Waitng for a delivery so time to post.

I have always wondered how road car aceleration times relate to claimed power to weight ratios and the latest Porsche 911 Turbo data makes me wonder all over again.

The main benchmark for all supercars has long been the McLaren F1. It was timed by Autocar magazine ( whose tests are pretty rigorous ) at 0- 100mph in 6.3 seconds with 0 60 in 3.2 seconds. that is with 627 BMW bhp and 1138kg kerb weight.

the new Porsche 911 Turbo claims 0 - 100 mph in 7.0 seconds with 0 - 60 mph in 3.4. This is with 1595kg and 493 bhp.

working with the assumption that 0 - 60 is as much about grip as power that gives 3.1 seconds from 60 - 100mph for the McLaren and 3.6 seconds for the 911 Turbo. That speed increment should be pretty much pure power versus weight so (with 100kg added for driver + test gear)

1) McLaren 627bhp/1227kg = 511 bhp/tonne = 3.1 seconds.

2) Porsche 911 Turbo 493/1695kg = 291 bhp/tonne = 3.6 seconds.

A lot of power, weight and aceleration time data I would ignore but I do think both Autocar and Porsche have historically quoted solid data ( as I think would McLaren and BMW) so how come a McLaren F1 with a 76% better power to weight ratio than a 911Tturbo can only reduce the 60 to 100 time by about 14%

Available traction is by far the critical factor/uncontrolled variable. Tire size, type, tread compound, inflation pressure, tread temperature; traction control effectiveness and calibration, switched on or off; road surface type, preparation, and temperature; driver skill. (Most cars produce better times with the TC shut off.) I have long held that any 0-60 time under four seconds or so is sort of a dice roll. Cars of this performance are essentially traction-limited. If the Porsche and F1 were tested at different locations by different drivers, that could account for a good part of the .3 second difference in 0-60 times, maybe all of it.

I hope I wrote it clearly but I was not talking about 0 - 60 times, clearly they are traction limited at those P/W ratios.

The 0.3 second difference is between 60 mph and 100 mph where traction is probably not limiting but also aero drag deltas are ( for such powerful cars) not significant.

So 60 to 100 in 3.1 seconds for the McLaren versus 3.6 seconds for the Porsche still seems to me a very small gap for such large power to weight ratio differences between the cars.

Again if the power, weight and acceleration numbers came from flaky people I would not bother but Porsche etc. do not usually make false claims.

Isn't 14% a big advantage?

a turbo 4x4 with wide rubber, like 911 turbo, will handle a full throttle, launch control, launch. Which really means that you are launching with the engine at 4000-5000 RPM, and something like 0.8 bar of boost, at the start, and will have no problem putting it down to the track in first gear, which is not something that can be said for any street tired RWD car.. I think you are seeing the obvious example of traction limited performance..

for example, we have a local drag chevy malibu, with 1070 HP big block (documented) which in reality is a sub 9 sec car, yet can not get under 11 sec 1/4 mile on our shitty pavement..

I do not beleive that the McLaren F1 was traction limited between 60mph and 100mph, again I am not asking about launch aceleration but in gear acceleration from 60 mph to 100mph.

If you look at the actual data recorded on the F1 by Autocar it accelerates from 60 to 80mph faster in second than third gear. If it was traction limited then the second gear times should equal the third gear times due to excess wheelspin in second gear. Similarly from 80 to 10 when third gear is used the thrird gear time is quicker than the fourth gear time.

Also the F1 had a 58% rear bias unladen. Given a relatively high CofG by race standards it probably put 70% of 1227kg on the rear wheels under full power. With about 900kg on the rear wheels and tyres that could posibly give 1.2 CF on forward thrust I am not sure a lot of wheelspin would happen.

Mac is pretty accurate with his post #2. Plus with various launch control programmes that limit the HP/T ourput your answers for low speed accelerations get pretty meaningless in corelating HP outoput, weight and acceleration.

