Landracing Forum
Tech Information => Technical Discussion => Topic started by: Rex Schimmer on December 03, 2015, 05:01:44 PM
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Well it is winter time (at least if your are north of the equator) so I think we need to do a little thinking/calculating/guessing about something we have kicked around before but because of the latest trend in fuel funny car we might masticate on this one again. It seems the funny car guys have made a move to zoomies with a more laid back sweep and they claim that the increase in forward thrust from this change is one of the big reasons that they are suddenly going faster and quicker. From what I have read they have changed the header angle from 38 degrees from vertical to 45 degrees and this would increase the horizontal thrust by about 13%, which if the claimed total header thrust of 4000 lbs is accurate, mean that the horizontal thrust of the headers increased from 2460 lbs to 2830 lbs. That is an increase of 370 lbs and at 300 mph this would be the equivalent of approx. 300 hp.
So how does this apply to Bonneville? I am thinking of the 911 CFR of Cummings,Beck, Davidson and Thornsberry, which runs what is in reality a small top fuel/funny car fuel motor. If you use the weight of the car, 6500 lbs, and assume that they have approx 60% of the weight on the drive wheels and use a salt coefficient of friction of .5 and use the classical James Watt definition of horse power it then calculates that they are able to put down about 1560 horse power at 300 mph, a number that I feel is a little low to drive this car at 300 mph. If we assume (guess) that they are able to produce 1/2 of the header thrust that a 500 cu.inch top fuel motor, 2000 lbs of thrust, and looking at pics of the car assume (guess) that the headers are at 45 degrees then the horizontal vector of the header thrust would be 1414 lbs and at 300 mph this would be equivalent to 1130 horse power! Which would make the total power of 2690 hp! Now that should make that "barn door" roadster go 300 mph!! Note that I am not taking into account the vertical component of the header thrust which would also be 1414 lbs of down force which would increase the rear tire load based upon the ratio of the header position to the wheel base of the car.
So what am I saying is that header thrust can be a huge contributor to top speed especially if you are running copious amounts of nitro, which, when used in excess, acts like "rocket fuel" actually burning in the exhaust to provide additional thrust. I am not sure that cars like Danny Thompson, Treit and Davenport and others that are running fuel have considered this as a supplement to their total drive package but at the speeds they are planning to go it can represent some pretty huge horse power.
Any way it is cold and rainy where I live so I think we need some more comments on this.
Rex
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Why would they produce half the thrust of a funny car if they are not producing half the horsepower of a funny car? Also what Cd did you use for your hp required calculations?
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. . . and you forgot to add the Davidson component. Dave sits pretty far back, so the weight of his balls add to the traction coefficient -- something not usually figured into these formulas.
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That was a good one, Jim.
Franey
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I've thought about this, on and off, for many years -- specifically with respect
to its application in drag racing.
Since dragsters have, for decades, been accelerating horizontally at more than 1G,
it's obvious that the coefficient of friction between the rear tires and the pavement
is more than 1. So it made sense, to me, for the exhaust to be as vertical as possible
when leaving the vehicle, provided, of course, that traction limitations were the thing
keeping the car from going any faster/quicker.
At Bonneville, for the more powerful wheel-driven vehicles, traction certainly limits speed
attained, so it seems to me that having the exhaust exiting vertically would help more than
aiming it toward the rear.
I suspect that the amount of exhaust thrust from a wheel-driven vehicle is significantly
less than was implied earlier in this thread. Here's why: Only a severely-limited amount
of combustion takes place, resulting in a severely-limited amount of exhaust thrust. Why
is the amount of combustion so limited? Because the desired traction is not there. Remember
that a funny car's horizontal acceleration is about 4G, so the coefficient of friction must be
at least 4. This is about 8 times what's available on the salt, barring the use of clever things
like spoilers, which I strongly encourage. I think the Carbiliner is, in the foreseeable future,
going to impress us all by going well over 500 mph while using "negative lift" provided by the
airfoils that contain the rear axle(s).
While putting out limited horsepower (because of traction limitations), one is only burning a
limited amount of fuel; therefore, the exhaust thrust is going to be limited also. A dragster
can get away with lots of all this stuff -- power/fuel consumption/thrust -- because there is
so much traction available on a drag strip.
For Bonneville, I think it would be helpful for a fast car to use some of its horsepower --
okay, maybe a LOT of its horsepower -- to generate downforce at the driving wheels. And
then maybe less ballast (or mass in general) would be needed to keep the car "on the ground."
