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View Full Version : What affects dyno #'s?


Michel
01-15-2010, 11:14 AM
Would bigger/heaviers rims and tires for example show lower numbers on a dyno? What about flywheel or differential upgrades?

Can't seem to find a consistent answer!

thanks *wave*

Dr. Flyview
01-15-2010, 11:23 AM
Anything that affects performance on the streets will affect performance on the dyno, so yes to all the above.

dbworld4k
01-15-2010, 11:36 AM
I want to say "no," to all the above seeing as none of them alter the amount of power made at the flywheel, but Flav's post makes me think twice.Anything that affects performance on the streets will affect performance on the dyno, so yes to all the above.

You might have a point.. seeing as a Dyno measures how quickly the vehicle's tires can rotate the drum. If that is the source of measurement, then I can see how heavier wheels/diff/etc can make a difference in the numbers outputted by the dyno. However, this would be more along the lines of how much HP is being "effectively" produced at the ground, seeing as none of the mods listed by Michel actually change the amount of power then engine makes, but only how it is delivered.

Hmm... I'm a bit confused here lol.

hockeyfan27
01-15-2010, 12:10 PM
You may find that the mods you referenced would change where the power happens in the rpm range, but not an increase in HP.

Stephanie
01-15-2010, 12:15 PM
You may find that the mods you referenced would change where the power happens in the rpm range, but not an increase in HP.

+1... That would be my guess too, but...

Hmm... I'm a bit confused here lol.

+1. LOL

Dr. Flyview
01-15-2010, 12:45 PM
Remember at the dyno you're measuring wheel horsepower, so delivery counts.

hockeyfan27
01-15-2010, 01:02 PM
Lightweight Flywheels

From the BMW Digest

... Lightening the flywheel cannot result in increased steady state horsepower as measured by a dyno. After all, the flywheel is not an energy creating device. However since a flywheel does absorb and store some of the energy generated by the engine during acceleration, a lighter flywheel does result in increased transient state energy delivered to the rear wheel, and some therefore measurable acceleration improvement.

But the Dr. makes a good point re: wheel HP (as i'm sure the quote from above is referencing an engine dyno)

put me in the confused pile.

King Luis
01-15-2010, 01:05 PM
Remember at the dyno you're measuring wheel horsepower, so delivery counts.

true, but air pressure and temp (and a few other things not with the car) effect the final output, BUT, when corrected (which most dynos should do) none of that will matter.

here's a good link on what i'm trying to explain.
http://wahiduddin.net/calc/calc_cf.htm

dbworld4k
01-15-2010, 02:12 PM
In any case, I'm pretty sure that differences made from wheel/diff/tires are *so* small, you'd be arguing peanuts.

Dr. Flyview
01-15-2010, 02:13 PM
true, but air pressure and temp (and a few other things not with the car) effect the final output, BUT, when corrected (which most dynos should do) none of that will matter.

here's a good link on what i'm trying to explain.
http://wahiduddin.net/calc/calc_cf.htm

"This Correction Factor Calculator determines the dyno correction factor which is to be multiplied by the actual dyno data to make the resulting corrected readings independent of the effects of temperature, barometric pressure, humidity, and altitude."

Ok, so what. We're talking about mods to the car. External conditions are corrected for either way, stock, or with the mentioned mods. I'm not too sure on the lightened flywheel (that may just help throttle response, how fast revs go up and down, etc), but anything which you put on the car that either makes the car quicker or slower should affect the wheel horsepower or torque (ex. shorter geared differential would increase the wheel torque), EXCEPT for weight changes.

e30_kid89
01-15-2010, 02:21 PM
Don't forget also what gear you do the dyno pull in also affects the numbers.

dbworld4k
01-15-2010, 02:24 PM
^Yup, you want the 1:1.

Michel
01-15-2010, 02:26 PM
In any case, I'm pretty sure that differences made from wheel/diff/tires are *so* small, you'd be arguing peanuts.

i disagree! i felt a very noticeable drop in acceleration when i went from winter 17" rims to the summer 19's with 235/265 tires. As for the diff, you would be the first know Anuk, when you put in the m3 diff, did you notice a big difference in acceleration? hahah :rolleyes:

I would think that if these mods are accounted for on a dyno, then they would make a difference.

but im def. still in the confused pile of people!

richie_s999
01-15-2010, 02:33 PM
everyone will agree that power is lost going from the crank to the rear wheels through the driveline, so anything you do to lighten or take away friction (i.e. lightened parts or better lubricants) and even proper balancing of drive line components will affect the numbers somewhere in the dyno charts.

