foot brake launch?

[daturbosix]

ive heard alot about using your footbrake to build upwards of 10psi of boost at the line. why does my car loose traction with the rears at only 4psi? i cant hold much more than that.

 

[azgn]

all depends on a bunch of stuff...how good your brakes are (front & rear), how much power you are making, what the stall speed is on your converter...on a stock car, I doubt you could hold 10 lbs of boost with a D5 without some help in the brake dept since you would be well by the stall of the converter

 

[tim87tr]

S10 rear wheel cylinders with D5 and 28x9 MT Drags (actually 27") and you can hold 10 lb boost for an instant before mashing the pedal and quickly releasing the foot brake when the converter flashes (much higher than stock 2400). My car needs boxed lowers with upgrading bushings before I try another 10 lb launch . My times were with a 5 lb launch or less, and the following technique:

Launch Technique

 

[daturbosix]

alright i read the whole post, but im kinda lost on one term you used..."(converter flashes high)" <<<whats that!? you said after you feel the tires spin a little the "converter flashes high"...what does that mean?

 

[tim87tr]

Maybe someone has a more technical explanation, but as I understand it, the stall of the converter is when the TC engages to transfer power to the rear wheels. Although the stock D5 is rated at about 2200 to 2400 rpm, I have read posts related to direct scan (scantool) readings that the converter reaches into the 3000 rpm range before allowing the rear wheels to turn, much like aftermarket TCs. Using the launch technique in the thread, I would imagine that is what my car is doing, launching at a much higher rpm when I mash the pedal.

Maybe someone else can add to this since I'm new to drag racing (34 total passes). For a stock setup, there doesn't seem to be hardly any cars at the dragstrip that launch like the TRs

 

[UNGN]

As tim said, hold the same 4 pound you normally hold on the foot brake, then when ready to launch just mash the accelerator to the floor while still on the brakes. The turbo will spool and the engine will bog slightly. When you release the brakes an instant later you will be near 10 psi and climbing.

Steady state, holding 10+ psi at the line, waiting for the lights to come down is pretty hard on the trans.

Taller tires and better rear brakes will allow you to hold more boost than you currently are.

 

[denn454]

I was going to ask about that. I can only hold 0-1 lbs of boost before the passenger wheel starts to go up in smoke, I know I need a posi (I have one on its way) but shouldn't I still be able to hold more boost? I have a vacume brake system, that in my oppinion sucks, I have to really put my foot into it just to stop the car. I've checked for leaks and everything sems to be working right, just not good. Is that normal w/ out a posi?

 

[Priz]

I'm running recap tires with a #45 compound on them. I've been trying to leave by bringing the boost up to about 1-2psi, then getting the second light(staged) lit up on the tree. I hold it there until the last yellow lights up then I mash the gas and dump the brake. Plain and simple, for me at my track, the end result of this is going almost completely sideways. I'm just wondering if I hold it still and do that, as you guys have said with keeping the brake on until the engine bogs, if it will make a difference or if I'll just go sideways earlier. Any suggestions are appreciated.

 

[d0n_3d]

with a stock brake setup and stock d5 converter with the stock turbo and injectors i was able to launch at 10 lbs of boost on 26x8.5 slicks...just heat up the slicks real good before staging...

do what was said earlier...stand on the brake and boost it up to about 5 lbs...then right before the light goes green mash the pedal and the boost will shoot up to 10 lbs or so and then let go of the brake right away when it turns on the last yellow light

i was able to run 12.2 @ 112 with this launch and many 12.4-12.6 passes after that...

 

[denn454]

i've gotten the wheel problem straightend out kinda untill i can get the posi in (lots of leg pressure, not fun but it works) so i've been able to keep the wheels from spinning while i'm building boost, but i can only build 2# and it takes a LONG time to build it up. I know a big exhaust will help, and i plan to do that some time soon (mine is starting to rust out ) but is there anything else i can do or is this normal?