.
The piston speed is also what helps low rod ratios eat up cam duration, and allows for extra duration that would otherwise make the engine lazy at low rpms.
I, personally, like higher rod ratios, and refuse to go below 1.6, but I also like small intake port heads that bring plenty of velocity on their own.
I have also seen a trend of relatively tight LSA's in the short rod ratios. That may be just a trend of the times, overall, and strokers with low rod ratios happen to be popular right meow, but it is a pattern that I have noticed nonetheless.
Shorter rod ratios help lazy heads, due to the piston speed... and everyone is super dickhigh on LS3/7 heads these days, so strokers make sense.
The piston speed is also what helps low rod ratios eat up cam duration, and allows for extra duration that would otherwise make the engine lazy at low rpms.
I, personally, like higher rod ratios, and refuse to go below 1.6, but I also like small intake port heads that bring plenty of velocity on their own.
Last edited by DavidBoren; Jun 20, 2019 at 04:49 PM. Reason: Don't type good...
Launching!
Joined: Mar 2019
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From my understanding, the lower rod ratios actually allow more ignition timing advance than higher rod ratios, due to how fast the piston accelerates away from TDC. The lower rod ratios are said to be more forgiving to tune.
I have also seen a trend of relatively tight LSA's in the short rod ratios. That may be just a trend of the times, overall, and strokers with low rod ratios happen to be popular right meow, but it is a pattern that I have noticed nonetheless.
Shorter rod ratios help lazy heads, due to the piston speed... and everyone is super dickhigh on LS3/7 heads these days, so strokers make sense.
The piston speed is also what helps low rod ratios eat up cam duration, and allows for extra duration that would otherwise make the engine lazy at low rpms.
I, personally, like higher rod ratios, and refuse to go below 1.6, but I also like small intake port heads that bring plenty of velocity on their own.
I have also seen a trend of relatively tight LSA's in the short rod ratios. That may be just a trend of the times, overall, and strokers with low rod ratios happen to be popular right meow, but it is a pattern that I have noticed nonetheless.
Shorter rod ratios help lazy heads, due to the piston speed... and everyone is super dickhigh on LS3/7 heads these days, so strokers make sense.
The piston speed is also what helps low rod ratios eat up cam duration, and allows for extra duration that would otherwise make the engine lazy at low rpms.
I, personally, like higher rod ratios, and refuse to go below 1.6, but I also like small intake port heads that bring plenty of velocity on their own.
Given the example engine in your OP, the difference between the two engines at 16* ATDC (point of peak combustion pressure) is just .0017". Your bearing clearance is more than that. You most likely will never see the difference on a dyno or at the track, and most likely no difference in wear over 200k miles. You certainly wouldn't see enough difference to make up for the extra weight of the taller block.
Thread Starter
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From: Portland, Oregon
I have absolutely no interest or intention to ever build anything big enough to ever warrant a tall deck block. I have used the 1.6 rod ratio limit to determine that a 3.9" stoke, 6.25" rod, 1.040" piston, and 9.24" deck is where I am willing to go for a naturally aspirated engine. Keeping it square with a 3.9" bore makes for a 373ci motor I have mentioned in other threads. Everything else will be small stroke/long rod, low displacement, and boost.
But I do see that the difference in the previously given example is less than 1* at the cam for the two given points of importance that you listed. It does take a tremendous amount of difference between rod ratios to make the cam change, all else being equal.
But I do see that the difference in the previously given example is less than 1* at the cam for the two given points of importance that you listed. It does take a tremendous amount of difference between rod ratios to make the cam change, all else being equal.
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Launching!
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I have absolutely no interest or intention to ever build anything big enough to ever warrant a tall deck block. I have used the 1.6 rod ratio limit to determine that a 3.9" stoke, 6.25" rod, 1.040" piston, and 9.24" deck is where I am willing to go for a naturally aspirated engine. Keeping it square with a 3.9" bore makes for a 373ci motor I have mentioned in other threads. Everything else will be small stroke/long rod, low displacement, and boost.
But I do see that the difference in the previously given example is less than 1* at the cam for the two given points of importance that you listed. It does take a tremendous amount of difference between rod ratios to make the cam change, all else being equal.
But I do see that the difference in the previously given example is less than 1* at the cam for the two given points of importance that you listed. It does take a tremendous amount of difference between rod ratios to make the cam change, all else being equal.
