Drag Racing!

Why don’t they make drag cars rev to a certain RPM and just rub the clutch the whole way down the strip, instead of connecting the engine at lower, more inferior RPM and worrying about torque and horsepower curves?

one could have a bigger engine rev to a higher peak torque (15k-20 RPM’s?) and just use a hardcore torque conversion system, so that one could rev it the full 15k and torque/rub the car forward off the line all the way down the strip, with the engine at full capacity the whole time.

sort of like using the high revved engine as a constant generator, outputting the same horsepower and dipping into this seemingly endless amount of horsepower just enough to not peel out at all times.*

seems that the issue in drag racing shoud not be power but torque conversion.


i forgot to mention a couple things:

that this theory would also do away with having to switch gears!!!

also, this torque conversion system i speak about i presume would need to be made either by fluids or by planetary gears. obviously there would be too much heat and energy lost from so much rubbing, using a metal-on-metal system.

That’s why they use torque convertors, at least most classes/racers use them.

(Sorry, I didn’t mean to be so abrupt. Let me do a follow up now that the hampsters in my PC will let me.)

I don’t follow drag racing, but I’m pretty sure that all of the top classes allow the use of automatic transmissions and torque convertors that are smaller than what would be used in a stock car. The smaller torque convertors are less efficient at lower RPMs, but they allow the drivetrain to slip until the engine reaches a higher RPM.

Above that higher RPM, the convertor acts to convert any slippage inside the convertor into torque. This allows the engine to turn at a more constant RPM as the input to the transmission changes RPM.

Racers who use manual transmissions have to feather the clutch and gas as they launch, but they lose power by slipping the clutch. On the other hand, the “slippage” in a torque convertor is at least partly converted back into torque.

If we’re talking about Top Fuel and Funny Cars (mechanically, they are identical), then that’s basically what they do. These cars use a pneumatically actuated multi-stage centrifugal clutch system that is “programmed” by the crew chief in the pits. The complex timer system runs it’s course and is fully engaged about 1/2 to 2/3 of the way down the dragstrip, though this varies depending on the setup. It also has a series of fingers that are actuated by centrifugal force at different RPM’s.

Top Fuel/Funny Cars don’t change gears, either. A Top Fuel car doesn’t have a real transmission per se, just a reverse gear. It’s otherwise a direct 1:1 drive. However, the tires are designed to expand with rotational speed by centrifugal force (they are about 6 inches larger in diameter at top speeds than when at rest), and this effectively provides an “upshift” of sorts.

A Top Fuel clutch uses 4 or 5 carbon friction plates, and temperatures can reach 1000+ degrees on a run. How do they cope with such conditions? They replace them after every single run. (Plus, they still sometimes blow up anyway. :slight_smile: )

cornflakes, torque converters and automatic transmissions are almost never used at the top levels except in breakout classes, where consistency is more important than top speed. That’s because manual transmissions are lighter and have less mechanical losses than an automatic, and this means more power to the wheels (and at the very top levels, transmissions are basically unnecessary). Plus, there are limits to how much power you can put through a fluid coupler like a torque converter (you run into cavitation problems at some point). Even in classes such as Pro Stock where the driver shifts a transmission, the clutch is manually controlled by the driver.

(As an aside, Pro Stock and similar cars have an interesting shifter arrangement. The transmission has individual levers for each gear, and the driver just grabs and pulls the lever for the gear they wish to change into. This is a much easier, faster, and more consistent way to shift than trying to bang through an H pattern.)

It occurs to me I ought to explain what a “breakout class” is. Breakout racing is a form of handicapped racing. The competitors predict how fast they will run (their “dial in”), and the driver who comes closest to their dial without running faster (“breaking out”) wins. If one driver breaks out, the other wins automatically. If both drivers break out, the driver who ran closer to their dial-in wins.

Thanks Jet.

Jet Jaguar: what do you mean by cavitation?

i guess this question boils down to why we can’t use fluids to do this conversion job and instead use carbon plates!? there’s gotta be a way! what are the problems that they run into trying this?

thanks for your reply.

JJ: also, when those tires expand like you said, do they make them like that on purpose in order to gain that upshift? wouldn’t that be a tradeoff between more surface area in contact with road and larger “gear”? i guess it’s a positive trade-off?

since you said they indeed do “rub” 1/2- 2/3 down the strip, why don’t they just “rub” all the way down, while revving the engine a little higher? i guess that’s my main question.

thanks in advance.

they would like the clutch to not rub at all. the problem is that if the clutch is set to lock right off they would just spin the tires.

the torque converters that race cars use do not really rub. they have higher stall speeds. this means that it allows the engine to rev higher before sending the power through the trans to the wheels. in a normal passenger car, at idle, in gear, the car might begin to move a bit. when ya step on the gas you will notice that the engine rpms will flash to about 1500 rpms then the car will begin to go. a race car may use a 3500 stall converter. this does not mean that the car will act as if it is in neutral until 3500 rpms.

btw, even the cars that shift try to keep the rps high, and even during shifts they will operate in a somewhat narrow range. launch at say 3500 to 5000, rev to 6500, drop to 5000 at each shift, and back to 6500. this would be a fair estimate/example for a 700 horse power car running a high 9 second 1/4 mile.


