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wildbillnc
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Discussion Starter #1
First off welcome back VTF :thanks:
Ok at the risk of sounding dumb I have to ask about this and hope someone can explain to me the difference of torque and fast revving. I have heard people say it is important to have a fast revving engine but I thought torque was an indication of how fast a bike would rev am I wrong ? and please explain what is needed to make the engine rev quickly?
 

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WILDBILLNC said:
First off welcome back VTF :thanks:
Ok at the risk of sounding dumb I have to ask about this and hope someone can explain to me the difference of torque and fast revving. I have heard people say it is important to have a fast revving engine but I thought torque was an indication of how fast a bike would rev am I wrong ? and please explain what is needed to make the engine rev quickly?
I see you have Hillside stage 3 heads, I'm guessing you have a "fast revving" motor. I use to have a bike with #'s almost the same as your but I am gonna guess that your bike would get from 3k to 6k at least a second faster than mine..
I don't know the actual I don't know why or how but just wanted to post something, I am so happy VTF is back up!
 

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adlerx said:
I see you have Hillside stage 3 heads, I'm guessing you have a "fast revving" motor. I use to have a bike with #'s almost the same as your but I am gonna guess that your bike would get from 3k to 6k at least a second faster than mine..
I don't know the actual I don't know why or how but just wanted to post something, I am so happy VTF is back up!


ok adlerx, thats enough VTF for you back to your cave.:harhar:
 

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The quick answer to that is VE (volumetric efficiency) equals velocity equals torque so the trick is to get the port to the right size to do a smooth transition from when it takes over after the valve opening of .2562 Ld ratio all the way through the rpm's of your engine and have it sized to the rate of change of your cam. The better the VE through out the rpms the quicker the rev seems to be.
 

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And I am guessing here, but low intake velocity due to poor port design or valve sizing will cause the motor to be slow reving.
 

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wildbillnc
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Discussion Starter #6
I agree that intake as well as exhaust velocity make a quick revving engine but isn't torque a result of a quick revving engine??:whatever:
 

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Also lightening your rotating mass will give you a faster rev. Trying to spin 30 pound flywhels quickly is not an easy job. Shave a few pounds of and make that engine real snappy!!!:chopper:
 

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Hey Randy and guys. Great thread.

How about the effects of changing gearing though?? Just wondering.
 

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YES
Those same high VE numbers translate to higher torque numbers. The optimized motors have more average torque under the curve. In other words if you took a data point at every 50 rpm in the operating range and recorded the TQ number at WOT and averaged that these numbers the result would exceed a motor with lower VEs. Personally I believe this concept can be overexerted if the focus is primarily getting the motor to have the fastest obtainable rev times, just my opinion. Some riders may prefer a little less torque at the lowest speed ranges in exchange for more up top. A larger than stock very high torquer can be a chore to ride hitting bumps and little blips of the throttle the motor is jumpy. So my own bike has modest CR (read not as prone to ping) and a little later intake close so that I can accomplish this but it gets the job done.
 

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DEVILDOG said:
And I am guessing here, but low intake velocity due to poor port design or valve sizing will cause the motor to be slow reving.

YES it can cause the motor to Rev slower. :yikes:
 

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MaxHeadflow
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"No we have seen bikes with similar torque and hp that were 3 seconds quicker or slower reving."

Randy,

I need some help understanding this. First I assume that when you are talking about reving in seconds, you are talking about rev time on the dyno going from say 2k to 5.5K. Using a interial dyno (no brake), if the torque and HP are the same then as far as I san tell the only way one motor can produce a quicker rev time is if the gearing, (including tire diameter) is lower. For instance, the run time of a 124 hp motor takes about 4 seconds to got from 2500 to 6K in 4th but it takes 6 seconds to do the same thing in 5th..


Do you have any run files that we can look at that shows this?

As far as quicker reving, if you are taking about under load, seems to me the motor that make more torque throught the RPM range will be quicker reving. Also reducing rotating mass will keep the power from flowing into the flywheels and allow it to go to the rear tire. Years ago, I used to lighten 4 stroke dirt bike flywheels to achieve this.


