TonyBurns

iTrader: (3)

Oh and just to rub salt into your wounds mr cosworth, look who posted the 13th post on here, long before you thought your super supra was gods gift (and considering that yours is tuned a slightly bit more than mine, it also looks like subaru build a more powerful car )

Tony

Mark_S

Why dont you **** off back under the rock that you crawled out from under. No one here is interested in your opinions or your lardy arsed car that is fine as long as you're not asking it to take corners in the wet. Please find another forum to pollute, rsoul.

Cosworth427

Someone can say WRX's in standard form don't make more than 215 HP, as it is implied that we are talking about WRXs, and not the "WRX STI". I'm pretty sure you're aware that this thread isn't about the spec-c, but chose to be pedantic about it because there are opinions on this thread that do no match yours.



You quoting extrapolated crank HP/tonne in my face is a sign of someone who doesn't know anything.

Cosworth427

The biggest loss in HP from crank to wheel is the tire itself, so doubling the contact patch raises the overall loss, as well as the extra loss from the 2 extra differentials on a 4wd car. I think you are implying that the front axles will always be less loaded than the rears, leading to a reduced measured torque at the fronts. But on the other hand, there are countless front engined awd, rear engined awd, traverse mounted awd, inline mounted awd, and the loss shouldn't be any worse than (crank HP - 10 HP) - 16%

Even if the whp figures are suspicious, the charge temperatures from the boost levels used from the tiny turbochargers and small intercoolers (typical among the turboed 2 litre engines, i.e cars such as EVOs, Cosworths, Vauxhalls Turbos etc) is enough to prove that the extrapolated crank HP figures are exagerated.

I'm not going to refute that AWD is demands less from the tire compounds, which gives more grip per tire when you take laterial acceleration and wheel torque when powering out of a corner. We're just talking about straight-line acceleration, a motorway or long straight situation where traction advantage of AWD is diminished.

Here's a question, why should all cars carry the extra weight, inefficiency and complexity just so that most road drivers can use 90% of the power in the rain, and yet lose 20% performance in the dry? When most of the time, roads are dry?

I can list a dozen of obvious things regarding the mk4 supra (and mk3) that shows it was never meant to be a race car. The car is front engined, front biased, with double wishbone suspension, with a huge surge of torque delivery, the worst combination for traction. The traction problem can be helped with suspension work, but it makes a bad handling car or just too uncomfortable for the road. But the car is stable at high speeds and doesn't change course from every bump or change in camber it experiences. It is a GT after all.

crush her

Its a chest wig chariot

john banks

Thanks. I agree with you, for a standard WRX the AWD is a blunt to straight line performance, it could be RWD most of the time, but it makes it a very easy car to drive if you don't go over the relatively high limits where an average driver is going to be caught out. But is it really losing 20% over the RWD car? My losses at c.90 mph were 13% last time it ran on a Bosch rolling road. Other rollers which pull the tyres down a lot have shown losses up to double this, although they were going to slightly higher speeds. I know which I believe from the road based WHP measurements which can also work well on 2WD cars. You would think even a 328i under acceleration would leave a standard Classic Scooby if you based things on rolling road WHP measurements and weights of the cars. But even when standard, my 215 BHP classic steadily pulled away from 328s once rolling. Similar with countless other warm or hot hatches. A classic Scooby will still do 60-100 in 9 seconds. Not quick, but enough to give even the best hot hatches a real problem despite the apparent close WHP:weight ratios based on roller results. I know particularly the bugeye WRXs lost this advantage because of weight, whilst the hot hatches got quicker as well.

When you double the power output on a Scooby it is a nice platform to work with. Then I feel the losses seem more reasonable for the gains you get in stability under power, whether in a straight line or a bend, wet or dry. With say 350 WHP in a 1270 kg car, I have full traction in 1st gear in the dry (if I don't launch it with more than 2000 RPM), and fairly good traction in 2nd gear in the wet, even though my gears are short. With a Supra I would need something like 212 BHP to each rear wheel to match that, presumably you wouldn't have full traction until you were in 3rd gear based on videos I've seen of 500 BHP RWD cars?

People that have converted their Imprezas to RWD don't seem to report a gain in straight line performance from doing so. Perhaps the conversion retains the drag of some of the unused AWD driveline though?

Cosworth427

The 20% remark is just a guess when you take into account of the extra hp loss and the extra weight of AWD hardware. I can't give precise estimation of how much potential performance you'd lose, but you will lose acceleration performance just by having AWD over RWD.


The 328is make around 160 at the wheels, and weigh atleast 150 kilos more than a classic scoob.