either you are wrong, or you don’t understand my point and jet jaguars point here.

trust me, you wanna rub, in order to:

  1. NEVER have to switch gears.

  2. NEVER for one second have to be dealing with any lower output than peak horsepower.

  3. be testing the friction between the road and tires AT ALL TIMES.

you described multiple RPM’s in your reply. those different RPM’s have different power outputs. my point would be to always have a pool of constant peak horsepower ready whenever it’s needed by having the engine rev higher at a constant peak RPM, have one BIG gear and be testing the friction between tires and road the whole way down the strip.

jet jaguar said that’s what the TOP FUEL cars do, but only 1/2-2/3 down the strip. i am wondering why they don’t make the engine rev a little higher and rub it down the strip the whole way. (if you are gonna do it at all, why not do it the whole way?)

Fuel, cavitation is where a fluid is decompressed quickly enough that it transitions into a gas state. Jet Jaguar was saying is that if torque convertors were used the transmission fluid would be compressed and decompressed enough in the convertor that gas bubbles would form. In this case, I think the froth of bubbles and oil wouldn’t transfer the engines power efficiently.

If by rub you mean “slip”, here is your answer.

When the clutch is slipping it is generating heat. Generating heat ALWAYS uses up some of the energy that could be doing useful work. So, the idea is to make the clutch slip for the amount of time it takes to get the car to a speed that locking up the clutch still leaves the engine in the fat part of its torque curve. Every bit of slippage beyond that is wasting power.

In top fuel, the amount of heat generated in the clutch packs during the second or two that they slipping is enormous. This brings another problem, it would be difficult to design effective, light weight clutches that could resist completely melting down if they were allowed to slip the entire race. In fact, as it is today, those clutch packs are typically completely destroyed by the heat and must be replaced every run.

I thought the reason for sliping the clutch for the first part of the run is because the engine made more power than the available traction. I saw or read this somewhere, is that wrong?

Well, that is somewhat true. It needs some qualification to be accurate.

These cars have one gear in them, and that gear is design to allow the car to run 340 mph (or there abouts) before blowing up. This makes the top gear in most road going cars look like nothing. Now, keep in mind, that this is the gear they are leaving the line with. One of three things are going to have to happen until the car gets up to a speed that the engine car happily with the clutch fully engaged. The tires are going to have to spin, the clutch will have to slip, or the engine will have to die or lugged under optimal rpm.

This gearing is so tall (high) that the car would easily run hiway speeds at idle. So, the clutch cannot be simply engaged right off the line. Something has to give, clutch, tires or engine.

A truer statement of the facts is that these engines make so much power that using a shorter (lower) gear to get off the line with makes them nearly uncontrollable (i.e. very difficult not to just smoke the tires off the car). So, it was determined that they could get better/more consistent times by using one gear and slipping the crap out of the clutch to leave the line.

This has only come about in the last few years. If I remember correctly Gene Snow pioneered the use of the one speed clutch only top fuel car. This would have been around 1986 or so. He was much more consistent off the line, but was loosing if the two speed (what was state of the art at the time) car really hooked up well off the line. Over the next 7 or 8 years, other people tried the idea off and on as well. Finally, as the horsepower continued to creep up, the single speed car became more and more popular. Now it is consider the way to go and nearly (if not all) everyone uses that set up. All it took was so much power that a lower gear was a liability and the ability to deliver as much torque as the tires could use even in “high” gear right from the line.

From what I have read, a top fuel dragster makes around 6000 hp at around 6000 rpms (torque, who knows?), and runs the quarter mile at over 300 mph in about 5 seconds.

A multi-speed transmission reduces available speed at lower gears, but multiplies torque.

At 3:1 gear ratio, 6000 rpms are reduced to 2000rpms, but a torque figure of 6000 foot/pounds (pound/feet?) are increased to 18000!

I don’t think many drivetrain components can handle that kind of twisting power.

The current time record is 4.477 seconds and the speed record is 332.18 MPH, both of which are mind-boggling. That’s 0-100 in less than a second! They have little wires connecting the chin of the helmuts to the center of the chest harness because if their heads are slightly off center when they start, their necks would get twisted by the force of the acceleration.


Occasionally there is a tire failure because the axels spin so fast that they rip away from the rest of the tire, leaving only a tattered bit of rubber on the hub. If you watch a slow motion shot of the starts you can see the outside of the tire strain to catch up to the wheel hub.

so these cars do have multiple gears, instead of one big 1:1 gear?

could you describe this transmission? how far down the strip does the car go before the clutch is fully engaged? and then the golden question of why don’t they rub the clutch not just 1/2-2/3 down the strip but the whole way down?

No, they have one gear and a clutch. That’s it. (today, but not so in the past)

Actually, what they have isn’t rightly called a transmission. It is simply clutch packs driving the differential.

As I already pointed out, the reason they don’t slip the clutch the whole way down the track is because when the clutch is slipping they are wasting horsepower heating up the clutch.

:smack: of coure a gear that will run 300+ MPH can’t be fully engaged off the line. Now I remember the context of how I got that information. A dragster mechanic was talking about how the clutch had to be set up to slip just enough to use all the available traction depending on track conditions for the day. From the way he was talking available engine power was not a factor (because it was more than enough).