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.
Example:
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 (S-series.org)
Horsepower Calculator from 1/4 Mile Trap Speed & Weight (C.P.R Parts.com)
HP Calculator from Weight & Trap Speed & ET and Trap Speed from Weight & RWHP (Mustangworks.com)
HP Calculator from Weight & Trap Speed & ET and Trap Speed from Weight & RWHP (Fbody.com)
Convert 1/8 Mile ET & MPH to 1/4 ET & MPH (Chevyasylum.com)
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.
Ed www.edgesz28.com
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.
Example:
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 (S-series.org)
Horsepower Calculator from 1/4 Mile Trap Speed & Weight (C.P.R Parts.com)
HP Calculator from Weight & Trap Speed & ET and Trap Speed from Weight & RWHP (Mustangworks.com)
HP Calculator from Weight & Trap Speed & ET and Trap Speed from Weight & RWHP (Fbody.com)
Convert 1/8 Mile ET & MPH to 1/4 ET & MPH (Chevyasylum.com)
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.
Ed www.edgesz28.com