usually these improvements will come in the midrange of the charts, which are where it is most important for actual driving, high HP numbers at full rpms are ather misleading when looking at how a car will actually deleiver the power to the ground.

dbworld4k
01-15-2010, 02:38 PM
i disagree! i felt a very noticeable drop in acceleration when i went from winter 17" rims to the summer 19's with 235/265 tires. As for the diff, you would be the first know Anuk, when you put in the m3 diff, did you notice a big difference in acceleration? hahah :rolleyes:

I would think that if these mods are accounted for on a dyno, then they would make a difference.
Right, but with wheels and diff you are changing how quickly the car moves, not how much power you are making. A dyno doesn't measure acceleration times, but rather how much the car can turn the drum. The difference you notice in 17s vs. 19s is because of rotational mass being dropped, this difference is pretty much felt only when moving from a stop (with respect to wheels). That doesn't have anything to do with power made, but rather, delivery.

Again, I don't know what verdict to give here. Now I'm starting to confuse myself even more LOL!

richie_s999
01-15-2010, 02:43 PM
Right, but with wheels and diff you are changing how quickly the car moves, not how much power you are making. A dyno doesn't measure how acceleration times, but rather how much the car can turn the drum. The difference you notice in 17s vs. 19s is because of rotational mass being dropped, this difference is pretty much felt only when moving from a stop (with respect to wheels). That doesn't have anything to do with power made, but rather, delivery.

Again, I'm still confused on what verdict to give here. lol.



well said, Dyno's are a great TUNNING TOOL, but unless the tunner/builder of car can take those numbers and use them and factor in car weight and gearing ect its really just a power chart and has nothing to do with real world performance

Michel
01-15-2010, 02:54 PM
Right, but with wheels and diff you are changing how quickly the car moves, not how much power you are making. A dyno doesn't measure acceleration times, but rather how much the car can turn the drum. The difference you notice in 17s vs. 19s is because of rotational mass being dropped, this difference is pretty much felt only when moving from a stop (with respect to wheels). That doesn't have anything to do with power made, but rather, delivery.

Again, I don't know what verdict to give here. Now I'm starting to confuse myself even more LOL!

well said!

Dr. Flyview
01-15-2010, 03:21 PM
Right, but with wheels and diff you are changing how quickly the car moves, not how much power you are making. A dyno doesn't measure acceleration times, but rather how much the car can turn the drum. The difference you notice in 17s vs. 19s is because of rotational mass being dropped, this difference is pretty much felt only when moving from a stop (with respect to wheels). That doesn't have anything to do with power made, but rather, delivery.

Again, I don't know what verdict to give here. Now I'm starting to confuse myself even more LOL!

This is wrong Anuk. Where's your physics knowledge Dr.? :P

Whether the car pushes the road (to propel forward) or the drums, it's the same thing. The drums just measure how hard the wheels CAN push. And when the car has to push a bigger wheel, the actual torque that pushes onto the road surface/drum is less b/c more of the power goes to spinning the heavier wheel.

richie_s999
01-15-2010, 03:22 PM
This is wrong Anuk. Where's your physics knowledge Dr.? :P

Whether the car pushes the road (to propel forward) or the drums, it's the same thing. The drums just measure how hard the wheels CAN push. And when the car has to push a bigger wheel, the actual torque that pushes onto the road surface/drum is less b/c more of the power goes to spinning the heavier wheel.

don't most dyno's do runs with the wheels already spinning?



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Dr. Flyview
01-15-2010, 03:25 PM
IT doesn't matter if the wheels are already spinning, they still have to do work to accelerate, i.e., spin even faster under acceleration. Going from 0-5km/h is the same as 20-100, they're both accelerating. Of course, the difference in power loss due to the bigger wheels may be much bigger at 0-5km/h than 20-25km/h.

richie_s999
01-15-2010, 03:27 PM
IT doesn't matter if the wheels are already spinning, they still have to do work to accelerate, i.e., spin even faster under acceleration. Going from 0-5km/h is the same as 20-100, they're both accelerating. Of course, the difference in power loss due to the bigger wheels may be much bigger at 0-5km/h than 20-25km/h.

true weight is weight, i agree, thats why dyno numbers are only useful for power numbers and don't show real performance of a car

Michel
01-15-2010, 03:39 PM
this is starting to feel like that thread about the plane taking off on a treadmill...

richie_s999
01-15-2010, 03:44 PM
this is starting to feel like that thread about the plane taking off on a treadmill...

