BMW turbo street car Drag Racing 101 Good Info [Archive] - Bimmerfest - BMW Forums

: BMW turbo street car Drag Racing 101 Good Info

05-22-2007, 12:13 PM
This was written on another forum, by (MR.BLONDE aka KENNY). I am just passing the info for all you FI enthusiasts! I hope the info comes in handy. :thumbup: --------------------------------------------------------------------------------

Kenny's ideas on drag racing a BMW turbo street car.

I've assumed the following:

+ BMW turbo street car (not street/strip cars)
+ Street radial tyres with street rims (no front runners, etc)
+ Pump fuel
+ Manual transmission

Setup For Drag Racing - Weight Reduction
First setup you car before going to the strip.

Your first task is weight reduction. Ensure you get to the strip with 1/8th tank of gas. That means allowing enough gas to get there with 1/8th left. Better to arrive with too little gas and then topping it up a tad from your gas can, which is of course filled with the same unleaded gas you have in your tank. A properly mixed slug of octane booster (or unleaded race gas) will give you some peace of mind too. Remember to put the booster in first to allow the gas to mix with it properly in the tank.

Every quart of fuel weighs 1.5 lbs.

Next drain all the water out of your washer botle. My M Coupe takes 5 litres in the washer bottle which is 11 lbs you don't need.

Then remove the spare tyre, toolkits and anything else from your trunk and from inside your car. Anything that can be eaily removed should go (CD Stackers, carpets, lose stuff, etc).

Now decide if you want to go to the trouble of removing your rear seats and passenger seat. Believe me when I tell you it will make a big difference. On average they say every 100 lbs is a tenth (0.1 off your ET).

Setup At The Strip - Tire Pressures
Now you've driven to the strip and been through scrutineering, get out your trusty pass journal and tire pressure gauge.

Ensure you've got 1/8th tank of gas - you don't want to starve for gas during a pass. Now hook up your compressor and increase your front tires to 55 psi. If you don't have access to a compressor then swing by the closest place to your drag strip and do this before you arrive at the strip.

You increase the pressure in your front tires to minimise rolling resistance. If you happened to have an alternate set of narrower wheels/tires then you'd fit the narrowest practical set for race day.

The rear tire pressure is more involved, because you've got to try a range of pressures and see what results in the best 60' times. The optimum setting changes as the weather conditions change, so this is a bit of a black art. That's what your Pass Journal is all about. This is where you record your car setup and the results after each run. That way you can make changes one at a time and make some sort of analysis in order to optimise your setup.

Generally speaking you should drop your rear tire pressure 4 psi from standard and then work your way down by half a psi at a time until your 60' times get worse.

Remember that fine adjustments on street radials is less of a factor than when you graduate to drag radials or slicks. However it is still well worth your while to get close to the optimium.

Your Pass Journal will have an entry for every pass you do, including the following information as a minimum:

+ Tire pressure
+ Launch RPM
+ 60' result
+ My comments

More on how to use your Pass Journal seek the optimum setup in a later post.

The Burnout - How Long?
If you are a beginner or feel a bit nervous about the upcoming pass, then simply drive around or through the water box and don't worry about a burnout. You can add the burnout to your later passes as you become more comfortable.

Remember that street cars using street radials are very different to a Top Doorslammer. You're not going to do a half track burnout and wow the crowd. Street cars that do long pointless burnouts are basically wasting everyone's time. Don't be one of them!

Now street radials like a touch of heat in them to get optimum traction. But too much heat and their traction is reduced. However if you don't spin the tires at all then you've still got some water and grit on your tires from the water box (unless you managed to drive around it).

My advice for street radials is to drive through the water box and come to a halt. When signalled by the official, dial up the revs and drop the clutch. Wait until your tires smoke a tad and then button off. We're talking less than a 2 second burnout here. More than that and you risk cooking the rubber (ie reducing traction until they cool).

Realise that drag radials and slicks are made totally differently to street radials. These special tires require a lot more heat to work. Street radials don't.

The Burnout - How To Do One?
Now you know how long to let the burnout go for, exactly how do you perform a decent burnout?

