The hp calculators vary quite a bit, it sems like about 30%. The question I have is is there anyone out there that had an engine dynoed and checked the results with a 1/4 mile hp calculation? My car is 4200 lbs with me in it. 13.23@102.5 mph. I figure around 400hp but I have found it to be as low as 350 and as high as 430. How do I know which to use?

Dave, I plugged in your numbers to virtualengine2000.com calculator and got 400hp, ET, but at 4000# wt. Guessed you were near 200#! But no I've not dynoed an engine to compare. I think your in the ball park!

The hp calculators vary quite a bit, it sems like about 30%. The question I have is is there anyone out there that had an engine dynoed and checked the results with a 1/4 mile hp calculation? My car is 4200 lbs with me in it. 13.23@102.5 mph. I figure around 400hp but I have found it to be as low as 350 and as high as 430. How do I know which to use?

Are you driving a belly tank or a barn door?

those calcs. give you RWHP, how much do you add for flywheel HP?

The 65 Biscayne weighed 3950 and I should weigh 195 but tip the scales at 235. So I guess I do have a belly tank or two. So I figured 4200,

The 65 Biscayne weighed 3950 and I should weigh 195 but tip the scales at 235. So I guess I do have a belly tank or two. So I figured 4200,

http://www.rodnrace.com/images/gallery/p-38bellytank42.jpg

Belly Tank

http://24valve.com/images/truck3.jpg

Barn Door

It takes more power to push a barn door down the track...

Dont get taken by all those bogus internet calculators out there,
the late model croud has twisted them silly.
Heres a real calc - gives flywheel & rw:
http://www.wallaceracing.com/Calculators.htm
also is accurate compared to a real engine dyno!

I ran my numbers on the chart and came up with 375 or so. I remember having a 1996 Buick roadmaster with 265hp and it weghed probably 4400 with a driver and it was suppose to run a 15.0 1/4 mile. The formula is just about exactly right on that estimate.
Thanks for the help. Now I know what a belly tank is too.
Dave

Those calculators think that all our cars have ideal gearing and traction and therefore are very inaccurate. Use the old Mopar formula after adjusting the trap speed by multiplying your current trap speed by 1.006. The current way NHRA & IHRA measure trap speed gives your speed at 1287' instead of 1320'. The first clock is 66' before the line and the second is "AT" the line unlike the old days when the second clock was 66' past the line, giving a true 1/4 mile speed. The new formula is: trap speed x 1.006 x (.00426 squared x car weight = appx. H.P.). Speed is a much better indicator of your car's horsepower than E.T.

This one is pretty accurate to the time slips and dyno sheets I have on my car.

http://www.dsm.org/tools/calchp.htm

I have found this one to be pretty accurate depending on the combination. Although the car was not used on a chassis dyno, the motor was dynoed. The motor made 608 hp. The car weighted 3200 lbs with the driver. The best 1/8 mile time was a 6.88. 6.88 * 1.57 = 10.8. if you assume a 20% loss to the rear wheels, that comes out to 486 at the wheels.

The calcuator comes up with a 1/4 time of 10.9... not too bad
http://www.race-cars.net/calculators/et_calculator.html

On the other hand, it ran a 6.21 on a 100 shot which is around 9.75 in the 1/4. If you add another 80 hp to the rear wheels, the calculator only comes up with 10.37. I think the extreem addition of power down low starts to throw the calculator off. The motor was a 358 that made 608 hp @8000 RPM but only 460 ft lbs. I have also found this to be the truth wtih stroked, lower RPM big blocks. They tend to run a little faster than the calculator states they should.

Adam

The last two posted are some of the bogus ones I'm talkin about, they used the same hp formula but say its at the wheels when its really at the flywheel. Dont know how all these people get them wrong or try to change the laws of physics. Next they'll have a new speed for the speed of light;)

Harold
The Wallace racing one does exactly what you say (new mph measurement),
and uses the original Mopar formula from the looks of the hp #s.
plus it gives both flywheel and rw hp.

