Piston speed
Thread Starter
Race Director
Joined: Nov 2000
Posts: 16,416
Likes: 1
From: NC,USA
Piston speed
Hi all
I am starting to think about high reving engines... but the limiting factor is rods and pistons breaking. I read something that explained that it was piston speed that breaks these... they have to much stress because they go from 0 to over 100 in 4 inches twice every time the crank turns 1 time.
so how do you reduce the piston speed? I was thinking that a stroker of some type might do it.
Anybody on here know anything about this stuff Gkull? Ganey? Monty?
Thanks for the help guys... i hate it when i think, it ends up costing money in the end :jester
ZD
[Modified by ZD75blue, 9:50 AM 11/8/2001]
I am starting to think about high reving engines... but the limiting factor is rods and pistons breaking. I read something that explained that it was piston speed that breaks these... they have to much stress because they go from 0 to over 100 in 4 inches twice every time the crank turns 1 time.
so how do you reduce the piston speed? I was thinking that a stroker of some type might do it.
Anybody on here know anything about this stuff Gkull? Ganey? Monty?
Thanks for the help guys... i hate it when i think, it ends up costing money in the end :jester
ZD
[Modified by ZD75blue, 9:50 AM 11/8/2001]
Burning Brakes
Joined: Feb 2001
Posts: 866
Likes: 1
From: Peabody,MA,USA
Re: Piston speed (ZD75blue)
Stroking increases the distance the piston must travel and the speed also increases. The best thing to do is to buy a book and find the specs on a high reving engine. I think a light weight valve train would make the most difference.
Le Mans Master
Joined: Feb 1999
Posts: 5,877
Likes: 1
From: Park Ridge IL
Re: Piston speed (ZD75blue)
I've alwasy used 4500 feet/minute as my recommended maximum sustained piston speed, but I've seen others quote 4000 feet/second. I've enver had a problem with 4500 in either automotve or motorcycles.
To find feet/minute, just multiply stroke times 2, multiply the result times rpm, and then divide the result of that by 12.
4" x 2 = 8
8 x 7000rpm=56,000 inches/minute
56,000/12=4667
A longer stroke will obviously increase piston speed, rpm remaining constant. Destroking will decrease piston soeed, rpm remianing constant, but it allows you to turn higher rpm before you reach 4500 feet/second.
Realistically, if you have good bottom end components - forged crank, rods, pistons, rod bolts, main caps, etc - the valvetrain is typically going to be your limiting factor, as mentioned before. As rpm increases, you must increase valve spring pressure, use lighter, more expensive components (titanium valves and retainers, shaft mounted rockers, etc).
Also, remember that as rpm goes up reliability and durability go down in the long run. For the street, and even some levels of drag racing, super high rpms are unnecessary. For reliability/durability purposes, it is generally better to increase displacement, rather than increase rpm, in order to increase power.
To find feet/minute, just multiply stroke times 2, multiply the result times rpm, and then divide the result of that by 12.
4" x 2 = 8
8 x 7000rpm=56,000 inches/minute
56,000/12=4667
A longer stroke will obviously increase piston speed, rpm remaining constant. Destroking will decrease piston soeed, rpm remianing constant, but it allows you to turn higher rpm before you reach 4500 feet/second.
Realistically, if you have good bottom end components - forged crank, rods, pistons, rod bolts, main caps, etc - the valvetrain is typically going to be your limiting factor, as mentioned before. As rpm increases, you must increase valve spring pressure, use lighter, more expensive components (titanium valves and retainers, shaft mounted rockers, etc).
Also, remember that as rpm goes up reliability and durability go down in the long run. For the street, and even some levels of drag racing, super high rpms are unnecessary. For reliability/durability purposes, it is generally better to increase displacement, rather than increase rpm, in order to increase power.
Thread Starter
Race Director
Joined: Nov 2000
Posts: 16,416
Likes: 1
From: NC,USA
Re: Piston speed (Ace77)
i take it that the best way to go faster (higher rpm) is to beef up the bottom end... and the rods/ pistons and higher compression valve springs
Anything else about this that i need to know?
ZD
Anything else about this that i need to know?
ZD
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Re: Piston speed (Monty)
Using Monty formula my 383 with a 3.750 stroke 7500 rpm = 4687.5 feet per minute. Not a problem with high end crank and rods. For pistons you have to buy the lightest possible. Your piston weight becomes a terrific load when going from a dead stop at TDC and BDC. You also have to run tiny 1/16th inch rings so they don't tear up the piston lands. JE pistons are @ $75 each. I use titanium intakes to keep valve weight down.
Monty is right about C.I. the bigger the motor the less rpm required to make a given power level. So I always recommend 3.875 strokers to get 393-396 ci out of 350 blocks.
Monty is right about C.I. the bigger the motor the less rpm required to make a given power level. So I always recommend 3.875 strokers to get 393-396 ci out of 350 blocks.