Let's start from standstill, waiting for the official to motion you to start your burnout. Select the appropriate gear (1st, 2nd or 3rd depending on your horsepower, the higher the gear the more power required to turn the tyres over). When the official signals to start your burnout, dial in some revs and drop the clutch. You can dance your left foot over to touch the brake a tad and balance the revs via throttle with forward movement via brake (takes a bit of practice).

Now if you continue to creep forward and start to contact your tires with dry sticky track then you're obviously going to put more strain on your driveline.

When you've got some heat into the tires, just let off the brake and /or drop revs a tad and your car will creep forward. Get off the gas and allow the car to roll gently towards the staging lights.

How To Pre-Stage
You're rolling gently towards the staging lights. Keep your eyes on the Christmas Tree and look for the White Pre-Stage light to blink on. The instant you see this light come on you nail the brakes and come to a total halt. You're not yet fully staged - just pre-staged. The second Full Stage light is still unlit.

I recommend you now wait until your opponent is at least Pre-Staged or even on Full Stage before you bump in to Full Stage. Once either competitor is on Full Stage, the other racer has only 20 seconds to fully stage or the starter may disqualify you. In practice at a street meet the starter will rarely disqualify you, but they may come over and bang on your side window to hint that you're holding things up!

Once you are pre-staged you must now focus on that Full Stage light with all your concentration. Because as soon as you fully stage, the starter can trigger the Christmas Tree at any time.

How To Bump On To Full Stage
You're at a halt with the Pre-Stage light up and your competitor is fully staged.

Now apply your ebrake gently and dial up your pre-determined launch RPMs. You've already thought about what RPM you're going to launch at before you lined up. If this is your first pass, then just choose whatever starting point you think appropriate.

Your clutch is in, your RPMs are steady at your launch point and your ebrake is slightly on. Now you ever so gently tease your clutch out - looking for the friction point where the RPMs start to dip showing you the clutch is dragging your engine just slightly.

With your hand you are holding the ebrake just slightly on. This is a delicate balancing act, ensuring your engine is at launch RPMs, your car rolls forward ever so slightly and your rear brakes are just slightly on to ensure you move very very slowly on to Full Stage.

This is called bumping on to Full Stage and is not easy to do. If you roll forward too far you will Redlight by crossing over the start beam. No big deal, it's all practice and you'll do better next time.

Once you bump on to Full Stage you pull your ebrake up firm so your car comes to a dead stop. But you don't allow the ebrake to ratchet on lock, you keep it on but not locked by pressing the button as you hold it.

If you get it right you will find yourself fully staged, motionless on the start line and most importantly, you are solidly locked on your launch RPMs with your clutch just at friction point, dragging against your ebrake.

You are not looking at your tacho to check your RPMs. You are not looking at your ebrake. You are not looking at anything other than the Christmas tree.

The Launch
It's been covered in detail in many other places so I'll just state it here. Because of human reaction time and because of the vehicle reaction time of your car's suspension and drivetrain, you do not launch when you see the Green light on the Christmas Tree. You launch the instant you see the last of the three Amber lights come on.

Option One
You're at steady launch RPMs with the clutch at friction point and the ebrake held on but not locked. As you see the last Amber come on, you release the ebrake fully (holding the button in) and flatten the gas while modulating your drivetrain using your clutch. Because you're launching a turbo car you flatten the gas. On an NA car you wouldn't do that.

Option Two
You've got your ebrake held on but not locked. You pulse your gas pedal so your RPMS peak at redline and drop slightly but stay in the power band of your engine (revving your engine quite hard). As you see the last Amber come on, you release the ebrake fully (holding the button in) and flatten the gas while modulating your drivetrain using your clutch. This is your attempt to build some boost off the line. Negatives are it's harder to get a consistent launch. Up to you - maybe try it both ways and see what you prefer.

Whichever option you choose this is not easy to do and takes many repetitions to get a good result. This is where it's all at - your reaction time versus your opponent and how well you can launch your car without wheelspinning.

You must slip your clutch mercilessly to get an optimum launch. Sometimes you may find you have not let the clutch out fully before it is time to change up a gear. This is OK.

Wheelspin on launch or upshift will cost you precious time. The optimum launch is at 10% wheelspin. This is very difficult to achieve and again comes only with much practice and experience. It's all good!

