Understanding Dynos...

[stephenevans99]

After my recent dyno day I was left leaving a little bewildered as to exactly what went on - it was my first session even watching a car on a dyno. The owner of the place writes technical bulletins for various publications and he sent me a link to an article he wrote a few years back giving a fairly easy to undertand explanation of the various dynos and the results:

FAF258.tech.pdf

[mitchyboy01]

After my recent dyno day I was left leaving a little bewildered as to exactly what went on - it was my first session even watching a car on a dyno. The owner of the place writes technical bulletins for various publications and he sent me a link to an article he wrote a few years back giving a fairly easy to undertand explanation of the various dynos and the results:

FAF258.tech.pdf

Interesting read mate thanks

[LiamT4]

So in closing, its whp thats the most accurate and bhp is calculated from the whp....................something a few people on here have been saying for ages.

[mitchyboy01]

So in closing, its whp thats the most accurate and bhp is calculated from the whp....................something a few people on here have been saying for ages.

No not at all!

What this should tell us is that the yanks are "wrong" when they harp on about WHP - no two dynos will give the same power but they should ALL give the same flywheel figures if the prevailing conditions are identical. Gear choice - it affects wheel power but not flywheel power if drag is being measured accurately. The only comparable constant is fly power. It would be interesting at the next dyno day (wherever that may be) to run a few cars in 3rd 4th 5th (6th) and see what happens - the engine power isn't altering but drag will change hugely with the varying wheel speed. If the dyno is accurate at measuring anything it should show equal fly power every time with very different WHP.

[LiamT4]

No not at all!

What this should tell us is that the yanks are "wrong" when they harp on about WHP - no two dynos will give the same power but they should ALL give the same flywheel figures if the prevailing conditions are identical. Gear choice - it affects wheel power but not flywheel power if drag is being measured accurately. The only comparable constant is fly power. It would be interesting at the next dyno day (wherever that may be) to run a few cars in 3rd 4th 5th (6th) and see what happens - the engine power isn't altering but drag will change hugely with the varying wheel speed. If the dyno is accurate at measuring anything it should show equal fly power every time with very different WHP.

Did you miss the part that explains that transmission losses can never accuratly measured on a rolling road? Or the part that says "go by what the rollers measured at the point of measurement"

Some people seem to forget that its the bloody wheels that are in direct contact with the dyno, so if you §§§§ up the whp reading (like using the wrong gear for example) than your even less likely to get an accurate flywheel measurment.

and it does explain how different gears effects the whp result and thats why you need to use a gear thats as close to 1:1 as you can get.

[mitchyboy01]

Ok so how does a maha dyno give hugely low whp figures (remember frankies 198bhp?) and still comes up with a fly power comparible to every other dyno?

[mitchyboy01]

1.      The drag curve is exponential. The faster you go the steeper the curve. This how you ended up with a very high drag figure, your car will have hit over 140mph. 2.      The wheels figure is meaningless without a drag curve. If I ran your car in 4th gear you may have 140bhp/120mph at the wheels. If I then ran it in 5th  gear you would have around say 125bhp/145mph. The engine is still producing the same power so why are the two figures different? It is because of our old friend the drag curve. At 120mph there is a far lower drag than at 145mph so the wheels figures will differ. 3.      Some dyno’s do not have the ability to measure drag so they are forced to try and calculate it (dyno dynamics etc.). In my experience this calculation is always flawed for the simple reason that the drag curve rises exponentially. The favourite way they calculate this is by adding a percentage to the wheels figure. This can never work as the drag of the tyres is not influenced by the power of the engine in any way. A good little check you can do with your graph is to look at the peak power rpm at the wheels. Because of the shape of the drag curve this will always be at a lower point than the peak power rpm at the flywheel. If they are the same then the calculation used is fatally flawed and must, by the laws of physics, be giving you incorrect 

So in theory its all a load of §§§§§§§§ and everyone has a different opinion on the matter and has been debated to death!

[LiamT4]

Ok so how does a maha dyno give hugely low whp figures (remember frankies 198bhp?) and still comes up with a fly power comparible to every other dyno?

Maybe you should get in contact with the people who make it and ask them......and at the same time ask which gear is best to use on a 5spd car, 4th or 5th......

I could try and give you some information on how dynos work, but as someone has already posted a link to something that provides a plenty of information regarding the subject, i don't think i need to.

