Piston speed question............
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Joined: Dec 2001
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From: Ft. Lauderdale, FL, USA
Originally Posted by briannutter
To speak VERY generally, the top end of the motor (including cam) determine the power. The displacement you put under that top end determines where the power is made. The better the heads/manifold become, the more displacement you have to put under it to place the rpm range down to a point where the valvetrain is stable, reliable, and the valves aren't floating. When you have a displacement rule as in Cup, Pro Stock, or F1-you basically up the rpm and horsepower as you make cylinder head and valvetrain advancements. Compared to horsepower, torque actually remains pretty close to the same year to year..it just moves up the rpm scale. Back to LS land, the newest heads these days can enable even the 500ci to pull strongly through 7000rpm with medium sized hydraulic cams and a factory intake, so bigger is better for most of us. If you're gear limited or have a self imposed cam duration for driveability, even more so.
Lets' create a average target piston speed of 5100..no big deal. The 3.622 stroke has to turn 8700 to achieve it, the 4" turns 7900rpm and the 4.5" is about 7000. This is a bit of a generalization, but let's say we've got a killer drag race top end on a 3.622 stroke that makes peak power at that 8700rpm. the 4 and the 4.5" engines will make about the same peak power at the lower rpm that coincides with the equal piston speed. This is why you would want a lot of displacement with a valvetrain limited engine.
For theoretics and little else, the peak compression load on the rod occurs at about 30 deg. abdc and peak tension remains at tdc through any of the common rod/stroke combinations for these 3 engines. In reality, with a given piston speed and rod ratio, the SHORTER the stroke-the higher the PEAK tension and compression loads will be. Using the same #'s from above, a 3.622 stroke at 8700rpm, a 4" stroke at 8300 rpm, and a 4.5" stroke at 7900rpm have about the same PEAK COMPRESSION loads. The 3.622 stroke at 8700rom, the 4" stroke at 8150, and the 4.5" stroke at 7700rpm, all have the about the same PEAK TENSION loads. In my opinion, these peak loads are more important than average piston speed (and certainly more than rpm) to reliability.
My basic point is you don't hear of pins getting yanked out of the bottoms of pin bosses or rods snapping in 1/2 very often with decent aftermarket parts, so increasing stroke and even piston speed can actually help the overall reliablity of the engine. One of the above posts made a very good point about the sleeve length being the limitation. A call to a knowledable piston manufacturer can help determine how much stroke can be put in your block. The taper on the piston has a break point. The break point must be above the bottom of the sleeve at bdc to keep the piston from cocking to avoid obscene amounts of skirt wear. When you see a piston that is severely worn in a long stroke engine, Bubba usually say's "that durn bad rod/stroke ratio ate up my piston". In reality, the piston manufacturer didn't do a good job working with the engine builder to determine proper skirt taper and maximum stroke.
-Brian Nutter-Wiseco Piston
Lets' create a average target piston speed of 5100..no big deal. The 3.622 stroke has to turn 8700 to achieve it, the 4" turns 7900rpm and the 4.5" is about 7000. This is a bit of a generalization, but let's say we've got a killer drag race top end on a 3.622 stroke that makes peak power at that 8700rpm. the 4 and the 4.5" engines will make about the same peak power at the lower rpm that coincides with the equal piston speed. This is why you would want a lot of displacement with a valvetrain limited engine.
For theoretics and little else, the peak compression load on the rod occurs at about 30 deg. abdc and peak tension remains at tdc through any of the common rod/stroke combinations for these 3 engines. In reality, with a given piston speed and rod ratio, the SHORTER the stroke-the higher the PEAK tension and compression loads will be. Using the same #'s from above, a 3.622 stroke at 8700rpm, a 4" stroke at 8300 rpm, and a 4.5" stroke at 7900rpm have about the same PEAK COMPRESSION loads. The 3.622 stroke at 8700rom, the 4" stroke at 8150, and the 4.5" stroke at 7700rpm, all have the about the same PEAK TENSION loads. In my opinion, these peak loads are more important than average piston speed (and certainly more than rpm) to reliability.
My basic point is you don't hear of pins getting yanked out of the bottoms of pin bosses or rods snapping in 1/2 very often with decent aftermarket parts, so increasing stroke and even piston speed can actually help the overall reliablity of the engine. One of the above posts made a very good point about the sleeve length being the limitation. A call to a knowledable piston manufacturer can help determine how much stroke can be put in your block. The taper on the piston has a break point. The break point must be above the bottom of the sleeve at bdc to keep the piston from cocking to avoid obscene amounts of skirt wear. When you see a piston that is severely worn in a long stroke engine, Bubba usually say's "that durn bad rod/stroke ratio ate up my piston". In reality, the piston manufacturer didn't do a good job working with the engine builder to determine proper skirt taper and maximum stroke.
