About the WHP "rating"

[Vigge]

About the "WHP" rating

Often when one reads an internet discussion forums cars have a tendency to be rated by wheel power, or more like the peak wheel power figure which is then used to benchmark drivability. Also person who have limited experience of powerful FWD cars often shout out that a FWD with more than 200whp does nothing else than spins wheel etc.

To rate cars by their peak "whp" would of course not be a problem, if the power band for a motor would be "fixed" to a certain shape. Then the peak WHP value would just scale the curve up or down, but as we know in real this is not the case and the following tries to explains in more detail why and also how the whp, tq, gearing etc. link together.

Lets us first look at a bone stock Viggen which will roughly produce the following figures when put into a dyno (blue curves)



and with stage III kit the power output is normally percentually lifted through out the rpm registry and resembles the red curves in the same picture.

The plain curves themselves may not open up to all, how the link to performance and traction and a good known approach is look into the calculated acceleration curves in various gears, since the limit of traction can be expressed as a function of acceleration [G's], normally around 0.5G for FWD cars.

For the bone stock Viggen the calculated acceleration curves are shown in the picture below as a function of speed [km/h]. The "height" difference in curves for different gears are due to gearbox, which multiplies the crank shaft Tq with the total gear ration (drive shaft Tq curves www.stcf.net/slip/slip1.gif).



From the chart we can note the following.

1) acceleration in any given gear is the highest at peak tq rpm/speed, not at peak hp

2) tq from the bone stock motor is enough to spin wheels in 1st but in 2nd requires one to pop the clutch, a bump in the road etc. to get the wheel spinning, since steady state acceleration falls shy from the traction limit.

Lets us now add the stage III acceleration curves to the same plot and we get



And we can note that 2nd gear acceleration also exceed the traction limit and driving WOT in this gear will result to wheel spin on steady state. From the curves we can also see that for instance in 3rd gear the acceleration begins to fall quit heavily after 110km/h mainly due to std turbo which is beginning to run out of breath and by rev limit the reduction in acceleration is around 50%. It should be remembered that in modern turbo cars the turbo is mainly used to up the low/mid rpm torque and not the peak whp. This is easy to understand when its known that an N/A engine can produce up to 100Nm/liter, 100bhp a liter, meaning that if the Viggen would be without tbo figures would read 230Nm/230hp at the best. Also turbos have quit a narrow operation range, and if the unit has been chosen to kick in around 2000rpm it will not carry effectively past 5k rpm. Anyhow there isn't much that can be done to raise the Tq curve and higher rpm's without modifying the Hw, but if wanted more "grunt" can be taken into use in the are where the stage III tq curve is "flat". Personally drove around in the early days with a "balls-out" stage IIIx and its curves print out in the same plot as follow



from the curves we can note that the lower rpm grunt was increased over a "normal" stage III and 3rd gear pushed closer to the traction limit. This type of tune creates the feeling that the car has lots of power, but when run up the gears one does not use the rpms where the extra Tq was obtained, so no improvement in that type of go.

At this point based on the curves we do see that 2nd gear traction has already been lost with the raised tq but 3rd gear there is plenty of marginal to work with until you begin to spin the wheels on steady state acceleration. So lets swap some HW and move on

to a 350+whp set-up (my ex. 3071 with std cams etc, dynoed to 412hp/527Nm)



Thus the whp increased by 100whp or so it did change much in terms of traction since the Tq did not exceed the tq max. obtained with the stage IIIx set-up. From the graph we can however note that acceleration when running up the gears was greatly improved by moving the rpm registry upwards, but since tuning is a newer ending process the request was for more.

Often when you move forward from this type of bhp level the restriction in the engine HW will begin to pop up (cams, tbo etc) and if not taken care of the breathing at higher rpms will be limited and will set the roof how high the whp can raise. Of course if the tbo is of good side and you just keep upping the boost you can get the "big" peak numbers like in this manner

http://genuinesaab.com/gallery/album...d_Run9_001.gif

but drivability due to high tq will not be the greatest not the running up the gear performance since the higher rpm grunt is missing.

Next up are the curves in the same plot form my 500bhp set-up and they print out as follow against the 412hp curves. Tq in this set-up was mapped against traction limit in 3rd gear





So with bhp once more lifted by nearly 100whp the 3rd acceleration curves remain beneath the traction limit, but instead of just "touching" the limit at 100km/h the remain close to it for a much broader rpm/speed range, but as we can note there still is margin to go at higher rpm's where the GT3071 was inadequate to pump enough air anymore. So Hw swap once more to current Holset Hx40 set-up and those curves plotted in same graph.