With enough available horsepower being used to provide traction, then aiming the exhaust
rearward should help accelerate the car horizontally.
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Aerodynamic drag should also be considered. I see a lot of cars with side exit exhaust and can't see how expelling all that hot expanding gas isn't causing drag. Our exhaust has always been rear exit. It was originally pointed upward, but on the advice of someone whose identity escapes me, we reoriented it rearward. The idea being that the hot expanded gas would cool when mixing with the air coming off the spoiler and create a negative pressure pocket, thus reducing downforce at the rear. The car was faster with the pipe pointing back AND felt more stable at speed. We're working with 900cc and maybe 100rwhp so for it to make a few mph difference means something.
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The ratio of engine power to exhaust force can be manipulated. Cacklefest engines are being built now for the highest noise/power ratio.
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Imfoley79 asked: "Also what Cd did you use for your hp required calculations?"
The method that I used does not require knowing anything about the cars aerodynamic characteristics, it only requires what the surface coefficient of friction is, my guess for Bonneville is .5 which is high but it is a level that cars like the 911 roadster need to go fast, you need a good estimate of the car's weight on the drive wheels and the cars terminal speed. If you multiply the car's drive tire load times the coefficient of friction you come up with the maximum amount of forward thrust that the tires and provide. In the case of the 911 roadster I used 60% of a car weight of 6500 lbs which equals 3900 lbs on the drive wheels, multiply that by the coefficient of friction, .5, and you get 1950 pounds of thrust, which is the maximum forward thrust (force) that the roadster can apply to the salt. Using Watt's definition of horse power: HP= 33,000 lbs raised one foot in one minute, and doing some math to convert 1950 lbs of thrust exerted at 300 mph
(1950 lbs thrust)(300 mph)(5280 feet/mile)/ (60 minutes in an hour)(33,000)= 1560 horse power. This is the amount of horse power that the 911 roadster can apply to the salt based upon my assumptions. Although a salt car definitely does not have the engine load at the hit of the throttle that a top fuel car has, once it begins to gain speed the engine load is on a very steep rise due to aero drag. Obviously the fuel application on the roadster is not at the same rate as a top fuel car but they do use a considerable amount of fuel. I asked Beck one time why he only ran to the 3 mile marker and his answer was that he only had 30 gallons of fuel and he did not have enough to go further!! So they do consume copious amounts of fuel and also note that Danny Thompson's twin motor liner carries 60 gallons and he consistently runs in excess of 75% nitro.
Rex
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A good top fuel engine uses >1 gal nitro/sec. Will need a big tank.
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You folks do realize... That header angle was brought to light by a DSR driver saying on national TV, it was the reason why they were faster.. which means in race car land..... don't look at my CLUTCH controller... and realize the amazing 60 ft, 330, & 660 times I'm running... sure it helps at 320 mph, but the DRAG race is OVER by then... I'm not a drag racing genius... but I know one.. .. and I did stay at a Holiday Inn once.. though.. :-D
In nostalgia Top Fuel, some cars are looking for down force, and running headers vertically..
As for header angle.. in Land speed.... I'm with you... but, only cause it looks cool.. (which means I'm lying) :cheers:
bob
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Harkening back to the relationship T = me * ve
where T = thrust, me = mass flow rate at the exhaust, and ve = exhaust velocity,
Assuming 100% nitromethane consumption at Jacksoni’s 1gallon/sec rate, fuel density of 9.5 lb/gallon and an assumed 200 ft/sec exhaust velocity (12,000 ft/min) we get--
T = (570 lb/min)(12,000 ft/min) = 6,840,000 lb-ft/min^2 = 59 lbf thrust. A reasonably believeable figure.
Where does the alleged “4,000” pounds of thrust come from??
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Beats me. I tossed it out as anecdotal when I realized 3000lbs thrust is more than a healthy 289 Ford with a c4 and 3.70 gears could put to the back tires at it's torque peak. I have trouble believing a Top Fuel dragster could out accelerate a decent street car using exhaust thrust alone.
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I have a Nike Ajax rocket engine made by Aerojet General. If I remember right, it has 2600 lbs static thrust and burns 10.6 lbs of fuel/oxidizer per second. I doubt that header thrust would come anywhere close to this.
Regards, Neil Tucson, AZ
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Watch this you tube of Jeff Diel when the left side headers come of his car at about the 600 foot mark. https://www.youtube.com/watch?v=BxEBwPQFfpw
Tell me what you think almost blew his car completely over if not the thrust from his exhaust.
