BMW M5 Forum and M6 Forums banner
1 - 18 of 18 Posts

·
Registered
Joined
·
114 Posts
Discussion Starter · #1 ·
I'd be curious to know how do you perform to do times under 6 sec for 0-60 mph lauches. The big difficulty is to start the car properly, and I'm not sure to know how to.

Wheelspin is big, and when you try to avoid it your launch is not enough fast.

I've read that to perform good 0-60 mph with the new Audi RS4 for example, they had to run to 6500 rpm first and then let the car start like a drag !

What is the optimum rpm level to reach before start moving the car for best acceleration ??
 

·
Moderator Emeritus
Joined
·
10,698 Posts
Im sure thats very healthy for the audi's clutch!


Im curious also about this. from reading others comments it seems like you should rev it to around 1200-1500 rpm and let it go. The engine makes a very sweet sound from around 4k rpm on with nearly full throttle.

ME
 

·
Registered
Joined
·
5,239 Posts
totally different animal than a turbocharged, small-displacement engine. There is so much torque you don't need high revs, and wheelspin will slow you down.

Best possible recipe:

0) Pick a cold day.
1) Get as far below sea level - like Death Valley - as you can.
2) Remove all excess weight from car.
3) Run it down to one gallon of gas.
4) DSC off, sport mode on
5) Turn the AC off!!
6) Turn all possible electrical accessories off (and make sure your battery is fully charged.)

GO!

7) About 1600 RPM to start, no more - or you'll just smoke the tires. A LITTLE squeak is OK.
8) Shift fast, no higher than 6800 RPM (first, the tach won't keep up with the engine's speed and you'll hit the rev limiter, and second, someone once calculated/posted the optimum shift points and 1st was 6800, the rest of the gears were about 6500.)

I'm sure there's an optimum tire pressure too, but I have no idea what it is.
Lemme know how you do....
 

·
Registered
Joined
·
2,009 Posts
Drink five beer and sidestep the clutch at about 6500 RPM.

KIDDING...kidding...

Alps
 

·
Registered
Joined
·
114 Posts
Discussion Starter · #5 ·
I remind you that the RS4 is a FWD, so start at 6500 rpm is possible without having a large wheelspin, and of course with the M5 even a start at 2000 rpm is not possible w/o wheelspin.

About sea level, do you know how it changes the car power. I do my tests frequently at 900 m ASL, that's around 3000 feet ASL ; is that many % of power I lose compared to a test at 100 ASL ??
 

·
Registered
Joined
·
5,239 Posts
Originally posted by Tom:
<snip>
About sea level, do you know how it changes the car power.<snip> ??
Many moons ago I used to be a marine mechanic, and I went through one of the major manufacturer's service schools two years in a row. Since I worked at 6200 feet we had lots of questions about effects of altitude. The rule of thumb was that you lose about 5% of your power per thousand feet of altitude. This was assuming proper carburation. I'm really not sure if this number is accurate but based on my experience it is pretty close.

Note that the altitude the car sees isn't the actual distance above seal level - it also takes into account temperature and humidity. Pilots refer to the "effective" altitude as "density altitude". On a hot day at Tahoe (6200 feet) density altitude could be as high as 9000.

/G
 

·
Registered
Joined
·
114 Posts
Discussion Starter · #8 ·
Thanks for the info !

So, at 3000 ft ASL, we can say that on average, with regular air pressure decrease from 0 ft to X ft, I lose ~15% of power !!

That's 340 HP instead of 400 HP ! Or 0-125 mph times of 19.5 sec instead of 17.8 sec !! That's what I thought it should be.

I have to do proper tests at low altitude levels (hard to find here in Switzerland, where the lowest points are higher than 1000 ft ASL.

Perfect meteorological conditions are : very high air pressure (H zone), low humidity (?) and low air temperature.
 

·
Registered
Joined
·
90 Posts
The loss in power is obviously due to the density (or lack of) the air entering the car. All other things being equal, you would definitely get less power with less dense air. I would think that you would also naturally run richer in the engine. What I wonder is that with advanced engine management in modern cars it can somehow compensate for this or not (all things do not have to be equal from valve timing down the line)? I would think that if it were possible that it would be done. Any thoughts?