LOL

think the easiest way to sum things up is

yes weight will affect numbers on the dyno, but the numbers don't always reflect the actual driving of the car on the road


um can a plane take off on a treadmill????????? :eek:

hockeyfan27
01-15-2010, 03:55 PM
So it would stand to reason that whatever performance difference was realized on the dyno as a result of lighter rims/tires would be doubled when driving. (dyno only tests 2 wheels)

is there a dyno that bolts directly to the wheel hubs?

richie_s999
01-15-2010, 04:01 PM
So it would stand to reason that whatever performance difference was realized on the dyno as a result of lighter rims/tires would be doubled when driving. (dyno only tests 2 wheels)

is there a dyno that bolts directly to the wheel hubs?

yes abbey dyno is one of them....

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Michel
01-15-2010, 04:08 PM
So it would stand to reason that whatever performance difference was realized on the dyno as a result of lighter rims/tires would be doubled when driving. (dyno only tests 2 wheels)

is there a dyno that bolts directly to the wheel hubs?

I think Dynapacks also bolt on the hub, but they get much higher readings than, for example, a Dyno Dynamics.

Which leads to this; maybe mods such as lighter wheels/diff etc. will be read by certain dyno types and not others? possible? does anyone care? lol

dbworld4k
01-15-2010, 05:08 PM
This is wrong Anuk. Where's your physics knowledge Dr.? :P

Whether the car pushes the road (to propel forward) or the drums, it's the same thing. The drums just measure how hard the wheels CAN push. And when the car has to push a bigger wheel, the actual torque that pushes onto the road surface/drum is less b/c more of the power goes to spinning the heavier wheel.
Yup, that's precisely what I was running through my head, but then I was debating it against all the times I've read on forums that wheels/tires don't affect dyno readings; hence my confusion as to why it does/doesn't matter. I guess misinformation can be a powerful thing...

Red_Rocket
01-15-2010, 06:04 PM
The weight of drivetrain will affect RWHP numbers but i believe that after the acceleration is done they do a coast down to measure the resistance of the drivetrain to better predict flywheel horsepower.

craigIS
01-15-2010, 06:27 PM
i think that any part of the car that is used to drive the car, such as wheels, drive-train, diff, trany, affect how quick the car can move. with bigger wheels, more power should be used trying to driver the larger wheel, but if that can be accounted for, it shouldnt affect the readings. with a diff, or trany, on a dyno, you should get a slower or faster time if your running 0-60 or the 1/4 mile because those parts dont make HP, they deliver it differently, so you wouldnt see a difference in HP on the reading.

Mystikal
01-15-2010, 07:29 PM
Holy crap, like 50% of what's been posted in here is wrong or misleading.

But instead of going through that, let's revisit this:

Dyno's are a great TUNNING TOOL

Aside from the spelling error, this is key. Even on back to back runs, with identical temps, everything controlled...the car will make different numbers. So there's really no point calculating what does and what doesn't affect them (by the way just about everything does, from wheel size to tire weight to fluids), as they are inconsistent anyway.

Dynos are for tuning and part testing. You can't compare the numbers to anything really, besides other runs done on the same day at close to the same time (hence, tuning/testing). That makes all of the concerns here irrelevant.

Dr. Flyview
01-15-2010, 11:14 PM
In short, I was right :P

fitter527
01-16-2010, 12:28 AM
you can have all the power in the world,its how you deliver it.
i like how my car feels in the corners.im gettin off topic.sorry!!!
every little factor affects dyno #S.

aaron320i
01-16-2010, 02:30 AM
Everything from your transmission to your differential to the weight of your wheels will effect dyno readings.

True engine hp can only be measure on a engine dyno, where the engine is taken out of the car.

You may have a 300hp engine and lose about a 100hp through the tranny and everything after, leaving you with 200hp. To the wheels.

Another BM
01-16-2010, 10:33 PM
no a plane will not fly on a treadmill..

thats an easy one

richie_s999
01-17-2010, 12:58 AM
no a plane will not fly on a treadmill..

thats an easy one

um are you sure............MYTHBUSTERS FTW!!!!

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Another BM
01-17-2010, 01:53 AM
but the plane is moving forward....

on a treadmill you stay in one spot.. how would the wind cause lift under the wing if you are not moving????

Another BM
01-17-2010, 01:54 AM
the big sheet they used was not matching the planes speed.. you can see the plane pass the cones

Dr. Flyview
01-17-2010, 03:01 AM
haha. The plane pushes against the air, hence it doesn't matter what the wheels do, it pushes forward against the air and goes its own way anyway. The wheels on the conveyor belt are just going twice the speed of the usual.

Another BM
01-17-2010, 03:08 AM
yeah i know, but what i'm saying is the plane is not staying in the same spot..

the plane is in fact moving forward.
the forward force is causing lift

Another BM
01-17-2010, 03:09 AM
lift can not be created if no force is under the wing

NOTORIOUS VR
01-17-2010, 03:31 AM
Even on back to back runs, with identical temps, everything controlled...the car will make different numbers.

