ok, here goes......

 

ive been having a big argument today...

 

all those rolling road bhp figures, are they an estimated figure at the crank, or are they actual at the wheels figures.

 

plenty of people seem to be saying that all the figures you see [eg, like with the zed rolling road day etc] are calculated in the following manner.....

 

get an 'at the wheels figure' eg 250bhp,

then add 10%/20% [or whatever the power loss may be through the drive,gears whatver] to get your at the crank figure - and then print that in the magazine etc for your bhp figure.

 

is this how it is done????

 

or am i a big idiot with a big idiot sign on my head that says '45iq at the crank'.

 

what. hello,

 

ben

 

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That's correct mostly mate, figures you saw in the Jap Perf mag were at the rear wheels!

 

Flywheel? Depends on whether it's an Auto or Manual so anywhere between about 10 and 25 percent loss via the trasmission.

 

 

 

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so you are saying......... that........

 

jeff's 250, er, the 346 or whatever, and all those figures were pure and simple an at the wheels figure? is that right.

 

so to then get to the at the crank figure you need to calculate in the loss through the drive and all that so.........

 

in which case you could add 15-20% to that on the manuals for an estimated crank figure,

 

and perhaps 30% or more for an estimated auto at the crank figure.???

 

which would give jeff a figure of ,..hold on....let me get the mind going......er, 23, 46, ....shit!! where;s the calc.....

 

er 325 bhp at the crank!! is that right.

 

bet i am wrong again.

 

 

but surely if 250 is the at the wheels figure ...then you have to do this, adding 20 0r so % on...

The crank figure is interesting but largely academic to me and tends to lead only to discussion about how much is lost through different transmisisons etc. The only figure that really matters is what touches the road.

Its not as simple as that to work out. Different clutches, flywheel, diff even propshaft can make you lose power. 4WD cars lose more power from the crank to the wheels, because there are more drive shafts and 2 diffs.

 

Stuart

Maybe at the "crank" on the printout maybe not.

It depends whether the printout machine has built in %age adjustment for PAC, or if indeed it does actually give a PAW, printouts tend not to say which.

The ONLY way to get a PAC, is to take the engine out and run it on an engine dynamometer (this is the 'brake') read brake horse power. Unfortunately, this doesnt take into account running any ancilliaries either.

 

Check whether printouts are PAW, or they may have already been upped 10-20% by the machine, then you see it and add even more on to get crank power.

Dont forget, the transmission losses are all estimates, too, frictional resistence in one gearbox over another etc arent exact, and people just assume xx% for a RWD car, which you shouldnt do, but there is no other option, as you can real;ly diagnose the transmission losses for your particular car

Three diffs stuart in a proper 4wd

I agree with Pringram on this, it does not matter what the flywheel figure is as its is irrelevant. Wheel horespower is what counts.

 

Our rolling road day was wheel horespower and according to the dyno man the general figure to add to this is between 12% for a mnaul and 15% for an auto, but that is just a general figure.

 

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YOU LAG YOU DRAG

I watched some Scoobys and Evos on a 4X4 rr day. Quite interesting. The way I saw it was the machine measured the power transferred to the rollers right up til it goes past the power peak at whatever revs that is. The operator then cuts the throttle and clutch-in free wheels. The machine measures the rate the turning wheels slow down and from that calculates the drag in BHP due to the transmission losses. Then adds it back in to give the theoretical flywheel figure.

 

On scoobys I saw as much as 70BHP lost in the 4WD transmission on powerful ones 250bhp+. The higher the rpm for peak power the higher the transmission losses. Logical cos they rise with rpm of wheels etc.

On some 2 wheel drive cars I saw around 40BHP transmission loss. Seems to concur with above figures here.

On auto Zs with OD locked out I am not sure if the torque converter stays locked up at full throttle but I thought it might at high rpm - if it does stay locked up the transmission loss should be the same as for a manual in 4th. Otherwise it'll count slush box losses in the coverter too.

Other infuences are that the ICs dont get enough airflow from the big blower to simulate high speed so power can go down cos of hot ICs and turbos.

Willie

Transmission loss also varies between front and rear wheel drive. Front is more efficient (asuuming a front engine with crank running across the car) because the direction of drive is the same, whereas with rear wheel drive (front engine, crank runs longways) the drivetrain is turned through 90 deg in the diff.

 

I suppose the only really important thing is to compare like with like. So calculated crank power figures on different days on different dynos will leads to %age variations. Warren managed to get a whole buch of people together on same day, same dyno and we only measured power at the wheels, so no excuses and no bullshit !

Originally posted by AndyP:

Warren managed to get a whole bunch of people together on same day, same dyno and we only measured power at the wheels, so no excuses and no bullshit !

 

LOL

 

Which still didn't seem to stop the non-believers that the SE chip out performed a JWT chip back to back! Accusing you, honest Mr P, of frigging your boost imbetween runs! Which would have been very hard to do in a few minutes...hear those welding tips are a twat to get to? ROFL

 

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[This message has been edited by Timmy_Turbo (edited 31-05-2002).]

yup only way to do it is on the day.

Pity no one belives the output i made iether?

 

What do you mean didn't believe it ? We heard it detting for ourselves ! LOL

i know and stil 120 more than your excel spreadsheet

Hmmm, well I'm getting a new one done in a couple of weeks, with my manual box, so it will be interesting to see a more direct comparision. Would be great to be up there with Grant, if that's the case I shall be very happy. May have to raise boost above 12psi to get there though ....

Andy do you recon yull notice a big difference? And if you get near Grants figure would that be the difference between manual and auto loss?

(not starting anything)

Just it will be interesting to notice.

The car feels 'snappier' with the manual box, but and it's obvious that the auto box loses a fair amount of power through heat. With the manual the transmission tunnel stays fairly cool, with the auto you get gently cooked ! This will probably be the most direct back to back test anyone has done and whether I get to Grants figure will be a bit of a challenge, mainly because I have auto turbos, so more midrange and less top end. If you compared our graphs previously I had something like 60bhp more in the midrange, so it will be interesting to see how that compares manual to manual.

However, I still have that extra 2-3psi boost disadvantage compared with him - so theres all to play for. Wanna come over and impartially spectate ?

I'll be up for another run Andy when you get a date for the RR booked.

 

Let me know.

 

P.S. I think were gonna have to wait for the darker nights to come around again before we do that headlight testing we were planning.

 

Grant

Would be interested in a dyno run, too. It would be interesting to see the before / after difference - all I have had done is to junk the OEM exhaust and fit cat back Mongoose. Feels snappier but is this just psychosomatic?

Or would I have to go back to the same dyno I did a run on before? (Millways in Andover - 258 bhp @ 6416 rpm - I assume at wheels)