-Brian Nutter-Wiseco Piston
The new block I'm having done is using the 5.800" long Darton dry sleeves in an LS7 block with a 4.125 stroke. It can support up to a 4.250" stroke but at that stroke I'll be pushing the limits again and the piston will come out of the bottom again. I'm using a 4.155 bore x 4.125 stroke...447ci, and even if I went with a 4.185" stroke (454ci) its just too much, the piston will rock too much at BDC. I'll give up cubes for longevity. Another oil burning engine will drive me friggin mad.
I think my builder mentioned getting the pistons from you.
.
Last edited by Quickin; Oct 12, 2006 at 06:59 PM.
Originally Posted by Quickin
I'm using a 4.155 bore x 4.125 stroke...447ci, and even if I went with a 4.185" stroke (454ci) its just too much, the piston will rock too much at BDC. I'll give up cubes for longevity. Another oil burning engine will drive me friggin mad.
I think my builder mentioned getting the pistons from you.
.
I think my builder mentioned getting the pistons from you.
.
Originally Posted by DONAIMIAN
Im confused on why your droping 3k on an LS7 block only to get it re-sleeved. Why not just get an LS1 or LS2 block and get it done.
FormerVendor
Joined: Nov 2001
Posts: 3,065
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From: Houston, Tx.
Damn I missed this thread till now! Brain summed it up very well and with the awsome cylinder heads of the LSx series engines more stroke generally creates more power which is simply all good. None of these hydraulic roller cam LSx engines are going to turn much over 7000 rpm anyway with the long runner manifolds and hydraulic roller valvetrain so bigger stroke is simply going to up the torque everywhere down low and still make slightly to a lot more power up top.
First though you need to make sure the parts you are using are done right especially with a big stroke engine but otherwise the more the merrier.
First though you need to make sure the parts you are using are done right especially with a big stroke engine but otherwise the more the merrier.
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Joined: Feb 2006
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From: Cleveland
the numbers I ran the numbers....worst case scenario is 4.250 stroke, 6.125 rod, .995 compression height. Piston will be at 0 deck with a standard deck block. The piston will have our gf ring package with a couple tweaks and will handle a "touch" of nitrous with the land thickness available, but I'd prefer not to if you can hold yourself back. The ring package will have a good life because it's steel. The sleeve length of 5.800 is long enough to support a "certain" skirt shape of ours. The piston will drop out of the bore at bdc of course, but the piston itself will still be at full diameter when it's at bdc. As long as the bottom of the sleeve is touched up with a cartridge roll, the piston skirt should be pretty happy. Our Zylan skirt coating would be a nice touch as well.
It's conceivable that your present piston is poorly designed for this app and will have excessive skirt wear, but I'd bet the main problem is the ring pack. It may have a little lower tension than it should, but we'll know more when we talk. When you pull the present pistons, you'll see a very distinct line on the lower 1/3 of the skirt where its very shiny and skinned up below that point.
Good luck, Brian Nutter -Wiseco Automotive
It's conceivable that your present piston is poorly designed for this app and will have excessive skirt wear, but I'd bet the main problem is the ring pack. It may have a little lower tension than it should, but we'll know more when we talk. When you pull the present pistons, you'll see a very distinct line on the lower 1/3 of the skirt where its very shiny and skinned up below that point.
Good luck, Brian Nutter -Wiseco Automotive
wow, brian that was very well explained.
so is it more important, for a reliable motor, to concentrate more on the avg tension than peak? as of now the list for these motors go: biggest stroke, longest rod, and best ring combo possible to acheive a reliable, powerful motor
so is it more important, for a reliable motor, to concentrate more on the avg tension than peak? as of now the list for these motors go: biggest stroke, longest rod, and best ring combo possible to acheive a reliable, powerful motor
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Yes, but maybe the key points for the original poster are buried a little. To recap:
Brian, would using a smaller pin--.866, .827--help? Would running the piston out of the block absorb some of the top land height? Does material--4032 vs 2618 make a difference with a sleeve length challenged piston? BTW, what is the top ring down height of the recommended piston?
David
- piston speed/max load issues won't be a problem unless you are running your stroker over 7500rpm;
- the valvetrain will probably end up as your limiting factor (any 7500rpm hydraulic roller street engines?); and
- working with your piston supplier to ensure you have a piston designed for the sleeves in your block and stroke is critical, especially in a long stroke motor. Otherwise you can have an assortment of oil, wear, and possible failure issues.
Brian, would using a smaller pin--.866, .827--help? Would running the piston out of the block absorb some of the top land height? Does material--4032 vs 2618 make a difference with a sleeve length challenged piston? BTW, what is the top ring down height of the recommended piston?
David
Last edited by DavidNJ; Oct 14, 2006 at 11:36 PM.
FormerVendor
Joined: Nov 2001
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From: Houston, Tx.
The stroke and the height of the ring stack is the main factor in how much skirt is left in the cylinder at BDC.
If you have a 5.500 cylinder and are zero deck and have a 4.000 stroke and a .750 tall ring stack then you have .750 of skirt left in the cylinder.
