Summit pistons, rod length vs boost
Thread Starter
Joined: Sep 2007
Posts: 4,188
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From: woodbine, md
Summit pistons, rod length vs boost Gathering parts for a 6.0 for the Hornet, looking at 2 Summit rod/piston kits. They appear to be the same comp except diff rod lengths, so the pin is prob raised in one set.
So would the longer rod length be more suitable for boost, better side angle.
Plan on using the stock crank, 317 heads.
#PR360053, 4.005, 6.098 rod
#PR36L0052, 4.005, 6.125 rod.
So would the longer rod length be more suitable for boost, better side angle.
Plan on using the stock crank, 317 heads.
#PR360053, 4.005, 6.098 rod
#PR36L0052, 4.005, 6.125 rod.
@Summitracing hey Brian B got your ears on, love to hear your thoughts. I still plan to get with you on my LC9 build too.
Here is how it was explained to my by Joe (summitracing on the forum) when I was looking into my purchase.
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The SUM-PR3603012 uses the stock length of 6.098" for the rods.
The SUM-PR36L03012 uses the common longer 6.125" rod length.
Either of these is good for 1,000 whp out of the box.
What does that difference mean?
Shorter rods: Shorter rods mean the overall height of the block can be shorter, which means the overall weight of the block can be lighter. The engine will typically pull more vacuum at low RPM, which means better throttle response and low-end torque (good for street performance and everyday driving). Spark timing can be advanced a few degrees for some additional low-speed torque, and the engine is less prone to detonation, which can be a plus in turbocharged, supercharged or nitrous applications.
Longer rods: Using longer connecting rods with the same stroke reduces the side loading on the pistons, which reduces friction. It also increases the piston dwell time at Top Dead Center. Holding compression for maybe half a degree of crankshaft rotation longer at TDC improves combustion efficiency and squeezes a little more power out of the air/fuel mixture. Typically, an engine with a higher rod ratio will produce a little more power from mid-range to peak RPM.
The pistons are different to accommodate the associated rod length. Our pistons will be .005" in the hole at TDC. This is no matter of 6.098" or 6.125" rod length. Factory is around .004-.005" out of the hole at TDC.
Let me know if you need any further assistance and I'll be happy to help.
Thank you,
Joe
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----------------------------------------------------------------------------------------------------------------------------------------------------------
The SUM-PR3603012 uses the stock length of 6.098" for the rods.
The SUM-PR36L03012 uses the common longer 6.125" rod length.
Either of these is good for 1,000 whp out of the box.
What does that difference mean?
Shorter rods: Shorter rods mean the overall height of the block can be shorter, which means the overall weight of the block can be lighter. The engine will typically pull more vacuum at low RPM, which means better throttle response and low-end torque (good for street performance and everyday driving). Spark timing can be advanced a few degrees for some additional low-speed torque, and the engine is less prone to detonation, which can be a plus in turbocharged, supercharged or nitrous applications.
Longer rods: Using longer connecting rods with the same stroke reduces the side loading on the pistons, which reduces friction. It also increases the piston dwell time at Top Dead Center. Holding compression for maybe half a degree of crankshaft rotation longer at TDC improves combustion efficiency and squeezes a little more power out of the air/fuel mixture. Typically, an engine with a higher rod ratio will produce a little more power from mid-range to peak RPM.
The pistons are different to accommodate the associated rod length. Our pistons will be .005" in the hole at TDC. This is no matter of 6.098" or 6.125" rod length. Factory is around .004-.005" out of the hole at TDC.
Let me know if you need any further assistance and I'll be happy to help.
Thank you,
Joe
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TECH Enthusiast
Joined: Feb 2020
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.027 " difference in rod length is nearly nothing and would have a very small effect on dwell and side loading. Not like going from 5.7 to 6" on a gen 1 . That extra .027" for the rings COULD result in stronger ringlands though , if they capitalized on that. Not an LS expert here , just many years of engine work. I wouldn't over think that very small difference
.027 " difference in rod length is nearly nothing and would have a very small effect on dwell and side loading. Not like going from 5.7 to 6" on a gen 1 . That extra .027" for the rings COULD result in stronger ringlands though , if they capitalized on that. Not an LS expert here , just many years of engine work. I wouldn't over think that very small difference
Thread Starter
Joined: Sep 2007
Posts: 4,188
Likes: 292
From: woodbine, md
Wouldn't the pin just be higher in the scraper ring area, the top and 2nd ring distance be the same for the longer rod? Usually I have just bought forged pistons, used gen 4 rods, but was looking at a piston/rod package, where two diff rod lengths are avail., end up with same comp. Both are out of stock now, as about everything is now, lol, otherwise I prob would have ordered whatever was avail.
I thought about building a 403 stroker, but years ago I pulled a 6.0 out and did this, didn't think it picked up as much as I thought it would, running 12 lbs boost.
I thought about building a 403 stroker, but years ago I pulled a 6.0 out and did this, didn't think it picked up as much as I thought it would, running 12 lbs boost.
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Joined: Jan 2008
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From: Ohio, Georgia, Nevada, Texas
Hi there, the idea behind the two different rod lengths was actually based on the 6.098 version have a .945 diameter rod instead of the .927 rod used in the 6.125 rod. The .945 pin is a bit heavier but it's offset by the .027 in. shorter rod. It's also 9% stronger than the .927 pin. On either piston, we have all the room in the world for our .300/.185/.080 in. target ring land thickness without having the pin enter the oil ring groove. It's our 4 in. stroker rotating assemblies that use the 6.125/.927 rod exclusively and that is due to it's ability to max ring land thickness with the groove lock spacer and the fact that rod is designed with a different shoulder over the big end to give the clearance necessary to clear the bottom of a cylinder with the added stroke. Hope this helps!
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Wouldn't a longer rod be more beneficial to increase piston dwell time for more efficient combustion or is that apply to NA setups mostly?
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Increased dwell can/will promote increased cylinder pressure therefore increased potential for detonation. May not be a problem with sufficient fuel, etc. Lot's of discussions on the dynamics of rod length on YB.
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Joined: Jan 2008
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From: Ohio, Georgia, Nevada, Texas
Rod length theory is fun to thing about....whether long or short is better on what stroke (Power vs. Intake). Certainly never sacrifice cubic inches for rod length. The actual differences even between extreme differences is very slight. We've seen this done many times on a variety of engines and there are MANY different factors that change that are much more important. Still, if you want to watch an interesting video, Engine Masters Season 5 Episode 54 does a deep dive.
Thread Starter
Joined: Sep 2007
Posts: 4,188
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From: woodbine, md
Thanks Summit, I think I will go with the 6.098 package, and the ghost cam. I ran the stage 2 in my truck, and it wasn't happy being in lockup around 50-55, 1900 RPM, so I figured a little less cam would be better.
I recall reading an article years ago in Hot Rod where they built a Gen I 355 SBC and used a 6 inch rod with a static compression of 11.5:1 and were able to run it on 87 octane fuel while making good power because of longer rod.
The theory I believe was the additional piston dwell time provided the needed combustion time for the poorer quality fuel to burn, does that apply to FI, that's a discussion I'd love to be a fly on the wall for.
I'll have to join YB and check out those discussions/debates as well.
The theory I believe was the additional piston dwell time provided the needed combustion time for the poorer quality fuel to burn, does that apply to FI, that's a discussion I'd love to be a fly on the wall for.
I'll have to join YB and check out those discussions/debates as well.