When a scooby has 350 hp at the wheels, it is quick by most standards. A standard blobeye UK STI (this car of the thread) isn't anything like that.

A supra increase in weight is LESS than the increase in engine size. Many STI boys call Supras big barges, lardy etc, but what they don't realise is their 2 litres have to carry 1470 kilos of Impreza, and have more of whatever power that motor pushes out is lost because of AWD. Whilst the 3 litres in the Supra carries 1540 - 1600 kilos. That's 513-533 kilos per litre, vs 735 kilos per litre in the Impreza. Even the light weight classics are more burdened than the Supra. Around 617 kg per litre.

Engine capacity relates to torque through the entire RPM band. Sure, you will have the odd member here say "m8, my classic PPP makes 290 lbs of torque at 3500 rpm". But has anyone seen how low the torque is below that? There are plenty of driving situations where you have to drive with RPMs well below 3.5k. How many people have complained about the on/off power delivery of cars like this? You never that have problem in a Supra, 300zx, or even a Skyline.

Also, a larger engine means they can drive larger "colder" turbos without being undrivable, so even more HP per PSI increase is possible.

john banks

Mine is 525 kilos per litre, but it is a modified (2457cc) classic and agree not on the topic of the thread. As they come out the box, especially the 2.0 1470kg STis that spool up quite late, I agree they have too much weight to carry for each litre, had I not gone for the bigger engine I would have not kept or enjoyed the Scooby as much. One of the great things I think about the Supra is that it has similar weight per litre (which has massive benefits I'm only too familiar with), but also the sequential setup as well. All nice to make it more normally aspirated. However, as a potential car for me following the Scooby, there would be little gain in power band "width" considering that the extra half litre is offset by a proportional increase in weight, because of this I would have to put proportionally more power and torque through one instead of two axles. I do agree on what you're saying about the standard cars though, but there is very little that is standard that would be aspirational in terms of performance now except for silly money

automodellistagt

lol why am i not suprised

Pavlo

c427,

What john is trying to get at when comparing RWD and AWD RR figures is, that an AWD car on the rollers has to overcome the losses associated with the front tyres, just as it does on the road (although we agree the RR exagerates these losses). However when you run a RWD car on the rollers, especially a front engined car, there is no power being lost via from the contact/deflection of the front tyres as they roll.

The end result is that with the front engine/RWD format like a supra or BMW will get RWHP from a rolling road which are overly flattering. So in your example of the 328is with 160hp at the rear wheels, how much of that HP does it have to give up just to drive the front wheels? Remembering that we have already agreed that tyres make up a significant proportion of the losses...

260 hp at the wheels on 11psi That's not bad, my Impreza needs as much as 14psi to do that!

Paul

Cosworth427

I've already explained this, there is an increased loss at the wheels but not as dramatic as it is assumed to be. Plenty of AWD cars that are rear engined, mid engined, front inline mounted engined, front traverse mounted engined lose no more than (crank HP - 10 HP) - 16% Porsche 911 Turbos, Evos, Audi turbos, the list goes on make enough whp to match or even beat the rated crank HP.

The overestimation of transmission loss I'm pretty sure roots from overestimating the peformance per PSI of boost from the small turbo chargers and small intercoolers used to run 2 litre engines. 15 psi of boost from a turbo of the size found in a stock WRX, Cosworth, etc pushes around 0.0023xx lbs of air per hour. Which is at 5700 RPM with 15 psi held, around 260 HP at the flywheel. That level of boost, you'd have some tuning companies quoting 300 HP at that boost level, and overestimating the transmission loss to make the atw figure appear to be "spot on".

DrJP

Further to the 2nd post in this now lengthy debate, Cosworth 427 you talk complete rubbish, go back to supranet.co where people will be interested in your opinion

Cosworth427

Really, to call yourself a "Dr" you should be participating in this discussion as it involves transmission losses, adiabatic heating of inlet air, torque reaction and tire loading. It's far from rubbish.

Pavlo

Lol,

The over estimation of losses is just what it says. You can take a car to 2 differenr rolling roads (different manufacturer) and get 2 different wheel HP figures, but using the drivetrain loss estimation they will often come up with similar crank HP figures.

You assume that the the power measured by the Rolling road is the power at the wheels, it is not a safe assumption at all, and can be wildly out. What the dyno actually measures is power transmitted to the dyno, less tyre losses. And running a steel belted radial tyre on 2 small rollers cab rob a lot more power than a nice flat road (depends on roller configuration).

Anyway, you completely missed my point, and where on earth did you come up with 0.0023xx lbs of air per hour?

