Homemade Engine Dyno

[Mike Laws]

Mike,
Thanks for coming and sharing your knowledge.

On an ID when you change the weight / spin time. Do you see the same type change in HP from an engine that you would see on a brake dyno lets say going from 300 RPM sec to 600 RPM sec?

Stan

Stan: Similar but less of a difference. (If that makes sense.)

Mike,
Thanks. I asked because I might be looking at it wrongly. The way I see it if I double my weight or cut my weight in half that is all what I will call external weight while my internal weight / rotating assemble while only a very small part of total weight does not change. On the brake dyno I am not sure how the change in MOI of the rotating assemble effects how the brake works.

Stan

Stan: (You probably know all of this, but in the spirit of this thread, please give me a pass for using an analogy.) In this example; weight does not matter. Only MOI. There are collateral circumstances involving a change in MOI of the rotating assembly (friction, part deflection, piston speed, etc.), however let's talk only about MOI. In the practical sense; we machined a steel flywheel to bolt onto a harmonic balancer. We made pulls with and w/o the steel flywheel (same engine) on the ID and on a Superflow. The differences were apparent on the ID, but not so on the SF. So transfer this data to the race track. The engine made the same torque with or w/o the flywheel bolted to the balancer. We never installed the flywheel to the balancer in the car, however it was apparent that E.T. would have been quicker w/o the flywheel, despite the same torque(force). Does this answer your question?

[Belgian1979]

2 identical engines except for rotating assembly MOI. Will both make same torque (force)? Will both accelerate at the same rate?

Depends on the cars' weight...
Assuming that the 2 engines have the same cubic inch displacement and purely theorethically you would not have any negative cam influences, the amount of pressure on top of the piston is the same. One has a longer arm, but the other has more pressure acting on a bigger surface.

IMO, no difference in that situation.

In practice however you can never tune these engines the same, you can never have the same cam in these engines, so a valid comparison is not possible..

When testing both engines irl with different car weights, transmissions, rear ends, frontal area,... a comparison is uterly useless.

But I don't see how this relates to what I said about an inertia dyno versus a brake dyno.

What if the 2 engines has the same bore, stroke, and rod length and the difference in MOI is due to difference in the weights of the crank, rods and pistons?

Stan

Stan, first, I do not understand what the abbreviation of MOI means.

Evidently if the bob weight of the crank/rods is different you will have a difference in mass and since F=m*a due to increased mass it will accelerate slower.
However, still fail to see the relevance in testing this combo on a brake dyno (I realize that I should have mentioned that I have a brake dyno fitted to a rolling road in mind not an engine dyno) versus an inertia dyno. I mean you create a resistance with both that you accelerate and in essence the resistance you create with your dyno sort of resembles the car's weight, roll resistance etc. In both cases you record the time to move it from one rpm point to the other and based on that you calculate your power. Assuming that you do testing on both sytems, and try to improve the tune of the engine and let's assume that these are equal with only difference in stroke and bore, you should get similar results.
Again, in view of the fact that you cannot maintain a steady speed with an inertia dyno, what is the difference between that and a road test ?

If you tune on a brake dyno, basically you find the best setup at each rpm/load point and it should provide optimum power overall when doing a acceleration test on the bench. (for clarity's sake I refer to a brake dyno fitted to a rolling road type of test bench. Not a pure engine dyno, where there is no drive train etc.).

[BCjohnny]

MOI = 'Moment of Inertia'

I've used PMI, which is possibly technically incorrect, and is an abbreviation of 'Polar Moment of Inertia'

But we're all referencing the same phenomenon

Get a foot long piece of heavy rope and swing it above your head in circles, now get a twenty foot piece and do the same thing ........

[BCjohnny]

Interestingly the subject of flywheel 'weight' has not been addressed, unless I've missed it, so I've had a look around at other diyers musings

This guy kind of echoes what I've said

Every few years I get the urge to build an inertia dyno for testing engines. So it’s cold out and my brain is wandering. The flywheel for this dyno needs to have an adjustable mass to simulate different vehicles. This will work from the polar moment of inertia. Depending on the diameter, the flywheel could weigh over 1000#. The larger the diameter the less weight it will be but safety is a concern. This could be spun 10000 RPMs.

