madmacslemon

Why do some rolling roads give different readings?, some seem to be on the conservative side whilst others over estimate.
Surely they all must be calibrated the same?.....example....when i first got my scoob(my97 sti type R) i had it put on the rollers, now the only mod was a Jap spec "cat" back exhaust, after three runs it showed as 298bhp and 275lb/ft torque. Well chuffed!.....anyway recently i had a full de-cat pipe, freeflow panel filter, fuelling altered and a major service, now i wasn't expecting massive HP gains but around 10bhp would have been nice, three runs on the rollers again at a different place......300bhp 263lb/ft torque, slightly miffed, i mean was the first place way out, or the second, how do we really know what our cars are putting out? I ponder.

NotoriousREV

Anyone that deals with rolling roads will tell you the same thing. The numbers are unimportant, the shape of the curve is everything.

If you are going to compare numbers, use the same dyno each time, they will still read different every time (due to temp/air pressure/humidity differences) but they'll be more comparable than using 2 totally different machines on different days.

madmacslemon

I see, so how does the curve shape compare to power output?
ill go check my readings!

NotoriousREV

Well, if you're after peak power bragging rights,it doesn't. If your tuning your car to make it go faster under particular conditions it relates to driveability:

Example 1:



Example 2:



Both have peak power of 230bhp and peak torque of 217 lb/ft. Look at the shape of the curves. The first graph has a more linear curve, so it will accelerate smoothly from low revs.

Example 2 at 2500 rpm is making about 10bhp less than Example 1, so it'll feel more sluggish when off boost, by 3000 rpm it's making 10bhp more and by 3500 they're roughly matching eah other. Notice, the numbers are only a guide used to illustrate the curve shape! I would say car 2 "feels" faster due to the relatively poor low-end but stronger top-end, it definately makes more torque than car 1 but revs less, car 1 will be a smoother drive.

You should have the dyno graphs for your pre and post mod runs, compare the shape of the 2. Have your mods given you top-end at the expense of low down power? Have you gained or lost mid-range in comparison?

Before modding you should plan what you want to acheive (for a road car you want reasonable low down response a nice fat mid-range and a good top end, for a track you can ignore the bottom end and concentrate on mid - high revs).

HTH

[Edited by NotoriousREV - 12/17/2002 11:49:16 AM]

chrisp

Heres my graph from Nobles and I am pretty please with the curve. the graph suggests that the torque starts to really come in about 2,500-3000rpm which is exactly what it does on the road. Makes the car very driveable which I what I wanted.




Cheers

ChrisP

Pavlo

nom,

the graphs for cars '1' and '2' above are rather misleading as the scales are totally different!

Rolling road curves are not very good for comparing cars run on different rolling roads, the peaks may be farily accurate, but different places set the loading on the road differently, affecting the way the boost comes in.

Same RR on the same day is a useful comparison, or at least same RR in the same ambient conditions.

Paul

NotoriousREV

I wouldn't say that the above examples are misleading, it's just that one graph starts at lower rpm and bhp levels, but it's hardly a massive leap to ignore everything below 2500 rpm on that graph. The difference in curve shape is still blindingly obvious (especially on the torque curve)

madmacslemon

Cheers guys most useful, after the mods the curve shows much more lower end grunt the car is much more drivable in the lower rev range.

NotoriousREV

And hopefully this reflects how the car feels? So your extra mods may not have freed up much top-end bhp, but have inmproved driveability.

Single dyno runs aren't really that useful unless you have horriffic troughs or flat spots. It's only when you compare it that it beomes useful, but omparing is a minefield due to all the different factors. Even 2 runs 5 mins apart won't give the same numbers.

With the race bikes, we've been down the road of scratching our heads over numbers produced and then found ourselves lapping consistently faster than bikes that have more power on paper. Dyno runs a just a tool for checking your work, not the be all and end all

madmacslemon

yes it feels like a new car, turbo spools up better, just seems much easier to drive, pulls all the way through now, as opposed to the massive kick of boost then nothing.
thanks for all the info.