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Old 08-15-2010, 08:22 PM   #1
Soulofdarkness
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Default Launch Control for GC Impreza?

Hello all! I'm looking to set up a launch control system in my 99 2.5RS for Rally-X. All I have found so far is Bee*R but I don't know I want to go the route of flames out the exhaust pipe !

Anyone have any other ideas of systems I can use?

Thanks!
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Old 08-15-2010, 09:51 PM   #2
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I don't know of any LC systems that work with stock ECU's (other than the Bee*R). Flames out the exhaust is just a visual thing....you're going to attract alot of attention when using it already.....who cares?

Standalone ECU's will almost always have this function, and usually integrates flat-foot-shifting as well. A standalone will also allow you to wring more power out of that motor than any piggyback will. They're completely worth the cost and time to install.

Jay
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Old 08-15-2010, 10:29 PM   #3
Soulofdarkness
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Looks like I can't use a stand alone EMS in my Rally-X class (PA)... So that takes any of those out of the question... Looks like the Bee*R is my only option.
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Old 08-16-2010, 12:36 AM   #4
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Just find the perfect launch RPM, feet can be just as effective
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Old 08-16-2010, 12:55 AM   #5
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Old 08-16-2010, 11:59 AM   #6
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It's not really needed. I think launch control mainly exists for cars that spin up the tires way too easy, like 500hp rwd cars. Launch control is more like ABS for the throttle, and a normal traction control system would include this to some degree. For an awd car, launch control is much less of an issue. We can put a lot of power down to the ground really easy, and the car will just shoot off.

I think you might better invest time in getting used to slipping the clutch a little.

My launch control:
1) Sit clutch in at idle till I'm waved to start my lap.
2) Keep clutch depressed, and gun the throttle.
3) Wait for revs to climb towards 5k-6kpm.
4) Still holding gas pedal at the floor, let out clutch partially to start applying power to the drivetrain, before you ever hit the rev limiter (no reason to do so).
5) Modulate clutch pedal as needed to keep the engine up in the power band during take off. Mainly you're preventing the revs from dropping and bogging the car on take off. Slipping too much is a waste of time also. You're basically modulating rpm via the clutch pedal instead of the throttle.
6) You should be fully engaged into 1st hopefully somewhere between 4k-5k and where you're making peak hp. If you're low, you're most likely bogging some and you can afford to slip the clutch a little more. If you're high, you can let out the clutch a little quicker. It takes time to get used to how you let out the clutch and keep the engine rpm where you want it.
7) Once fully engaged, let the engine wind out and then toss it into 2nd gear. You'll be in 2nd gear for most of the course or back to 1st on one or two very tight corners. Heel-toe is useful to get back down to 1st. It is important to wait and let the revs get towards 5k or so to let you easily get into 1st gear. That extra wait time is the difference between having it go into 1st nicely or into 1st with a lot of grinding. It's a very low gear, lower than you think, and you pretty much need to redline the engine to have it fall in at speed.

I'll note that I have around 150 launches this way for auto-x and rally-x over the last several years. My Forester has 100k miles on it with the stock clutch still. These car's don't make a lot of power. You're very hard pressed with spin tires without shock loading the drive train which isn't healthy for anything. Even the turboed versions have a tough time putting down enough power to necessitate any outside device controlling power delivery. You'll break the drivetrain before you really make enough power to necessitate any form of launch control, and that's already 300-350 ft-lbs. for a 5mt and darn close to chewing gears and snapping drive axles if you're not careful, not to mention already requiring a beefier clutch. Even at this level, you are not lighting up all 4 tires. If you are spinning excessively, you're probably using the wrong tires to be competitive anyways. I just don't see a need for electronic control outside of needing to swap to a 6mt because you're making too much power for the 5mt to support. I would like to think that 300hp is the minimum you might start looking at some form of launch assistance. Even then, why? What would it do that you can not? I understand a traction control system that uses the ABS system to control wheel speed, but again you might first op for better tires for the surface and upgrading the open and mild viscous diffs to some beefier limited slip or locking form to get power to all the tires better.
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Old 08-16-2010, 12:38 PM   #7
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A 2 step rev limiter is not launch control. I've mistakenly described it as such as well. What we've been discussing is just a simple 2 step limiter. As BRR mentions, launch control covers more than just the rpm at which the motor is limited to when taking off. It also covers how quickly the motor or how quickly the wheels are allowed to accellerate....up to the normal rev limit.