Now I'm making a few assumptions here, one being that the intention is to test 'V8s', but as he's in NY it's not an unreasonable one

Maybe someone with one of these inertia calculators can tell us the varying weights and diameters relative to testing, say, a 400 lb ft / 500 bhp 'V8' between 3000-7000 rpm at a rate gain of 300 rpm per second ......?

[Stan Weiss]

Stan: Similar but less of a difference. (If that makes sense.)

Mike,
Thanks. I asked because I might be looking at it wrongly. The way I see it if I double my weight or cut my weight in half that is all what I will call external weight while my internal weight / rotating assemble while only a very small part of total weight does not change. On the brake dyno I am not sure how the change in MOI of the rotating assemble effects how the brake works.

Stan

Stan: (You probably know all of this, but in the spirit of this thread, please give me a pass for using an analogy.) In this example; weight does not matter. Only MOI. There are collateral circumstances involving a change in MOI of the rotating assembly (friction, part deflection, piston speed, etc.), however let's talk only about MOI. In the practical sense; we machined a steel flywheel to bolt onto a harmonic balancer. We made pulls with and w/o the steel flywheel (same engine) on the ID and on a Superflow. The differences were apparent on the ID, but not so on the SF. So transfer this data to the race track. The engine made the same torque with or w/o the flywheel bolted to the balancer. We never installed the flywheel to the balancer in the car, however it was apparent that E.T. would have been quicker w/o the flywheel, despite the same torque(force). Does this answer your question?

Thanks Mike

Stan

[Stan Weiss]

Interestingly the subject of flywheel 'weight' has not been addressed, unless I've missed it, so I've had a look around at other diyers musings

This guy kind of echoes what I've said

Every few years I get the urge to build an inertia dyno for testing engines. So it’s cold out and my brain is wandering. The flywheel for this dyno needs to have an adjustable mass to simulate different vehicles. This will work from the polar moment of inertia. Depending on the diameter, the flywheel could weigh over 1000#. The larger the diameter the less weight it will be but safety is a concern. This could be spun 10000 RPMs.

Now I'm making a few assumptions here, one being that the intention is to test 'V8s', but as he's in NY it's not an unreasonable one

Maybe someone with one of these inertia calculators can tell us the varying weights and diameters relative to testing, say, a 400 lb ft / 500 bhp 'V8' between 3000-7000 rpm at a rate gain of 300 rpm per second ......?

This is what I get from my calculations for 400 lb weight @ 12" radius and 300 RPM sec acceleration rate.

Stan

Code: Select all

__Weight_=_400.0#____Radius_=_12.0_Inches

____________________________Rear_____Aero_____Rolling____________Rear_W__Accele___Time___Rate
__RPM______MPH___Velocity___Wheel___dynamic___Resist.__Elapsed____Horse__ration__Differ_Rev_Per
__________________ft/sec___Torque__Drag_-_HP____HP______Time______Power__in_G's__ential___Sec

____0.0____.000_____.000_____0.00______.000____.000____.0000_______0.00__0.0000__0.0000_____0.0
__500.0__35.700___52.360___390.57______.000____.000___1.6667______37.18___.9764__1.6667___300.0
_1000.0__71.400__104.720___390.57______.000____.000___3.3333______74.37___.9764__1.6667___300.0
_1500.0_107.100__157.080___390.57______.000____.000___5.0000_____111.55___.9764__1.6667___300.0
_2000.0_142.800__209.440___390.57______.000____.000___6.6667_____148.73___.9764__1.6667___300.0
_2500.0_178.500__261.799___390.57______.000____.000___8.3333_____185.92___.9764__1.6667___300.0
_3000.0_214.199__314.159___390.57______.000____.000__10.0000_____223.10___.9764__1.6667___300.0
_3500.0_249.899__366.519___390.57______.000____.000__11.6667_____260.28___.9764__1.6667___300.0
_4000.0_285.599__418.879___390.57______.000____.000__13.3333_____297.47___.9764__1.6667___300.0
_4500.0_321.299__471.239___390.57______.000____.000__15.0000_____334.65___.9764__1.6667___300.0
_5000.0_356.999__523.599___390.57______.000____.000__16.6667_____371.83___.9764__1.6667___300.0
_5500.0_392.699__575.959___390.57______.000____.000__18.3333_____409.02___.9764__1.6667___300.0
_6000.0_428.399__628.319___390.57______.000____.000__20.0000_____446.20___.9764__1.6667___300.0
_6500.0_464.099__680.678___390.57______.000____.000__21.6667_____483.38___.9764__1.6667___300.0
_7000.0_499.799__733.038___390.57______.000____.000__23.3333_____520.57___.9764__1.6667___300.0
_7500.0_535.499__785.398___390.57______.000____.000__25.0000_____557.75___.9764__1.6667___300.0
_8000.0_571.199__837.758___390.57______.000____.000__26.6667_____594.93___.9764__1.6667___300.0
_8500.0_606.899__890.118___390.57______.000____.000__28.3333_____632.12___.9764__1.6667___300.0
_9000.0_642.598__942.478___390.57______.000____.000__30.0000_____669.30___.9764__1.6667___300.0
_9500.0_678.298__994.838___390.57______.000____.000__31.6667_____706.49___.9764__1.6667___300.0
10000.0_713.998_1047.198___390.57______.000____.000__33.3333_____743.67___.9764__1.6667___300.0
10500.0_749.698_1099.557___390.57______.000____.000__35.0000_____780.85___.9764__1.6667___300.0
11000.0_785.398_1151.917___390.57______.000____.000__36.6667_____818.04___.9764__1.6667___300.0
11500.0_821.098_1204.277___390.57______.000____.000__38.3333_____855.22___.9764__1.6667___300.0
12000.0_856.798_1256.637___390.57______.000____.000__40.0000_____892.40___.9764__1.6667___300.0