You can prove it to yourself by using food old well known laws of motion (F=ma and distance =Vot + 1/2 at^2, etc) in a computer programme and play until you understand the the limitations od reality.

Rather than talk 0 to 60 times that are highly problematical go out to quarter mile times and speeds. At that level the answers become far more rational.

A couple of good rule of thumb formulas that give quite good data answers over a very wide range of factors are:

quarter time in seconds = (200 * pounds / hp)^.333

quarter top speed = ((hp/pounds)^.333)*234

I have played with 100s of pieces of real data sets over a long time that validate the relative accuracy of these formulas. When they donn work is when the HP claims are wild. The formulas also will give you a reality check on hp claims when you know weight and performance.

Regards

Aero drag, power curve, shift points, testing environments, rounding. These will all mess with your comparison a little bit.

But to big thing to me is that the Porsche numbers seem a tad on the ambitious side.

Porsche horsepower claims also seem a tad conservative, in general (with the notable exception of the 993RS road car). Also I expect the tested turbo had the sport chrono pack, which delivers launch control and a temporary overboost function. Given that the old engine ran 1.0 bar overboosting to 1.2, and the new one delivers more hp with less boost (0.8 bar) there's some considerable room for figure-flattering temporary power increases. One of you genuine engineer types can work out what 50% more boost might deliver in a 500hp at 0.8bar 3.8l motor at 9.8:1 compression on 99 RON fuel. I believe the PDK transmission in the turbo is specific to the model, and took longer to develop because of the need for a high torque rating.

1 download lapsim
2 make a model of an f1
3 make a model of a Prosche
4 compare their straight line performance 60-100mph

5 then have an /interetsing/ discussion.

Or you could look at it this way:
How come a Porsche with 43% worse power to weight has 16% less acceleration from 60-100?

EDIT. Anyways you should use torque to weight ratio - right McGuire?

I'm going to guess that the average HP as the cars go from 60 to 100 is less tilted in the McLaren's favor. If anything because it's a N/A engine and the Porsche is boosted. The Porsche is bound to have a broader powerband (i.e. power within X% of peak over a given percentage of max RPM).

BTW, if you look at it from a kinetic energy standpoint, the Porsche has the more powerful engine.

K=1/2M(v2^2-v1^2)

K(m)=716260 J
K(p)=989454 J

Divide by the acceleration times and this give us power calculations of

P(m)=231050 W or 310 hp (average over 3.1 seconds)
P(p)=274850 W or 368 hp (average over 3.6 seconds)

Now both of these numbers are pretty rough and tumble, but they do give you a point of comparison. The driveline losses, rotational kinetic contributions, aero losses, etc. make these number not worth a whole hell of a lot, but they are interesting none the less.




Right......

How about we _don't_ go down that road.

Seems a no brainer to me.

The Porsche under load with turbo is producing significantly more torque than the normally aspirated McLaren.

Unlike an aspirated engine a turbo doesn't have a set torque figure at a set rpm, it is also load dependent. A turbo can have a poor 0-60 time compared to it's 60-100 time because of lack of time/load in lower gears to produce full boost not to mention a drivers inability to get a good 1st to 2nd gear change as the boost comes on in 1st gear.

FWIW The Ariel Atom 300hp version is one of the fastest 0-60 cars you can get but that time is acheived by starting in 2nd gear as it has enough grunt to launch it and the time taken for for a 1st to 2nd gearchange damages its overall time.

Whoops - looks like we just went down that road Fat Boy!

Maybe so but I think you will find that the figure quoted by Porsche (for both torque and power) is the highest value i.e. steady speed on a dyno.

I will qualify that by saying, if the Porsche can be full-throttle shifted, it may overboost briefly after gearchange and thus exceed the torque specification momentarily.

Corrected.

The theoretical limit for the extra boost is 2.2/1.8 (ratio of absolute boost pressures) = 1.22 ie 22% power increase or 602 hp from a 493 hp base.

