The hypoid (curved bevel) gears in the diffs and the gearbox (bevel or 'cross-cut') gears will together account for some of the transmission loss. As you up the engine power so the greater the side loadings on the diffs and gears and this consumes more power so the power loss is not a fixed figure. Theoretically if the gears were all straight cut then you wouldn't get the side loadings but this is only possible on competition type straight cut gearsets in the gearbox.

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Don
GT4DC Chairman
1994 Toyota Celica GT-Four ST205WRC JDM 269bhp @ 0.9bar
1994 Toyota Celica GT-Four Special GT 590bhp @ 1.8bar
1989 Van Diemen RF88/89 Formula Ford 1600
2008 Nissan Patrol GU 3.0L ZD30DDTi 154bhp

The long and short of it is that it's in the dyno operators interest to make their customers feel good. I've seen dynos slated and clubs go elsewhere for dyno days because people get 'lower power figures' from that dyno.

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If at first you don't suck seed, try drier grain.

The conflict of interest is quite obvious.
When I departed the OC eons ago the upward trend was apparent with a new magic tree the only requirement for making 300 HP at the flywheel on basically stock cars. A shiney Feu Orange was almost a guaranteed 310
I still remember the original buzz when Mysh's car which was mildly tuned made 301 @ 1.2 bar. Everyone thought it must have been a real Monday morning special to make that kind of power

Ive found my local dyno to be good for plotting trends and before/after comparisons but when you become a dyno slag and jump about between different machines then who can you realistically trust?

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FN2 Civic Type R w/Mugen Parts
Toyota Celica GT4 ST205
Toyota Celica GT4 ST185

I always trust me/dataloglab/MS excel/statistical analysis



















Unless I'm drunk/bored/mischievous/CBA/tired/drunk/distracted/hungover or drunk
So basically not me

+1.

Dyno days are only really interesting for comparing whp figures between similar cars run under similar(ish) conditions.
For a GT4 I would guess a reasonable transmission loss of 40hp @300bhp. And 275~280bhp @ 1.1bar with OEM setup.

There are different ways of "predicting" flywheel horsepower from the measured wheel figures. Each type of dyno will have different losses associated with the assembly as well as the actual quoted "transmisison loss" which is just passed to the user as a transmission loss. Thinking of transmission loss' as gearbox, tx box, propshaft, driveshaft, wheel bearing, binding brakes, tyre/roller interface friction and roller bearing friction and you get a bit more of an insight on how the wheel figures may vary depending on different parameters. Running those fuel saving tyres at 50psi with an unladen car (no fuel or junk in car) will reduce friction and improve your measured at wheel figures. In fact many dyno dynamics rolling road operators (when mapping and paid sensibly) will increase tyre pressure to around 50 psi to obtain calibrated results and reduce the chance of tyre blow-out for extended periods.

The measured power at the wheels is also predicted using a series of geek type equations and as such wheel/flywheel horsepower is only useful for comparing to other cars using the same dyno or your car on the same dyno before / after mods/tuning.

Dyno dynamics rollers are typically operated in "Dyno Shoot out mode" which just applies a 25% IIRC correction to the measured wheel figures - transmission loss' are not measured on the run down in this mode (although they can be in full dyno mode). Every dyno dynamics rolling road day I have been to has been operated in "shoot out mode".

Some of the old dynos used a calibrated weight as the dyno roller and the wheel bhp is predicted from some maths depending on how quickly you accelerate the roller - load can't be varied like the newer eddy current brake setups (NB eddy current brake may not be the correct term - I may be getting confused with my 1960 Garrard record player!? god don't you love old age and lack of sleep).

On dynos where transmission loss is measured it is still only an estimate and the transmission is loaded in reverse and the rotational/angular acceleration of the drivetrain is at a different rate and hence the energy used to speed up/slow down the wheel assembly/dyno roller is different.

In summary -
Q) what is the drivetrain loss
A) only by measuring on an engine dyno and comparing the difference to a specific wheel/hub dyno and car combination can you know what the transmission loss is for that combo.

Using accelerometers and measured vehicle weight/drag is much more accurate in practice and a few people have suggested that 1/4 mile terminal speeds are a much better indicator of power even though you use a wide rpm range - go figure.

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JP
GT4DC Treasurer

Grey St185 RC
Silver BMW E46 M3
Multicolour yawn Honda VFR400
Silver Honda Civic Type-R

In theory out of gear coast down calculations should allow for moderately accurate prediction of rotational losses I guess but wouldn't allow for losses in tooth friction

I think the dyno dynamics use a water cooled eddy current brake setup, as do most modern comes
I know Charlie's and Dave Walkers gurgle like my tummy a after a vindaloo during a run

ah good - eddy current brake was right then - it is also used on my Garrard 401 as a speed control/stabilisation device so my confusion was confused by its duality.

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JP
GT4DC Treasurer

Grey St185 RC
Silver BMW E46 M3
Multicolour yawn Honda VFR400
Silver Honda Civic Type-R

so have we come to the conclusion that wheel bhp on dynos is relatively accurate?

We all know flywheel can be whatever you want as percentage things vary massively

You lot maybe have but I haven't Wheel BHP varies a lot depending on dyno type used.

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JP
GT4DC Treasurer

Grey St185 RC
Silver BMW E46 M3
Multicolour yawn Honda VFR400
Silver Honda Civic Type-R

haha, Im not but I was just wondering as i have lost the whole topic somewhere. haha

But with regarding dyno dynamics on a normal shoot out graph would they all be relatively similar to the next dyno dynamic?

I have no idea on how the dynos work so have no idea at all

Interesting info JP, confirms what I suspected that even wheel horsepower is not an accurate comparison unless vehicles are tested on the same machine. In fact if they all need different calculations from the wheels to produce a flywheel figure then surely the calculated flywheel figure is a more accurate comparison between cars on different dynos than a WHP figure would be? Assuming they have both done their calculations fairly accurately.

For reference the Interpro rollers we used the other day did actually calculate the losses, it is not a fixed percentage but a measured amount which varied quite substantially as a percentage between cars on the day.

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GT4 no 10 - I must be mad :help:

^^^Click the banner for my Black Treacle project thread ^^^

Now back home and mapped to 531bhp & 411lb/ft - that's enough power for now!

Unless you remove the engine and bolt to a dyno directly, dynos are measuring power at the hubs or wheels.

So that's the figure you cross relate to another dyno run on another day or dyno. Its had the least amount of "fiddling" done to it.

--
Darryl

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--

Darryl

Darryl - read Diceman's post above and see if you still think that. The at the wheels figures will be different on different dynos as the losses that are calculated also include the losses of the dyno itself etc. so at the wheels figures are also meaningless for comparison unless try are done on the same dyno.

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GT4 no 10 - I must be mad :help:

^^^Click the banner for my Black Treacle project thread ^^^

Now back home and mapped to 531bhp & 411lb/ft - that's enough power for now!