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DavidT

How do rolling road numbers translate to the road?

I've always wondered. Plenty of us have been on MRC's dyno now and it has the biggest cooling fans I've ever seen, but how much more performance do you get on the open road at the same ambient temperatures? I got 477ps & 698nm on my last MRC visit, what do people think my car is doing on the road as an example?
VasP

it depends how much timing gets trimmed on the dyno due to inlet temperature. Of course something that can be measured in a lab is how much more air gets digested by the engine on an open road, and how that compares to the big fans.
jimbo

log the iat,s , maf and timing on the dyno and on the road. if there is no difference in those figures then there will be no difference in the power and vice versa.
MathiasS

I did 3 back to back VBOX runs when my car was running good in 3d gear with around 22-25 outside temp

400,405,398 wheel hp

My MRC graph showed 395 at the wheels with 28 temp @ MRC
supersi

i think you would get more on the road as those fans no mattter how big are not flowing air like at 100mph or more, dont and are not creating enough pressure directly over IC cores like on the road and maybe to the airbox. my freind owns a motorbike shop and sets up race bikes he has just bought extra fans for his dyno with flexable duct hoses witch he puts to ram air box to get as much pressure in there as if on the road , most superbikes have these airboxes, he said is a noticeable difference in power and he can map the bikes closer to there optimum as if on the track

maybe mrc should try a fan with a flexy hose and duct to straight to IC'S ? maybe even more important on the SMIC's as do work on pressure more so than the FMIC
Parptoot

MathiasS wrote:
I did 3 back to back VBOX runs when my car was running good in 3d gear with around 22-25 outside temp

400,405,398 wheel hp

My MRC graph showed 395 at the wheels with 28 temp @ MRC


I'd say they are quite accurate then if these figures are so close. It also depends on your data input, the more accurate you can be with that the better the figures.

I think A80avant has done a lot of work with power measurement using a PerformanceBox and is able to produce graphs!
supersi

dyno testing note how close they place fans over IC's ! they even remove rear bumper


Espen W

MathiasS wrote:
I did 3 back to back VBOX runs when my car was running good in 3d gear with around 22-25 outside temp

400,405,398 wheel hp

My MRC graph showed 395 at the wheels with 28 temp @ MRC


Mathias: are you using the Pbox power algorithm developed by one of the Bimmerboost guys?  If so, I seem to remember that program that takes rolling resistance into account, so results will not be quite comparable (read higher) with a dyno that reads true whp, like a correctly calibrated Mustang dyno.
Parptoot

The PerformanceBox and DriftBox both have a power calculation mode so there's no need to run aftermarket data gathering programmes, if that is what you mean?

This does of course assume you have loaded in the latest firmware versions into whichever unit you have.
Espen W

I know about the power calculation mode, but a lot of folks run custom made programs that use the raw Pbox acceleration data as input.

Here is an example: http://forum.e46fanatics.com/showthread.php?t=726271

Note that he is correcting for rolling resistance, so the final number will be higher than the awhp from a correctly calibrated, load type dyno.
Doug

With the amount of cooling we get with our fans there is insignificant timing differences due to heat compensation or heatsoak. You struggle to walk in front of our fans and there is only so much air you can get through intercoolers. We have to switch one fan off on some cars as the engine gets too cold.
Espen W

That sounds like a proper setup, Doug.
I wish more dyno operators would have a setup like that.
AliGod78

They're pretty big!

Graham

My latest MRC dyno result was acheived on the 5th run of being on the dyno. I can only imagine on road performance to be marginally better, if after 5 runs it was making the anticipated numbers. That's on stock S4 intercoolers too.
mattyb

On my last visit to the MRC rollers, I logged my intake temps on a private runway on the way up there, it was a full on fifth and into top upto 170 mph.
I have stock side mounts with water injection, 34 on road and 54ish on dyno.
I feel the dyno likes front mounts.
Hope this helps.
supersi

The rs4 smic work on positive pressure at inlet duct to force air through core and low pressure on exit gills draws air through so are probably not very efficient on a dyno, like a FMIC ?
mattyb

I think your prob right there Si, mine clearly work reasonably well on the road hence 34 but not so good on rollers with mid 50's
Doug

Matt:

24 to 45 in June at 19deg ambient
14 to 38 in November at 13deg ambient
upto 50 in January at 16 degC with too much boost.

It is never going to be as good on the road but fans won't compensate for being on the limit of intercoolers
mattyb

Fair point Doug, I have since found my n75 was funny, less boost up top now.
mattyb

Doug, David did ask about the comparison between road and rollers, and as these figures were under the same over boost conditions on the same day I think my point is a valid one. Would you not agree ?
mattyb

Doug don't get me wrong here I believe the cooling on your dyno is top draw, just an observation about the diff between road and rollers.
Doug

Yes - i agree.

But not many cars are pushed with your mods on the limits of intercoolers - you see how sensitve your car is to overboosting.
mattyb

Back to Mr T's original question, "How much more performance on the road".
If we were to say a drop of 20 degrees on the road, what do we think that would equate to power wise ?
Espen W

mattyb wrote:
Back to Mr T's original question, "How much more performance on the road".
If we were to say a drop of 20 degrees on the road, what do we think that would equate to power wise ?


On a NA engine (all dyno correction factors are developed for NA), a 20C difference, in this example, from 45C to 25C means 3.7% difference in power.
Here is an online calculator for the SAE dyno correction:
http://www.csgnetwork.com/relhumhpcalc.html
Doug

20ps?
VasP

Those calculations are only taking into account the air density/humidity without the fact that the map of the car changes with inlet temperature ie timing etc. So it is not 100% correct i think
Espen W

Doug wrote:
20ps?


Approx 20hp on a 500hp engine, less on less powerful ones.
SAE/DIN might have an algorithm for timing changes built in, but they are probably based on air density only (as Vas mentions above)
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