A McLaren F1 will spin the tires hard through first and second and well into third gear (which is over 100 mph). Which is hardly surprising as it has 620-odd hp, weighs ~1250 kg, and runs on road-spec rear tires ~12 inches wide. Think about what you are saying. On the McLaren, 60 mph is approximately the bottom of 2nd gear. You don't believe an F1 will spin the tires in 2nd gear? Seriously? What, are you kidding?

Of course it will. Most any high-powered sports car or GT can do that with no trouble at all. With the TC switched off they'll jump straight out sideways if you let them.

The sorts of inferences attempted here cannot be drawn on power/weight ratios from acceleration stats. There are too many variables, most of them related to traction. On any given weekend at any given drag strip you can see stock production cars with inferior p/w ratios outrunning cars with superior p/w weight ratios. For any number of reasons, including chassis geometry and tune, static weight distribution, tires, gearing, and driver technique. Many production cars will accelerate quicker with a full fuel tank or sandbags in the trunk. How does that square with the ongoing depiction of the relationship between p/w ratios and acceleration?

To me, all these specs show is that proportionately, the Porsche can put more of its power on the ground. (Another practical, old-timey expression some will have trouble with, but is perfectly accurate.) That is not surprising either, due to the Porsche's more conservative power/weight ratio. It's got both less power and more mass than the F1, so of course it will hook up what it has more efficiently. The F1 can do the quarter-mile in the 11.50s, not bad for a production road car, but a decent drag car of similar power and weight on drag slicks will run in the 9.50s. So yes, the McLaren's acceleration is traction-limited, no question about it. By a quite a bit. Two seconds in 440 yards due to wheelspin is significant, isn't it?

+1

Sorry, but that statement is incorrect in this case, as you calculate not what the engine delivers, but what HP the car can transform into forward thrust. The 4WD system and higher weight enable the Porsche to get more power 'to the street', but it also needs more power to achieve similar performance to the F1.

factory Overboost features are usually not active across the whole powerband but rather around the toque peak..

my point exactly... Also, the Nissan GTR shows how fast you can accelerate by using superior grip and seamless power transfer of a twin clutch box..

Thank you all the comments.

I would agree that the F1may be traction limited above 60mph but I would still question whetehr it is still spinning the wheels at 90mph+

However Autocar have just published a road test of the new Noble M600 here

http://www.autocar.co.uk/CarReviews/RoadTe...-4.4-V8/243971/

It is an interesting test in its own right but it also supplies some more data on the F1 vs 911 turbo discussion above.

Firstly the Noble has a 4.4 litre turbo engine of 650 bhp ( versus 627 bhp for the F1) and it weighed 1305kg (kerb) versus 1127kg for the F1. So its power to weight ratio is slightly less than the F1.

It did 0 -60 in 3.5 seconds and 0 - 100 mph in 6.8 seconds ( versus 6.3 for the F1). So 60 - 100 is 3.3 seconds versus 3.1 forthe F1. That means that its 60 -100 mph time is almost exactly in ratio to the laden power to weght ratios ( 11% lower P/W versus 6.5% slower time). So given the data is only to the nearest tenth the fit is spot on.

Hoever we now have two cars whose 60m - 100 times are seriously out of line with their power to weight ratio versus the 911 claims so with three data points ( and asuming Porsche are not over claiming what is the conclusion.

Now the Noble has a turbo like the 911 but NOT four wheel drive. So I come to the same suspicion as I had when I first askerd the question - 4WD is more important up to 100mph for supercars then power to weight ratio. Obviously it is more important fom 0 to 60 mph but in the real world nobody spends much time doing 0 -60: they do however , if they drive a supercar fast frequently do 60 to 100 hence my obsession with that time.

I understand all the formulae and Lapsims etc. but I have been trying to use real world observed data to draw any conclusions.

Two other thoughts , firstly it is progress that a tiny company with 20 staf can build a car to match the mighty McLaren F1 on performance., . Secondly just to re- confuse everything see this video

http://www.youtube.com/user/autocar?blend=.../43/vD9AW-EB3vo

The GT2 has 523 bhp and weighs less than the new Turbo, so it has a better power to weight ratio but has the virtually same 60 to 100 time 3.7 secons versus 3.6). It can also answer the 0 - 60 question as it does not have 4WD which costs 0.3 seconds, again assuming all Porsche's data is accurate.

