Calculating HP from 1/4 mile time and weight... 2

[mtrehy]

MG Maestro 1/4 mile speed is 12.48 sec and 127 mph - I estimate the weight to be 2200lb.

Please can someone give me a hp calculation and workings out.

Thanks

Regards...

 

[ivymike]

here's an idea... try searching on yahoo for "calculate horsepower from quarter mile" and see if you come up with anything...

 

[patprimmer]

Programs like Desk Top Dyno 2000 or the Moroso slide rule will do that for yoy, but you need exact figurers including weight with the driver aboard, as raced to get accurate figurers.

I will run the numbers for you tomorrow when I have more time

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[zimbali]

Here we go:

v*v= 2*a*d (V squared = 2* acceleration*distance)
you have v=127 mph and d=1/4 mile, so you can find "a"

We all know that F=m.a where F is force and m is mass. You now have "a" and "m" and you will have F. This is the average force that engine produced in 1/4 mile drag.

Total work done in this distance is W=f.d. If you divide this work into the time (12.48) you will have work done in each second, which is POWER. Then remember that 1bhp= 736 watts (if I remember correctly !! ).

I wish it helps.


Cheers


You can live in your car, but you can't drive your House!

 

[CRG]

If only engineering were so simple, or perhaps, you assume that the force required overcoming air resistance is not substantial. Is full power supplied instantaneously from a stop? I don’t think so. Energy is transferred in heat due to tire and clutch slippage. No consideration for driver reaction time? This is not my field of engineering; however, it seems like this problem is more complex than just finding average acceleration.

 

[patprimmer]

CRG

The simple method, while not perfect, is more accurate than you might expect.

1) Zimbali's statement does say AVERAGE power, so change in power during the race is not relevant to that statement.

2) Our car launches at 5000 rpm, and finishes at 8000 rpm, but it accelerates very quickly (about 1.6 seconds I think) through low gear to 8000 rpm it only drops to 6500 on the change, so it is close to maximum power for 90% of the trip.

3) If it is set up right there is not much tyre slip, but the torque convertor does slip and loose energy to heat. It should be corrected for.

4) Driver reaction time is not an issue as the timer starts as the car moves, not as the green light lights up. We could sit at the starting line for a minute, lose the race by about a minute and still record an 8 second run if that is what the car actually did.


Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[CRG]

Is air resistance insignificant for the last half of the run?

 

[EricFahlgren]

Well, that depends completely on the car. For a grocery getter that's going to turn a 20 second time at 70 mph, almost the whole run can ignore aero effects. On the other hand a top fuel car hits 100 mph in about 1 second, so for over 80% of the run it is clearly to your benefit to consider aero drag.

 

[antti]

I have fiddled around with a few drag calculators of my own, and have an iterative one that seems to be OK if some figures are twiddled appropriately. I basically take an average power level (say 90% of actual rwhp) and then iteratively determines the thrust avalible at the rear wheels depending on the current velocity. Subtract the aero drag and friciton drag (small) and that gives you the accel for that time step. But there are many difficulties. For example, if speed is approching zero thrust becomes infinite - obviously not true given the realities of the clutch/torque converter, tyres, etc. Also, what tyres are being used, what is the driving wheels and weight dist, etc, etc. So my fudge is to set some g-limit from launch and calculate if the thrust avalible (due to "average" power) exceeds that level, and chose the lower one. Anyway, g limits of between 0.5 to 0.7 for cars seems to give OK results. It also would be nice to know what the power limited top speed is to try and access CdA and so forth.

Anyway, I have no idea what sort of car you are talking about, nor how much grip it has, but trying to match up those numbers, it would have to have the best part of 300 rwhp to pull that kind of speed. And if it really is that light, it seems to launch fairly poorly.

 

[PSlem]

ET=1.05 + 4.99 x cube root(WT/HP)
MPH=10 + 221 x cube root(HP/WT)
If we solve for HP from MPH we get 315 HP
From ET we get 183 HP but this is not accurate unless suspension is ideal.
Ideal relationship of ET to MPH is
ET=1.05 + 1102.79/(MPH-10)or 10.48 sec.

 

[zimbali]

It's not easy to find out the EXACT hp with such a method, that's for sure...!!