[LiamT4]

1.      The drag curve is exponential. The faster you go the steeper the curve. This how you ended up with a very high drag figure, your car will have hit over 140mph. 2.      The wheels figure is meaningless without a drag curve. If I ran your car in 4th gear you may have 140bhp/120mph at the wheels. If I then ran it in 5th  gear you would have around say 125bhp/145mph. The engine is still producing the same power so why are the two figures different? It is because of our old friend the drag curve. At 120mph there is a far lower drag than at 145mph so the wheels figures will differ. 3.      Some dyno’s do not have the ability to measure drag so they are forced to try and calculate it (dyno dynamics etc.). In my experience this calculation is always flawed for the simple reason that the drag curve rises exponentially. The favourite way they calculate this is by adding a percentage to the wheels figure. This can never work as the drag of the tyres is not influenced by the power of the engine in any way. A good little check you can do with your graph is to look at the peak power rpm at the wheels. Because of the shape of the drag curve this will always be at a lower point than the peak power rpm at the flywheel. If they are the same then the calculation used is fatally flawed and must, by the laws of physics, be giving you incorrect 

So in theory its all a load of §§§§§§§§ and everyone has a different opinion on the matter and has been debated to death!

Did you read the part in the op original link that explains why using the "coast down system" is also flawed?

I'll say it again - if you mess up the whp figure, by using the wrong gear for example, then you are never going to get an accurate bhp reading.

[mitchyboy01]

Have a read of the MAHA dyno thread. There are about 10 different opinions on the same subject. Seems like there is no definitive answers. People very knowledgeable on the subject can't agree and each argument has numerous eveidence to back it up! Im not saying you are wrong as I dont really care anyway. I just don't agree that whp is the holy grail.

[LiamT4]

Have a read of the MAHA dyno thread. There are about 10 different opinions on the same subject. Seems like there is no definitive answers. People very knowledgeable on the subject can't agree and each argument has numerous eveidence to back it up! Im not saying you are wrong as I dont really care anyway. I just don't agree that whp is the holy grail.

neither do i, but i also don't believe people should be looking at bhp readings from a dyno and thinking that they are 100% accurate, because there are more variables in working out the bhp than the whp.

[smithy]

Frankies got 300 bhp flywheel figuires on an unmapped car too on a maha dyno weldone lol.what I am going to do is when my car is finished to the stage I'm doing at mo .i will take the car to a hub dyno ,then a dyno dynamics and lastly a maha dyno .they will all measure whp but only 2 will measure bhp fly and we will see what the figuires are .but if I'm not mistaken isn't a hub dyno the most accarute or am I wrong again like so many peeps keep telling me and I think they only read whp but as said I might be wrong again lol.

[WOODY T5]

well i dont know nothing about them but my car goes like stink.

[stephenevans99]

Some excellent posts above from folks who obviously know a lot more than me....

I was very disappointed with the results from my day at the dyno, but after speaking with the guy at the garage he confirms some of the things mentioned above such as the bhp on the rollers is far from being 'holy grail' & that depending on the particular dyno how the session can be manipulated to give better results........I wish they manipulated mine...lol

[LiamT4]

Frankies got 300 bhp flywheel figuires on an unmapped car too on a maha dyno weldone lol.what I am going to do is when my car is finished to the stage I'm doing at mo .i will take the car to a hub dyno ,then a dyno dynamics and lastly a maha dyno .they will all measure whp but only 2 will measure bhp fly and we will see what the figuires are .but if I'm not mistaken isn't a hub dyno the most accarute or am I wrong again like so many peeps keep telling me and I think they only read whp but as said I might be wrong again lol.

Unless you are willing to take your engine out and fit it to an engine dyno then yes, they are supposed to be the most accurate, although i'v noticed that the losses on the hub dyno's seem to be similar to many twin roller dynos anyway. But at least they take out the tyre variables.

[LiamT4]

well i dont know nothing about them but my car goes like stink.

Some excellent posts above from folks who obviously know a lot more than me....

I was very disappointed with the results from my day at the dyno, but after speaking with the guy at the garage he confirms some of the things mentioned above such as the bhp on the rollers is far from being 'holy grail' & that depending on the particular dyno how the session can be manipulated to give better results........I wish they manipulated mine...lol

On the road is where it really matters.

I'v always thought that 1/4mile terminal speed is a good indicator of power. The time itself is to do with many things, but you can't get a high terminal speed without having genuine power.

[smithy]

On the road is where it really matters.

I'v always thought that 1/4mile terminal speed is a good indicator of power. The time itself is to do with many things, but you can't get a high terminal speed without having genuine power.

Not always about power ether it's about gearing too.you can have high power but if you haven't got te gearing to suit then the is no point unless you want to go 180mph though lol

[smithy]

Unless you are willing to take your engine out and fit it to an engine dyno then yes, they are supposed to be the most accurate, although i'v noticed that the losses on the hub dyno's seem to be similar to many twin roller dynos anyway. But at least they take out the tyre variables.

All the hub dyno operators I speak too say they only give whp figuires

[smithy]

well i dont know nothing about them but my car goes like stink.

I agree its what happens on the road that counts

[LiamT4]

Not always about power ether it's about gearing too.you can have high power but if you haven't got te gearing to suit then the is no point unless you want to go 180mph though lol

There are lots of variables on the 1/4mile, more than what a lot of people think, but i don't want to go off topic.

All the hub dyno operators I speak too say they only give whp figuires

Sure the one that people went to last year gave flywheel figures as well.