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Old 05-24-2006, 03:31 AM   #1
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First time using a trans. brake, what RPM to launch at??

I will be my first time launching with a trans. brake this weekend and was wondering on everybodys thoughts as to what is a good RPM to launch at??? I am running a TCI TH350 and a 8" 4000 stall?? I've been told to drop it at 3000-3500 rpm which will easliy pull the wheels. The car already has dragged the bumper once using the foot brake so I'm am eager to see what the brake will accomplish. I also switched to a 27" tire so that should help with my times a little ( from a 26" where I was at 7000rpm @ 3/4 track ) I'm hoping that my new trans and wheels, plus the Pro Billet Dist, shifter, and Demon carb will get me into the 10's. Well see huh?
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Old 05-24-2006, 09:18 PM   #2
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Re: First time using a trans. brake, what RPM to launch at??

What rpm did you launch at when footbraking?
Where does the engine make it's peak torque?

I only launch my car at 4500 rpm.
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Old 05-25-2006, 02:49 PM   #3
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Re: First time using a trans. brake, what RPM to launch at??

hit the button, put the pedal to the floor, and then release the button. you going to try this out at scc tomorrow?
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Old 05-25-2006, 02:53 PM   #4
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Re: First time using a trans. brake, what RPM to launch at??

Brakes are FUUUUUUUUUN
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Old 05-25-2006, 07:44 PM   #5
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Re: First time using a trans. brake, what RPM to launch at??

Quote:
Originally Posted by toby
hit the button, put the pedal to the floor, and then release the button.
That's the typical way to launch depending on the converter's stall speed. The engine rpm will only go as high as the stall speed. If the converter stalls at 3000 and you want to launch at 4000 off the transbrake, it's not going to happen unless you change the converter.

Also when launching off the transbrake, the carb's accellerator pump shot has already brought the engine up to rpm. When you launch, it's all jets so there shouldn't be a bog. The only bog you may get is when you put your foot to the floor once you set the transbrake.

Your foot is to the floor on the throttle pedel and nothing is touching the brake pedel. Your finger is on the button. When you see the last yellow light come on, let go of the button and hang on.
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Old 05-25-2006, 08:53 PM   #6
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Re: First time using a trans. brake, what RPM to launch at??

So what you are saying is that the engine will not rev. past the 4000 stall I have? I thought it was like a manual trans? So then with the trans brake activated, I just put it to the floor and it will not rev. to 7000??? I dont know how but I guess well see huh? And yes, if I can get the damn new carb to stop leaking I will be at SCC tomorrow night.
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Old 05-25-2006, 09:11 PM   #7
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Re: First time using a trans. brake, what RPM to launch at??

Your trans brake should be hooked in with your 2/3 step so whatever pill you have for staging rpm is all that the motor will rev to when the brake is activated. Too many variables, even if your stall is rated for 4000rpm, it's possible your motor will make suffiecent tq that it may actually flash or "stall" to 4500 give or take. For example my TCI 10" convertor is "rated" at 4600 but I can easily footbrake it to 4600 and it actually flashes to 5200, so it's all dependant on what the engine puts out in relation to guts of the covnertor and the stator/fin angle that it has.

Most bracket racers will leave stage/stutter anywhere from 3000-3500rpm on race day and I've found this to be the case with mild sbc to triple nickels and higher cid's, when going for show boat wheelies or trying to max out 60's they'll go as high as it will let it for max umph on release. If your just going to have fun with it leave at whatever rpm you so desire, once you settle down and get serious with a bracket racing mentality you need to start paying close attention to how staging rpm effects your R/T's, if your red then lower it a few hundred and see where your at, then once you get it consistantly in the .520's or .020's then start looking at other area's to fine tune into the .51? or .010 lights and if your feeling good then find what changes will drop you in to the .50? or .00? territory for when the going gets tough in the later rounds.

Also-keeping all the wheels on the ground will net faster time slips then the wheels up will since your not wasting your energy going up in the air, but rather your pushing the car forward instead like a rail. (not to mention more consistant R/T's and timeslips)
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Old 05-25-2006, 10:12 PM   #8
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Re: First time using a trans. brake, what RPM to launch at??

Ya. What IHI said.

I like pulling my wheels a bit on launch because my car is so nose heavy. I want a little extra weight transfer but if you watch a Super Gas car launch, they keep the front end down as much as possible. If they pull the front wheels, it affects their times. They also have large tires to grab the ground.

My converter is rated at 5700. My old engine stalled around 5700. Although I haven't done a non 2 step stall test with the new engine, I already know the stall speed is higher. Watching my playback tach, I can see the engine rpms don't drop below 6100 all the way down the track so the stall speed should be close to that.

The rated stall speed of a converter is only a guess. The true stall depends on how much torque is put into it. The same converter behind 2 different engines will stall at different speeds.

Doing a stall speed test with a transbrake is also a good way to do some carb jetting. Disconnect the 2 step and do a stall test on the transbrake. Fatten the carb up a couple of jet sizes and try again. If the stall speed increased then the extra fuel means the engine liked it and made more power. Keep fattening up the carb until the stall rpm stops climbing. You should always start by going fatter so you don't risk going too lean. If fattening the carb up doesn't change the stall speed then start going leaner.

Start with the primary jets until the best selection is made then switch to the secondaries and try again.

Jetting the carb by using the transbrake saves a lot of trips down the track trying to do the same thing but nothing beats a good dyno tune.

At least by using the transbrake without a 2 step, you'll know the true stall speed of your converter behind your engine. Stall means the converter has reached it's point where it's in 1:1 fluid transfer. Since the tranny is locked, the engine is fighting to overcome the hydraulic forces in the converter and stalls at the rpm limited by the converter. Because of this, there's no way and engine will rev to 7000 if the converter stalls a 4000 unless it's got some terrible slippage in it.

When you release the transbrake, the power is allowed to leave the tranny, the converter stops stalling and they engine rpms start climbing until you shift or let off the throttle.

As for the carb, change the o-rings in the needle and seats. I have to do that regularly because the alcohol affects them. I guess it just depends where your "leak" is.
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Old 05-25-2006, 11:36 PM   #9
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Re: First time using a trans. brake, what RPM to launch at??

It's a brand new Mighty Demon 825 and Ijust took it out of the box, put it on the car, hit the FP switch, it is leaking in the secondaries, it's just running off the boosters. I've been told the float is stuck and to tap it with a wrench, if that doesnt fix it then take the bowls off and "set the flap" or something??? They say they get stuck because the tossing around when shipped? I dont really know, I had an LS1 car before this and tuning was simple, just plug in the lap-top and go, but I decided to build a carb'd race car just to see what I could get with spare parts Ihave around the garage. I put it together, and I actually got it in the 11's using only 2 gears of my busted 3 speed. I have the new trans, and frame tied now so hopefully it will get me into the 10's ??
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Old 05-26-2006, 08:13 PM   #10
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Re: First time using a trans. brake, what RPM to launch at??

Leaking out the boosters is normally too much fuel in the bowl. Since it's a new carb, there's little chance it has a bad o-ring. The float opens and closes the needle and seat. If there's a piece of dirt stuck in the needle, the float can't close it. Loosen the float adjustment screw and unscrew the needle out. Make sure it moves freely. The other option is to pull the bowl off. Hold it upside down so the float drops down and closes the seat. Blow into fuel inlet. The weight of the float should have the needle closed. If you can blow through it, the needle isn't seating and needs to be fixed/cleaned etc.

Basic carb maintanence.
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Old 05-26-2006, 08:13 PM
 
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