SLMotorsports29
New member
does anyone have any plans or thoughts or pictures on how to build a engine dyno? Ive heard of people doing it just not seen any finish product pictures.
Wanting to build a engine dyno
- Thread starter SLMotorsports29
- Start date
Unfortunately the website from TDK Motorsports is no longer. It was the bible for inertia dynos. I captured and saved the math calculations required. Here they are. We built an inertia dyno and it became the best tool in our garage.
The formula for determining the torque is:
Torque = PM * rpm per second / 9.551
Where "PM" represents the Polar Moment of Inertia of our inertia dyno's flywheel.
If you don't know the Polar moment of Inertia for the flywheel (and your flywheel has a constant thickness cross-section) we can calculate it with the formula:
PM = (W * r^2) / 32.16 / 2
where "W" represents the flywheel weight in pounds and r is its radius in feet. (the formula for weight of a steel disk can be found in the "FAQ" page) Once you have the torque, it is easy to calculate the horsepower with the standard formula:
Hp = Torque * rpm / 5252
Keep in mind that the rpm in the last formula must be the average rpm during the sampling period.
Say our example uses a 10 pound flywheel, 8" in diameter (thus it would have a Polar Moment of Inertia of .017 foot-pounds-second^2). If the engine was able to accelerate this flywheel from say 4,800 rpm to 5,200 rpm in 2/10 of a second (a rate of 2,000 rpm per second) that would represent a torque of 3.6 pound feet. Since our above example had an average rpm of 5,000, it produced 3.4 Hp during the test.
"I" = Inertia
I=1/2MR^2
Where: I=inertia M=weight of the wheel in pounds divided by 32.2(the constant for gravity) Weight= volume of wheel in cubic inches (radius squared x pi x thickness) x .2833(weight of steel/cubic inch) R^2= radius squared
In this formula, the diameter is the multiplier and holds the biggest deciding factor. If you pick a couple of numbers and run through this formula, you will see that changes in diameter add up quickly. Example: Our wheel measures 24.5" in diameter x 1" thick, approx. weight = 133lbs Running through the formula you'll find that "I" equals approx.310
Now, try a wheel 18" in diameter... With a little backwards math, you'll find that in order to get the same weight in lbs. the wheel will have to be approx. 1-7/8" thick. If you can handle this then run the numbers through the formula again and you will find that the "I" of this wheel is only 168! What this means is that you'll have to spin the wheel nearly twice as fast to achieve the same loading on the engine.
Try the other direction... 30" wheel 1/2" thick Approx. weight only 100lbs Approx. "I"= 349. Now you can actually spin the wheel slower with the same results.
Once you start thinking about this, something like a flywheel from a car quickly becomes less attractive for a couple reasons...
1. Weight/diameter-most car flywheels are only 12-15" in diameter and approx. 1" thick. Most weigh less than 50lbs. You'll have to spin these really fast to get much of a load on the engine, this requires a major gearing change, which means you'll have to go out and buy several sprockets/chains etc instead of using a larger wheel which allows you to use standard kart parts.
2. Be careful with automotive flywheels (and some others for that matter), some of them are not "neutral" balanced. Many manufacturers use flywheel/balancer combinations that are counter-weighted to help balance the internal assemblies. A Chevy 400 is one example, most Fords also are "externally" balanced. Ford had 2 systems, one used 50oz.of weight, the other used 28.5. Spinning up a flywheel with 2-3 lbs. of counterweight could be really exciting!
3. Most automotive/truck/tractor flywheels are made of cast iron, assuming you'll be rummaging through someone's junk pile to find a flywheel, be careful, there is usually a reason something is in the junk pile. You don't know for sure what is wrong with it. Having seen the damage caused by an exploding cast iron flywheel, I can tell you that, you don't want it to happen to you!
How do I calculate the weight of the flywheel?? The formula looks/works like this:
Pi x R^2 x thickness x weight of steel/cubic inch
Or Pi(3.14)x wheel radius inches squared = area of round wheel in square inches
x thickness of the wheel in inches = volume in cubic inches x .2833(weight of steel(lbs) per cubic inch) = weight in pounds.
The formula for determining the torque is:
Torque = PM * rpm per second / 9.551
Where "PM" represents the Polar Moment of Inertia of our inertia dyno's flywheel.
If you don't know the Polar moment of Inertia for the flywheel (and your flywheel has a constant thickness cross-section) we can calculate it with the formula:
PM = (W * r^2) / 32.16 / 2
where "W" represents the flywheel weight in pounds and r is its radius in feet. (the formula for weight of a steel disk can be found in the "FAQ" page) Once you have the torque, it is easy to calculate the horsepower with the standard formula:
Hp = Torque * rpm / 5252
Keep in mind that the rpm in the last formula must be the average rpm during the sampling period.
Say our example uses a 10 pound flywheel, 8" in diameter (thus it would have a Polar Moment of Inertia of .017 foot-pounds-second^2). If the engine was able to accelerate this flywheel from say 4,800 rpm to 5,200 rpm in 2/10 of a second (a rate of 2,000 rpm per second) that would represent a torque of 3.6 pound feet. Since our above example had an average rpm of 5,000, it produced 3.4 Hp during the test.
