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:

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[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:

21302

http://i24.photobucket.com/albums/c2/stephenevans99/screen-capture-6.jpg (http://s24.photobucket.com/user/stephenevans99/media/screen-capture-6.jpg.html)

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.

[AndysR]

In my opinion/experience a dyno run is better at highlighting a problem and disappointing an owner than it is at producing accuarate power outputs. I took from my rolling road session that the AFR's were safe the boost was consistantly solid as soon as it hit full boost and the power graph was as smooth as you could ever want, the later being something I expected by the way the car drives, after all the car is auto so the power reading will never be correct. Even before the run I did know that it's very good at putting what power it does make to good/effective use, a fact that was later highlighted when I ran the car down the 1/4 mile at Avon, in fact I was more shocked at the results it achieved over the 1/4 than I was at the rolling road day. I don't think I will be rushing to get to another rolling road day as I see little benefit in them other than providing ammunition for boasting rights and pub conversations.. Just my opinion on the subject of rolling roads... lol

[LiamT4]

In my opinion/experience a dyno run is better at highlighting a problem and disappointing an owner than it is at producing accuarate power outputs. I took from my rolling road session that the AFR's were safe the boost was consistantly solid as soon as it hit full boost and the power graph was as smooth as you could ever want, the later being something I expected by the way the car drives, after all the car is auto so the power reading will never be correct. Even before the run I did know that it's very good at putting what power it does make to good/effective use, a fact that was later highlighted when I ran the car down the 1/4 mile at Avon, in fact I was more shocked at the results it achieved over the 1/4 than I was at the rolling road day. I don't think I will be rushing to get to another rolling road day as I see little benefit in them other than providing ammunition for boasting rights and pub conversations.. Just my opinion on the subject of rolling roads... lol

Probably the way i look at them to be honest. Fault finding, good afrs, smooth power curve, etc...... dyno days make for a good day out though.

The looks on some peoples faces as there car is screaming towards the red line is priceless lol

[AndysR]

Probably the way i look at them to be honest. Fault finding, good afrs, smooth power curve, etc...... dyno days make for a good day out though.

The looks on some peoples faces as there car is screaming towards the red line is priceless lol

Agreed! I love a good dyno shoot out from a viewing perspective, I was at one today, but as for running my own car on the dyno again I doubt I will bother tbh..

[LiamT4]

Agreed! I love a good dyno shoot out from a viewing perspective, I was at one today, but as for running my own car on the dyno again I doubt I will bother tbh..

Not even to check for any possible issues?

I'd like to do one in my car, just to see how smooth the curve is and how that compares with turbo'd cars.

[stephenevans99]

Stuart Sanderson who wrote the article has 60+ other Tuning articles available to download as PDF's on his site.

Motorsport Developments Technical Articles Link (http://www.motorsport-developments.co.uk/stus.html)

[V70 Graham]

The looks on some peoples faces as there car is screaming towards the red line is priceless lol

Don't know what you mean Liam :cool:

[wegal]

A Dyno is a useful tool to compare things with, not car Vs car, but on your own car, so long as you use the same dyno and the same operater. Run the car, make a change run the car again, make a change etc etc. It will give you an indication of what difference ( if any) your mods have made.

Beyond that its just a number on a bit of paper

[smithy]

I've just read the last page of that article and that will give something to think about .

[stephenevans99]

The article mentions the Coast Down method.....

"Most systems use the coast down method. This system has the operator depress the clutch and allow the transmission to come to a halt on its own while the system measures its resistance to rotation. The system measures this resistance and creates what you might call a negative bhp graph. This resistance is then simply added on to the power we measured at the wheels/hubs and given to us as flywheel power. As an example, if we make 200 bhp on the power run, and our rollers measured that the transmission consumed 30 bhp when slowing down, it would give you a power reading of 230 bhp at the flywheel. Simple..."

He then goes on to mention how figures can be manipulated by gently touching the brakes during the coast down to give greater resistance and increase the predicted flywheel power.

Anyone heard of a dyno operator doing this?

[mitchyboy01]

A Dyno is a useful tool to compare things with, not car Vs car, but on your own car, so long as you use the same dyno and the same operater. Run the car, make a change run the car again, make a change etc etc. It will give you an indication of what difference ( if any) your mods have made.

Beyond that its just a number on a bit of paper



Yeah this exactly. It's a great tool for comparing your own figures to gauge how well new mods are working and as a diagnostic tool to ensure everything is as it should be.

[Wobbly Dave]

For better or worse I have always used the same dyno - as I am primarily interested in the delta value, the changes I have made.

The dynojet at HLM has both inertia & load cell functions & the single roller has only a single tyre deflection. I've used it on all my major modifications on the silver sh_1t box since I've had it.

[smithy]

For better or worse I have always used the same dyno - as I am primarily interested in the delta value, the changes I have made.

The dynojet at HLM has both inertia & load cell functions & the single roller has only a single tyre deflection. I've used it on all my major modifications on the silver sh_1t box since I've had it.

I use the same rollers but for comparison reasons I will go to merlon motors on there maha rollers next time the is a dyno day

[AndysR]

Not even to check for any possible issues?

I'd like to do one in my car, just to see how smooth the curve is and how that compares with turbo'd cars.

I'd consider it as an option if I felt the need but it wouldn't be my first port of call if there was one...

Being a V8 and auto the torque curve should be smooth and flat as a witches.....

[jdavis]

Only ran at one rolling road. That was in my 2.0 zetec'd mk3 fiesta and despite a dead throttle position switch and a major misfire it produced a lowly 130bhp. Not sure of whp. Operator wanted to abort the run due to misfire but told him to carry on. A standard 106 gti Got 140+bhp at the same session yet my fiesta wiped the floor with it on the road. In my opinion. The opinions above capture it great, rolling roads are good for diagnostics and pub ammo. That's about it. Want to know how fast your car is go to a 1/4 mile or the track.

[turbo-tuner]

This topic has come up many times over the years. A couple of people have already mentioned that it's a tool best used for measuring changes between modifications, and you can minimise the variables by sticking to the same dyno. But here are a few other summary points to take into account -

1. Single roller dynos have less tyre drag than twin roller dynos, therefore single roller dynos normally show significantly higher WHP.
2. WHP + Drag losses = Engine Power (normally measured at the clutch). This applies to ALL chassis dynos. However, results can vary due to many other variables. Drag losses increase exponentially with increasing speed / rpm.
3. Other variables also affect your power, including load, fuel type, ambient temp. Ambient temp can be factored into the final readings using a standardised correction formula.
4. Dyno loading can be a problematic variable to deal with (load is normally applied using eddy-current retarders). Too little load and the engine doesn't work hard enough (not enough boost) to generate max torque and power. Too much load and you get excessive intake temps which cause the ECU to reduce the power by lowering boost and/or increasing the fuel mixture more than normally required and/or retarding ignition timing.
5. The size of the dyno fan makes a significant difference to the peak power. The larger the fan, the better it is at keeping the intake temps down during the dyno run.

Diesels tend to perform much better on dynos than petrols, because they are not as sensitive to high intake temps.