Mmmmm... that's not entirely true either. I've seen it many times that cars have pulled almost identical numbers back to back (I'm talking 0.2HP difference) even up to 4+ runs.

So there's really no point calculating what does and what doesn't affect them (by the way just about everything does, from wheel size to tire weight to fluids), as they are inconsistent anyway.

Dynos are for tuning and part testing. You can't compare the numbers to anything really, besides other runs done on the same day at close to the same time (hence, tuning/testing). That makes all of the concerns here irrelevant.

For the most part I agree with that... tuning is the primary purpose and too many people get hung up on peak #'s where in reality theres so much more you can learn from a dyno sheet. But you can relatively compare numbers from a car on the same dyno even on a different day. It gives you an idea of what's going on and the correction factors will correct for environmental changes.

But that said, rolling road dyno numbers should never be compared from one car to another. There are just too many factors, from environmental, to dyno calibration, to mechanical, dyno type/make and setup, etc...

In regards to the original post. Yes a larger, heavier wheel will effect dyno numbers. More rotational mass + a slightly different rolling diameter (usually anyway) means you're also changing your final drive ratio. All of which effects numbers on a rolling road.

Really, the only way to get proper HP/TQ numbers is from an engine dyno.

hockeyfan27
01-18-2010, 11:48 AM
yeah i know, but what i'm saying is the plane is not staying in the same spot..

the plane is in fact moving forward.
the forward force is causing lift

what is causing the forward force? the wheels (which are affected by the conveyor) or the prop (unaffected by conveyor).

Another BM
01-18-2010, 11:50 AM
the prop is causing the forward motion....

i dont get what you getting at tho

hockeyfan27
01-18-2010, 12:14 PM
The force applied to the wheels (by the conveyor) will never affect the forward force the propeller is applying.

The wheels freely spin on their bearings. If the bearings were bad and allowed significant friction only then would the prop would have to work harder to achieve take-off velocity. If the bearing are working well, the conveyor is moot. All of its force goes into spinning the wheels, not deceleration the plane preventing take off.

Look at it this way.

You are trying to push your dead car back into your driveway. (assume level ground) you put your hands on the trunk and walk forward. Your shoes exert a force on the asphalt in the opposite direction that you are pushing; car moves forward.

NOW, strap on a pair of roller-blades. You put your hands on the trunk and walk. The wheels under your feet just spin, your car stays put. The bearings in the roller-blades absorb 100% of the force. It doesn't matter how fast you run, or even which direction you run; the roller-blade wheels are a disconnect between the force you exert and the asphalt. same as the landing gear in a plane.

If you put that plane in a wind tunnel where the wind is moving the opposite direction (a tail wind) then the plane stays put and no lift is generated.

Another BM
01-18-2010, 01:21 PM
i understand what you are say, but the myth was can a plane take off on a threadmill...

a treadmill only has so much room.. the plane has to move forward to take off..

If a plane needs 100 ft to take off, then it will always need 100 ft to take off.

no?

and remember i'm talking take offs from land, not destroyer ships

NOTORIOUS VR
01-18-2010, 01:24 PM
i understand what you are say, but the myth was can a plane take off on a threadmill...

a treadmill only has so much room.. the plane has to move forward to take off..

If a plane needs 100 ft to take off, then it will always need 100 ft to take off.

no?

and remember i'm talking take offs from land, not destroyer ships

I don't think they were talking about distance to take off with the treadmill series... I think they were just trying to make the plane stationary using a treadmill, but since the motor drives the prop and not the wheels the plane will always move forward.

Michel
01-18-2010, 01:53 PM
hahahah, this thread is a mess

Another BM
01-18-2010, 02:06 PM
I don't think they were talking about distance to take off with the treadmill series... I think they were just trying to make the plane stationary using a treadmill, but since the motor drives the prop and not the wheels the plane will always move forward.

exactly my point..

the plane cannot take off if it is not moving forward..
so the fact of the matter is..

if the plane is on a traditional treadmill that is scaled for a plane lets say 25 ft long it will pull itself right off the treadmill..


lol i think we're all saying the same thing in different ways.

but never the less it was still an interesting topic, and no one freaked out call each other names

Stephanie
01-18-2010, 02:51 PM
hahahah, this thread is a mess

+1 ... So much for finding a consistent answer! :P

NOTORIOUS VR
01-18-2010, 02:57 PM
lol i think we're all saying the same thing in different ways.

but never the less it was still an interesting topic, and no one freaked out call each other names

agreed lol