Paul

john banks

I think I'm using about 45 lb/minute, but I could be orders of magnitude out and powering a rocket to the moon

Cosworth427

It isn't as safe assumption, but as far as Imprezas are concerned, their whp readings are consistent across the board, give or take 5-10 hp. And in terms of performance, it is far better to measure what you can make at the wheels than what you make at the flywheel.


Sorry, I should have stated 0.0023xx lbs of air per litre from the intercooler outlet.

john banks

I've had my car in the same gear on two different rollers and had 50 BHP loss on one and 100 on the other, but similar flywheel figures. And even if they were consistent, consistency is not the same as validity.

Why when we have agreed that they are flawed when comparing AWD and 2WD? Flywheel figures are also compensated for air temperature and pressure, whereas the rollers I have seen do not do the same for wheel figures (and yes I know the compensations can be fudged). Flywheel figures also attempt to account for gearing, but coastdown losses are flawed as well.

I still maintain that rollers are pretty much useless for shootouts, and should be used as a mapping or fault finding tool.

I also think that turbo lag is a far bigger real world performance killer than AWD losses, which on a car with what I would call reasonably exciting acceleration (say >300 BHP/ton) are IMHO made up for by the traction advantages even in the dry. However, you don't easily get 300 BHP/ton in a road friendly daily driver with creature comforts very easily without turbo or supercharging or you end up with a huge and heavy engine in the front with poor weight distribution, or you put it in the middle or at the back and ruin practicality.

Pavlo

c427,

0.0023lbs/hour/litre is still way out. Can you please explain exactly what you mean, are you refering to mass airflow into the engine (which is the same mass airflow out the turbo, or anywhere between).

I agree with john, expect that I do have more confidence in coastdown losses for telling us something useful.

Anyway, you still seem to be labouring under the illusion that the rolling road measures the same output from the drive train that is transferred to the road in normal use.

Out of interest what sort of power at the wheels would you estimate is required to get a 1270kg car (inc driver) a 10.9@126mph 1/4 mile result, don't worry if you don't have an estimate, but I am icurious if you do.

Paul

Cosworth427

The rollers measure the multiplied torque at the wheels, and divide by the overal gear ratio i.e overall gear ratio = 4th x final drive. The dyno tuner should know the ratios of the car being tested.


Because the possible flaws (human error or machine) in rolling road tests also apply to RWD/FWD as well as AWD cars.

You and Pavlo should remember that even though the front axles of a 200 HP AWD car received less weight load, the loss in aggregate traction is no where as bad as a 200 HP FWD car, and the torque reaction (car squating towards rear) is no where near as dramatic as a 200 HP RWD car.

Suppose you have a 51, 49 torque split front and rear, this only means the torque reaction (car squat) is only half as bad as a RWD car that applies 100% torque to the rear wheels.

Bubba po

Please, God.... make them stop...

Cosworth427

0.0023xx lbs per litre is an approximate unit of air mass for every litre consumed by a turbo charged engine with 15 psi boost of a given turbocharger and intercooler size. For example:

Target Brake Specific Fuel Consumption. 0.50 lbs per HP per hour.

Air mass per litre @ 150 deg F = 0.002350 lbs (example only)

1.998 litres x 5700 rpm / 2 = 56943 litres (Remember 2 litres consumed every 2 engine cycles)

56943 litres x 60 minutes = 341658 litres per hour

15 psi boost is 103% increase in atmospheric psi.

341658 ltr/h + 103% = 693565.74 ltr/h

Now find how much air mass consumed in an hour:

693565.75 x 0.002350 = 1629.8 lbs per hour

Assume you have a 12:1 air/fuel ratio

1629.8 / 12 = 135.82 lbs of fuel per hour

Now apply Brake Specific Fuel Consumption. B.S.F.C

135.82 / 0.50 lbs = 271.6 HP

271 HP - pumping losses in intake, throttle body, port and valve, Compression ratio etc etc, say 6% meaning 94% efficiency.

271.6 - 6% = 255.3 HP

This calculation just demonstrates the relationship between boost, post intercooler air temps and the power it in theory should make at a given B.S.F.C, engine size and RPM.

[/QUOTE]

Again, the "illusion" also applies to FWD and RWD cars, yet no dramatic losses on the dyno chart nor road for these types of cars.

ET of 126 MPH? ...hmm. That would be around 340 - 360 whp. With around 300 hp at the wheels per ton without driver. Drag Cd and frontal area must be reasonable though.

john banks

I was under the impression that the dyno measured power at the wheels (this is the variable it reports real time) and then went on to calculate flywheel power based on coastdown losses and correcting for atmospheric pressure and temperature, and then deriving flywheel torque from an RPM pickup, entering the gearing, or telling the dyno when the engine is at a given RPM so it can work it out. But if it doesn't know gearing it can tell you the power at the wheels and plot it against road speed. At least the MAHA rollers appear to function that way. Do I misunderstand?

