blower motor shift point - peak power point vs. clutch lock up

Will, Where in relation to clutch lockup in low gear do you blow the tires off? Does it happen right at lockup? Norm

 

can you guys figure in drive shaft speed ( and or tire spin at the hit), all this talk with higher rev shifting points for low gear, is tire spin part of this ??? more tire psi , more hit on the wheelie bar????.
thanks vic

 

Will, have you ever noticed that every so often you'll make a glass smooth run with a really good set of incrementals, ET and MPH, yet the computer shows the engine RPM to be way ahead of the driveline RPM? It's obvious to think about what it might have run if it hadn't slipped the clutch so much! But, when you do bring the engine and driveline speeds together, the ET hardly improves and you only pick up a couple or three MPH.

This started happening more than occasionally with the latest BA-5 stuff. The only way this could be possible is for the engine's torque curve to be fairly flat, because if its torque fell significantly in the clutch lockup range, then slipping the clutch would compromise performance.

I think that being aware of this widens the clutch tuning window. Any comments?

Norm

 

You've probably all noticed the abrupt jerk that happens when elevators finish inching to achieve floor level alignment. They aren't going very fast, but that jerk is always there, and it's quite noticeable.

Speed, which enginerds call velocity, is the time rate of change of position, as in miles per hour. We're all so familiar with this one in our daily lives that it doesn't need any explaination

Acceleration is the time rate of change of velocity. As drag racers, we're familiar with it also, but many people who don't get their adrenaline fix from it don't understand it as well.

Jerk (no kidding) is the time rate of change of acceleration. Now this one, while common, gets by almost everyone. Jerk is what sloshes the coffee when an inexperienced driver in the family car doesn't feather the brakes at the end of the stop.

If you pull up some runs on the computer, look at the engine RPM graphs. Note the shape of the transition between the relatively slow increase in engine RPM preceding low gear clutch lockup and the nearly vertical rush to the shiftpoint following it. Some of them make a very sharp transition, and others make a smooth, radiused transition. If you correlate this to what the car did, you'll find that if the transition is smooth and radiused, the car usually made it through clutch lockup without shaking the tires, even on marginal tracks. In contrast, while you may have gotten away with the sharp transition at lockup on really good tracks, it's death on marginal tracks. On marginal surfaces, if it hasn't knocked the tires loose before clutch lockup and the transition is sharp, that's the instant it usually starts. I believe that jerk is what's responsible for causing tire shake at clutch lockup.

I'm worried that if this gets any longer, you'll think I'm the jerk. I've come up with a cool way to control this transition, and I'll spill it out a little later.

Norm

 

Will, I hope when i get my alcohol car my blower makes plenty of snot!!!! Lol, J/K, informative post. Learned a lot from Norm Drazy over the last few weeks. Have a great holidays everyone!!

 

Still waiting...

-chris

 

Wow! Did I ever sink the hook with that one! Just call 1-900-555-JERK . Just kidding! But from the amount of E-mail I've gotten asking me to cough it up, it does make me wonder. Sorry for the time it took me to post this. I had some other things to clear up first.

Remember that surprisingly good run that was as smooth as glass with absolutely no tire shake, but the engine RPM was a mile ahead of the driveline? Of course, you always want a bigger fish, so you threw some counterweight at it, knowing that you can't go too far without shaking the tires. When you've brought the engine and driveline RPM curves together as much as possible, the ET hardly changed at all, and if you were lucky, you might have picked up all of two to three MPH. This first started happening with the latest BA-5 stuff, which implies that it has a very flat torque curve in the 8800-9200 RPM range. These loose runs are usually considered to be a writeoff, and therefore the computer data usually escapes careful scrutiny, but there's a nugget to be gleaned: On a run like this, the engine RPM almost always makes a nice, radiused transition through clutch lockup, and that means minimal jerk. Maybe that's why it made it through lockup without a quiver. The trick is to get it to do this without slipping where you don't want it to.

A centrifugal clutch should theoretically catch the engine RPM at the stall speed following the hit of the throttle, and then hold it constant until the clutch discs catch up with the floaters at lockup, where the engine RPM curve goes nearly vertical. Unfortunately, its behavior is little more complicated because the engine RPM gradually drifts upward during this interval. The coefficient of friction between the disc friction facing material and the mating steel surfaces apparently decreases with increasing temperature. Its cause may be that the friction between the disc material and the mating steel surfaces fades with increasing temperature, as any time you blow through the clutch, which is an exaggeration of this low gear RPM drift, you don't even have to remove the clutch can cover to know it involves temperature.

Air density affects the horsepower per square inch applied to the clutch interface, and track conditions affect not only how much and how long the clutch slides before lockup, but also how much it must be set to slide. The difference in RPM between the floaters and discs, which we'll call the clutch differential RPM, the engine torque (less any torque used to accelerate the engine, blower, and clurch), and the time the clutch spends sliding, and the total clutch friction area, all determine the rate at which the friction interface temperature increases. In good air where the engine makes more power, it's higher than with bad air, assuming the same size clutch discs are used in both places. On a good track, you almost can't knock the tires loose, so you can frequently get away with a really sharp-cornered transition at clutch lockup. Here, it's easy to underestimate how much static and counterweight is required, and if you do underestimate it, it will blow through the clutch by brute force. On the other hand, on a bad track, avoiding tire shake at lockup is like walking on eggshells with static and counterweight settings, also making it easy to blow through the clutch.

Varying the surface area of the discs to control the horsepower per square inch lets you control the friction interface temperature rise rate. This adds another dimension to clutch tuning that's just as important as adjusting the static and counterweight settings. For some reason, many teams select full-faced or cut-down discs and always run them, regardless of the track and atmospheric conditions. You can see why they are successful under some conditions, bur not on others.

A good track combined with good air demands full-faced discs. This keeps the friction interface temperature from running away, with obvious consequences. Using cut-down discs in the opposite conditions brings the friction interface temperature up to where they begin to fade just before lockup. You don't have to choose between one extreme or the other, as mixing full-faced with cut-down discs gives you four choices.

Les Davenport's Jetsize software calculates relative density which determines relative engine torque and relative horsepower. Start by selecting a good run made on a moderately good track under average weather conditions in which the clutch lockup proceeded smoothly, and calculate the total clutch surface area used on that run. Then, when you're setting the car up at a diffwerent track, use the relative density to proportion the total clutch surface area up or down in order to keep the horsepower per square inch, and therefore the temperature rise rate, constant.

Now that you have a calculable reference point that gets you into the ballpark, start fine-tuning it to get the desired slide into lockup. The transmission ratio, track conditions, and low gear leanout size all affect its behavior. Taller trans ratios, larger low gear leanout jet sizes, and stickier tracks all require you to hedge the total clutch surface area upward, as they all tend to either increase the horsepower per square inch, or they affect how long the clutch slides before lockup. You'll also find that there are conditions when you can get by with a more aggressive lockup than others.

Now that you've taken the clutch to within an inch of giving up, how do you get it to live through second gear and high? Revving it up to 9800 to 10,000 RPM not only applies more plate load, but since it isn't slipping after lockup, also provides more time for the friction interfaces to cool down by conducting the heat from them into the much cooler interiors of the floaters and the flywheel and hat facings. These parts are all steel and have a high thermal conductivity, so this is a fairly rapid process. After it recovers, it's ready to accept more heat input when it unlocks after the shift and yet quickly lock up at the bottom of the next gear. To show that this really fits in this thread, I believe it's this and not the peak horsepower point that may be the benefit of revving it up.

Norm

 

Norm,
Are you referring to Les's clutch software
Dale