What would happen if....

You launched your car in 4th gear and just left it there through the entire run? What would your MPH be at the end of the track? Would it be the same as your MPH if you raced normally?

This is a very interesting question to ponder. People claim that quarter mile ET isn't a good indicator of the power your car is making but MPH is. I think this example illustrates that you can agree to that arguement only to an extent, such that other conditions are not changed significantly. Once the conditions change, such as gear, transmission, drivetrain, you cannot accurately compare the power level of two cars based strictly on quarter mile MPH.

Thoughts?

BTW if anybody thinks that this is a thread strictly intended to argue with Jesse, it isn't. It's a legitimate question that I feel like brings up a legitimate point. I'm also fully aware that the answer could be that the MPH could be damn near identical. If this is the case I find that this is a very interesting thing to ponder, and it proves the point that MPH is a good indicator of power under any and all conditions.

 

I think the MPH will be very close.
How much time do you spend the first 100 feet?
How much does the MPH increase the last 100 feet?

 

The consideration would be the amount of power that the car is putting to the ground. You put a lower amount of power to the ground so you don't accelerate as quickly. However since you don't accelerate very quickly the engine remains in that RPM band longer. Many cars have a very "peaky" power curve in which they only make good power at high RPM's. So it would be putting less power to the ground over a longer period of time because the lower power doesn't accelerate the car as quickly.

If the MPH ends up being close you could begin to consider the total power applied to the track as a function of the integral of horsepower plotted vs time. Does the lower power output over a longer time net a larger area under the curve than does a much higher output over a much shorter time? If so how does that reflect the MPH at the end?

It's really pretty fascinating if you think about it, or maybe I'm just an engineering geek. Maybe I should contact Mythbusters.

 

Larry
code5coupe

 

I do not know the arithmetic behind this. My car will lose quite a few mph if I accidently shift into 4th gear on a 1/4 mile run.

 

I think it depend on the tq band and cam rpms

 

If we focus on that statement alone, then it it easy to explain.

The first 60' has a very big impact on ET. A bad or a good driver can make a huge differance in ET.

On the rest of the 1/4 mile it is much easier to make consistent runs. The differance between a good an a bad driver will be much smaller.

If you loose half a second in the start, that differance will increase all the way to the finish linebecause you will have a lower MPH at every point of the track.

The MPH is different ... it will rise fast at low speed and rise slow at high speed. If you loose 5 MPH the first 60', that differance will shrink during the rest of the 1/4 mile.

If you compare a perfect run, and a run with a bad start.
If you put these numbers into horsepower calculators, ET to HP and MPH to HP, and compare the results. Then the MPH to HP will have smaller differance between the HP numbers.

ET depends more on the driver and traction at the start.

 

I always run in 3rd (regular D) so I can't help you there...I do know the car makes whatever HP it makes..no more, no less...

Last Thursday I had 3 runs at the 1/8 mile track...my mph was 82.12, 82.12 and 82.42...on the last run I had an awful short time spinning like crazy out of the hole...I normally trap a little over 80 mph but the DA was a tasty +1200 (welcome to Florida) so I made a bit more power...

 

Here's teh deal, we've found that when you spin off the line, you usually end up with a higher MPH than if you hooked off the line. That's been the case with me,DaveT,Panuzzo,Mackey,BobT,BobS,JimK and about every other regular at E-Town.

The theory is that since the tiress spun, you are higher up in the RPM sooner. I never really looked into it any further, but the timeslips tell all.

Now as far as your experiment of startig in 4th. If you took out the clutch slippage it may bring some interesting results. I can tell you that in my car, I'm not shifting into 4th until the 1000' mark, so that only leaves 320 ft in 4th gear and I cross the line at about 5000 RPM, so I still have 1000 RPM to go.

Only way to find out is for someone to do it. But I think, trying to get the car rolling in 4th is too much abuse, so you may want to try short shifting and running it that way, but you may lose too much ground at the start due to the car trying to get moving. How many feet would it take before you reached say 2500 RPM? and hopefully not slipping the clutch?

 

Nathan, I like the question too, because it gets to a lot of mis information in drag racing. Here is what I know:

For several years I raced:
1. A 1971 GMC 3/4 ton pickup with a 496 inch "grunt motor", about 470 Hp
2. A 1987 Pontiac GTA with 383" engine, about 375hp
3. A '32 Bantam altered roadster bracket car, 505" BBC, injected on alcohol, about 700HP

During all three cars, I used Quarter Jr computer simulation extensively for gear ratios, tire sizes, weight changes (related primarily to the altered roadster). This program includes variables for weather, cubic inches, weight with driver, type of induction and fuel, HP/torque maximum RPM's, transmission gear ratios/converter size and stall rpms, rear-end gears, body cross section and airodynamic factor, tire width/ diameter and degree of both tire and track "stickyness". Essentially every significant variable we face on the drag strip.

