Basic launch questions on 2-step RPM and air pressure...

Went out this weekend to see if I could get my TD down our local track. Our track is an old runway that has been topcoated, and is not the greatest, although some of our racers get wheels up launches.

I initially pulled some timing out and it spun at every part of the track, so for the next run I pulled some more base timing, and 4 degrees extra for the first second of the run. At the hit, the tires squatted, then stood up and just whisped smoke, at around the tree or the 60 I pedaled and it grabbed, then came loose again at the top end. When I went back and looked at the tracks, I saw some basketballing around the christmas tree. I honestly don't know if it happened before or after I pedaled, but I definitely felt it shudder during the run, and I think it was after the pedal.

You can see the video of the run here:

http://www.youtube.com/watch?v=IKblVn3bn1Q

I basically have three options to get it to stick at the hit. Timing, 2-step RPM, or tire pressure. I know what timing does. But the other two are more of a mystery to me.

What is the effect of adding/reducing tire pressure? I'm at 5.25 in the video.

What is the effect of raising/lowering the 2-step RPM? I had it at 3000 in the vid.

I think if I can get it to stick at the line it will set the chassis and probably stay stuck up top without changing much from where it is.

Thanks for any help.

-Scouder

 

I hate to be the first to reply to my own thread, but I just got some more info. A friend took a video from behind the car as I launched and it plainly shows the wheelie bar slapping the ground hard, and launching the car into wheelspin. So I believe that's why it came loose even though I had a bunch of timing out.

I would like to discuss the two questions above though, because once I raise the wheelie bar I'll still need to know how to tune the launch.

-Brian

 

Try to keep it simple to start with. Leave from idle. If running a PG trans all hell seems to break loose with higher rpm launches. Set it up soft with 0%OD 28 deg timing and a good burnout and it should go 1.10' 60ft. ,4.80' 1/8th , 7.50' 1/4. Then gradually work up from there..best of luck

 

boy, that's a lot of rain barrels. How well does that work?

 

We'll find out the first time somebody whacks 'em. Our track is all volunteer, non-profit, so it's the best we could do (free). We are hoping a shallow hit will deflect the car and a steeper hit will decelerate it. They are also a sound buffer for the houses in the background.

-Brian

 

So leaving from idle is softer on the tires than leaving on the brake?

Is it possible to leave too soft, and not set the chassis on the tires?

Thanks,

-Brian

 

To answer your question, how about some info:

1. What 60' does it run on a good track?
2. do you have a data logger?
3. What are you goals -

A. You are willing to bracket race it going 7.30's 1/4 mile with a 1.12 60' if that's what it takes to get down your local track.
B. You are looking for mid 5's and have no interest in running soft.


If it is more like A, I think I would recommend rigging up a air throttle control so you could control throttle opening.

GK http://www.motorsportsinnovations.com/

 

I am sure that a few racers here are tired of me preaching about too much overall 1st gear ratio...but with a P/G and a 1.80 low 1st gear ratio.....sometimes it's hard to calm down the launch....and taking out timing and using an air throttle controller helps, but what is your overall 1st gear ratio (trans X rear)?

 

In answer to some of the questions above:

I don't have a 60' yet. I have launched the car three times, and am still working toward a workable combo. The wheelie bar was hitting the ground, which has been fixed, but I won't get to the track again until July 17.

I run a RPM logger with Engine RPM, shaft rpm, fuel pressure, fuel flow, oil pressure, boost, and 8 egts.

I have no interest in 5 second runs. 1.1 60's would be great.

I have a two step, currently launching at 3000.

I have an air throttle, holding the blades at about 1/3 open while on the two step.

I also have the 8973 for timing control.

My first gear total is 1.8 * 3.9 = 7.02.

My next run will be 26 degrees flat for the whole run, 5.25 pounds in the tire (34.5-17-16 Goodyear), 3000 on the step.

Does leaving from idle hit the tires harder or softer than the transbrake?

I have heard that reducing air pressure in the tire increases contact patch, but there is a point that it "cups" in some way that reduces contact patch. Is this true? Is there some kind of tattle-tale marks on the track when it does this?

-Brian

 

For what it is worth. Once I was helping a team that had transbrake problems and were forced to leave at an idle. The car produced it's best 60' time on that run. Our guess was it didn't hit the tires as hard and hooked up better.

 

If you have the air cylinder, then I'd keep it at 1/3 throttle for say 2 seconds and open it slowly so that it's a full throttle in 3 secs.

With your logger you will see the boost at various RPM's. With my blower higher RPM = more boost which = more power. I launch at 3300 which is under 4 lbs of boost.

GK http://www.motorsportsinnovations.com/

 

from what I have witnessed with a converter and blower combo, you HAVE to have a msd 6 shooter or some type of timing control system. the converter is a multiplier and you have to kill power with timing retards. I have seen guys have the timing retarded as much as 10 degrees leaving the starting line and then ramp the timing back in smoothly. you will never calm it enough with fuel. not trying to argue with others but have the blades open fast and smooth. air cylinders opening slowly are to inconsistent due to air pressure and can hurt reaction times. eliminate inconsistencies and control things that can be controlled reliably. just my two cents.

 

In my experience leaving from idle really works too. I had my best 60ft(1.03) and my friend had a .997. Its hard to cut a light like that but to start out try it. My theory is it gives the tire time to wrinkle up and grip whereas if you whack it with 3000rpm plus, especially on a marginal track such as yours it just going to jump up and spin before the tire has a chance to squat down and work. Try it and see.....

 

I took the second cylinder off of my air throttle. I only use it to hold the blades at 1/3 while I'm on the 2-step so I don't drown the engine. As soon as the brake releases it jumps off the 2-step and slaps the blades wide open instantly.

I guess I'll just keep pulling timing with the 8973 till it holds through the 60' and then start fine tuning from there. I'll probably experiment with a launch from idle as well just for grins.

Thanks guys.

-Brian

 

BINGO we have a winner!!!