What I suggested was a simple method to find an estimate of the range of engine power. As patprimmer said it is an AVERAGE, and there is "no way" that you can find the maximum hp from its average, if you don't have the hp curve itself!!

It was also assumed that air resistance is low enough to be neglected, but it's not a good assumption especially after 50-60 mph. And about the the transmission loss and other things like that, I may say that we try to measure (or estimate) whp which is wheel horse power, and is the same if you take your car to a dyno shop. It is some few hp less than engine bhp which that LESS is a function of bhp and how efficient the transmission is.

And PSlem,
do you have any idea where your formula comes from?


Cheers


You can live in your car, but you can't drive your House!

 

[PSlem]

Mine came from an old article showing how the original dyno programs from RSA? worked. I remember they were different from earlier ones as they came out right after NHRA dropped the 66' timing strip after the finish. I'd written them in my Rule Book so don't have data, but I would think this is RWHP which would account for things like aero drag. Figure 25% for driveline losses. A couple other formulae
RGR=(RPM x TD)/(310 x MPH) for getting ideal rear gear. Again I forget what RPM you should cross finish at as related to HP and TQ curves. And lastly LGR=(TWXMPH)/HP for manual and LGR=(TWXMPH)/(1.8xHP)for autos where LGR is low gear ratio and TW is tire width. 1.8 allows for torque multiplication in convertor. Use 60% of tread width for treaded tires.

 

[evelrod]

I was digging through some old files this weekend and happened upon some old time slips from 1963 (the last year that I did any dragracing). Car was a stock ~2900 lb. 1961 Corvette 283cu.in/270hp. with a 4:44 and "Bruce" recap slicks---12.27 @ 105.57 mph!
Yeah, I'd say the MG in question launches rather poorly!!!

Rod

 

[GregLocock]

The Maestro (a car on which I did some development work many years ago) is front wheel drive, possibly explaining the poor launch. Personally I have a hard time believing the quoted speed after a launch like that. I would be interested to know what diff and so on they are using.

Rod, you'll be glad to know the Maestro was released with the A series engine in, bored and/or stroked to just under 1.3 litres. You had to wring it out to make it fly, but the good chassis made it reasonable fun through country lanes.

My performance model says it would need about 350 hp at the flywheel, but it doesn't handle standing starts gracefully so i've no idea how accurate that is.



Cheers

Greg Locock

 

[mtrehy]

http://www.an-racing.co.uk

This is the car in question - the reason for my original post was that I am very sceptical about this vehicle. on 2 fronts.

1 - I find it hard to comprehend that a front wheel drive vehicle of this minimal complexity could use 400bhp.

2 - They are only using a rover k-series....

 

[zimbali]

I assumed a drag coeef of 0.4 for the car (from the picture I saw from the internet it should be somewhere there) and frontal area of 1.8 m2. the average air resistance power got 28.5bhp. And the net average of work done was 181.5 bhp. So engine did 181.5+28.5+6.5; where I assumed 6.5 for average friction power (just a guess).
So the average engine power is 216bhp.
My assumption for the maximum power is not more than 290bhp as long as you only cover the whole RPM range in gear ONE and when you change the gear you come somewhere in 3500-4000RPM and you have at least 4 times more power than in 800-1000RPM. So the maximum is not roughly 2 times more than average and my feeling says the maximum shouldn't go further than 300bhp.
I did all this calculation to say that maybe the car is not that powerful (400bhp) as you might have imagined. And remember when this car was introduced to the market the "motto" was that it can beat a Ferrari!!! so it shouldn't be a very simple car.


Cheers


You can live in your car, but you can't drive your House!

 

[zimbali]

P.S. If you use a racing diff., RPM even won't fall that much as I assumed.

 

[ss1227]

how about mph x mph /et x weight = hp

 

[willeng]

mtrehy

This works for us:

ET= the cube root of (WEIGHT/HP) x 5.825

POWER OR WEIGHT FROM ET

HP= weight/(ET/5.825)cubed

WEIGHT=(ET/5.825)cubed x HP

MPH= the cube root of (HP/WEIGHT) x 234

POWER OR WEIGHT FROM MPH

HP= (MPH/234)cubed x WEIGHT

WEIGHT= (234/MPH)cubed x HP

 

[patprimmer]

Willeng

Your figures work for me, and come pretty close to a known example

Regards
pat

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