And with this set-up so far the tq in the mid range has been limited to same figures as before, but up at higher rpms raised and from the curves we can see that the acceleration lies at traction limit from 100km/h to 150km/h J and the aim is to have it carry on all the way to 170km/h (7k rpm).

Comparison of the current to std Viggen



and we do note that the all day drivability on WOT has not suffered that much, thus more than double whp. In first the Holset set-up provides more tq to drive shaft from 30km/h, and in second 55km/h, in 3rd 85km/h and in 4th 110km/h .

All torque curves and rpm vs. speed for all of the set-ups



and yes "theory" works with practice as well, here is a in-car film form this summer

http://www.stcf.net/viggen/lpr_ov.wmv

where no wheel spin is seen on after 2nd gear. Rougly 460whp was in use and the curve did look like this.

http://www.stcf.net/viggen/lprs1.gif

and as for the a conclusion to rate a cars drivability based on a whp figure will not work alone, nor will a a fb-ft limit work either since saabs do come with diffent gearboxes with different gear rations etc. Hopefully this post shined some light that it is possible to have tons of whp, thus no more show with spinning wheels than what is the case with a stage III set-up.

[SaabLV93]

excellent and thorough write up vigge. i love how you always back up what you say with hard data...and not just conjecture or hypothesis. thanks for this.

[RITmusic2k]

Great writeup and interesting insights you revealed there! Can I borrow some of your hardware that you're not using?

EDIT: Also, I just looked up your Hx40 turbo. According to Cummins, that model is intended to be used on 7-10 liter bus engines!??!?!! Where do you keep that thing, in the trunk?

[Sap]

best thing i've read all week, great post vigge!

[al aero]

Great post.

Sticky maybe mods?

a.

[John Z Williams]

Some nice comparisons there, but as you have said, mapping can have a big influence on the how the car runs, some people like tons of torque and spinning the wheels.

It is possible to short shift a car with high torque and run faster than winding it out where a smaller turbo is not efficient, I proved that myself on the drag strip.

Finally, even a car with loads of torque past the traction limit can be driven by a good driver with some good right foot control to keep the car within traction limits and still go faster and with combined short shifting, make an aggressive high torque mapped car run fast.

You made many good points and the bottom line is many people want to know what the peak bhp is, not what the drivability is or where the peak bhp is made, just what that peak power is.

I think its very hard to get around that simple fact, people want to know numbers...

my 2 cents,

John

[Vigge]

Originally Posted by John Z Williams

Some nice comparisons there, but as you have said, mapping can have a big influence on the how the car runs, some people like tons of torque and spinning the wheels.

yes we all have different taste, but the above just one example that it is possible to run big whp figures without suffering from drivability problems in 3rd and higher gear, which for many is an equation that does not add up when we speak of a FWD car.

Originally Posted by John Z Williams

It is possible to short shift a car with high torque and run faster than winding it out where a smaller turbo is not efficient, I proved that myself on the drag strip.

from the drive shift torque curves or better yet from real acceleraiton curves it is really easy to see if it is worth winding or not, but usually with a bigger turbo that does not "die" in the high end you need to be making tremendous tq at mid range to achieve a likewise acceleraiton in a higher gear.

Originally Posted by John Z Williams

Finally, even a car with loads of torque past the traction limit can be driven by a good driver with some good right foot control to keep the car within traction limits and still go faster and with combined short shifting, make an aggressive high torque mapped car run fast.

its possible since the traction increases with slip to some degree, but the falls on it nose.
http://www.racelogic.co.uk/?show=Tra...l-How_it_Works

but the hard part without traction control is to control the slip to be small enough in order to not loose on traction compared to when there is noslip. Also to have the engine produce the max tq as rpm "rush" up rapidly when traction is first lost becomes an issue.


In my car I have played around with the slip issue a few times and have found that its not much slip the normal tires allow before the acceleration suffers.
Here are two runs, in the other slip 5% and it the other allowed to increase to 25% and as one can see the acc curve takes a nose dive pretty much directly as the slip begins to increase




How much that is due to traction and how much to boost which dips with rpm rush is hard to say, but here you can see the MAP, rpm etc curves for both of those runs.
http://www.stcf.net/viggen/sutsut.jpg

Here though is a datlog from one fast saab running ~12s/210km/h (1ft roll) on street with big slics and has hard time to manage .5G on average in 3rd gear


On first and second gear the acceleration is very good (0-100km/h < 5s), but in those gears it easier to control since there the engine will always deliver "plenty" tq thus rpm hops and map sinks... Here are a few runs from mine


Sadly though far too few people have enough interest to look into these issues, so it not really possible to develop the discussion to deeper levels than feeling based.

[John Z Williams]

So, what kind of fuel octane do you have in Finland? Do you have any idea what level of octane of the fuel they run in Sweden or wherever maptun is located? I swear their timing maps are as aggressive as my w/m injection map that I showed you, its nuts!

