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Discussion Starter · #1 ·
Ok... I don't know if this is even a tech question but here goes: What exactly is the formula to convert Horsepower to Rear Wheel Horsepower? I heard one way is to calculate by percentages: 20% for automatics and 15% for manuals. The other way I heard is it's about -60 HP for automatics and -30 HP for manuals...... There are other methods I've heard of but I can't remember them right now. What exactly is the method or which method is the closest to being accurate? I'm just interested so if anybody can let me know I'd greatly appreciate it...
 

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That's because there is no exact formula. Use the rough 15/20% value you have stated above. The loss in rwhp over the engine hp has many variables (temperature, humidity, sea-level, gas type, parasitic loss, transmission, rotating mass, etc.) that it becomes impossible to produce an exact equation. The best way to tell is to get a dyno run which also gives you a rough idea of your rwhp.

 

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Ok, thanks..... My last question regarding this. It is about a 15-20% loss but what if i say, put on a CAI that supposedly adds 10 hp? Does that mean that in reality it's giving me 8 rwhp? Or does rwhp still increases by 10 because it is something I'm adding to the car? Thanks.
 

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Selling performance products is a numbers game. I would think that if a company is advertising that a particular product adds horsepower, that it is engine horsepower and not RWHP. That way they can show a higher number. The other item is that they have no idea what transmission you are running, what your temperature is, what is your elevation and what existing mods are done to your vehicle that might affect this product. Keep in mind they are going to advertise the absolute highest possible gain. I would say that this holds true for most products, but perhaps not all.

Thanks,

Chris B.
 

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As Chris stated it is a BIG numbers game. Comes down to cost per hp. The best advice is to get a before/after dyno of the mod(s). The down side is, not everyone has the time or money to dyno every performance mod so you are stuck using the often over-inflated manufacture numbers. HP numbers are a pain since everyone wants to know what you are "putting down" but the number is so relative to that car & the circumstances of the dyno run that it's becomes somewhat meaningless. The real numbers to use are you 1/4 mile times. There is no "gray" area with these. :winks

 

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another thing peole forget about is gear ratio. If you swap out your 3.55s to a set of 4.10's they dyno thinks that your engine is making more power because of the torque multiplecation. This to will effect the Fly wheel HP vs. the Rear Wheel HP as long with the previous mentioned items.

Though a chassis dyno reall rates the Useable or Effective HP that you car is getting to the wheels. Then the chassis has to do the work to use that to make the car move.
 

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And then on top of theat there are differences in dynos. A Mustang dyno will give lower numbers than most Dynojets because it takes into consideration more real world variables like wind and rolling resistance. The faster you go the more resistance a Mustang dyno will give. I've been on both with the same setup. Mustang dyno gave me 302 rwhp while a Dynojet said I have 352rwhp. That's a pretty significant difference, I think.
 

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5twenty,

I didn't even think about there being a difference in Dynos, but that makes sense. It is like everything else, it depends on how you caculate it. The other variable is where the engine makes its horsepower. You could theoretically build a small block V-8 to make 500 H.P. at 2500 RPM, and have lousy 1/4 mile times. That's because you could never stick that horsepower to the ground at such a low RPM. That engine would be great in a rock crawler however. Rearend gears and transmission ratios become much more important as horsepower increases. I think that the new mustang gt could benifit greatly from a 6 speed manual trans. The 5 speed is just a little to far between gears. Rear end gears would help, but would hurt highway cruising. The 6 gear would make all the difference. That may be a future swap!
 

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cbgexec, That's why I don't put my HP numbers in my sig. They mean nothing. fin1 said it, ETs tell the story. You can have 500 HP but if you can't put it on the ground then I'll beat you in a 1/4 mile.
 

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5twenty said:
And then on top of theat there are differences in dynos. A Mustang dyno will give lower numbers than most Dynojets because it takes into consideration more real world variables like wind and rolling resistance. The faster you go the more resistance a Mustang dyno will give. I've been on both with the same setup. Mustang dyno gave me 302 rwhp while a Dynojet said I have 352rwhp. That's a pretty significant difference, I think.
That makes no sense. Wind Resistance and Rolling resistance does not affect RWHP. Those issues count against ET though.

Caluclating that in would require you to determine the Coefficient of Drag of you car, your total frontal area, the coefficient of friction of your tires and the road, your total weight and weight tranfer rate.

Just try to do that. It requires a 3rd order differential equation.
 

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Kyle F said:
That makes no sense. Wind Resistance and Rolling resistance does not affect RWHP. Those issues count against ET though.

Caluclating that in would require you to determine the Coefficient of Drag of you car, your total frontal area, the coefficient of friction of your tires and the road, your total weight and weight tranfer rate.

Just try to do that. It requires a 3rd order differential equation.
While it's true wind resistance and rolling resistance do not effect rear wheel horsepower, those factors must be taken into account when MEASURING rwhp. The Mustang dyno does exactly that. All those variables (wind, rolling resistance, CoD) and more are taken into account on a Mustang dyno, this makes for a more true, measured horsepower reading. But more important than a true HP measurement is that by simulating real world effects of air resistance (especially), etc. you can make your TUNE work better in the real world.

http://www.mustangdyne.com/performancetuning.htm
 

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To put it simply, a Dynojet is an "inertia" based dyno. The rollers on it are a fixed weight, and the software calculates hp/tq by looking at Engine RPM and the amount of time it take the vehicle to accelerate the rollers to a given speed. This does not take into account the weight of the vehicle.

A MD (Mustang Dyno) is a "load" based dyno. This system takes the weight of the vehicle into account and determines HP/TQ by a number of different factors. Acceleration rate, Roller resistance, load rate, etc. This dyno is better at simulating the load the rear wheels would see on the street, so it is easier to tune with.

But, like anything else, the numbers are subjective, and a dyno IMO should only be used as a Testing tool, to determine if your tuning/mods are making a differnce in HP. For example I have 440RWHP/420TQ, and only managed at 12.74 at 113 because of stock tires spinning through 1st, 2nd, and part of 3rd. But I've seen an 05 with CAI, tune, 4.10's and real good tires run 12.65 at 105 because he could hook up. His MPH shows he had less hp then me, but he got all of to the ground, where it counts. Hopefully with good tires, I'll be in 11.50 range, but fin1 is correct, 1/4 miles times are the true proof. HP is only one aspect of a whole package. Dynos should be used for testing tuning only, not "Dyno Racing"
 

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Wolfpac said:
Ok, thanks..... My last question regarding this. It is about a 15-20% loss but what if i say, put on a CAI that supposedly adds 10 hp? Does that mean that in reality it's giving me 8 rwhp? Or does rwhp still increases by 10 because it is something I'm adding to the car? Thanks.
You can use the 15/20% rule on that too. Only backward. Take 15-20% off what they say and then you have possibly a real number.

Remember when headers gave you 50 more hp and a carb gave you 50 more hp? Pretty soon you had a car making 450 horses.... oh no wait. Many are simply making it up. Throttle body change? That was one lonely and expensive horsepower! In fact, an actual *horse* would have been cheaper!

And horses get great gas mileage.

If they don't say that it will get you x number of extra horses on your specific engine [and the ad will generally have either the dyno sheet to prove it or a link to one], they are giving you a HP number that still smells like where they pulled it from.
 
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