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Old 09-02-2008, 09:07 AM
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How Low RPM Can a Dyno Read?

How low can they go? Typically you see pulls from 2500-3000 and up. Can they effectively measure below that?

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Old 07-23-2011, 05:57 AM
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Found this thread via a Google search and would like to bump it up...

My engine/dyno guy said he couldn't load my engine up much below 2,500rpm because the engine dyno (Superflow) wouldn't be able to hold the torque. He said this was because the water brake the dyno uses wouldn't be shifting sufficient volume of water to hold the motor.

On another forum I post on, there's a guy arguing that this is rubbish and the only reason a dyno can't go lower on a particular motor is because the MOTOR doesn't develop enough torque to fully load-up the dyno.

Who is right? Please explain.
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Old 07-23-2011, 08:36 AM
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This article doesn't answer your question, but it does provide a fairly good description of how the water brake and other dyno's work.

http://www.dyno-dynamometer.com/how_dyno_works.htm

Bruce
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Old 07-24-2011, 05:17 PM
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Quote:
Originally Posted by 1ownerT
How low can they go?
Zero rpm.
That is how they are calibrated.

For example, take a ten foot arm, bolt it to the frame of the load absorber part of the dyno, hang a ten pound weight off the end, and the dyno torque reading is then adjusted to read exactly 100 Ft/lb.

You can calibrate it for any torque reading by using a different arm length or weight, but the principle still applies.

Some types of load absorbers such as eddy current or water brake cannot hold very high torque at very low rpm.
The dyno itself will read torque an Hp accurately right down to zero speed.
You just may not be able to hold the engine speed down at very low rpm, even at full load.
Something like a big turbo diesel truck motor can make a whole bunch of torque down at 1,000 rpm, and some dynos just cannot put enough load on the motor at that very low speed to hold the engine back at full throttle.

Last edited by Silver Shadow; 07-24-2011 at 05:36 PM.
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Old 07-24-2011, 11:16 PM
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Quote:
Originally Posted by 1ownerT
Typically you see pulls from 2500-3000 and up. Can they effectively measure below that?
I'm thinking it has to do with the usable powerband of the engine. Since many gas engines don't start making substantial power below 2000-2500 rpm is there really any reason to measure torque that low? This is especially the case in most performance engines. I've seen semi truck engines that max out at 2100-2200 rpm on dynos.

Quote:
the only reason a dyno can't go lower on a particular motor is because the MOTOR doesn't develop enough torque to fully load-up the dyno.
I don't think a given engine should fully load the dyno? If it did that then you couldn't put a more powerful engine on that same dyno. The dyno needs to fully load the engine. At least that's my take on it.
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Old 07-24-2011, 11:24 PM
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Quote:
Originally Posted by Blazin72
I don't think a given engine should fully load the dyno? If it did that then you couldn't put a more powerful engine on that same dyno. The dyno needs to fully load the engine. At least that's my take on it.
Yup spot on .
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Old 07-25-2011, 05:16 AM
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Quote:
Originally Posted by Silver Shadow
Some types of load absorbers such as eddy current or water brake cannot hold very high torque at very low rpm.
The dyno itself will read torque an Hp accurately right down to zero speed.
You just may not be able to hold the engine speed down at very low rpm, even at full load.
This was the reason given to me by my engine/dyno guy when we were dynoing my 383... I was curious to see torque numbers below 2,500rpm, but he said the dyno wouldn't be able to hold the engine that low. Torque at 2,500 was 400lbft. A guy on another forum I post on has said that's rubbish and the only reason my motor couldn't be measured below 2,500 is because it's over-cammed and doesn't develop enough torque to load-up the dyno. Well, I don't think it's over-cammed if it's developing 400lbft at 2,500.
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