What I can say for sure is the program is extremely accurate once the torque and Hp curve are known and the other variables listed are input. Accuracy in ET and MPH to under .005 seconds et and 1/2 mph terminal speed.

As this all applies to your question, once the power curve (what RPM max torque and HP occur), the other variables such as weather variations, weight, gears, etc can be changed. What typically results is variations in ET but very consistent, similar terminal MPH. I am talking about anticipating changes in the computer simulation, for instance a gear or tire change, or shift RPM and terminal MPH. The latter tends to stay very much the same, whether one winds the crap out of the engine (I shifted anywhere from 6,500 to 7,500 to see what would happen ) or not, the mph tends to stay fairly constant, barring engine HP changes. But ET can vary significantly. Although I did not make many changes in the two door cars, the Qtr Jr was very accurate when compared to actual results on the track with them as well.

edit: I recall one time accidentally setting the stall switch at 3,000 (lowest on the dial) instead of my usual 4500-5000 stall. The roadster had a terrible reaction time, but the MPH and et were same as a normal launch. Maybe this helps with your question about what would happen when launching in high gear.

 

Well I made a run where I screwed up my 1-2 shift and recovered and went into 4th after a delay.........

My speed was 103.47.......run before that pass was 110.8, two runs after that were 111.16 & 111.12......my 4 runs of the night...........

BTW this is with 4.10 gears so I am crossing the finish in 4th somewhere around 6100 rpm @ 111mph

 

To launch in 4th....



Smoke. Lots and lots of clutch smoke.

 

I would think not , it would take to long to get the weight of the car moving, and by that time you would be to far into the 1320 to mph.

 

That's what I would think!!

 

You will not get the same MPH and it won't be even close especially in a semi heavy street car. An example is very simple. Get on your 21 speed bicycle and start out in first gear and peddle 200 feet starting out in low gear and shifting as you increase crank speed. Then try to start out in 21st gear and try to even peddle. Your car is not any different.

However, if your car is designed more for torque than horsepower you will see a much higher MPH in 4th gear runs because you will be able to make better use of your power in the lower rpm range which is exactly where your engine will spend most of it's time.

Still question this. Put a powerglide in a heavy street car make a run using both gears and then put a turbo 350 3 speed in the same street car and make a run using all 3 gears and the 350 will run a faster MPH. You will run a higher MPH with the 350 because you car will be able to utilize the power faster which will in turn get your wheel speed and engine RPM into it's power band much faster. This example is a good one even though the powerglide actually takes less power to operate due to less rotating weight but the 350 will still be faster.

 

I agree, You woudln't see anywhere near the same MPH..

I back it up with my experience.. My car picked up around 4mph in the 1/8th just going from 3.23 to 3.73. My 00 Z picked up 2 mph from 3.42 to 4.10's.

 

If I shift to 4th - I can kiss the run goodbye.

Just a note - I just picked up a Rossler built 700r4 and Mr.Rossler told me to put it in third and floor it. He specifically said - do not use 4th.

 

If you do this comparison in a light weight car the MPH will be much closer than if you do this comparison in a heavy street car.

 

Yeah, the others have it right.... mph would be "wayyyy" down....

What I have discovered with "my" set-up, is that if I just spin the tires 1/2 or even 3/4 through 1st gear, my mph will be un-effected, although et will obvisouly be hurt hard. However.... if my car spins all the way through 1st and into the beginning of 2nd, my mph starts shooting down pretty hard. However, I have heard of others not losing mph even spinning into 2nd gear.... thus, I think a factor to this has to do with the torque curve of your particular motor and rpms you come out of your spin at.....

 

here is my 2 cents i think it would be close possibly within 3-5 mph but that is in a auto left in 3rd and a stick in 4th
but to compare hp numbers there is also differences going to happen a stick car will mph 2-4 mph more than a auto car everything else being equal
then a street tire car should say radial tire will also mph 1-2 more than a slick will also
so for best mph the best combo would be a stick with street tires but i would hate to see the et in that particular application tho
so the worst case scenerio is a auto car with a slick mounted to it but it is also the most consistent (lol)