_Averages__________________390.57________________________________464.79__________1.6667__300.0

[dynoflo]

i should have mentioned as to the question of controlling the 300 rpm acceleration rate that we get it close and the computer program does the rest. i have a steel 168 tooth flywheel with a srtipped clutch disc bolted to it that the brake drives from. after using flex plates to start engines it is quite noticeable that it is harder to control so i just stick to heavy flywheel. on external balanced motors i have to use flexplate. if i could send picture of dyno sheet you could see how program works.

[brentry]

20200710_170808.jpg[/attachment [attachment=0]20200710_170754.jpg

[BCjohnny]

brentry

Thanks, couple of clarifications from the second graph ......

Total weight for steel mass weight is 2523.19 lbs @ 24" OD x 20" L

Acceleration rate is 4000 rpm spread over 9.46 secs = 423 rpm per sec gain

'Total Gear Reduction' ratio is 2.1

So, is it simply a question of dividing the mass weight by the TGR ratio to obtain the required direct couled mass ? If not can you run the analysis again with 1:1, as in a direct coupled engine dyno ?

Also we seem to be at a 423 per sec rpm gain, not critical as it's gives an indicator but any chance you can get t closer to 300 rpm ?

Otherwise, if it's simply a case of extrapolating the figures by dividing the mass 2523 lbs by the TGR ratio 2.1 = 1201 lbs, multiply by the percentage difference in rpm gain (423 over 300) and we get ~1693 lbs total mass weight for a 24" steel 'disc' @ 300 rpm per sec gain for a 400 lbs ft engine ?

Or does the 'required inertia' have to be factored in as well (1083.81 over 1281.34) giving 1432 lbs ?

Probably too many assumptions in my calcs to be valid, but regardless that's one helluva chunk of steel spinning at 7000 rpm

Cheers, John

[Stan Weiss]

brentry

Probably too many assumptions in my calcs to be valid, but regardless that's one helluva chunk of steel spinning at 7000 rpm

Cheers, John

John,
I will have to run some calcs later to see what weights I get. But they are not spinning the weight at 7000 RPM. Do the calcs. 7000 RPM, 2.1 reduction, 24" diameter wheel, 238 MPH.

Stan

[brentry]

Ya i can run #s again .
That was just a quick example

[Stan Weiss]

OK using the second sheet posted. My weight works out to the current inertia.

