Can some one help me understand the difference between dynos and why people get such different figures depending on wether its a hub dyno or a rolling road (if thats the two types). I thought i new the difference but after reading this thread

https://passionford.com/forum/general-car-related-discussion/265594-dennis-is-at-it-again.html

Im confused.


Why do you get different figures? Why is there more than one way to test the power, wouldnt one type of dyno make it easyer?

 

 

Mike Rainbird has the only accurate Dyno in the world all the rest are useless & operated by cheating bastards, picked that knowledge up from this Forum.

 

Ok then, so far hobnobs and a bloke with a huge chip on his shoulder have yet to shed any light. Anyone?

 

Dyno's are only as good as the operator.........

 

http://www.motorsport-developments.c...AF258.tech.pdf

 

 

Everyone can give Mike a slagging in Jest ( with some its a bit more ) but i'll bet 99% of the users would love his car sat on their drive. The thoughfullness of the spec is amazing, and it's not been done with no knowledge

 

Dyno's are affected by the diamiter of the wheels/tyres - just like a bike when you change gears on the front cog it affects what goes on at the back wheel - so the bigger the front cog the harder it is to rotate the rear wheel - if you got 19" rims on the dyno you'l get a different power reading to that of a car with 15" rims

a hub dyno gets around this by taking direct drive from the hubs, therfore not being affected by the rolling ratio's of the wheels.

 

i have always found this was the simplest explanation

you have an engine and the manufacturer states it produces 100 horsepower in the handbook

this power is the power the engine makes

when you add an alternator to it and a power steering pump and an ac pulley etc this power is sapped from the engine power (parasitic losses) so you get brake horsepower

this way of measuring it's power is by bolting it to an engine dyno so the flywheel figures are what's being measured

if you add a gearbox to the equation and diffs and driveshafts, these deduct more power (transmission losses) and you get a smaller reading at the hubs (if you were using a hub dyno which bolts to the hubs)

then you lose more power (and accuracy) if you use a rolling road due to the tyres adding to the losses, this is then calculated back through the transmission to give you a best guess estimation of engine flywheel power

if you have the same car, tested on the same days with different mods this is the only realistic way to measure like for like on the rolling road

the problems in the other thread have occoured due to different interpretitions by some on the engine dyno side of things

engines produce heat, the rooms they are tested in need to have some sort of cooling, the cooler it is up to a point ehmore pwoer it will make, the more air flow it has the mroe pwoer it may make, etc, etc

the TUV bit is a set of standards that are set that mean that everyone and everything is equal when they are tested so you can compare like for like

one dyno was set up with these rules in place, others were not so you coulnd't directly relate one to the other
this same dyno was, in subsequent times, checked over to the same level as the TUV standards but for one omision, no one from TUV did the checks so it was not certified, even though the same checks were made to ensure it was at the same level (then it got technical and bitchy so i gave up)

so there you have it, unless you have the same paramiters for all your equipment across all the sites, you will never get a direct like for like comparision

 

Thanks for that

 

it took me so long to type that up i was beaten into 6th place

 

What.

 

bling rims cost you power

 

Dyno's are also only as good as the calibration method the person that does this and the frequency of calibration

 

You can only measure torque at the wheels, everything else is calculated and some of it is estimated on a hub dyno or RR

 

So lets say for arguments sake you had built a 3dr and with the spec you built it to were expecting say 400 bhp. Would you choose one type of dyno over the other to test the power because that would give a higher figure or am i still getting the wrong end of the stick?

 

but i didn't say "meausring torque" did i

but would you not accept that the amount of torque produced at the wheels can only give you wheel torque/power whereas measureing the torque at the flywheel will give you a more accurate reading of what the engine is actually producing as you don't have to take into consideration all the losses throughout the drivetrain?

 

only if you want to go to the pub and brag about it

it's about how accuratly the figures are obtained if it's just figures, the whole point of doing this is to make sure the engine is running right at the right places and to measure all the bits that need measuering to make sure they are doing what they should be doing when they shoudl be doing it

it's like asking you to carry a bag af cement up a flight of stairs, yu can carry the cement bag yes, you have lifted it yes, but going up the stairs you have a heart attack and die, if you were checked out before you strated the lift this may have been apparent and something could have been done to avoid it, and i think that's the best way to describe the use of the dyno

 

If the dyno Cylinders are lets say 17" in diamiter and you go on there in your car with 15" wheels - you'll get a power reading of XXX

Then you take the same car, on the same dyno, with the same setup, except change your wheels to 19", then you'll get a different power output reading - this is becuase the mechanical drag that the dyno uses to convert to tourq has been affected by the increased rolling diamiter of the wheels.