If your break point like Brian says is .725 down then you are still good.
Now if you have a 4.100 stroke and could crunch your ring stack to .650 tall you'd still be at the same place as far as the skirt was concerned at BDC.
You are right though that if you have a head gasket that is good at a thicker size you could run a little taller ring stack and it's done all the time.
If you have a 5.500 cylinder and are zero deck and have a 4.000 stroke and a .750 tall ring stack then you have .750 of skirt left in the cylinder.
If your break point like Brian says is .725 down then you are still good.
Now if you have a 4.100 stroke and could crunch your ring stack to .650 tall you'd still be at the same place as far as the skirt was concerned at BDC.
You are right though that if you have a head gasket that is good at a thicker size you could run a little taller ring stack and it's done all the time.
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Verdad Gallardo TECH Regular
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From: Cleveland
reply to David NJ I like that bit about "sleeve length challenged piston"..I'll have to use that sometime with your permission 
I think you've got it, yes a smaller o.d. pin gives us a little more available land material..I'm comfortable with 22mm in a 5100fpm motor if everything is designed right, that does get you .030 worth of ring land. That being said, I can count the number of LS people that have done that on one hand though.
Although we can run some top lands on LS's down to .175".... (without making a model) I think the average big stroker guy with big valves and big cam are going to need about a .230 top land minimum...that's what I used as a factor.
As for running the piston positive deck, it's fine if your intake manifold seal is properly addressed from the thicker head gaskets. BMW's and a lot of other o.e.'s run .030 positive w./ .065" gaskets all the time (even if they're not burdoned by extreme strokes or rod lengths). I'm not sure of their reasons to do this, but I've considered that the trapped air/fuel in the crevice area of the gasket might serve as a heat barrier between the combustion and the top ring.
The 4032 vs 2618 does affect skirt taper, it's not as much as you might think.
Thanks for your interest. -Brian Nutter-Wiseco
I think you've got it, yes a smaller o.d. pin gives us a little more available land material..I'm comfortable with 22mm in a 5100fpm motor if everything is designed right, that does get you .030 worth of ring land. That being said, I can count the number of LS people that have done that on one hand though.Although we can run some top lands on LS's down to .175".... (without making a model) I think the average big stroker guy with big valves and big cam are going to need about a .230 top land minimum...that's what I used as a factor.
As for running the piston positive deck, it's fine if your intake manifold seal is properly addressed from the thicker head gaskets. BMW's and a lot of other o.e.'s run .030 positive w./ .065" gaskets all the time (even if they're not burdoned by extreme strokes or rod lengths). I'm not sure of their reasons to do this, but I've considered that the trapped air/fuel in the crevice area of the gasket might serve as a heat barrier between the combustion and the top ring.
The 4032 vs 2618 does affect skirt taper, it's not as much as you might think.
Thanks for your interest. -Brian Nutter-Wiseco
5100 fpm!!!
I thought that even 5000 fpm was reserved for aluminum rods and pure billet goodies, No?
What are you guys saying is safe for a car that will see at least 50k mi between rebuilds with say Eagle forged/ARP rods and quality pistons?
for fun:
3.622” LS1:
RPM FPM
6600 3984
6800 4104 safe(street)with good rod bolts
7000 4225
7200 4346
7454 4500
7729 4666
8282 5000
4.000”
RPM FPM
6000 4000
6300 4200
6500 4333
6750 4500
7000 4666 LS7 sits here stock
7500 5000
8200 5466 Tosto Drag machine
I thought that even 5000 fpm was reserved for aluminum rods and pure billet goodies, No?
What are you guys saying is safe for a car that will see at least 50k mi between rebuilds with say Eagle forged/ARP rods and quality pistons?
for fun:
3.622” LS1:
RPM FPM
6600 3984
6800 4104 safe(street)with good rod bolts
7000 4225
7200 4346
7454 4500
7729 4666
8282 5000
4.000”
RPM FPM
6000 4000
6300 4200
6500 4333
6750 4500
7000 4666 LS7 sits here stock
7500 5000
8200 5466 Tosto Drag machine
TECH Regular
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From: DFW, Texas
Take the Ferrari 360 for example. A high-horsepower, low torque car. It is a 3.6L V8 with very little stroke. The motor is capable of higher rpms because of the decreased stroke. Of course this is all relative to the quality of parts you put in a motor.
FormerVendor
Joined: Nov 2001
Posts: 3,065
Likes: 6
From: Houston, Tx.
Originally Posted by GIGAPUNK
5100 fpm!!!
I thought that even 5000 fpm was reserved for aluminum rods and pure billet goodies, No?
What are you guys saying is safe for a car that will see at least 50k mi between rebuilds with say Eagle forged/ARP rods and quality pistons?
I thought that even 5000 fpm was reserved for aluminum rods and pure billet goodies, No?
What are you guys saying is safe for a car that will see at least 50k mi between rebuilds with say Eagle forged/ARP rods and quality pistons?
