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Old 08-11-2005, 08:55 PM
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Horsepower & Performance Formulas/Calculators

We know that there are formulas that are used to estimate 1/4 mile performance based on a known RWHP and race weight based considering ideal conditions. The formulas are:

Estimated 1/4 Mile ET = ((Weight/RWHP)^0.333)*5.825
Estimated 1/4 Mile MPH = ((RWHP/Weight)^0.333)*234

On average, the difference between 1/8 mile and 1/4 mile ET is a factor of 1.55 to 1.57, and the difference between 1/8 mile and 1/4 mile trap speed is a factor of 1.24 to 1.27. If you study a number of time slips, you will see that this holds true for the vast majority of the cases, probably 95% or better. (See my study here)

So if you only have an 1/8 mile track to measure performance you can convert your 1/8 mile trap speed to a 1/4 mile value and then estimate the horsepower required to have achieved that 1/8 mile performance. The formulas are:

Estimated RWHP for 1/4 mile = (MPH/234)^3*Weight
Estimated RWHP for 1/8 mile = (MPH*1.27/234)^3*Weight

And it is estimated that RWHP is between 80% to 85% of flywheel horsepower due to drivetrain losses including but not limited to the transmission, rearend gears, bearing, the wheels themselves turning, etc.

If you have a 3650# car (w/driver) that ran a 7.99 @ 85.6 mph in the 1/8 mile, you would need approximately 365 RWHP, which would be 430 HP at the flywheel (considering a 15% drivetrain loss). This is approximately 12.54 @ 109 mph in the 1/4 mile. That's pretty fast for most street cars.

Here are a number of decent online horsepower calculators:

Horsepower Calculator from 1/4 Mile Trap Speed & Weight (
Horsepower Calculator from 1/4 Mile Trap Speed & Weight (C.P.R
HP Calculator from Weight & Trap Speed & ET and Trap Speed from Weight & RWHP (
HP Calculator from Weight & Trap Speed & ET and Trap Speed from Weight & RWHP (
Convert 1/8 Mile ET & MPH to 1/4 ET & MPH (

So what's my point?

I read all the time people saying that based on their build (reading too many magazines), they have XXX HP. The next time you hear or read someone saying that they have a 475 HP 350 SBC, maybe just snicker and say "If you say so". You know the typical story ... "I have a 10-1 355 SBC with Vortec heads, Comp 280 or XE274 cam, Performer RPM intake, 750 carb, headers, dual exhaust, 2500 stall, 3.73 gear ... approximately 475 HP." I mean, after all, that's what the magazines build, so surely that's what everyone is getting, right?

If they really had that much horsepower, the typical street/strip car with halfway decent traction would be running low-to-mid 12 second 1/4 mile times at over 110 mph. How many street/strip cars do you see running that fast without a 125 or 150 shot? Not many. But if building a 475 HP 355 was so easy, as Chevy High Performance, Car Craft, etc. would have you believe; you would see a lot more 12 second street cars.

The next time someone tries to tell me how much horsepower they have, I want to see a timeslip to prove it.


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Old 08-12-2005, 06:04 AM Moderator

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Yea i just sit back and laugh anymore.....

I had a customer take his engine to another builder last year because this guy told him he will do a "full port job" on his cast iron heads including a valve job for $300.00....... So he gave the guy the whole job,,, When i did the engine it went 425hp on the dyno with a cam that was a tick on the big side. So now the magic part, with the "full port job" and a correct cam the engine is now going to make 525 hp....... I can wait to see this happen.....BTW the head porter does not even have a flow bench!!!!

People love to believe hyp over tried and tru results.....

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Old 08-12-2005, 08:12 AM
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SBC, 350hp. Headers +35 hp. Mystery cam, +30 hp. Whatever intake, +20 hp. Tornado fuel saver, +12 hp. Misc racing stickers, +50 hp. Look on their face when it can't get out of it's own way, priceless. lol
Just the other day this kid was trying to tell me his NA 4 cyl. Honda Prelude (VTEC) makes 300+ hp and ran 12's at 140 mph. NOT
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Old 08-12-2005, 09:11 AM
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I like it when you ask what parts they used, and they can't remember.
I had a friend who was supposedly having someone rebuild the LT1 in his 94 z-28. He would constantly ask me how much horsepower my engine had, and laugh and say well mine is gonna have 650. Id ask him whats going in it, the only parts who could remember was fuel rail and fuel injectors. This was about 2 years ago, I saw him again a couple weeks ago. And he told me He can't wait to get it back from being rebuilt, which apparently its been in the shop for almost 2 years now!

But what about us guys that have estimated guesses at how much horsepower we have? I dont really see anythign wrong with it, as long as its not some miracle engine that gains 15 more hp by using octane boosters. Heck I am one of those guys, but I dont go about braggin and saying "my cam is bigger than the one in the magazine's build, so I should be making over 500hp." Instead I tell people when they ask me, that I would GUESS and say 375-400 from a built sbc 350.

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Old 08-12-2005, 12:36 PM
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I just tell people that it makes more than it used to. Which is without a doubt,the truth.I had a 200 V6 when I started.I have pretty much surronded myself with guys who race or who know what the hell is going on,so I do not hear that as much as I used to.If I ever get a rear end under my car I will have it dyno'd.
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Old 08-12-2005, 01:41 PM
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The Tornado fuel saver is Not A +12 hp Gain


Last edited by Mustangsaly; 08-12-2005 at 01:51 PM.
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Old 08-13-2005, 01:57 AM
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"The Tornado fuel saver is Not A +12 hp Gain "
Your darn tootin its not, its good for at least 25hp. I had a customer bring in a Mazda 626 for brake's, and he had a honest to goodness "tailpipe turbo" (a little fan blade that spun from the exhaust gas's) he claimed that "it promoted faster exhaust evacuation, and increased both hp and tq by at least 50" I just smiled and said "wow, thats something" and walked away. Anybody know where I can get me a "tailpipe turbo", if it's good for 50hp on a small v6, then its gotta be good for at least 100 - 200hp on the sbc in my truck, wait, I have duel exhaust, so I need like two, thats gotta double the power increase or better
Gotta love these kids, they keep bringing me money ... and a good laugh

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Old 08-13-2005, 06:01 AM
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Back in the fifties, an empirical equation for quarter mile speed was developed by George Wallace, of the Vehicle Performance Laboratory at Chrysler. This was used by the Ramchargers and others within Chrysler. It was [225(HP/#)^0.318]. This is not as convenient as the equation given, since many calculators have a cube root key and the 0.318 power might require some extra keystrokes. It is, however, a more accurate "fit" to the data. If you exclude aerodynamic effects, the cube root is actually to be expected, so the deviation from the cube root could be considered an aerodynamic "fudge."

The horsepower is at the flywheel.
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