------------------
 

·
Registered
Joined
·
255 Posts
Since power is proportional to how much fuel and air you get into the cylinders, not much can be done to combat the lack of air density at higher altitudes in naturally aspirated motors such as the M5's. The mapping of the fuel injection will compensate for the lower air density at higher altitude by reducing the amount of fuel delivered to each cylinder in order to maintain the correct air to fuel ratio based on conditions such as load, throttle opening, temp, etc. Less air means less fuel means less horsepower. The only way to get around this is by forcing more air into the cylinders by supercharging, turbocharging or maybe by adding nitrous oxide with additional fuel.

Regards,
Doug
 

·
Registered
Joined
·
5,239 Posts
Doug - well said.

A related altitude anecdote from the boat side - say you have a 100hp outboard motor. Makes 100hp at 5000 RPM. Let's also assume for simplicity sake that the engine's recommended max RPM is 5000 and it makes its most power there. (This is typical for the older 2-stroke outboards I used to work on.) SO: You choose a propeller that allows the engine to go 5000 RPM, wide open.

Now, you bring the boat to Tahoe - 6000 feet, 30% power hit (more, because we didn't re-jet the carb to ensure best possible mixture) - now you have, best case, 70hp at 5000 RPM. But with less power all up and down the band, the engine will never get to 5000 RPM, because the prop was designed to absorb 100hp at that speed. So the engine only runs 3800 RPM, makes pitiful power (probably on 50hp at this RPM) and the boat performance sucks badly. Double whammy.

There is a partial remedy - you put on a prop of lower pitch, allowing the engine to get back to the 5000 RPM it is happy at - now you are back to 70Hp. But as Doug points out, without forced air induction (turbo or supercharger) there is no way to get the rest of the power back.

Fortunately with manual transmissions we are in control of the gear ratio and therefore the RPM by picking a gear - so we don't have to re-prop. (a boat has a gear ratio too - effectively set by the prop pitch and diameter - but it only has a one-speed transmission)

If this is too far off topic let me know. I always find this kind of stuff interesting. (For me, ANYTHING with a motor in it...
)
 

·
Registered
Joined
·
114 Posts
Discussion Starter · #12 ·
I'd be curious to know if turbocharged engines are then less sensitive to air pression variations, as maybe the turbocharger can better compensate ?
 

·
Registered
Joined
·
5,239 Posts
Yes, and no.

A turbocharger can ram more air in, so you can have more properly mixed gas/air to increase power until the engine explodes. It is perfectly reasonable to get sea level power at altitude via forced air induction - turbocharged airplane engines deliver sea-level power at 10,000 feet.

BUT there is a tradeoff in autmotive applications where, unlike airplanes, acceleration matters.

In order to make sea-level boost at altitude, you need a bigger turbocharger than you would need simply to make max designed power at sea-level. The bigger the turbo, the greater the turbo lag, and therefore the worse the acceleration.


My '82 Turbo RX7 demonstrated this really well. It was a FAST car at sea level - enough power to begin accelerating and get the twin-scroll turbo spooled up pretty fast. But on a hot day at Tahoe, unless you were willing to rev way up and sacrifice the clutch to get a good start, you could read a book waiting to get from 1500 RPM in 1st up to 3500 or 4000 where the turbo first became effective.

On the other hand, the stuff I hear about chipping the Audi S4 suggests that it has lots of excess boost to spare. So who knows.

BTW, Superchargers are mechanically driven - no "lag" - but you still wouldn't design one capable of a lot more boost than you needed - you'd just waste power turning the thing.

Cheers
 

·
Registered
Joined
·
226 Posts
Greg do you side step clutch? Let it out slow or fast? Heard clutches are weak. Also
when you grab second are you at 6800rpm or coming up to it? Lift throttle? What is approx speed at 1-2 shift point? And since engine has so much torque does it really matter how many revs at start and shift points? Other user input welcome. Jet Black/Black got on the boat July 1 projecting
arrival July 10 in USA 1 week to Indy. Can't
wait. Had to sell all my toys to justify spending 70k, hope its worth it.

GO!