I use LinkECU's version on my autocross Impreza. It's great for a consistant launch rpm....but depending on traction available, it can and will frag 1st gear or make just enough power to give me my 10% wheelspin without bogging.

I find it's alot easier to "dump" the clutch when your right foot is already planted, as opposed to holding the launch rpm with your foot at partial throttle.

Use it at your own risk....that's for sure. We're on tranny #3 this year......


Jay Storm
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Old 08-16-2010, 10:51 PM   #8
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But what power are you putting down? And how much slipping of the clutch are you doing? Is the clutch even stock?

I've driven my NA Forester for quite a few years. My bro has a FXT that I've also used on a number of auto-x and rally-x events on a couple different turbos. I find it hard to wreck these trannies without blatantly shock loading the drivetrain, especially with stock clutches. Now aftermarket clutches can be a bit more tricky to slip nicely repeatedly.

Dumping the clutch is never a nice thing. You're very likely to bog most of the time and fatigue the whole drivetrain excessively. It should always be a controlled slip and a very smooth delivery of power. If done right, it's no more harmful than stepping on the gas pedal in gear but you're always putting down peak power doing so, basically a CVT if done right.

Example of me launching. At the start gas to the floor and I don't let up. I wait for the engine to wind up and let the clutch grab. I hold the clutch at a point and let it partially slip for a couple seconds until the car speed matches the revs. The hold point is learned and where it's needed to put the engine rpm where you want it despite being at WOT. You can almost count to 2 where the rpms just hold as the car accelerates up to the held rpm. I don't let out the clutch fully till the car's up to speed of the motor.

The whole goal of launching is to output as much power to the ground as possible as efficiently as possible. One part of this is reving the engine to build up rotational inertia and get the motor up to the power band. This gives you excess usable energy to work with. The second part is keeping it in the power band as you take off. Keeping your right foot pegged provides the most power output as opposed to dinking around with launch revs (only useful if you can spin up the tires and need to limit output). Being a NA section with cars not making more than 120-160hp to the ground, there's just no reason to limit motor output outside of winter conditions. Even then it's fun to just light up all 4 tires from a stop and remains quite mild on the drivetrain. If I was driving a fwd or rwd open diff car of the same power, I would opt to pick an appropriate launch rpm to keep me from just winding up one tire and going nowhere. I'd still do the same approach, but I would limit what I rev to to keep the one tire from breaking free on launch. My bro has an old 318i with open rear diff, and that needs to be launched around 4k to keep from lighting up a tire at the start, but that's on merely ok all-seasons. If I can spin up a tire while I'm accelerating, then I would need to also limit throttle input. Our Subarus just can't output that kind of power on any surface with decent grip. Even if there is tire spin, you may opt to let it spin if the surface is deformable and may benefit from the excess tire rotation. On a high grip surface like asphalt, there's no point. My bro's FXT currently puts down 270 ft-lbs and it won't spin up the tires on launch on asphalt. The limit is still power and not traction, so slipping the clutch is still necessary to keep the engine in the power band during launch. You could shock the tires free on launch if you simply pop the clutch or you can bog the engine if you just let out the clutch too early and don't let the car get up to speed first. Now this isn't that big of a problem with a TD04 and quick spool, but it might be a rather big deal if you're running a 20G or larger and you have to putz up like a shmuck from 2k to your boost range because you let out too early.

I don't know. Both my bro and I are on our 4th year of auto-x and rally-x and neither of us are light on our cars at all and do not launch lightly either. Both are on stock trannies.