I created a formula for predicting ET and another one for predicting MPH, which were based on 15 or so real cars, making good passes at the 1/4 mile drags, where the flywheel hp and the actual weight were known. They usually work out reasonably close for most cars, when a good pass is made. Its easy to go slower than predicted. Of couse it is no substitute for the real thing, but it is pretty good for project planning or bench racing comparisons. Check it out and see what you think:

ET= weight/ crank hp, then take the cube root of that, then multiply that by 6.1

MPH= crank hp/weight, then take the cube root of that, then multiply that by 225.4

The hp calculators vary quite a bit, it sems like about 30%. The question I have is is there anyone out there that had an engine dynoed and checked the results with a 1/4 mile hp calculation? My car is 4200 lbs with me in it. 13.23@102.5 mph. I figure around 400hp but I have found it to be as low as 350 and as high as 430. How do I know which to use?

Using my equation above, your 4200 lbs with 412 crank hp, should go 13.23. So based on my equation, your 400hp estimate was darn close.

A few years back when my 3900# barge ran consistent 13.1's....I took the car to a chassis dyno. It put down 313HP at the wheels. That was pretty close to what some of the calculators said it should have done also. So, I'd say you are a bit over 325 at the wheels, maybe 375-390 at the crank. My car is also a stick-shift like yours. So I am not using that bogus 25% automatic drivetrain loss at this power level.

1/8 mile MPH /192= X
Xcubed x Weight =HP
this is for rwhp and is pretty accurate with the cars I have dynoed.
I can tell ya 3800lbs w/675 fwhp is good for 9.93@134mph w/a 60ft 1.38
and 470 fwhp @3800 is 11.20's@118mph w/mid 1.50's 60fts this is a 70 chevelle

Hiya JIM hows it going?

I don't believe the percentage loss stuff since a % of a larger number is always greater. Understood its a guesstimate, but a standard number is better. A setup with a TH350 might eat up 75hp whereas a TH400 or 700R4 may eat up 85hp because of design/power requirements. Those numbers have no basis, but are probably close based on my finger in the wind. ;)

Mike

The last two posted are some of the bogus ones I'm talkin about, they used the same hp formula but say its at the wheels when its really at the flywheel. Dont know how all these people get them wrong or try to change the laws of physics. Next they'll have a new speed for the speed of light;)

I an not sure what you are saying. I used RWHP in the calculator. The motor made 608 hp and I took away 20% to estimate the horsepower at the wheels which came to 486 hp. Maybe I missunderstood you, but it seems to me you are saying I was using flywheel hp in the calculator.


I don't believe the percentage loss stuff since a % of a larger number is always greater. Understood its a guesstimate, but a standard number is better. A setup with a TH350 might eat up 75hp whereas a TH400 or 700R4 may eat up 85hp because of design/power requirements. Those numbers have no basis, but are probably close based on my finger in the wind. ;)

Mike



I thought the exact same thing. As it was explained to me, as you increase the power of the motor, friction force between all of the parts increases since all of them are pressing together with a greater force.

Adam

I thought the exact same thing. As it was explained to me, as you increase the power of the motor, friction force between all of the parts increases since all of them are pressing together with a greater force.

Adam

Thats interesting, i didn't think about increased frictional forces. I wouldn't think that a 1000hp motor would lose 200hp while a 100hp motor would lose 20hp. (very different setups, but same theory.)

I can see losing a bit more power with the higher output motors because the rest of the system must be beefed up to withstand the power, but to use a percentage just didn't seem right.
Wow, my first time I've ever been wrong :)

I thought the exact same thing. As it was explained to me, as you increase the power of the motor, friction force between all of the parts increases since all of them are pressing together with a greater force.

Adam

Friction force (F) is equal to, the coefficient of friction between the two parts (u), times the force pressing them together(N). For purists, the u here represents the symbol for mu, and N represents the Normal force. So, in equation form we can say:

F=uN

It was explained correctly, from the textbook standpoint. However the % idea may or may not be super accurate, but it is likely fairly close, and will get you in the ballpark. It would be very interesting to do a real world back to back test. It wouldn't be that HARD to do, but would take some time. All you need is say an engine dyno'd 502ci, 502hp BBC and say an engine dyno'd 540ci, 750hp BBC, or something similar, for example. You get the idea. Then chassis dyno one and record both the actual difference from the engine dyno and the % difference from the engine dyno. Rinse and repeat with the other engine, and record both of its differences as before. Then look at the results. Did you end up with the same exact hp difference, as if the % idea is wrong? Or did you end up with the same % difference? Or maybe some variation in between? It could put this question to rest once and for all. Maybe someone has already done this, if so, fill us in on the results.