Stumble on Launch off the 2-Step

Mike Gaines · Jul 17, 2016

I have been racing only using my footbrake (I do not have a transbrake). I apply the footbrake on the start line and bring the RPM up to 3000 just riding against the converter (5000 flash). The brakes will hold it at 3000 RPM.....and everything is fine with this technique (when I release the footbrake and floor the gas it LAUNCHES HARD,,,not stumble, no miss, no farts,,,,it just kicks butt and goes)....BUT....I do have a 2-Step Box attached to my MSD Ignition with an adjustable low end chip that I have set at 3000 RPM (just like the guys have who use it with a transbrake)....I have a toggle switch where I can activate the low end rev limiter on the 2-step which is activated off my footbrake brake light switch....and I want to be able to use this method to launch the car...activating the 2-step rev limiter by depressing the brake pedal, flooring the accelerator pedal and then the motor comes up to 3000 rpm when it starts dropping cylinders to limit the rpm to 3000....everything is fine up until this point.....but....when I release the foot brake to launch the car I get a fraction of a second "stumble", NOT a clean start. I am sure it is in the carb adjustment somewhere but I do not know where to start. It is a brand new Dominator 1050 that works perfect using the footbrake launch without the 2-step and works perfect going down the track (10.582 @ 126.33) but I need to get rid of the stumble when launching off the 2-step box...SUGGESTIONS PLEASE

RT K_line · Jul 17, 2016

If the carb work correctly when not using the 2 step then it's probably in the split second that it takes the brake switch to release as your flooring it. You might add a spring to the brake pedal to quicken the release of the switch. JMO.

Mike Gaines · Jul 17, 2016

Thanks... I will try putting a second, or stronger spring, on my brake pedal to see if this solves the problem....would never have thought of that.
Has anyone had similar problem and what did you do to solve it...it you don't have a problem with a "stumble" coming off the 2-step...did you have to re-adjust your carb in any way ????? Thanks for input.

Tex013 · Jul 18, 2016

Mike, you might want to check over on yellowbullet. I know when i try my 2 step my sixty is slow . It is i suspect because there is no pump shot left . If you had a O2 sensor you might see this . Are you still using a powervalve ?
You may not have enough rpm to pull enough fuel/air , try a bit higher rpm . Dont forget on the 2step you should be able to hold a higher rpm due to loss of tq

Tex

SUPERSTOCKRACER · Jul 18, 2016

Richen up your intermediate air bleed some. Sounds like your going lean.

dvw · Jul 18, 2016

You have a huge carb with no pump shot at low rpm with no air speed through the intake tract. Why use a 2 step?
Doug

PurpleBeeper · Jul 18, 2016

RT K_line said:
If the carb work correctly when not using the 2 step then it's probably in the split second that it takes the brake switch to release as your flooring it. You might add a spring to the brake pedal to quicken the release of the switch. JMO.

Well, I have a 2-step which I haven't hooked up, so I'm no expert.... but this sounds very possible & cheap/easy to try.... especially if the carb runs perfectly without the 2-step.

69 GTX · Jul 18, 2016

Make sure you have a good quality switch, don't use the OEM one. Make sure it has a short travel.

Mike Gaines · Jul 18, 2016

69 GTX said:
Make sure you have a good quality switch, don't use the OEM one. Make sure it has a short travel.

I will look for a good switch. thanks. mike

splicer · Jul 20, 2016

Lack of pump shot would be my guess as well....

On the foot brake and brought up against the convertor at 3000 rpm, you have lots of pump shot left when you floor it and release the foot brake.

On the 2 step, you've already floored it, the pump shot is gone, and all 4 barrels are wide open with nothing to cover a momentary lean spot.

Splicer.

Mike Gaines · Jul 21, 2016

splicer said:
Lack of pump shot would be my guess as well....

On the foot brake and brought up against the convertor at 3000 rpm, you have lots of pump shot left when you floor it and release the foot brake.

On the 2 step, you've already floored it, the pump shot is gone, and all 4 barrels are wide open with nothing to cover a momentary lean spot.

Splicer.

Thanks....that sounds like a perfect explanation. I am going to try the "2-step" launch just bringing the rpm up to 3100 (just where the low end chip starts dropping cylinders) and then I will floor it the rest of the way the instant I release the footbrake....this should test this theory which sure sounds reasonable to me....thanks

dvw · Jul 21, 2016

Mike Gaines said:
Thanks....that sounds like a perfect explanation. I am going to try the "2-step" launch just bringing the rpm up to 3100 (just where the low end chip starts dropping cylinders) and then I will floor it the rest of the way the instant I release the footbrake....this should test this theory which sure sounds reasonable to me....thanks

Why even use the chip? Put it in prestage. Set the RPM with your right foot. Bump it in using your left foot only. With practice you can be with-in 50 RPM.
Doug

Homstead · Jul 21, 2016

Not sure this is relevant to your situation but when my car got into the 10s it started to stumble like you describe. I tried float levels and pump shot adjustments but when I put in jet extensions it was fixed. I was on the two step but was using a trans brake as well.

Mike Gaines · Jul 21, 2016

dvw said:
Why even use the chip? Put it in prestage. Set the RPM with your right foot. Bump it in using your left foot only. With practice you can be with-in 50 RPM.
Doug

Many racers tell me I will have a better REACTION TIME if I use the 2-step since it eliminates 1 of the two actions you have to take to launch the car off the footbrake...1) lift the foot brake 2) floor the accelerator.
With the 2-step the gas pedal part of the launch is already done...just release the footbrake and "go".... does this make any sense.
Let me know your opinions.Thanks

69 GTX · Jul 21, 2016

dvw · Jul 21, 2016

Consistency is the name of the game. You can adjust R/T to make it quicker. Inconsistency is much harder to cure. Show me a grouping of R/T with-in .015 and that can be upgraded, even if it's .100-.115. Show me .005-.090 your in trouble. I've never driven a race car that couldn't get to .000 on a .500 tree off the foot brake. .400 tree is tough.
Doug

SUPERSTOCKRACER · Jul 21, 2016

NENDRA runs a .400 full tree. Talk about tough. Worst light last weekend was a .035.

dvw · Jul 21, 2016

SUPERSTOCKRACER said:
NENDRA runs a .400 full tree. Talk about tough. Worst light last weekend was a .035.

I've run the .400 full tree and hate it. I don't understand why any association does it. Doesn't fit well with any type of car. With our type of car you have to bump in deeper or guess. That being said I've run it 4 times, 2 wins, 2 runner ups.
Doug

SUPERSTOCKRACER · Jul 22, 2016

dvw said:
I've run the .400 full tree and hate it. I don't understand why any association does it. Doesn't fit well with any type of car. With our type of car you have to bump in deeper or guess. That being said I've run it 4 times, 2 wins, 2 runner ups.
Doug

Maybe one day, you could try a race with NENDRA.
And see how you would do racing against these racers.
These guys are all very good.
And see if the clocks are wrong at Milan.

dvw · Jul 22, 2016

Gave up on Milan. 2 weeks ago at 131 Motorplex it went 9.11 scubbing 8 mph with Eddy carbs @85degrees. It should roll next week at Norwalk at Monster Mopar.
Doug

Stumble on Launch off the 2-Step

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