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Thread Starter
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From: Portland, Oregon
It's just the lowest I am willing to go, personally. Arbitrarily set based on **** I have read. And it's the golden ratio, which is fun, but completely unrelated. I like rod ratios in the 1.73+ range... the factory 4.8 crank and rods are 1.92, which is pretty awesome, and any combination using that interests me way more than anything using a 4" stroke and 6.125" (or less) rod.
Launching!
Joined: Mar 2019
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A good bit of my oil analysis and research has been with a 632ci BBC. It has a 4.75" stroke and 6.7" rods for a rod ratio of just 1.41. It's been spinning 8000+ rpm for 10 years. A tear down to freshen it up last winter showed no signs of excessive thrust wear in any of the cylinders.
Whatever the application may be, the piston design absolutely takes priority. The rod gets whatever is left. A 10 gram difference in piston weight has a far greater effect on piston inertia, crank balance, and rpm than going from a 1.5 to 1.6 rod ratio. The distance from the top ring to the piston deck, skirt length, pin diameter, etc... and the stroke of the crank for the displacement and rpm, all take precedence over rod length. Giving up anything in these areas to try to build around a specific rod ratio is going to cost you power, longevity, or both. They all take precedence and the rod just gets whatever is left.
Whatever the application may be, the piston design absolutely takes priority. The rod gets whatever is left. A 10 gram difference in piston weight has a far greater effect on piston inertia, crank balance, and rpm than going from a 1.5 to 1.6 rod ratio. The distance from the top ring to the piston deck, skirt length, pin diameter, etc... and the stroke of the crank for the displacement and rpm, all take precedence over rod length. Giving up anything in these areas to try to build around a specific rod ratio is going to cost you power, longevity, or both. They all take precedence and the rod just gets whatever is left.
Thread Starter
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From: Portland, Oregon
I'm no expert on piston design, but I do like the asymmetrical piston designs available these days. And things like vacuum pumps, thinner ring packages, and coated skirts. Combustion chamber molds, gas ports, and large radius parallel valve reliefs, **** like that.
I'm undecided on coating the tops of the pistons or the inside of the combustion chambers... too worried it will flake, not sure if that's an actual problem, though.
Is there a reliable way to figure out how stable the piston will be?
The 3.90" stroke, 6.25" rod, has a 1.04" piston in a standard deck block... which will be fantastically low mass, but seems on the short side.
I'm undecided on coating the tops of the pistons or the inside of the combustion chambers... too worried it will flake, not sure if that's an actual problem, though.
Is there a reliable way to figure out how stable the piston will be?
The 3.90" stroke, 6.25" rod, has a 1.04" piston in a standard deck block... which will be fantastically low mass, but seems on the short side.
I'm no expert on piston design, but I do like the asymmetrical piston designs available these days. And things like vacuum pumps, thinner ring packages, and coated skirts. Combustion chamber molds, gas ports, and large radius parallel valve reliefs, **** like that.
I'm undecided on coating the tops of the pistons or the inside of the combustion chambers... too worried it will flake, not sure if that's an actual problem, though.
Is there a reliable way to figure out how stable the piston will be?
The 3.90" stroke, 6.25" rod, has a 1.04" piston in a standard deck block... which will be fantastically low mass, but seems on the short side.
I'm undecided on coating the tops of the pistons or the inside of the combustion chambers... too worried it will flake, not sure if that's an actual problem, though.
Is there a reliable way to figure out how stable the piston will be?
The 3.90" stroke, 6.25" rod, has a 1.04" piston in a standard deck block... which will be fantastically low mass, but seems on the short side.
Teching In
Joined: Jun 2008
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From: Columbus, Ohio
To compensate for rod ratio, figure cam timing based on piston position rather than crank angle. With a short rod, the piston moves faster near TDC and slower near BDC, so use a smaller cam centerline with long rods.
As for what ratio to use. If max power is the goal, stroke the engine to the max and use a short rod. Use strong parts and rev it anyway. If low friction, better fuel economy, or other goals are desired, then shorten the stroke and lengthen the rod. For boost, you don't want too short a piston. Space for rings is a good thing. Everything is a compromise.
As for what ratio to use. If max power is the goal, stroke the engine to the max and use a short rod. Use strong parts and rev it anyway. If low friction, better fuel economy, or other goals are desired, then shorten the stroke and lengthen the rod. For boost, you don't want too short a piston. Space for rings is a good thing. Everything is a compromise.