"I" = Inertia
I=1/2MR^2
Where: I=inertia M=weight of the wheel in pounds divided by 32.2(the constant for gravity) Weight= volume of wheel in cubic inches (radius squared x pi x thickness) x .2833(weight of steel/cubic inch) R^2= radius squared
In this formula, the diameter is the multiplier and holds the biggest deciding factor. If you pick a couple of numbers and run through this formula, you will see that changes in diameter add up quickly. Example: Our wheel measures 24.5" in diameter x 1" thick, approx. weight = 133lbs Running through the formula you'll find that "I" equals approx.310
Now, try a wheel 18" in diameter... With a little backwards math, you'll find that in order to get the same weight in lbs. the wheel will have to be approx. 1-7/8" thick. If you can handle this then run the numbers through the formula again and you will find that the "I" of this wheel is only 168! What this means is that you'll have to spin the wheel nearly twice as fast to achieve the same loading on the engine.
Try the other direction... 30" wheel 1/2" thick Approx. weight only 100lbs Approx. "I"= 349. Now you can actually spin the wheel slower with the same results.
Once you start thinking about this, something like a flywheel from a car quickly becomes less attractive for a couple reasons...
1. Weight/diameter-most car flywheels are only 12-15" in diameter and approx. 1" thick. Most weigh less than 50lbs. You'll have to spin these really fast to get much of a load on the engine, this requires a major gearing change, which means you'll have to go out and buy several sprockets/chains etc instead of using a larger wheel which allows you to use standard kart parts.
2. Be careful with automotive flywheels (and some others for that matter), some of them are not "neutral" balanced. Many manufacturers use flywheel/balancer combinations that are counter-weighted to help balance the internal assemblies. A Chevy 400 is one example, most Fords also are "externally" balanced. Ford had 2 systems, one used 50oz.of weight, the other used 28.5. Spinning up a flywheel with 2-3 lbs. of counterweight could be really exciting!
3. Most automotive/truck/tractor flywheels are made of cast iron, assuming you'll be rummaging through someone's junk pile to find a flywheel, be careful, there is usually a reason something is in the junk pile. You don't know for sure what is wrong with it. Having seen the damage caused by an exploding cast iron flywheel, I can tell you that, you don't want it to happen to you!
How do I calculate the weight of the flywheel?? The formula looks/works like this:
Pi x R^2 x thickness x weight of steel/cubic inch
Or Pi(3.14)x wheel radius inches squared = area of round wheel in square inches
x thickness of the wheel in inches = volume in cubic inches x .2833(weight of steel(lbs) per cubic inch) = weight in pounds.
Jamie Webb
New member
TDK website is on Performance Trends website now.
http://performancetrends.com/tdkmotorsports/index.html
http://performancetrends.com/tdkmotorsports/index.html
XXX#40
2A supporter
What happened to the hydraulic dyno plans that was on your site?
flattop1
Dawg 89
Don are you saying, you calculate your dyno pulls via a time vs rpm sample.
What are you using to do the sampling?
We used OnTrack Digital Performance software as a lower cost alternative to the Datamite software. It worked OK but didn't have the polish of Datamite. It did do a good job recording runs accurately, storing each engines pulls by date and time and allow you to lay one pull on top of another for comparison. It also had specific notes per run.
The calculations are for calculating the moment of inertia for the rotating mass of the flywheel, clutch, axle, sprocket and brake rotor assembly. We didn't include chain weight, clutch weight or axle bearing weight in the calculations. We didn't use any altitude or barometric pressure compensations although the software could do it. By the way; it is the calculation for the moment of inertia that allows you to cheat and have a 13hp flathead or 24 hp Yamaha. We worked awful hard at being accurate with the moment of inertia calculation and soon learned there is a lot of fudging about hp on the internet. We never saw a 12hp flathead yet our motors ran up front with the best of them.
There is no question regarding the value of the inertia dyno. Not only can you see every carb adjustment, pipe change or plug change you can also dyno tune your clutches for best performance. With the flathead or Animals you can easily see how one pipe is different from another making it far easier to gear accordingly.
We had a total of about $1,500 into ours.
DK
The calculations are for calculating the moment of inertia for the rotating mass of the flywheel, clutch, axle, sprocket and brake rotor assembly. We didn't include chain weight, clutch weight or axle bearing weight in the calculations. We didn't use any altitude or barometric pressure compensations although the software could do it. By the way; it is the calculation for the moment of inertia that allows you to cheat and have a 13hp flathead or 24 hp Yamaha. We worked awful hard at being accurate with the moment of inertia calculation and soon learned there is a lot of fudging about hp on the internet. We never saw a 12hp flathead yet our motors ran up front with the best of them.
There is no question regarding the value of the inertia dyno. Not only can you see every carb adjustment, pipe change or plug change you can also dyno tune your clutches for best performance. With the flathead or Animals you can easily see how one pipe is different from another making it far easier to gear accordingly.
We had a total of about $1,500 into ours.
DK
Jamie Webb
New member
https://web.archive.org/web/20060714201153/http://www.kartingtechinfo.com/tech/dyno.htm
All the info is still there. That pump is outdated now and not available, but any pump close to the same specs will work you will just need to adjust your calculations based on what you get.
SLMotorsports29
New member
Cool thanks for the info guys