 

You can still only measure torqueon any dyno be it hub/rr/or engine dyno. You cant measure Power.

 

Once fitted to a car, a wheel doesnt have a rolling diameter mate. The Tyre does, and that should be chosen so that the rolling radius is in fact exactly the same as it always was.

 

well nobodys gonna drive in on bare alloys are they -jeez'

 

Dude, the point Stu is making is that no matter what sized alloy you have the rolling radius will be the same as it always was so your theory about different sized wheels is not right.

 

Are you for real? That is my whole point...

Since:

Then the tyre's on both wheels is the same rolling radius anyway, thus the reading on the rollers is totally unaffected.
IE: You are totally wrong.

And your attempts at backpedalling are even more wrong, you see, i can see your ORIGINAL reply before you quickly edited, so have another one.

 

yet another person who cant accpet being wrong

 

i'm never wrong

 

Evidently you are in this case

 

but power is calcualted from dyno speed and torque (in kW anyway) so that statement isnt important

 

to be honest, i really dont know what i'm talking about when it comes to dyno's and secretly i dont think a lot of people do either, just nobody likes to admit it - i only comment in these threads to wind people up as it makes me laugh and gets me through my working day.

it's like you know someones a car arsehole the moment they say the words "Power to weight ratio".........thats the moment you stop listening to them and go make yourself a cup of tea and just hope they've p!ssed off by the time they get back LOL

 

This is why you don't know as much as you'd like to. The moment you talk to someone who understands what they are talking about you fook off and drink tea.

I always stop what i am doing and try to learn something.

 

Dont get me wrong, i know cars inside out, but when i'm at work and we get chatting about cars, i dont want to know there compression ratio's, their tyre offset and what there power to weight ratio is - stick to make, model, engine size and we'll get along just fine. i dont need to be bored with the shite

 

Thats the most interesting bit, you could have two cars of same make and model but understanding how one is faster or more powerfully due to modds and changes is interesting IMO. Knowing a car by make and model doesn't mean you know cars inside out.

 

old age must be catching up on you because mike doesnt have a dyno

 

when i got mine r/r i got my mate to put his new car on it first totally standard with 5k miles on the clock and the flywheel figure was within 1 bhp of what manufacturer stated so it cant of been a mile out imo.

 

So basically you're not interested in modified cars then


One 2.0 Sierra isnt the same as another

 

Dojj,
As an objective opinion, that is pretty much spot on . Obviously if I had said that, I would have been shot down in flames being accused of belittling Rod's pet tuner's figures .

Stu,
I think that you're being a little harsh, as although the rolling radius "should" be kept identical (for speedo accuracy etc), it is very rare that this happens.

The standard size tyre on the Sierras is 205/50x15 and the common upgrade is 215/40x17, which is an increase of almost an inch in diameter (and on the 2wds, lots of people fit 235/40x17s on the rear, which is nearly 2" bigger).

On the Escort, the standard tyre size was 225/45x16 and the common upgrade is 225/40x18, which is also an increase of almost an inch in diamter.

I think all he was trying to say is that these tthings can all have an effect.

 

i find threads like this very interesting as engines and dynos are my bread and butter, and i find the discussion about rolling roads etc (which admittedly i know considerably less about) can get very ammusing at times when people start spouting off about stuff they dont understand.

wikipedia its great if you dont know squat, people should use it more often, but never take it as gospel

 

I have been through the info in Stu's link and googled the sbject befor my post i was just after an explination i could get my head round as i have no clue about tuning, rolling roads, mapping etc. Think iv got the basics now so thanks guys

 

you? shot down in flames?
i didn't know you wre a pilot as well

i told it like i read it, minus everything else that went on as it turned into something women would so, bitch and moan about stuff when they all get together

i think i now understand the point stu was trying to make to nash but, if you have a bigger wheel, do you then make more or less power?
if it's judged by the torque figures, the bigger wheel will turn the roller with more torque at the same rotation, it'll just take longer to get there right? so even though the figures may be off, they shoudl still compensate to give you the right resluts?

i only say this because when i changed to the 3.14:1 diff and put big wheels (215/45/17) on the sierra, the top speed still stayed the same, but it was much quicker than my mates car who had more power, smaller wheels (195/50/15) and a 3.92:1 diff along the start finish straight at silverstone

how's that possible?
i had a dyno'd 124 lb/ft and 130 brake, he had 140+ lb/ft and 140+ brake

or was the dyno out due to the bigger wheels and diff combo on my car?