Dragy 60 - 130 Predictor of Horsepower ?

schmeky

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Wonder if using a Dragy for 60 - 130 could be a relatively accurate method of determining WHP?

 

engineermike

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Yes, well sorta. From a physics standpoint you can definitely calculate average power if you know the starting speed, ending speed, time, weight, Cd, frontal area, and rolling resistance. However, the power figure would be average over that time, not peak.

Some of us have had luck recently using Hptuners vcmscanner to log speed, rpm, and IAT and you can actually make some really nice power and torque graphs without even leaving the vcmscanner using user math parameters and basic physics equations.

DC634AB8-FD10-44C1-8FAB-DE830BC28858.jpeg
 

engineermike

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If you take a typical example of an s550 pulling a 7.0 second 60-130, the simplified math is:

(130-60) x 5280/3600 / 7.0 / 32.2 x 4200 x (130+60)/2 x 5280/3600 / 550 = 485 whp

I omitted drag so including it would push the number higher, but not to the ~650 it would take to hit 7 seconds. I’ve wrestled with this several times over the years; that the physics always seems to result in a smaller number. We had the same issue with the vcmscanner math.

The conclusion I’ve settled on is that a lot of power goes into overcoming rotational inertia of all the driveline parts stem to stern, which isn’t easily accounted for. Even dynojet applied a correction because they had the same issue.

https://www.motortrend.com/how-to/dynojet-chassis-dyno/

I’ve found with the vcmscanner, if I make pulls in 4th (a10 with 3.55’s) I have to multiply by 1.2 to get numbers equivalent to dynojet. The correction is smaller if I make higher gear pulls.
 

Kennysum1

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Like Mike said if you know some other factors it will get you close enough basically. Even a Dyno is not perfect. You can dyno 1 day and get 1 number and another day and get a different one. They should be close but you get the idea.
 

aleccesarenriquez

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It's a good metric of how quick a car is, but not necessarily horsepower. The problem with estimating horsepower from a time is that it doesn't take into account variable like weight, drag, auto or manual, traction, etc.

But for a typical mustang weight, auto, and assuming traction is fine, here is an average hp guess from my experience:

8s: 500s
7s: 600s
6s: 700+
5s: 800+
4s: 900+ (maybe even 1000 to really dig into the 4s)

Obviously that's not perfect and don't use it as a rule of thumb but it gives a general idea
 


Angrey

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I wouldn't say "accurate" but you could draw some conclusions.

Too many factors to be accurate (manual vs. auto, traction, starting gear and rpm, etc).

Given that some people can watch an episode of family guy in between shifts or that starting in the wrong gear or at the wrong rpm can have pretty large effects,

the most ACCURATE way to do it would be to do it JUST like a dyno pull. All cars start in their 1:1 gear and remain in that gear from 60-130. Assuming traction, the resulting numbers would be way more comparative.
 

Angrey

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I wouldn't say "accurate" but you could draw some conclusions.

Too many factors to be accurate (manual vs. auto, traction, starting gear and rpm, etc).

Given that some people can watch an episode of family guy in between shifts or that starting in the wrong gear or at the wrong rpm can have pretty large effects,

the most ACCURATE way to do it would be to do it JUST like a dyno pull. All cars start in their 1:1 gear and remain in that gear from 60-130. Assuming traction, the resulting numbers would be way more comparative.
But again, unlike a dyno which can correct for environmental conditions, even with isolating all the other factors, we'd still be left with the same car doing really well in TX in the winter vs not so great when in Colorado in the Summer.
 

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engineermike

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Any chance you can break down this formula?
Force = mass x acceleration
Acceleration = change in speed / change in time
Power = force x velocity
So,
Power = change in speed / change in time x mass x velocity

Conversions:
5280 ft/mile
3600 seconds/hr
550 ftlb/s / hp
32.2 is acceleration due to gravity
 

MKL_DS

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An oft overlooked benefit of the dragy is the g-meter. It tracks acceleration through the entire pull. If you see the car holding more g’s as you approach 130 mph then it did previously, then you are definitely making more power than it was before.
It also charts speed vs. time allowing you to see if it’s pulling harder in certain areas.
IMO it is a great tool for seeing performance gains, even ones that might normally be missed on the dyno.
 

vtknight

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It's a good metric of how quick a car is, but not necessarily horsepower. The problem with estimating horsepower from a time is that it doesn't take into account variable like weight, drag, auto or manual, traction, etc.

But for a typical mustang weight, auto, and assuming traction is fine, here is an average hp guess from my experience:

8s: 500s
7s: 600s
6s: 700+
5s: 800+
4s: 900+ (maybe even 1000 to really dig into the 4s)

Obviously that's not perfect and don't use it as a rule of thumb but it gives a general idea
So far - this is really accurate. I ran 5.15 with 860 whp. The car trapped 148+ MPH on that run. That said - I found out my tune started me in 1st instead of 2nd gear - so with some luck - I may be able to get that 60-130 into the high 4's. Hoping to get to the track this Saturday - weather permitting - to find out. I will report back.

515 60130.jpg


971_148.jpg
 

TMC GT500

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If you take a typical example of an s550 pulling a 7.0 second 60-130, the simplified math is:

(130-60) x 5280/3600 / 7.0 / 32.2 x 4200 x (130+60)/2 x 5280/3600 / 550 = 485 whp

I omitted drag so including it would push the number higher, but not to the ~650 it would take to hit 7 seconds. I’ve wrestled with this several times over the years; that the physics always seems to result in a smaller number. We had the same issue with the vcmscanner math.

The conclusion I’ve settled on is that a lot of power goes into overcoming rotational inertia of all the driveline parts stem to stern, which isn’t easily accounted for. Even dynojet applied a correction because they had the same issue.

https://www.motortrend.com/how-to/dynojet-chassis-dyno/

I’ve found with the vcmscanner, if I make pulls in 4th (a10 with 3.55’s) I have to multiply by 1.2 to get numbers equivalent to dynojet. The correction is smaller if I make higher gear pulls.
The number may be smaller since the math is assuming constant power. That 485 whp is assumed from start to finish. That could be more accurate for a Tesla with 1 gear, but internal combustion engines with multiple gears will have horsepower output constantly changing as it goes through the rev range and shifting gears. Most people will compare the math above to dyno's but only fixate on peak power. A more accurate comparison would be the average whp over that interval.
 

96gt4.6

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Not sure about the 60-130, or HP, but as far as vs the dragstrip, seems very accurate. This is the same pass, dragy was running in the car during the dragstrip run:

dragy1.jpg
 
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engineermike

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The number may be smaller since the math is assuming constant power. That 485 whp is assumed from start to finish. That could be more accurate for a Tesla with 1 gear, but internal combustion engines with multiple gears will have horsepower output constantly changing as it goes through the rev range and shifting gears. Most people will compare the math above to dyno's but only fixate on peak power. A more accurate comparison would be the average whp over that interval.
I agree with this conceptually. I had a car once that pulled 146 mph with only 680 rwhp because it made 680 rwhp all the way down the track. However, if you look at the power curves on these cars pulling 7.0 60-130, they don’t really below 485 hp at any time after the launch.
 

 
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