Stan

Code: Select all

__Weight_=_1281.0#____Radius_=_12.0_Inches____Rear_Gear_Ratio_=_2.1

____________________________Rear_____Aero_____Rolling____________Rear_W__Accele___Time___Rate
__RPM______MPH___Velocity___Wheel___dynamic___Resist.__Elapsed____Horse__ration__Differ_Rev_Per
__________________ft/sec___Torque__Drag_-_HP____HP______Time______Power__in_G's__ential___Sec

____0.0____.000_____.000_____0.00______.000____.000____.0000_______0.00__0.0000__0.0000_____0.0
__500.0__17.000___24.933___399.92______.000____.000___1.1820______38.07___.6556__1.1820___423.0
_1000.0__34.000___49.867___399.92______.000____.000___2.3641______76.15___.6556__1.1820___423.0
_1500.0__51.000___74.800___399.92______.000____.000___3.5461_____114.22___.6556__1.1820___423.0
_2000.0__68.000___99.733___399.92______.000____.000___4.7281_____152.29___.6556__1.1820___423.0
_2500.0__85.000__124.666___399.92______.000____.000___5.9102_____190.37___.6556__1.1820___423.0
_3000.0_102.000__149.600___399.92______.000____.000___7.0922_____228.44___.6556__1.1820___423.0
_3500.0_119.000__174.533___399.92______.000____.000___8.2742_____266.51___.6556__1.1820___423.0
_4000.0_136.000__199.466___399.92______.000____.000___9.4563_____304.59___.6556__1.1820___423.0
_4500.0_153.000__224.399___399.92______.000____.000__10.6383_____342.66___.6556__1.1820___423.0
_5000.0_170.000__249.333___399.92______.000____.000__11.8203_____380.73___.6556__1.1820___423.0
_5500.0_187.000__274.266___399.92______.000____.000__13.0024_____418.80___.6556__1.1820___423.0
_6000.0_204.000__299.199___399.92______.000____.000__14.1844_____456.88___.6556__1.1820___423.0
_6500.0_220.999__324.133___399.92______.000____.000__15.3664_____494.95___.6556__1.1820___423.0
_7000.0_237.999__349.066___399.92______.000____.000__16.5485_____533.02___.6556__1.1820___423.0
_7500.0_254.999__373.999___399.92______.000____.000__17.7305_____571.10___.6556__1.1820___423.0
_8000.0_271.999__398.932___399.92______.000____.000__18.9125_____609.17___.6556__1.1820___423.0
_8500.0_288.999__423.866___399.92______.000____.000__20.0946_____647.24___.6556__1.1820___423.0
_9000.0_305.999__448.799___399.92______.000____.000__21.2766_____685.32___.6556__1.1820___423.0
_9500.0_322.999__473.732___399.92______.000____.000__22.4586_____723.39___.6556__1.1820___423.0
10000.0_339.999__498.666___399.92______.000____.000__23.6407_____761.46___.6556__1.1820___423.0
10500.0_356.999__523.599___399.92______.000____.000__24.8227_____799.54___.6556__1.1820___423.0
11000.0_373.999__548.532___399.92______.000____.000__26.0047_____837.61___.6556__1.1820___423.0
11500.0_390.999__573.465___399.92______.000____.000__27.1868_____875.68___.6556__1.1820___423.0
12000.0_407.999__598.399___399.92______.000____.000__28.3688_____913.76___.6556__1.1820___423.0

_Averages__________________399.92________________________________475.91__________1.1820__423.0

[brentry]

I dont think 1.1 gear is realistic.

I would probably use at least 24" or bigger wheel diameter , if i ever did it. I probably would just buy a water brake and use the software i have. Unless i ran into some dirt cheap steel. Still some other issues to get around, doing it the steel disc way. Probably why it isnt done to often.
I have a bunch steel behind the shed i could build a engine frame from . I just need some type of load device to do v8s.

[Stan Weiss]

I dont think 1.1 gear is realistic.

I would probably use at least 24" or bigger wheel diameter , if i ever did it. I probably would just buy a water brake and use the software i have. Unless i ran into some dirt cheap steel. Still some other issues to get around, doing it the steel disc way. Probably why it isnt done to often.
I have a bunch steel behind the shed i could build a engine frame from . I just need some type of load device to do v8s.

What do you get with a 1.775:1 reduction? I get the same results but only need 1287.275 Inertia. At 7000 RPM 281.6 MPH What software is that?

Sorry missed the change in wheel size. Above 24" wheel

What do you get with a 1.775:1 reduction? I get the same results but only need 643.64 Inertia. At 7000 RPM 398.9 MPH

Stan

[brentry]

398mph would be in the severe red zone on wheel speed. If you could actually spin the disc that fast without exploding, u would save a bundle of money on steel. smaller disc height alot more safe rpm,but no interia. Program bases it off interia