TIA
Bruce
 

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Iron Butt, SS2000
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Lets keep it simple guys, he's trying to learn.

A little bit of an apples & oranges question.

Torque = twisting force (without regard to work being done i.e. crankshaft rpm)

Quick reving refers to when you whack the throttle, how short of a time from idle to red line.

All things being equal the more torque at any given rpm the quicker the engine will rev.

Who wants to provide the formula for Horsepower?
 

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Iron Butt, SS2000
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Bruce Woltz said:
"No we have seen bikes with similar torque and hp that were 3 seconds quicker or slower reving."

Randy,

I need some help understanding this. First I assume that when you are talking about reving in seconds, you are talking about rev time on the dyno going from say 2k to 5.5K. Using a interial dyno (no brake), if the torque and HP are the same then as far as I san tell the only way one motor can produce a quicker rev time is if the gearing, (including tire diameter) is lower. For instance, the run time of a 124 hp motor takes about 4 seconds to got from 2500 to 6K in 4th but it takes 6 seconds to do the same thing in 5th..


As far as quicker reving, if you are taking about under load, seems to me the motor that make more torque throught the RPM range will be quicker reving. Also reducing rotating mass will keep the power from flowing into the flywheels and allow it to go to the rear tire. Years ago, I used to lighten 4 stroke dirt bike flywheels to achieve this.


TIA
Bruce
You see it the same as I do Bruce.
@gree:
 

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Bruce Woltz said:
"No we have seen bikes with similar torque and hp that were 3 seconds quicker or slower reving."

Randy,

I need some help understanding this. First I assume that when you are talking about reving in seconds, you are talking about rev time on the dyno going from say 2k to 5.5K. Using a interial dyno (no brake), if the torque and HP are the same then as far as I san tell the only way one motor can produce a quicker rev time is if the gearing, (including tire diameter) is lower. For instance, the run time of a 124 hp motor takes about 4 seconds to got from 2500 to 6K in 4th but it takes 6 seconds to do the same thing in 5th..


Do you have any run files that we can look at that shows this?

As far as quicker reving, if you are taking about under load, seems to me the motor that make more torque throught the RPM range will be quicker reving. Also reducing rotating mass will keep the power from flowing into the flywheels and allow it to go to the rear tire. Years ago, I used to lighten 4 stroke dirt bike flywheels to achieve this.


TIA
Bruce
This really makes sense, except why did we use a larger(heavier) Harmonic balancer when hot rodding the Chevy smallblock?
 

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Iron Butt, SS2000
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route66paul said:
This really makes sense, except why did we use a larger(heavier) Harmonic balancer when hot rodding the Chevy smallblock?

I didn't know that was common practice. But don't be surprised by my lack of knowledge, maybe Bruce can answer better. My understanding of the purpose of the Harmonic Balancer is to quell vibrations that travel up and down the crank(this is starting to sound bad) so it does not try to tear itself apart or crack at a potentially weak area. Anyway, the correct mass of the balancer will vary due to changes in the mass of the flywheel, the crank, pistons, compression ratio, cam & perhaps other things. Harmonic vibes can be spooky to chase, sometimes what works does not make sense at first look.

I'm certainly not an engine builder Paul, so no way I'm saying you can take that to the bank. Just sharing the world as I have imagined it to be.
 

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MaxHeadflow
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1,344 Posts
I think I know...

"This really makes sense, except why did we use a larger(heavier) Harmonic balancer when hot rodding the Chevy smallblock?"

I'm no car guy. If my truck needs an oil change I take to the shop,, but I think I know the answer.. To keep the crank from breaking or at least to reduce harmonic activity in the twisting of the crank. It gives the front end of the crank a place to store energy to reduce torsional wipping. I think the Chevy guys still run lighter flywheels, don't they?


Bruce
 

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GoFast!
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Aren't we all Chevy guys, running our Chevy cams in our v-twins??
 

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wildbillnc
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117 Posts
Discussion Starter #20
I still don't understand!! It seems to me that two bikes on the same dyno the same load and the same everything then the one with the most torque would rev the quickest therefore making it the fastest revving engine and also proving that either torque is the result of fast revving or fast revving is the result of torque !! Either way they are related :rolleyes:
 
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