I see your point about weight transfer in the RWD car being worse than AWD on the dyno, but surely to compare RWD and AWD wheel power you would need to lock the rollers together for rotational speed as it would be on the road without slip, and let the RWD suffer the losses of pushing the front wheels around against their artificially deformed tyres?

Pavlo

just a note. Brake Specific Fuel Consumption is measured WRT to the flywheel HP and you do not subtract pumping losses, just the same as you don't subtract FMEP before calculating BMEP.

Also you very nice calculation and use of the magic figure of 0.00235lb/litre boils down to just assuming an inlet charge temp of 65ēC which isn't very accurate in itself.

Paul

Cosworth427

For a turbocharged engine, I don't believe that correcting for atmospheric temperature will give you any meaningful results, because a turbocharger builts absolute psi as set by boost control, regardless of whether actual atmospheric pressure is 13, 14.5 or 15 psi.

N/a engines on the other hand would be directly affected by the changes in atmospheric psi.

The dynos ultimately measures the wheel torque. Either by hydraulic brake or electric motor applying a torque load vs the wheel torque to hold the engine RPM. It extrapolates the "power" readings from the torque and RPM of the wheels or with RPM pick up hardware.

The coastdown measurement is supposedly used to measure losses, but I can't see how this is true when there is no power fed into the wheels when the measurement is done.


I don't fully understand what you mean here.

What I think you're trying to propose is to include the frictional drag of the front tires, to induce extra losses which if detected by the dyno software, would indicate greater transmission loss, but in reality any kind of external drag only reduces the car's performance, but torque/hp output remains the same. So how can the drag of the front tires affect the total amount of power transmitted from flywheel to the rear wheels on a rwd car?

Cosworth427

You're absolutely right there. I suppose my calculation (by no means gospel) can be rearranged to work out losses from induction and then apply the fuel consumption after.



65 c post intercooler temps from 15 psi boost far too high? The temps of air charge of my supra is around 150 F at 15 psi, and my compressor housing and wheel is significantly larger than those in most turbo'd 2 litres.

Pavlo

If you are seeing 65ēC at the throttle body, then you need a better intercooler or turbo or both. Even at TOTB in 30+ ambient temps I was seeing no more than 40ēC inlet temps, that was at a 22psi which will of course increase heating over 15psi even for a 100% efficient compressor.

You seem to be making a lot of assumptions, like all the owners on SN have 2.0 engines, they have small turbos, crappy intercoolers and that we all don't know what we are doing.

Paul

john banks

C427, my engine changes power substantially when the atmopsheric pressure changes. In reducing atmospheric pressure and achieving the same absolute pressure in the inlet manifold, the PR across the compressor wheel increases, the pre-intercooler temperatures increases, and the exhaust manifold pressure increases as the turbine wheel does more work to drive it all. Consequently the airflow through the engine reduces at the same boost level and the ignition needs to be retarded. Otherwise, why would turbocharged cars at altitude not perform as well as at sea level?

How do you propose on cars of different gearing that produce their peak power at different engine or road speeds, that you compare with another car if you don't do coastdown losses? Instead the WHP figure is presented without qualification usually.

Agree up to the torque/hp output remains the same... the measured WHP would surely drop because of the frictional losses for the front tyres - making it comparable with the poor AWD car that is doing the same. The flywheel figure would be corrected by the higher loss figures.... another reason to compare the flywheel figures IMHO. I don't know if you can lock rollers together in the way I suggested, but it seems a glaring difference to only have half as many tyres losing you power when comparing WHP and flywheel figures.

Pavlo

If we continue we could be getting dangerously close to some level of fact...

Pavlo

The power at the rear hubs will of course remain unchanged.

Seeing as this whole thread is based around the relative differences between AWD and 2WD cars, and their respective measured power outputs, don't you think this point is rather significant?

If we were to measure power at the hubs such as a Dynapack measures we would see the difference between 2WD and AWD diminish greatly as the power absorbed by the tyres (which doubles for the AWD on the dyno but not on the road) is eliminated.

John and I woul actually like to be able to make some comparisons of performance from one car to another, and one power level to another. It's a useful measure, so why wouldn't we choose to do this in the most technically correct manner?

I still don't understand what your point is, as you seem to be supporting both sides of various arguements without realising.

Paul

Cosworth427

And how many Imprezas out there use the standard turbo and intercooler? Equal if not more than those who use aftermarket turbos.

What set up are you using?