Clearly neither is traction limited above 60. The fact that the Turbo matches the GT2 above 60 suggests the actual power of the Turbo might be higher than claimed - perhaps benrapp was right about the test car having the sport chrono pack.

is the turbo in question a PDK equipped car? as that might be a factor too..

also, I'll be driving the new turbo in about a week's time so if you have any questions for porsche engineers fire away, I'll try to get some answers..

Why ignore tires? If the Mclaren's times are from 90s and the Porsche Turbo's from 2009, the improvements made in tires (and difference in tire sizes) will make quite a difference. Mclaren F1 came with bespoke Goodyears OEM, IIRC. Do they even make tires in that particular size anymore? Have most F1 owners upgraded/modified their car to accomodate today's enormous tires?

I'll add that the first time I drove on a track - a long time ago in a galaxy far far way - I was amazed by how much power to weight mattered and acceleration from a standing start did not. Although this may seem obvious to this crowd, I continue to be amused by folks who brag about 0-60 or 0-100 until their monster's four wheel drive advantage from standstill - for example - turns out to be a wee bit of an anchor on a track at speed.

0-60 IMHO is important if one lives between stop lights...

Yeah sure why not. Ask them whether it is true that one in six 996 (and probably 997) Carreras have had engine issues? Is the actual figure higher? What causes these problems in the engine, and why weren't these issues discovered during pre-production testing and rectified at that stage? How do they reconcile their ineptitude at building a reliable motor and a job title that says 'engineer'?

Also, do they secretly wish that they didn't have to build a nice handling car around a fundamentally flawed concept? Do they think their jobs would be easier when the engine is placed in the middle and not hanging somewhere out there at the back? Could they engineer a car with similar (if not better) performance than a 997 for less cost and/or with less engineering efforts if the engine was in the middle and not at the rear?

Finally, given the (hypothetical) choice, where would they prefer to work: a Porsche F1 program versus a 'normal' job of engineering Porsche road cars? Why?

it all depends, in my evo, which drags along the 4x4 driveline, I am handicapped on our track as it is way too fast for it to have any signifficant effect... but that is in the dry.. in the wet it is a whole different ballgame..
but, I agree that 0-60 is unimportant.. as you will likely destroy something in the driveline if you do it often... a real street start is a better indicator, like 5-60 or 5-100 mph...

1. I think that the crank seal issue affected only the 996..
2. Cayman S
3. I'd have to get them drunk first, this will be a press launch and they are usually too PC to talk openly... although the guys from ford team RS were really open at the focus RS launch..

Jesus, why does everyone have to be a ****?

I put disclaimers in because I knew that this was coming. Look, it was a back-of-the-envelope reality check. If you can't reconcile it with reality, go **** yourself.

I love a bit of righteous indignation!

Does accelerating from 0-100 and extracting from that a 60-100 time give a different result from cruising at 60 and flooring it in the appropriate gear and timing it to 100? My gut feeling is that there would be a difference, the second method possibly giving a slower time. But that's my guts for you...

oops! I forgot about driving in wet! While I was being certified by BMW a number of years back, I drove with a fellow who ran a very nice EVO...not stock. But I did do my mini proud...he was pleasantly surprised by its abilities. I cannot repeat his exchange while entering into the first turn, but he did try to apply the invisible brake while in the passenger seat. He did speak about the EVO's wet ability, I simply forgot. Nice rig!!!

That's basically my point in my post above. But I think this has to be taken on a car to car basis. If we generically analyze four wheel drive, from 0mph I think there is an advantage...from 60mph the advantage is less and from 100pnh even less. And this is striaghtline...four wheel drive has advantages elsewhere in a track environment - I'm guessing - but do they also go thru tires faster??? A high performance day at the track is also different from a full blown race where tire durability is a factor.