Rex
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I would imagine that having that hot expanding gas explosively vented UNDER the body shell rather than outside of it contributed to the problem. Like Earl's Crosley, which had no belly pan, if the car is a gaping void underneath and you lose negative pressure beneath the body it wants to lift off the ground, quickly. Obviously there is some significant thrust because that funny car wants to turn as well but not 2000lbs worth. That's just unreasonable.
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Basically what I'm saying is having the thrust directed between the track(hard solid surface) and the car allowed the effect of the push to be far more effective than venting the gas into the open air.
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hot expanding gas explosively vented UNDER the body shell
10000 sq inches of under body 1/2lb pressure = 5000 pounds of lift
wait.... now I understand Nascar bodies / front fender / wheel well configuration :-o
nascar sucks ...
their cars down
fans too...
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It seemed to me that I had read something about exhaust thrust being used in WW2 aircraft so I googled it and found this on Wikipedia: "Ejector exhausts[edit]
The Merlin consumed an enormous volume of air at full power (equivalent to the volume of a single-decker bus per minute), and with the exhaust gases exiting at 1,300 mph (2,100 km/h) it was realised that useful thrust could be gained simply by angling the gases backwards instead of venting sideways.
During tests, 70 pounds-force (310 N; 32 kgf) thrust at 300 mph (480 km/h), or roughly 70 horsepower (52 kW) was obtained which increased the level maximum speed of the Spitfire by 10 mph (16 km/h) to 360 mph (580 km/h).["
These motors produced up to 2,000 horsepower.
Tom
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So...... I have a question.......
If we disconnect the drive line on a fuel car............ point the exhaust toward the rear of the car............then hammer the throttle............ will it move?
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And by proper careful design of the ducting, the P-51 had useful thrust from the radiator cooling ducts rather than drag. We could use some of that sort of design ('cept prolly run afoul of the radiator rules. Oh Well. :))
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Dynoroom..
Austin Coil did that test.. way back in the 90's
still waiting for the ET....
Should be done next year.. though :-D
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I'm envisioning a LARGE flat bumper on a push-car staying no more than an inch from the exhaust tips to detect any movement :-o
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So...... I have a question.......
If we disconnect the drive line on a fuel car............ point the exhaust toward the rear of the car............then hammer the throttle............ will it move?
That was my question too. 3000lbs thrust should make it beat a 60's era muscle car to the 60' line.
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Mike,
Maybe the real question should be: If we take an enclosed container with one end open, and pump 100 gpm of 90% nitro into it at 500+psi and ignite it, will it become a rocket? Which is exactly what fuel motors are doing. The 4000 lb number is not mine it is the number that the crew chiefs, Jimmy Prock, Mike Green, John Medlen etc, are talking about.
Rex
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True that Rex :-)
Just asking a question. I learned a LONG time ago....... Never say never.
I also learned from a real smart racer the word..........
Deception. :wink:
Beside, I know more than a few racers who went through the trouble to run the exhaust all the way out the back of the car........ Maybe not for thrust though. :-o
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So...... I have a question.......
If we disconnect the drive line on a fuel car............ point the exhaust toward the rear of the car............then hammer the throttle............ will it move?
This happened to us at EL mirage with the 222 Camaro. We fired the car up and my brother took his time getting in the push truck. Troy was blipping the throttle and it moved several ft away from the push truck. We angle the exhaust back and up to get downforce and thrust. We set the hood on top of the car when warming up and have to hold on to it when blipping the throttle as it almost blew off once.
If a top fuel car has a cylinder go out it moves to that side from the thrust from other side.
JL222
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Somebody has to mention the Convair aircraft UA was flying when I started there. Two 2800 cubic inch blown hemi engines on race gas. ( more or less) With Exhaust Augmentation. Cool headers and cowling to dump the exhaust out behind the engines and provide thrust. Someone else can Google it and find out how much. Then look up Turbo compounding. 3350s
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When the late, great Don Vesco set his 458 MPH record a few years ago someone on this board indicated that maybe the location/angle of the exhaust pipe was illegal because it appeared(my word) to contribute to the thrust/speed of the vehicle.
I should have posted this in the Friday Funnies thread I think,
DW
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Large amounts of header thrust is unique to blown nitro engines that need long cam overlap to let liquid fuel cool the exhaust valves . That the nitro released it's oxygen in the headers and turned them into rockets was an unexpected bonus . A little later somebody tried aiming the thrust upwards for traction . Thrust comes from pressure being less on the open end of the tube than in the other directions .
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What has been the average increase in "mph" with the headers at 45* :?, the answer to this would be found there, :wink:,,,