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Discussion Starter #1
I know we have had loads of discussion on rolling roads on here before, but to quote from Guy's comment in the 'fastest R34 in the UK' thread:

We are however now moving towards dynos that measure power-at-wheels
If my understanding of the way rolling roads work is correct then measuring PAW will leave us unable to realisticly compare power between different models (R33 to R34 for example), due to the different gearing.

If all the runs were done in 4th gear for example, then an R34 is always going to return a higher PAW figure than an R33 due to the lower ratio gear of the 6 speed box. Does that sound correct?

Is the suggestion to use a 'fixed' transmission loss value for each model. i.e. 150bhp for an R33, 120bhp for an R34 etc and use this to 'calculate' a flywheel estimate?

The subject of rolling roads really annoys me, as I really want a reasonably accurate and relatively inexpensive way to measure (or more so compare) the power of our cars. It appears though that one does not exist. :(

Have we decided that PAW + fixed loss is better than having the RR calculate drivetrain losses?
 

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Nothing about this is 100%.
But using a dyno such as Abbeys which eliminates wheel size and wheel spin and the restrictions wheelspin place on ordinary rolling roads is a better estimation than most.

I believe the idea is usually to run an R34 or six speed gearbox car in 5th and R33 fivespeed cars in 4th, these two gears being 1:1, but not even that is even due to the significant difference in differential ratios between R33 and R34.

I think the best bet is forget other cars and concentrate on your own car and comparisons against your own car as you carry out modifications to it.

Also only compare your own car against other models which share the same transmission. Never try to compare an R33 against an R34, it is not a flat playing field.

My 2p worth
 

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Ged's quite right, forget the testosterone, concentrate on maximising your own car's output. There never will be a consensus on this issue IMO, too many variables but having a RR calculate losses isn't the way to go. PAW is the way to go, no arguement, it is what it is, you can apply whatever losses to that for an estimate of ATF figures but somebody won't agree with you.

I'm also reliably informed that even running a 33 in 3rd and 4th will only produce a difference of between 4% & 6% so near enough really.

My 2 bhp worth. :)
 

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Supraman
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I thought the best way to get PAW figures was to use the 1:1 ratio, for all cars, whichever gear it is. 1:1 in my car is 4th, despite having 6 gears. I'd imagine it's the same on the 34, and probably the same on the 33 with 5.
 

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Rolling roads measure torque at the wheels, not power. Providing the engine Rpm are measured correctly, Power figures at the wheels are calculated from torque measured. This then gives you a power figure at the wheels, which is comparable to other cars regardless of gear ratio. A Gearbox is a torque multiplier, not a power mutliplier, and so has no direct effect on power at the wheels except by it's efficiency.

The point with engine Rpm, I have seen RR used in a 'dyno shoot out' where the engine Rpm was calculated from gearing, not actually measured. All the cars measured were of the same type, however one had a modified gearbox with different ratios. Since power is a function of torque and Rpm, the torque figure measured was correct, and similar to the other cars, but the calculated power figure was incorrect, due to the Rpm used being incorrect. If the engine Rpm is not measured correctly, the calculated power figure is wrong.

The only issue this then leaves is whether one gearbox type or gear is more efficient than another. On a normal rolling road, tyre pressure can have a significant effect on measured power. In reality, there are so many variables as to make it almost impossible to do a like for like comparison between two cars even on the same dyno. This must be even more of a nightmare with the Skylines 4wd system!

An RR is a very useful setup tool, and we all like to see nice numbers. It's always an interesting debate as to why X has more power than Y with less mods, but there just seem to be too many variables to get a conclusive answer.
 

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interesting thread...
i wont make a comment on this one simply because i dont know enough about them....
keep the opinions/ideas rolling in...
 

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some rolling roads do seem to be able to accurately measure hp at the fly wheel, my car was rolling roaded and gave an estimated flywheel figure of 133 bhp, i had the engine taken out and dynod last week and it gave a figure of 135 hp.

i think the rolling road used the run down method to calculate the tranmission loss or something, not to sure goes right over my head
 

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Supraman
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Oops - I had my car RR'd in 4th instead of 5th. As my car shares a gearbox with the R34, and the RR operator told me that 4th was my 1:1, I was kinda assuming that the R34 would have 4th as the 1:1 also. However, a quick websearch shows I got it wrong.

How would that have affected my power/torque figures? Curses - I was feeling so chuffed about having loads of torque too :(
 

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Discussion Starter #10
Keith... it shouldn't have really effected the flywheel figure in theory as the run down should have calculated the drivetrain losses and then added that to your PAW figure.

The PAW figure would be different though. Although if what has been said above is correct then difference would not really be much.

What were your drivetrain losses BTW?
 

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Discussion Starter #11
30hp, is that all.

In fact forget that I just saw the link on your sig.
 

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Supraman
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This is my power chart - apparently only 30bhp lost which I always thought a bit low, and 314 at the wheels. Thing is, 350bhp is about right for my mods, (exhaust, *cough* valve), although it was a *bloody* hot day (34C!)

Hmm, I suppose really I should bung it on another RR with it in the correct gear. Bugger.
 

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See, here we go, 30bhp seems ridiculously low even for a 2wd car when GT-Rs are reckoned to lose 130bhp +....
 

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Supraman
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PeterE said:
See, here we go, 30bhp seems ridiculously low even for a 2wd car when GT-Rs are reckoned to lose 130bhp +....
Not as standard, surely!!!!!

I'm really baffled as my engine figures are, like I say, pretty much right. My torque figure seems believable as it does spike up to 1.1-1.3bar (depending on airtemp) at those rpms (when no.2 kicks in) before settling down to .9-1bar.

But I prefer to use wheel figures as they're what count, but being in the wrong gear has probably artifically boosted my wheel figures.

Ah, maybe it's because I was in the wrong gear that my transmission losses are so low? Seem likely?
 

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30BHP loss does look very low, it's hard to say. At the end of the day, the wheel figure should be correct, as that's where it's measured, so maybe the flywheel figure should be higher! Either that or you have a very efficient transmission - I've had cars with less than half your power and more than double your losses:(!!!
 

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Diff Ratio

As said earlier the R33 is 1:1 direct drive in 4th and the R34 is 1:1 direct drive in 5th.

The diff ratio on the R33 is 4.111 and the diff ratio on the R34 is 3.545 which means that the R33 wheel RPM is 16% less than the R34.

The circumference of the 245*45*17 R33 tyre is the same as the 245*40*18 R34 tyre, so at 8000 RPM and 1:1 the R33 is doing 149 mph and the R34 is doing 173 mph. The calibration of the rolling road may skew the values as the power is being read at different wheel RPM.
 

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I was at Top Secret today and watched Nagata san dyno test/map an R34 GTR. He used 4 th on every run he did. Why is that?:confused:
 
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