I just found my experiences amusing...the mini is often referred to as cute...although 218hp moving 2,500 lbs isn't genuinely quick, the 'cute' name doesn't ususally stay attached for long.

Its also worth bearing in mind when comparing the 991 turbo with the McLaren F1, that the cars were geared for different top speeds - 190(ish) mph (porsche) and 240(ish) mph McL F1. (Sorry I don't have the actual figures!).

I'm not saying that's the only reason - as the comments and opinions above testify - but it certainly helps the 911 achieve comparible 60-100mph acceleration times despite its apparent weight and power disadvantage.

drove a JCW mini, on slicks and with quaife diff and big brakes on track recently (stock suspension.. ) Boy was it FUN!, huge grip, massive pace..

In the case of the Porsche (or any turbo) - Yes. Did you watch the video above (RS2 v Corvette)?

No Sir, sorry Sir. I'll watch it tomorrow, it's my bedtime.

The top speed the cars are geared for is pretty much irrelevant. Both would have close enough ratios too keep them within say 10% of peak power at all times between 60 and 100.

I think you'll find McGuire is spot on with this one - its a case of traction limits.

0-60mph (96km/h) in 4s!

http://www.youtube.com/watch?v=dRn38s5wClY

Within 10% of peak power? Only 60 odd horsepower difference then? I'm not disputing that traction is a factor in this, but you can't ignore the effects that a set of gear ratios 30% longer than the 911 has on acceleration from 60-100mph.

Sure you can, the final drive ratio is irrelevant unless you are in 1st or top gear. The driver will choose the overall ratio best suited to the speed the car is at.

Don't engine characterisitcs play a role here too? Again, I have to refer to my humble experiences driving high production type cars. My mini 2,500 lbs - 218 hp and 193 lb ft torque...GTI 3,100lbs - 200hp 207 bl ft torque. Gearing aside, these two are very close in any battle. The GTI's torque curve is much more robust for much longer...so despite what might appear to be a clean kill on paper is obviously quite different in reality.

I don't have to power bands for either the F1 or the Porsche...

I seem to recall that, from the time Audi ran four wheel drive in super touring cars, that their four wheel driven cars were faster out of slow speed turns, had a slightly lower top speed on long straights, were more stable during braking and were easier on the tires than the two wheel driven competition.

Inspired! You may have to explain that one for me Gruntguru.

The last road car manufacturer I worked for used to spend quite a lot of time optimising the combination of gear ratio, final drive and tyre size to ensure that the best possible performance figures could be achieved. I'll have to let them know it was a complete waste of time because apparently it is irrelevant.

Have you ever bothered to work out what the actual force at the contact patch is from a given flywheel torque output?

Regardless, your point about traction is still relevant, I'm just amazed you believe the gear ratios mean nothing except 1st and top.

heh....now, where's my popcorn...

Speed Vision Challenge? My memory, which is not great lately, is that thier tires fell off toward the end of a race...track specific of course. But OMG, even if they qualified thre or four rows back, on occasion they would beat all to the first corner. Re the tires; the Audi is really brutal on its front tires...and brakes. I have an RS6 story but its too long and involves a public road.

That was more of a issue with that car, weight, tire combo. When the Audi's ran Trans-Am, they always came on at the end of the race when everyone else had burned their rears off.

You're only talking about up to maybe 3rd gear, but probably only second. Bigger gear splits towards the top of the gearbox would not make a difference. Having said that, there might be considerable difference at the bottom of the box, I have no idea.

We seem to think that from 60-100 mph, at least the McLaren is still traction limited, which may or may not be true. If it is the case, then gearing really won't be that important.

The 3.1 seconds from 60-100 gives an average acceleration of a tick over a 1/2g which should be able to stay hooked up. Now, average acceleration may not be all that important. If the acceleration at 60 may be about 0.6, then it might be an issue. More than likely, the tricky piece would be the shift from first to second. A ham-fisted driver could easily light up the rears.

When did they run TA? Which car...curiously missed that stuff.