Yes, you will punish your clutch. Yes, it will give off that awful smell of tortured components. Yes - it is the only way to optimise your launch.

Clutches are consumables and are meant to be replaced. Regular drag racing will severely shorten the life of your clutch, but so what? You replace the clutch and go on your way. Just like tyres.


Getting A Better Time
Your reaction time (RT) is all about how quickly you react on the start line. It does not factor into your ET at all. However the winner of each race is very much affected by the RT. So once you are comfortable with the process of doing the burnout, launching and shifting up through the gears I recommend you treat each pass like it's a race. The only way to get better is to practice like you race.

Drag racing is all in the launch. Every tenth (0.1 second) reduction in your 60' time is .15 to .25 reduction in your ET.

You can beat an opponent with a much more powerful car by getting a sharp RT and by getting a sweet launch.

Using Your Pass Journal
After each pass I recommend you drive back to the pits after collecting your timeslip. Get out of your car and get your Pass Journal out.

Now note down the vital pieces of information about the pass:

+ Launch RPMs: (eg 4500 RPM)
+ Tire Pressure: (eg 28 psi)
+ 60' time: (eg 2.150 seconds)
+ My comments: (eg Car bogged down, need RPMs)

Sometimes you can easily tell what you did wrong - too few RPMs on launch and you bog down. Too many RPMs and you get wheelspin or wheel hop.

Sometimes you have to use your 60' times to help you work out what's going on.

The important thing to remember is to only change one thing at a time. Otherwise you have no idea which of the changes you made created the different result, not to mention that the combination of changes hopelessly confuses things.

I recommend you start with your launch RPMs and keep your tire pressure at your standard setting (ie what you run on the street).

The rule of thumb is that you want to launch at your peak torque figure. Now this is not so easy to find, because as you launch your engine and drivetrain slow as the load comes on. So you have to launch at an RPM level higher than your engine's peak torque, so that as the load comes on then you are at your peak torque.

In practice, pick a fairly low launch RPM .. say 2000 RPM, and then bump it up 500 RPM at a time. Keep doing this until you get hopeless wheelspin on the launch. Then drop the launch RPM down by 250 RPM and try again. You're doing a sort of chop sort by way of hitting on the optimum launch RPMs.

And how are you evaluating what is a good launch? By examining your 60' times after each pass. You soon note that wheelspin or wheel hop means a ****ty 60' time and that bogging down results in a ****ty 60' time, too. But sometimes it all goes right and you get a stormer!

Before too long you'll have a good idea of the RPM band your car best likes on that day. Remember that different weather conditions will change what your car wants, but it will only change so far.

Let's say you finally settle on 3750 RPM as the best launch, the one at which you consistently get the best 60' times. Now it's time to fiddle with the tire pressures. So you dial in every pass at 3750 RPM and then you adjust your rear tire pressure down by 0.5 psi each pass until you again notice your 60' times get worse.

You should notice that the 60' times improve for a while and then get worse again. In any circumstance, you don't want to go much below say 10-12 psi less than recommended; that can be dangerous.

Once you've done this analysis you will have an optimum launch RPM and an optimum tire pressure for that day.

Now you will find that the optimum tire pressure will allow you to launch at a higher RPM! So you start the trial and error analysis all over again. Of course, for each pass you pre-determine what the launch RPMs will be so there's no thinking on the start line. And you record all your results in your pass journal when you get back. A pass journal may sound fancy but I use the back of a scrap of paper usually (see attachement image in post above) - whatever you prefer.

Also don't forget to factor in your improvement as a racer. Your first passes will be crappy compared to your most recently passes as you get the hang of it all.

It's all good fun! And it doesn't matter if you've got the most powerful car in the world or a stocker, the thrill of drag racing is there for everyone.

A good racer can beat an opponent with much less hardware at his or her disposal using the techniques listed above. And it doesn't matter if you win by an inch or a mile ... winning is winning ;-)

Thanx to (MR. BLONDE aka KENNY) for doing the reaserch, and writing this!!!!:thumbup:

03-23-2010, 05:12 PM
Bump..Some good info here.

03-26-2010, 07:17 AM

Excellent write up