Dyno results. Wheel HP vs Calculated Flywheel BHP

sleepyhead · Post by **sleepyhead** » Mon Dec 18, 2006 7:19 pm

Over in the mx5 forum it kicked off a while back regarding people quoting bhp figures. It started off with one poor bloke saying "I got my car dyno'd at x bhp after being tuned by x company." then turning into a semi-troll session of why actual wheel hp is more reliable than calculated flywheel figures as transmission losses cannot be accounted for accurately and that the flywheel figure can only be done on an engine dyno...(then I got lost in the world of dynojet vs mustag rollers in the states giving different results).

I'm no dyno expert...or have any knowledge of it.

So what are your comments/views on this? Is a calculated flywheel figure acceptable or can you only really work off the actual raw wheel hp reading?

Post by **Stuart** » Mon Dec 18, 2006 7:48 pm

Oh Christ!
I have lived in fear of this very subject rearing it's head on this forum :lol:

My opinion~ any dyno should only be used as a measurement of gains or losses against previous runs on the same dyno. There, does that cover my arse?:)

Now over to someone that knows what they are on about.

sleepyhead · Post by **sleepyhead** » Mon Dec 18, 2006 7:51 pm

stuartstaples wrote:Oh Christ!
I have lived in fear of this very subject rearing it's head on this forum :lol:

My opinion~ any dyno should only be used as a measurement of gains or losses against previous runs on the same dyno. There, does that cover my arse?:)

Now over to someone that knows what they are on about.

That's a pretty good answer

In SMF I found that you get people backing certain vendors and obviously their dyno results and it does degenerate into a trolling session! :lol: Here where there is a large mix I believe that there will be less biased comments

gary@apt · Post by **gary@apt** » Mon Dec 18, 2006 8:22 pm

i can give you my view and thats based someone that owns a dyno dynamics rolling road.

rolling roads all read power at the wheels and then convert to flywheel figures either by reading the run down and measuring the drag though the transmission or by a correction factor that dyno applys

theres many veiws out there as to who is right and whos wrong but best not to get into the battles

if we do a dyno day we give a few graphs

1,with power and torque @ flywheel
1,with power @ wheels and a boost curve plot
1,with air fuel ratio

it also can depend on how the dyno operator runs to the test too.

but as stuart says its either best to use dyno in before and after tests or a dyno day that were every one is run the same...

Mark · Post by **Mark** » Mon Dec 18, 2006 8:29 pm

Fly power is calculated and is only an estimate, its normally a pretty good one (depending where you go) but some will say even wheel power isnt accurate due to different tyre sizes, pressures and slip. This is why you now get hub dynos as they claim to give true power.

As an example this year i have been to 3 dyno days at 3 different RR's with the same spec car

RR1 recorded 247@wheels 309@fly
RR2 recorded 243@wheels 290@fly
RR3 recorded 267@hubs and estimated 307@fly but they didnt give that as a reading.

So therefore i say my car is around 250@ wheels and 300@fly if anyone asks. To be honest power figures are for the pub and i treat RR days as a day out with a bit of banter with fellow petrolheads, some people though take it all far to serious and if they get 300 on 1 RR and then a month later get 280 on another they get all shirty and start slagging the dyno :roll:

You also cannot compare 2 different cars tested on different RR's which is another problem you get, someone with a set spec might get 280 on a RR then someone with the same spec gets 300 on a different RR with the same spec, the first person then starts saying thats bollox as he only got 280 :roll:

Anyway power figures have the making of some of the finest forum fights so its best to try and avoid getting to hung up about them, its how it drives that matters

sleepyhead · Post by **sleepyhead** » Mon Dec 18, 2006 8:53 pm

w00t! This place has sensible answers! Fantastic!

There are too many people hung up over RR figures in the mx5 forums and all I wanted was some clear headed clarity. Thanks for your replies

Been much help.

It's definitely all about the drive!

PS Have you met people who can claim that from just looking at a dyno chart can tell when a car has poor throttle response and lag etc? :lol:

Miniwilliams · Post by **Miniwilliams** » Mon Dec 18, 2006 10:57 pm

"PS Have you met people who can claim that from just looking at a dyno chart can tell when a car has poor throttle response and lag etc?"

Yes this is true as on my graphs you can see where there is not much at the bottom end as the graph is low and flat, then I suddenly comes on boost and rises steeply, this is a classic turbo lag / low bottom end power. As friends with little lag have higher graphs then me at the lower end.

sleepyhead · Post by **sleepyhead** » Mon Dec 18, 2006 11:49 pm

But isn't that just on WOT?

What about when the boost is regulated by throttle position and/or revs/gear?

IWANT1 · Post by **IWANT1** » Tue Dec 19, 2006 3:45 am

a big can of worms this one :x

benh · Post by **benh** » Tue Dec 19, 2006 9:15 am

On a dyno, unless you are mapping, it is very unusual to make a run at less than WOT.

The thing is this:-

The only real thing you can compare is results from one dyno. You cant compare the results from one machine to another. Tuners use a dyno for two reasons- to get into all the areas of the map and tune each point accurately without having to worry about hitting a peugot 205 diesel on the ring road and to measure the results of their work.

We use an independant rolling road with a dyno dynamics machine. I was down there the other day talking to the owner and he had a brand new M3 on it. Rated bhp at the fly of 338. it made 338 according to the road.

I look at it like this, the more variables i can remove, the more accurate the numbers are likely to be, so if i map a car i do a before run, map it and then do an after run, which i try to do in the same day, and if possible in the same session. That way i know the difference which i measure is as 'real' as it can be.

I dont argue my cars have dyno charts which are high, or low, but i just say you cant really compare to other runs on other machines.

You can read, as has been said, about where the spool is on a turbo car, and how much power it makes relitively at different rpms, but you cant really read anything about time- ie how long it takes to respond from a throttle blip because there is no TPS trace normally to tell it from etc.

Cheers,

Ben.