camarokev400

Hey i was just going through my dyno sheet from last year and i noticed that my vehicle weight was plugged in at 3625lbs when actually i am closer to 3200-3300, how much affect would that have on numbers or does it not affect it.

ScottyRS

iTrader: (2)

It won't effect how much the engine/rear wheels are putting out, just how fast the car will move.

camarokev400

allright thats what i figured but i didnt know if the plugged in weight had a affect on the resistance of the roller or anything like that, thanks

AlkyIROC

HP is the product of power to weight. Lets say your dyno sheet says 400 hp for 3625 pounds. Based on the weight, the dyno knows how much energy is needed to make 400 hp. Now since your weight is probably closer to 3250, the dyno is giving a false reading. What it thinks is 400 hp is probably closer to the low 300's. Since you never said how much HP is on the dyno sheet, we'll just guess.

Want to have a fast car? Don't increase the HP, reduce the weight. It will run quicker since not as much HP is needed to make a light car as fast as a heavy car. You could pull a typical street engine out of a car and drop it into an altered and easily run 10's or maybe 9's. Trying to move 3500 pounds is a lot harder than trying to move 1500 pounds.

GuitarJunki17

iTrader: (1)

Hence the reason Matt (mw66nova) got that little 305 to run a mid 12 with a daily driver street engine.

rx7speed

I'm lost. thought horsepower was torque to rpm not power to weight.

AlkyIROC

Torque is produced on a dyno. HP is a calculated number from torque but proper weight is required to get accurate numbers. Torque is how much work is done. HP is how quickly the work is done.

A 3500 pound car with 400 hp will be slower than a 3000 pound car with 400 hp. It takes less effort to move the lighter car so the HP can do the work quicker.

Fast355

iTrader: (2)

The horsepower constant inserted into a Mustang Dyno makes the dyno more accurate. It is more for emissions/performance elvaluations with this dyno though.

rx7speed

agreed a 3500lb car vs a 3000lb car one will be faster then the other of course the winner being the the lighter one.

but if I take that same engine put it in my honda(2000lbs) put it in a 60's lincoln then take the engine out and put it on an engine dyno it should read the same horsepower (not counting drivetrain loss differences)


weight of the car shouldn't be part of the equation as far as how much horsepower or torque the engine makes but rather weight should be part of the equation as to just how fast the car is.

AlkyIROC

http://www.hardtail.com/techtips/hpexplained.html

http://www.hardtail.com/techtips/hpandtorque.html

Sonix

iTrader: (1)

I can't believe I read the whole thing..... Boy I need to get out more...

Apeiron

The car doesn't move on the dyno though, so the dyno doesn't care about the weight of the car.

Bristol

iTrader: (1)

So what happens to the whole weight constant when the shop ratchets the car down onto the dyno to keep the wheels firmly planted? Evry dyno I have gone to locally seems to ratchet the car down in place and it loads the rear suspension. Just curious....

pizza_guy

Lemme throw this out.

When putting and engine on an ENGINE dyno, do you use the weight of the engine to calculate HP?? Are all engine dynos wrong because of the unknown weight of the vehicle??

camarokev400

glad im not the only confused by the whole weight thing

AlkyIROC

Going back to the original post

If weight doesn't matter on a dyno then why was weight on his dyno sheet?

Apeiron

Either for reference, or because it attempts to predict drag strip performance based on the dyno numbers. The mass of the car isn't necessary for the dyno to calculate the torque and horsepower of the car. Only the mass of the drum is.

rx7speed

I don't know honestly but again let me ask horsepower/torque on the same motor tested in a 60's lincoln a 2000lb car and then the engine itself on an engine dyno.

do you really think it's going to come out different? (again not counting drivetrain loss)

the torque is rated in lbs/ft rating. this rating has nothing to do with the weight of the car correct? all that rating includes is how much force is put at a 1 foot point away from a central axis. no weight of the car it is propelling at all.

the formula for calculating horsepower is (tq*rpm)/5252. again nothing about weight put in there for the car. now you say that horsepower is the amount of work being done right? well it is. but not against the weight of the car itself but rather horsepower equals 33,000lbs-ft in an hour.
then if you do a bunch of math explained here Howstuffworks "How do you convert engine torque to horsepower?" you see where the 5252 number comes from.

again doesn't mention weight of the car just the amount of torque the motor puts out. which again is based upon the amount of force it puts out rated in ft/lbs.

camarokev400

there is the sheet, dont mind the torque reading thats like that because my rearend broke while on the dyno and it messed the rollers all up

Apeiron

Ok, the Mustang dyno is a bit of a different animal because of the way it can simulate loads on the car. I don't know exactly how it would affect the numbers on the sheet though.

Fast355

iTrader: (2)

Because it takes into effect the HP available to accelerate the vehicle, not true RWHP. It takes RWHP and subtracts HP to push the vehicle from it and gives a number. If you lie to a Mustang it will give different numbers. For example the numbers on my 5,500#, 25 HP @ 50 MPH G20 van gives a completely different reading than a 3,800 lbs F-body that only takes 13 HP @ 50. I have set the dyno at work up with both.

camarokev400

so from what i've been told the mustang dyno reads like 20-30 hp less than a dynojet?

rx7speed

I think I see a little as to why weight might be on there also taking a look at the mustang web page. Kinda like someone was saying but just blew right by me for some reason. the mustang dynos can also be used for emissions testing, mpg testing and so forth. with these you would need the weight of the vehicle to simulate what kind of gas mileage you would get right? or even emissions testing if you wanted to do testing of a car actually being driven rather then idle emissions.

but as far as true horsepower of the motor it doesn't mean much.