I just got back from the Vegas NHRA races this weekend... Awesome btw, but... Had a question... are those top fuel dragsters making boost at their 2000+rpm idle? You can really hear them, but I've always wondered at around 2200-2400rpm "idle" if there is boost being created. I know these things run upward of 60-70psi boost (insane) so I would think at 2400, it would. Any of you know and if so, how much?

Tony

 

yes probably...and they are high compression...like 18:1 or more?

 

No, 6.5:1! Check it out on wikipedia!

 

The superchargers are usually at around 35% overdrive. If you take into account that they are setup to make the maximum boost, 45.5 p.s.i., at around 8,500 engine rpm, that would be around 12,000 supercharger rpm. The blower would be turning roughly 3,500 rpm at idle, so I'd imagine that it is making about 10-12 p.s.i. boost at idle. Top Fuel motors are unique when it comes to compression ratios, 6.5:1 is basically an average of all eight cylinders. They use staggered compressions, one cylinder may only be 5.75:1 while another may be 7.5:1. It all depends on the way that boost is delivered to individual cylinders. You'll never get any definite answers from anybody on the Top Fuel subject. Engine builders are a pretty secretive bunch. I've talked to a couple guys over the years that tractor pull in the unlimited class and they basically use methanol versions of top fuel motors, so it's sort of an educated guess.

 

By definition (manifold pressure is proportional to engine load), they are not making boost at idle. If they were making boost, then the engine would be on the limiter or propelling the car forward. The butterfly valve is keeping the blower from making boost.

Diesel engines are the exception to this since they do not require a throttle air valve. Load is independent of manifold pressure.

 

That is awesome, thanks for clearing that up? Is there a reason why they have to idle so freakin high?

 

Also with the throttle blades closed very little air can come in. Its hard for the roots, or any style blower to make boost out of very little air, but once you open the blades up. It will be boosting. I've had the same question myself and this was basicly the answer an engine builder gave me.

 

I never knew that about the 6.5 or so to 1 comp. that is a trip. I guess under 50-60lbs of boost, 10,000 hp is achievable. cool! I need one in my old z28.

 

There is a guy at my track there in Texas that was testing an F4 Procharger for them. He was running 40 lbs of boost on a 19.0:1 compression motor nd running straight alcohol. He got to the line and took off. He let out just before the 330 marker, coasting the rest of the way, and still ran a mid 9 sec pass. That was awsome to see.

 

That certainly calls for the old BS flag.

 

Yep... 19:1 + 40psi don't match.

 

I dont know about 19:1 and 40psi but a lil under that is doable on Alky

 

That sounds like a diesel motor or something

 

Actually I've seen a show on (I believe it was Top Fuel, Either way) It said that they ran 14.1 compression with the huge *** blowers but they don't run on methanol.. They ran them on nitromethane... The stoich ratio is something like 2:1 or some crazy number... Just something I remember seeing, please do not quote me... I think it's interesting none the least... I really can't imagine going WOT in one of those things...

 

Actually they use different lenght rods to set compression in each cylinder. They can control the amount of fuel in each hole, but not the air. The blower has a tendency to blow more air into the front cylinders, hence the diff compression ratio's per hole. They use air dams to regulate the air flow and switching to rear-set blowers helped alot.

 

Well I told the story the same way it was told to me. Oh well.

 

they have a very low CR but they make up for it with the volume of fuel... since you cant compress a liquid thats how they build their CR.... they squirt tons of fuel on top of the piston and since it cant compress a liquid it like having a huge domed piston...

 

thats a good point! its probalby not that relivent in a gas car as the ratios are much less. but if you are running afr much lower (what are the approx afrs for one of these things?? 2-1??) then the fuel would dramatically increase the dynamic comp ratio of the engine.

Chris.

 

Are you sure he didn't blow the motor before the 330 marker?

Someone threw an extra digit in there somewhere - maybe 9.0:1 compression?

Jim

 

# One Top Fuel dragster's 500-cubic-inch Hemi engine makes more horsepower than the first four rows at the Daytona 500.

# A stock Dodge Hemi V-8 engine cannot produce enough power to drive the dragster's supercharger.

# With 3000 CFM of air being rammed in by the supercharger on overdrive, the fuel mixture is compressed into a near-solid form before ignition. Cylinders run on the verge of hydraulic lock at full throttle.

# At the stoichiometric 1.7:1 air-fuel mixture for nitro methane, the flame front temperature measures about 7000 degrees Fahrenheit.

# Nitro methane burns yellow. The spectacular white flame seen above the stacks at night is raw burning hydrogen, separated from atmospheric water vapor by the searing heat of the exhaust gases.

# Dual magnetos supply 44 amps to each spark plug. This is the output of an arc welder in each cylinder.

# Spark plug electrodes can be totally consumed during a single pass. After half-distance, the engine is dieseling from compression plus the glow of exhaust valves at 1400 degrees Fahrenheit. The engine is shut down by cutting the fuel flow.

# If a spark plug fails early in the run, un-burned nitro can build up in the affected cylinder and explode with sufficient force to blow the cylinder head off in pieces or split the cylinder block in half.

# In order to exceed 300 mph in 4.5 seconds, dragsters must accelerate at an average of more than 4 g's. In order to reach 200 mph before half-distance, the launch acceleration approaches 8 g's. A Top Fuel dragster reaches more than 300 mph before you have completed reading this sentence.

# With a redline that can be as high as 9500 rpm, Top Fuel engines turn approximately 540 revolutions from light to light. Including the burnout, the engine needs to survive only 900 revolutions under load.

# Assuming that all of the equipment is paid off, the crew works gratis, and nothing breaks, each run costs an estimated $1000 per second.

# The current Top Fuel dragster elapsed time record is 4.441 seconds for the quarter-mile (October 5, 2003, Tony Schumacher). The top-speed record is 333.25 mph as measured over the last 66 feet of the quarter-mile (November 9, 2003, Doug Kalitta).

# Putting all of this into perspective: You are driving the average $140,000 Lingenfelter twin-turbo Corvette Z06. More than a mile up the road, a Top Fuel dragster is staged and ready to launch down a measured quarter-mile as you pass. You have the advantage of a flying start. You run the Vette up through the gears and blast across the starting line and past the dragster at an honest 200 mph. The "tree" goes green for both of you at that moment. The dragster launches and starts after you. You keep your foot down, but you hear a brutal whine that sears your eardrums, and within three seconds, the dragster catches you and beats you to the finish line, a quarter-mile from where you just passed him. From a standing start, the dragster spotted you 200 mph and not only caught you but nearly blasted you off the road when he passed you within a mere 1320 feet.