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reving motor with no load questions

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MasonIsGreat35

Member
is it bad to rev a motor to high rpm without any load?
say if i put my minibike on a stand with wheels not touching the ground and revved to 7k, is this bad for the motor, and why would it be bad if it is

or if i reved the motor not bolted to any bike/kart and reved it high without load is it harmfull

reason im asking is i read an old post where someone was told not to free rev there motor or its a good way to blow hole in block, is this correct and ifso why exactly is it bad to rev high with no load compared to with load?
thanks for any help/info
 
J'-remy said:
raving a motor without load is a good way to throw a rod in any engine or run a piston into a floating valve. never understood why so many people rev motors at the track for no apparent reason.
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okay but why is it, what changes between reving with load and without,
what specifically causes failure during reving without load is what im asking
 
The rate of acceleration is much quicker, which is hard on valve train components, connecting rod, and piston, which can fail when revved without load. Rod bolts and wrist pins are especially susceptible. Oiling also doesn't occur as designed so you get wiping of the bearings in short order. Of course, a rev limiter helps, but I never recommend revving an engine with no load.
I'm sure that those who you see/hear revving engines on kart stands at the track are just as quick to rev the engines in their pick-up trucks in the same manner (at least it's got a forced oiling system.)


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🏁Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
Carlson Motorsports on Facebook
www.youtube.com
34 years of service to the karting industry ~ 1Cor 9:24
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
 
MasonIsGreat35 said:
okay but why is it, what changes between reving with load and without,
what specifically causes failure during reving without load is what im asking
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Dynamic loading and Load Moment changes .
A typical lap is 15 seconds . Reduce that by a third , too test .
Hold it wide open for five seconds .
After five seconds release the throttle.
Write down the data collected .
Report your findings .
 
CarlsonMotorsports said:
The rate of acceleration is much quicker, which is hard on valve train components, connecting rod, and piston, which can fail when revved without load. Rod bolts and wrist pins are especially susceptible. Oiling also doesn't occur as designed so you get wiping of the bearings in short order. Of course, a rev limiter helps, but I never recommend revving an engine with no load.
I'm sure that those who you see/hear revving engines on kart stands at the track are just as quick to rev the engines in their pick-up trucks in the same manner (at least it's got a forced oiling system.)


-----
🏁Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
Carlson Motorsports on Facebook
www.youtube.com
34 years of service to the karting industry ~ 1Cor 9:24
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
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that makes alot of sence thanks for the info its helping to understand greatly
so its the action of internal parts gaining rpm too fast since theres no load for it to work against? basically u dont want the motor to climb in rpm crazy fast?
so reving a vehicle in neutral is the same issue, or less so cuz of forced oiling?
 
Think about throwing a baseball .
Throw it 5000 times as fast as you can. In a minute .
Or as many time as fast as you can in one minute .
What do your wrist and elbow feel like ?
Wrist is the piston , elbows the big end of the rod .
 
Every revolution of the engine has the piston going up in the cylinder and instantly being yanked or forced down, changing directions. The faster this happens, the more force is being exerted on the piston, pin, rod and crank. And that is where engines usually break when they are over revved. And the oil isn't forced since there is no oil pump. Oiling is incidental and yes there is a tiny bit of drag from that skinny little dipper being drug thru the oil sump but it is a negligible amount.
 
mike97760 said:
Every revolution of the engine has the piston going up in the cylinder and instantly being yanked or forced down, changing directions. The faster this happens, the more force is being exerted on the piston, pin, rod and crank. And that is where engines usually break when they are over revved. And the oil isn't forced since there is no oil pump. Oiling is incidental and yes there is a tiny bit of drag from that skinny little dipper being drug thru the oil sump but it is a negligible amount.
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reving a motor past its intended rpm it was built for i understand, just dont understand as well why the same rpm under load is bad not under load,
from my understanding the reason is because it reaches that rpm faster than it would under load? and thats the issue with reving with no load is this correct
 
MasonIsGreat35 said:
reving a motor past its intended rpm it was built for i understand, just dont understand as well why the same rpm under load is bad not under load,
from my understanding the reason is because it reaches that rpm faster than it would under load? and thats the issue with reving with no load is this correct
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throwing rods happens a lot when free raving engines. that's why there is RED at the far end of tachometers that's why your auto shop teacher and race engine builder say DON"T DO IT. the question I have is... why do you want to rev your motor on the sand? because it sounds cool?
 
J'-remy said:
throwing rods happens a lot when free raving engines. that's why there is RED at the far end of tachometers that's why your auto shop teacher and race engine builder say DON"T DO IT. the question I have is... why do you want to rev your motor on the sand? because it sounds cool?
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i dont think your understanding what i asked, im not asking why you dont rev the motor to the maximum, (redline) im asking for example a motor is built with springs that shud allow 10k rpm max before valvefloat and all other parts are rated for 10k in this example, why would it be bad to rev to say 7k with no load versus with load?
i dont want to rev my motor on a stand, im simply wanting to know what is different in reving with load and without load
and if its harmful to the motor to rev without load, not asking about reving to redline.
maybe i misunderstood what free rev meant i assumed it meant reving with no load
hope this clears up any confusion i appreciate the help im not the best with wording so maybe there was some miscommunication in my question
 
on another fourm i found this info

"it isn't good for bearings,cams,connecting rods to rev any internal combustion engine while not under load.
They are meant to work under load,you get a free rev. vibration when revving while not under load and this does the damage,excessive free revving has also been known to crack pistons and cylinder heads."

