Air vs. Electric #1 stop?

[Bob Verwold]

I thought Steve (been 200mph) explained it pretty well.........If you have the air now why would ya switch?

[Chris Williams]

CO2 is an independent system from the electrical, so it's not dependent on the state of the battery charge, or subject to the spikes and fluctuations of the other systems that effect the electrical system. It's also very stable wrt temperature and is therefore consistent in the amount of pressure applied to the system.

Finally, only with an air stop system can you control both the speed of on and off the stop. On isn't such a big deal, as most people just go on the stop as fast as possible. But you really need to be able to adjust the speed it comes off the stop. With an air system you can come off the stop smoothly and at the exact rate you need to avoid wheel spin.

Hope this helps, let me know if you have any other questions.
Chris

[3258]

Chris, thanks for the technical reasoning on the air vs. electric, this is the kind of response i was looking for.

I understand that for competitive .90 racing using a under carb. stop vs. the in line throttle cable stop is a no brainer. What is it that makes the under carb stop more consistent than the throttle cable stop?

I would also like to ask your opinion (and anybody else who cares to comment) on the different types of under carb. stops. I see that there is a disc type stop that Dedenbear sells, and one that uses a standard rotating throttle plate.

[Chris Williams]

The problem with stops that control the throttle cable/line is that the line and the parts are flexible, meaning it's tough to get consistency run after run. Under-carb stops are built from a heavy billet piece and go to the exact same spot every time, regardless of the pressure you have on the pedal. Some people put a very hard stop (bolt or something) under the pedal to insure consistent pedal pressure, but to me that's a band-aid to make an in-line stop work.

Discs are better because they are more directly proportional. Imagine a 25% closure of the disc, and a 25% closure of the traditional blades. If you close it by 10% more, a disc limits flow by precisely 10%, but blades don't -- because the angle changes, some slips by at a higher rate, it's not exactly clear what 10% more is because of the angle, and the actual rate of flow changes. In all it's not exactly 10% more.

IMHO, the best stop on the market is the Dedenbear TS6. It's just a rock -- highly predictable, highly consistent.

[3258]

Thanks Chris. I am no fluid dynamics guru, but what i do like about the disc is that the cross section of the disc is always perpendicular to the carb venturies etc. I think that would lend to a more linear adjustment by the user than a blade that rotates through an arc. I could see where it could potentially make for an easier prediction in making adjustments to get that perfect .90

[Chris Williams]

You just said what I meant to say The angle of blades changes everything. Discs are really the current state of the art.

[Chris Williams]

Originally Posted by 3258

I could see where it could potentially make for an easier prediction in making adjustments to get that perfect .90

However, there are people who point out that you rarely adjust the amount of stop, just the time on the stop. So they say, it's all hooey, blades are fine.

Then the disc fans say, but the angle of the blades changes the dynamics so that it's more effected by atmospheric changes. So the disc is better.

And so it goes. Back and forth. What I do know is that, if you look at the setups of the folks who win the most in the .90 classes, the disc is by far the most common. And we love it.

[3258]

This brings up a couple questions I have. As you said "....you rarely adjust the amount of stop, just the time on the stop" When tuning for .90 racing, i assume that you adjust the amount of stop for a certain RPM?

So if I understand right, you have about 3 adjustments with an air system to run the perfect .90; The amount of stop, the amount of time you are on the stop, and the rate you come on and of the stop? I assume all these factors are based on how much under the index your car can run off the stop, atm conditions, track conditions, and the list goes on. Is there any rule of thumb to get a baseline place to start with this?

One thing I find interesting with the throttle stops design is that they are base plate designs, meaning they decided that it is better to throttle the air fuel mixture coming out of the venturi, rather than just throttle the airflow into the venturi. Obviously there was a reason because all of the manufacturers seemed to converge on that solution. I imagine it has something to do with the airflow of the venturi itself. I bet there has been some crazy things on cars throughout the history of superclass racing

[Chris Williams]

Typically, you check/set your "dead stall" (the amount of closure on the stop, which equates to an RPM) at the start of a weekend. It's most effected by altitude and general atmospheric conditions, so once a weekend is usually enough. We check ours, but it rarely needs to be adjusted -- once set, it's good.

There are a lot of opinions on what your dead stall should be. Some are fans of dropping just a little, and being on the stop longer, others feel just the opposite. I prefer to adjust it based on the time on the stop. My goal is from 1-2 seconds. You need time for the car to settle on that RPM, but not so long as the car starts to climb. Even when on the stop, a high-HP car will start to accelerate. You want to avoid that, as it detracts from consistency.

Once you find a good rate of on and off the stop, most people don't change it. Ever. You're trying to limit the number of variables. So you find a rate for each, where the tire spin is limited, and stick with it.

Then there are the key variables: time on and off the stop. Time on is another religious argument. We go on the stop immediately, at 0.0, but some consider that controversial. Others prefer something like 0.1 or 0.2. Some even like to mess with people's heads and go on the stop at the end of the track...

For most Super racers, you get a dead stall that works, a rate on and off that works, and a time on the stop that works, and stick with them. And just adjust the time off the stop. That time when you come off the stop is the key variable.

To initially set it, the best way is to run two runs in similar conditions, and have wildly different times, like 1.2 seconds and 1.8 seconds, and then do the math. You come up with a throttle stop ratio (e.g. 3:1, for every 3 hundredths of stop change, you get one hundredth of ET change). Now you know a number you can use to predict your stop time for the race.

Then you need to figure out how the weather effects your car. Many use density altitude, others use a horsepower correction factor, there's a lot of "special sauce" here as well. People will argue one vs. the other.

Then others, like me (a computer geek with 20+ years in the computer business) use computer programs to do the math. Some examples are Crew Chief Pro, my favorite Race Log Pro from ifamilysoftware.com, and I'm working on my own system for our use. They let you log the history of all your runs and make sophisticated analysis of the car's performance wrt the weather.

Long winded, hope this helps,
Chris

[3258]

Thanks for all the info Chris, this answered many of my questions. I am sure I will have plenty more questions in the future as i get deeper into this