7) About 1600 RPM to start, no more - or you'll just smoke the tires. A LITTLE squeak is OK.
8) Shift fast, no higher than 6800 RPM (first, the tach won't keep up with the engine's speed and you'll hit the rev limiter, and second, someone once calculated/posted the optimum shift points and 1st was 6800, the rest of the gears were about 6500.)

I'm sure there's an optimum tire pressure too, but I have no idea what it is.
Lemme know how you do....

[/B][/QUOTE]
 

·
Registered
Joined
·
5,239 Posts
Dick - before you get the wrong idea let me make it clear that I have not gone out and done timed zero-to-sixty tests - I can't tell you the best clutch technique. Some of the review articles described their preferred launch technique, though. I have the March '00 Motor Trand, C&D and R&T articles - I'll look to see what they said.

RPM DOES matter, because too much wheel spin is bad. I let the clutch out fairly fast, and the roll the throttle on fairly fast - maybe a half-second - but not instantaneous. This gives a launch with a little wheelspin. Its possible sidestepping would be faster but "not on my car" - I can't bring myself to beat it up like that.

Grab second by 6800 on the tach - yes, I lift - but because I refuse to beat the car up. You might get a better time if you didn't, and presumably the rev limited would protect the engine. You might also, however break the wheels free if this aggressive.
 

·
Registered
Joined
·
226 Posts
Originally posted by greg:
Dick - before you get the wrong idea let me make it clear that I have not gone out and done timed zero-to-sixty tests - I can't tell you the best clutch technique.

Thanks for the info. I agree I would never
want to abuse such an awesome piece of equipment. However, the car is made to cook, and I think it's important to know how to maximize it's ability without abuse.
My biggest fear is that I would come up against some of these bad boy mustangs in our area and because I didn't rev/clutch etc
properly or worse banged into the rev limiter in first gear this guy would think he
took out BMW's Most Incredible Driving Machine. I have been driving fast but smooth
for years and seldom lose an "out of the hole
drag race, but the day I first drove a 540I 6spd I felt like a jerk every time I shifted.
I own several bad boy street racers including a 67 Shelby GT500 Cobra Jet 428cu
4 speed 389 rear gear. And used to say "what the hell is a rev limter and why".
(Just an old guy I guess) I dont have my M5 yet and promise to take very good care of it, break it in correctly etc. but at some point in time I WILL try to kick a vette or late model cobra Mustang and would love to have Audi S4 in my road kill bag. Tx.
 

·
Registered
Joined
·
257 Posts
Tom,

I am not a drag racer, but know quite a few people who like that sort of thing. The first 20m will be the most important. In any case, your question is a bit too general. The weather, road condition, and tire pressure will affect performance. You would actually have to experiment with dropping tire pressure to get more rubber on the road. Then you would have to experiment with a combination of throttle and clutch position (they aren't on/off switches). Last, you need a method to time you run, maybe a G-Tech.

Many AWD cars do well in 0-60, as it is easy (although expensive) to drop the clutch at 6500 revs and let the electronics work out the rest... but this is only a temporary advantage.

Try a bit of clutch slip, maybe that will help.

Dan
 

·
Registered
Joined
·
5,239 Posts
I said I would review the articles to see if they described their launch technique. Well, I have all of them at home, where I constantly forget to do this, but I have the Motor Trend article here at work.

No comments on launch technique, but since it has been a while, I'll remind y'all that this test pitted the M5 against an Aston Martin Vantage, a Jaguar XKR, Mercedes E55 and Porsche Carrera.

The M5 had the fastest acceleration in 0-60, 0-100, quarter mile and full-mile tests (33.4 seconds @ 148.5mph)

..."As we rotated through the cars, one by one, our drivers came to the conslusion that, for long stints of extreme high-speed driving, BMW's M5 is the standout here. It rails down even lumpy, weathered asphalt like it was paved yesterday, tracking perfectly straight, with a feeling of complete security. Obviously, BMW's engineers have tuned the aerodynamics of this sedan with precision. Press the throttle with impunity, and this bad-boy Bimmer whips *** on every other car in this test in sheer accelerative force, even in our demanding one mile test."
 
1 - 18 of 18 Posts
Top