I just see launch control as a lazy man's approach on a low powered car or a nice tool on an excessively powered car. A 500hp Viper could very much use launch control, heck traction control, just a stupid excess of power in lower gears where slicks are of little help. A NA Impreza has little need. Even a turboed Impreza is questionable until serious power is made. Even then it'd pretty much necessitate a 6mt first and subsequently the better diff package coming with it that is better suited to get the power down. If I have so little traction available that I'm just spinning tires, I'll just let the clutch fully engage and start fiddling with partial throttle to control wheel speed. I just can't say I've been in such a situation short of snow/ice/mud. Even in loose dirt, there is typically enough traction available with appropriate tires to bog the engine.

This is just my point of view though.
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Old 08-16-2010, 11:05 PM   #9
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"gizzmo launch interface"..google it
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Old 08-16-2010, 11:57 PM   #10
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Thanks for the tutorial on launching a Subaru. This sounds a lot more effective than my sorry attempts at a launch. My only previous experience of trying to get a car to launch was in my '04 Civic. 4-5k and dump the clutch. Tried that once in the new Subie. Hammered out the first five feet and then it nearly died. Wasn't pretty.

I'm a little concerned with how much wear this causing on the clutch, though. They are cheaper than transmissions but still not cheap.
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Old 08-17-2010, 12:20 AM   #11
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Yep, that's why you slip it instead of dropping it. Think of the cluch pedal like an inverse brake pedal. You move the pedal to a certain spot, and it will grip a certain amount and transfer a certain amount of energy to the transmission. You just try and equal the bite amount to the power output of the motor. That keeps you from bogging. Clutches like brake pads are actually pretty durable. The big issue is heat build up, and that really requires repetition. It's an organic compound much like your stock brake pads. It can overheat and change behavior just like a brake pad. It's why some people have slip issues (fade) with drag racing. It sort of requires one run right after another though and a whole heck of a lot of slipping over a short duration, or blatant user error like resting your foot on the clutch pedal, sort of akin to riding the brakes.

Wear or lack of wear can also be influenced by gear shifts too. The better you rev match the engine to the tranny going from gear to gear, the happier your clutch and syncros will be. Just like your brakes, it is a wear item, and in time from use, it will need to be replaced. Will that be 50k miles, 100k miles, 200k miles? That depends on how you use it. Use causes some wear, always. Different types of use causes different amounts of wear. Good or poor use of the clutch will cause different amounts of wear. You should never be of the mindset that the clutch will last forever, especially with sport use.

Frankly, I'm quite surprised mine lasted 100k miles so far. I seriously expected to have to replace it by now. I actually want to, because a lightweight package will go in when it does.
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Old 08-17-2010, 11:27 PM   #12
Soulofdarkness
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Thanks for all the inputs. I'm currently looking for a JDM Mechanical Rear diff for my RS, but I'm spining my tires at my rally-x events easily. I would like launch control for a good controlled/constant start speed so I can see if I'm losing time in my launches or in the turns (Take factors out to find out where the weakness is).

As for loading slipping, I do that now. So that's not the issue I just want to be consistant.

I do have an aftermarket clutch in the car so it does hook up well on start but the RPMs are just a pain to keep constant when launching.
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Old 08-17-2010, 11:44 PM   #13
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In my situation, the motor is a stock cam/compression EJ22. The launch is a drag launch (ProSolo tree) where I have to keep the car staged and get off the line as quickly as I can without redlighting. Pavement use only and with 275 Hooseiers at all 4 corners.....clutch is an ACT HD with an organic disc and 9.5lb flywheel.

When using the launch control, I find it harder to modulate the clutch pedal when my right foot is already planted and I'm focused on the tree and getting a good light. WHn holding rpms at partial throttle, I find it easier to modulate the clutch.....I'm no pro....so I'm just kinda warning the OP for what I've experienced.

Dirt would be alot easier on the drivetrain, as wheelspin should soak up much of the shockload. That doesn't mean the trans will hold forever....

Jay
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