"The reasons Steve cited for not free reving a harley are all valid, there is however another reason of great importance for not doing this. Harley crankshafts are more complicated than say a small block chevy that most are familiar with. The major difference is that a Harley crank is made in multiple pieces, pre-twin cam were at least 5 pieces, and both pistons share a common crank pin. All of these pices must be aligned correctly for the motor to run smooth enough that it doesnt shake itself to bits. Free-reving and especially the rapid deceleration from said free-reving can cause 1 or more of the many parts in the rotating assembly to come out of alignment."

so is free reving holding the throttle at redline? or is it reving under no load, or is it both
i see they comment on vibration, this is one of the thoughts i had wud be an issue from no load is that correct then
 
Post #2
tells what happens . The rod breaks, the piston bends the valve .
MasonIsGreat35 said:
okay but why is it, what changes between reving with load and without,
what specifically causes failure during reving without load is what im asking
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Engine Builder's everywhere want to know specifically what happened at the time of failure .
------
Reving either under Load or free reving . Both have risk of failure.
I can't say I haven't done it .
I can also say I've had to pick up the broken pieces .
Just gotta accept the risk .
 
flattop1 said:
Post #2
tells what happens . The rod breaks, the piston bends the valve .

Engine Builder's everywhere want to know specifically what happened at the time of failure .
------
Reving either under Load or free reving . Both have risk of failure.
I can't say I haven't done it .
I can also say I've had to pick up the broken pieces .
Just gotta accept the risk .
Click to expand...
so there is no differance in short revs either under load or with load, say i just built a motor and without putin it on a kart or bike started it up and cracked throttle (Not to redline) but up there in the rpm, it wont be any differant from reving while on the bike riding?

is the post from the other form talking about vibrations from reving with no load correct and can cause damage?
 
will it throw a rod every time... no. will a motor that's capable of 10k rpm smack a valve at 7k... probably not
7k this is not breaking idle. is the oil hot? has the motor come up to temperature?
under load you reduce the chance of the motor taking off and reving beyond what its designed for. its your motor and your $$ if you want to test the waters by all means do what you like but know if its a presher washer/generator engine these things were designed to redline at 3500 not 7k. you are running the risk of turning your engine into a grenade. I have seen cylinders blow off of flatheads on the track under load. blowing up racing engines is not uncommon just search dyno fails on YouTube
again why do you want to rev your engine?
 
J'-remy said:
will it throw a rod every time... no. will a motor that's capable of 10k rpm smack a valve at 7k... probably not
7k this is not breaking idle. is the oil hot? has the motor come up to temperature?
under load you reduce the chance of the motor taking off and reving beyond what its designed for. its your motor and your $$ if you want to test the waters by all means do what you like but know if its a presher washer/generator engine these things were designed to redline at 3500 not 7k. you are running the risk of turning your engine into a grenade. I have seen cylinders blow off of flatheads on the track under load. blowing up racing engines is not uncommon just search dyno fails on YouTube
again why do you want to rev your engine?
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reason for wondering is just curiosity, ive in the past started my bike on a stand while tuning carb n after gave it a few revs to hear the new exhaust, not redline but few snaps of the throttle, and i read a post that reving without load can be harmfull to the motor and i wanted to know why(or if its true) its different than reving with load, and it sounds like the reason is u can accidently hit redline or over due to no load.

a specific example, i built a 196cc with headstuds billet rod/flywheel 22lb springs say its max is 7k rpm n idles at 1800 or around there, oils warm motors happy if i rev to 5 or 6k without load compared to with load the difference is without load u can accidently rev higher than intended? is that the issue, not being not under load itself but because its easier to surpass the rpm limits of the motor due to no load?

so as long as im not redlining or surpassing redline, or say holding at 6k rpm for a minute without load its fine and no issue with reving without load?
 
MasonIsGreat35 said:
reason for wondering is just curiosity, ive in the past started my bike on a stand while tuning carb n after gave it a few revs to hear the new exhaust, not redline but few snaps of the throttle, and i read a post that reving without load can be harmfull to the motor and i wanted to know why(or if its true) its different than reving with load, and it sounds like the reason is u can accidently hit redline or over due to no load.

a specific example, i built a 196cc with headstuds billet rod/flywheel 22lb springs say its max is 7k rpm n idles at 1800 or around there, oils warm motors happy if i rev to 5 or 6k without load compared to with load the difference is without load u can accidently rev higher than intended? is that the issue, not being not under load itself but because its easier to surpass the rpm limits of the motor due to no load?

so as long as im not redlining or surpassing redline, or say holding at 6k rpm for a minute without load its fine and no issue with reving without load?
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Not trying to be rude or anything, but you seem to want to spend an overly large amount of time trying to dissect something, that as far as I'm concerned, is completely irrelevant to successful racing...if you're trying to tune your motor, you will gain much needed information by finding a motor shop with a dyno, that way you can Rev up the motor when it does have a load on it, and in many instances, can get a detailed computer printout to find out VIP data like peak torque among other things...😉
 
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