John

[Vigge]

Originally Posted by John Z Williams

So, what kind of fuel octane do you have in Finland? Do you have any idea what level of octane of the fuel they run in Sweden or wherever maptun is located? I swear their timing maps are as aggressive as my w/m injection map that I showed you, its nuts!

John

You have the choices for RON 95, RON 98 or shell vPower which is RON 100+ and of course E85.
B205 can cope with far more timing due to its comp.ration 8.5 in case you a looking at a map meant for that engine.

[lysmask]

Originally Posted by John Z Williams

So, what kind of fuel octane do you have in Finland? Do you have any idea what level of octane of the fuel they run in Sweden or wherever maptun is located? I swear their timing maps are as aggressive as my w/m injection map that I showed you, its nuts!

John

Sweden has 95-98 octane (plus shell v-power 99), most tuners recommend 98. I've seen a couple of ign maps that i wouldn't drive on 99 octane in my car, but perhaps i'm a sissy..

I've got maptun about 100km's from here btw.

[John Z Williams]

Well, that would explain a lot, which one do you think they tune with, the 98 or the 100+?

I wonder how they expect cars in the USA that can get from 89-93 octane to run on those maps? Plus, here in the mountains, we only get 91 octane and then its mixed in with 10% ethonal in the winter and it actually seems to make them run worse, some places have "pure" 91 fuel with no ehtonal and it definetely seems to run better.

This is what I thought and is probably the biggest reason I have had to address so many of those tunes from M. There is no way you can tune a car on 98 octane and then expect it to run fine on 91 pump at elevation where the hw, ic's and other compents run less efficiently.

John

[Vigge]

Originally Posted by John Z Williams

Well, that would explain a lot, which one do you think they tune with, the 98 or the 100+?

I wonder how they expect cars in the USA that can get from 89-93 octane to run on those maps? Plus, here in the mountains, we only get 91 octane and then its mixed in with 10% ethonal in the winter and it actually seems to make them run worse, some places have "pure" 91 fuel with no ehtonal and it definetely seems to run better.

This is what I thought and is probably the biggest reason I have had to address so many of those tunes from M. There is no way you can tune a car on 98 octane and then expect it to run fine on 91 pump at elevation where the hw, ic's and other compents run less efficiently.

John

Most often the tunes are recommended for RON 98
RON 98 is 94 PON
RON 95 is 91 PON

[John Z Williams]

Yea, that is about what we calculated, around 95pon, so we were pretty close in our estimates,

John

[David in Tallahassee]

I don't have a conversion for you.


However,

There are three octane related "ratings" for fuel. Research Octane, Motor Octane, Anti-Knock Index. RON, MON, AKI

The rating system in Europe is based on Research Octane Number (RON).

Here in the states we use the Anti-Knock Index (AKI) which is the RON and the MON (Motor Octane Number) averaged.

Most race gas focuses on the MON.

When I was tuning Jags, we used to figure 98 RON was nearly identical to 93 AKI. However, that was before there was any ethanol content in the AKI fuels so how that affects things I couldn't begin to say.

D-

[comtrang]

Great information, Vigge.

Just wondering, what's the modification you spoke of after your Super 40 went in, that was reflected in another graph you gave? I don't recall you changing something after the Super 40.

[John Z Williams]

Originally Posted by David in Tallahassee

I don't have a conversion for you.


However,

There are three octane related "ratings" for fuel. Research Octane, Motor Octane, Anti-Knock Index. RON, MON, AKI

The rating system in Europe is based on Research Octane Number (RON).

Here in the states we use the Anti-Knock Index (AKI) which is the RON and the MON (Motor Octane Number) averaged.

Most race gas focuses on the MON.

When I was tuning Jags, we used to figure 98 RON was nearly identical to 93 AKI. However, that was before there was any ethanol content in the AKI fuels so how that affects things I couldn't begin to say.

D-

Yea, we were thinking the 98ron would be about equal to 95aki or the averaged two that we have in the US. If its actually closer to 93 usa standard, that would help explain why people at sea-level don't have as much trouble as us at elevation with crappy 91 and ethonal blend in the winter months. That and the loss of efficiency of the ic, heat dissipation, turbo having to work harder, etc..

John

[David in Tallahassee]

Originally Posted by John Z Williams

Yea, we were thinking the 98ron would be about equal to 95aki or the averaged two that we have in the US. If its actually closer to 93 usa standard, that would help explain why people at sea-level don't have as much trouble as us at elevation with crappy 91 and ethonal blend in the winter months. That and the loss of efficiency of the ic, heat dissipation, turbo having to work harder, etc..

John

Yep, that sounds like it fits.

D-