Drifting
Joined: May 2001
Posts: 1,329
Likes: 12
Re: Piston speed (gkull)
Hummm from that calculation, my soon to be 540 should not be revved above 6352 rpm ...so going after a solid cam isn't worth it at all since I could attain that kind of rpm easily with an hydraulic one??
Le Mans Master
Joined: Feb 1999
Posts: 5,877
Likes: 1
From: Park Ridge IL
Re: Piston speed (Lohkay)
No, I'm not saying it shouldn't or can't be revved past whatever rpm generates piston speeds in excess of 4500 feet/minute, I said it was the "recommended maximum SUSTAINED piston speed". Short bursts above 4500 feet/minute will usually not hurt anything, assuming everything is in good shape. Actually, I wouldn't even recommend sustained operation at 4500 feet/minute. Most recommend not exceeding 2/3's of redline for sustained periods of time. I guess it depends on what your idea of sustained is - minutes, hours, etc.
Thread Starter
Race Director
Joined: Nov 2000
Posts: 16,416
Likes: 1
From: NC,USA
Re: Piston speed (Monty)
I was thinking that the capability to go to high RPMs would be nice... but you wouldn’t necessarily keep the motor a 7 grand all the time, you would just be able to miss shifts... etc.
That is mainly my goal, and if I decide to use nitrous i can crank it up to 6,000 rpm without blinking an eye.
Thanks for the help and making me think about beefing up what I have got as apposed to going all out race style motor.
ZD :cheers:
That is mainly my goal, and if I decide to use nitrous i can crank it up to 6,000 rpm without blinking an eye.
Thanks for the help and making me think about beefing up what I have got as apposed to going all out race style motor.
ZD :cheers:
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Tech Contributor
Joined: Aug 1999
Posts: 14,384
Likes: 6,408
From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Piston speed (ZD75blue)
ZD -
For some of my ramblings on a similar related subject matter, check out:
http://forums.corvetteforum.com/zerothread?id=172555
For some of my ramblings on a similar related subject matter, check out:
http://forums.corvetteforum.com/zerothread?id=172555
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Re: Piston speed (lars)
Lars - I read your whole story. You left out part of the equation. Fuel burn rate. Monty is running a 4 inch stroke in preperation for running the slowest fuel avaliable Nitromethane He really needed a 4.5 stroke, but he couldn't fit it. :jester
Le Mans Master
Joined: Feb 1999
Posts: 5,877
Likes: 1
From: Park Ridge IL
Re: Piston speed (lars)
That is very interesting. Some of those concepts are familiar and some are new.
I couldn't help but run my current engine configuration through the formula's you quoted for peak torque @ 2000 feet/min and peak hp @ 2800 feet/min, I'm sure all the other gearheads did the same thing;).
In my situation, peak torque and peak volumetric efficiency, both of which are actuals from a Superflow engine dyno, peak torque occurs at 5100rpm and peak horsepower at 6800rpm.
Using your formula: (stroke * rpm)/6 =
(4 * 5100)/6 = 3400 feet/min @ peak torque (97.5 V/E)
(4 * 6800)/6 = 4533 feet/min @ peak horsepower (95.5 V/E)
I couldn't help but run my current engine configuration through the formula's you quoted for peak torque @ 2000 feet/min and peak hp @ 2800 feet/min, I'm sure all the other gearheads did the same thing;).
In my situation, peak torque and peak volumetric efficiency, both of which are actuals from a Superflow engine dyno, peak torque occurs at 5100rpm and peak horsepower at 6800rpm.
Using your formula: (stroke * rpm)/6 =
(4 * 5100)/6 = 3400 feet/min @ peak torque (97.5 V/E)
(4 * 6800)/6 = 4533 feet/min @ peak horsepower (95.5 V/E)
Thread Starter
Race Director
Joined: Nov 2000
Posts: 16,416
Likes: 1
From: NC,USA
Re: Piston speed (Monty)
wow thanks lars... talk about being the wealth of knoweldge
Thanks for the help with this... peak torque/ peak horsepower
:crazy: i just want it under my right foot :jester
Thanks for the link
and lars i dont have insomnia but would like to read the fuel milage paper.
Talk to you guys later, after i finsih under standing this auto math book... cross refrensing with power sevrets by smokey and horsepower book "how to make horsepower with chevy engines" i get a little confused, but in the end i might just understand enough to build one a these things
ZD :cheers:
[Modified by ZD75blue, 10:00 AM 11/9/2001]
Thanks for the help with this... peak torque/ peak horsepower
:crazy: i just want it under my right foot :jester
Thanks for the link
and lars i dont have insomnia but would like to read the fuel milage paper.
Talk to you guys later, after i finsih under standing this auto math book... cross refrensing with power sevrets by smokey and horsepower book "how to make horsepower with chevy engines" i get a little confused, but in the end i might just understand enough to build one a these things
ZD :cheers:
[Modified by ZD75blue, 10:00 AM 11/9/2001]
