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Discussion Starter · #1 ·
Ok guys so i went today to dyno my car. It was a DynoJet machine. This was my first time actually getting my car dynoed so i didn't no what was going on really. He pulled 425rwhp in 5th gear in M-mode,dsc off and 416rwhp in 4th gear. Ok so now the question is how do i know how much my crank HP i have? My car is stock and has no real performance modifications. Just a section 3 exhaust. Can you guys help me out?
I will post the graphs soon. Thanks
 

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To get crank hp, you can do one of two things:

1. Pull your engine out, and put it on an engine dyno
2. Guess

Thankfully, that's not really what's important. What is is you now have a baseline. Next time you get a mod, go back to the same place, get the same dyno operator, and try to get a dyno in the same conditions (strapdown pressure, hood position, temperature, humidity). That'll tell you how effective your mod is.

It's important to remember that the actual power reading means NOTHING. It's how much change in power your car reads after you do your next mod.
 

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Discussion Starter · #3 ·
To get crank hp, you can do one of two things:

1. Pull your engine out, and put it on an engine dyno
2. Guess

Thankfully, that's not really what's important. What is is you now have a baseline. Next time you get a mod, go back to the same place, get the same dyno operator, and try to get a dyno in the same conditions (strapdown pressure, hood position, temperature, humidity). That'll tell you how effective your mod is.

It's important to remember that the actual power reading means NOTHING. It's how much change in power your car reads after you do your next mod.

Well thats what i want to do. I already placed an order for a full RPI Ram Air Kit. So when i get it installed i will go back to the same place. After that i will get the pulley kit and so on. But how do i guess? i want to know my overall hp too.
 

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Well thats what i want to do. I already placed an order for a full RPI Ram Air Kit. So when i get it installed i will go back to the same place. After that i will get the pulley kit and so on. But how do i guess? i want to know my overall hp too.
There's really no good way to get a guestimate of crank HP. Vehicles don't have a static parasitic drivetrain drag number, so it's not something you can simply multiply out. There are also issues with trying to figure out out doing a coast down.

Honestly, your guess is going to be as good as mine, in terms of crank HP.
 

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Discussion Starter · #5 ·
There's really no good way to get a guestimate of crank HP. Vehicles don't have a static parasitic drivetrain drag number, so it's not something you can simply multiply out. There are also issues with trying to figure out out doing a coast down.

Honestly, your guess is going to be as good as mine, in terms of crank HP.
So theres no conversion to rwhp to hp your saying? I just have no idea how other people have been posting there dyno runs and showed number they were getting to the wheels and approx crank hp. I know they might of guessed but it has to be somewhat accurate, maybe a couple of hp off. Im new to this and its just too compliacted than i thought :sad3:
 

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So theres no conversion to rwhp to hp your saying? I just have no idea how other people have been posting there dyno runs and showed number they were getting to the wheels and approx crank hp. I know they might of guessed but it has to be somewhat accurate, maybe a couple of hp off. Im new to this and its just too compliacted than i thought :sad3:
There are "quick and dirty" conversions people have used to give approximate crank hp. However, they're really no better than simple guestimates.

For instance, someone may say:

I assume my exhaust will give me 5 rwhp, so my 425hp to the wheels after exhaust may mean 420hp to the wheels stock.

500 crank hp / 420 hp to the wheels = 1.19 multiplication factor.

So, if I go up to 435hp to the wheels after my next mod, multiply that by 1.19 and you get 518 hp to the crank after the next mod.

Again, that's not actually accurate, but it's sometimes done by people/companies.
 

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Discussion Starter · #7 ·
There are "quick and dirty" conversions people have used to give approximate crank hp. However, they're really no better than simple guestimates.

For instance, someone may say:

I assume my exhaust will give me 5 rwhp, so my 425hp to the wheels after exhaust may mean 420hp to the wheels stock.

500 crank hp / 420 hp to the wheels = 1.19 multiplication factor.

So, if I go up to 435hp to the wheels after my next mod, multiply that by 1.19 and you get 518 hp to the crank after the next mod.

Again, that's not actually accurate, but it's sometimes done by people/companies.
Ok I understand now. But just a question since you guys been working on alot of M5's. What were the rwhp numbers you guys got for stock M5's? dont you think my numbers were to high?
 

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Ok I understand now. But just a question since you guys been working on alot of M5's. What were the rwhp numbers you guys got for stock M5's? dont you think my numbers were to high?
415-425 rwhp is roughly what you tend to see for stock M5/M6 on most DynoJets. This being said, there is some variation as well. There are DynoJets that tend to read a bit below that, and others that will read above that.

Mustang dynos will tend to read closer to the 400 rwhp mark for a stock car. DynoDynamics dynos tend to read around the 380 rwhp range.

None of this really matters though. Dynos are notoriously bad in terms of accuracy. Dynos are precise tools, not accurate tools. That's a very important distinction to remember.

I think I wrote up the difference in a thread before.

Ah, here it is:

http://www.m5board.com/vbulletin/e6...ive-m5-dyno-run-ac-schnizter.html#post1837044

You have to remember that dynos (specifically chassis dynos) have great great tools, but they must be used properly. This is a very VERY common misunderstanding about them. You see it in virtually EVERY thread about dyno numbers on this board. This is separate from the issue of peak numbers meaning far less than the actual area under the curve (how the car is making power through the RPM band).

Properly used, dynos can be very precise tools. In other words, they have great "grouping". For instance, properly used, one specific BMW M5 may show 383-386 hp to the wheels, time and time again, as long as you're correcting for temperature, humidity, etc.

However, they're not very accurate tools. In other words, take the same BMW M5 to another properly used and maintained dyno, it it may show 420-425 hp to the wheels, time and time again.

Visualize this way:

High accuracy, low precision (NOT NOT NOT how chassis dynos work):



Low accuracy, high precision (this is how chassis dynos work):



This is why dynos are such great tools for measuring relative horsepower, but such absolutely poor tools at measuring absolute horsepower.

It's worth repeating:

Using a chassis dyno for absolute numbers (ie. I dyno'd 400hp, and I guestimate the drivetrain loss to be 15%, so my flywheel hp is 460) is ABSOLUTELY INCORRECT, and is a clear misunderstanding of how dynos should be used.

However, taking your car to one specific dyno and putting down 400hp, then making a change of some sort to your car (ie. exhaust) and putting down 410hp will allow you to reasonably accurately state that your exhaust change is netting you +10hp to the wheels, given your conditions (temperature, humidity, strapdown pressure, fan power & location, dyno operator procedures, etc) stay constant or are at least corrected for.

Here's more proof:

1 car, 6 different dynos (including 2 of the same manufacturer). The numbers are wildly different, eventhough we're talking about a car that's putting out just over 1/2 the HP our M5's are.

Turbo Magazine's Dyno Dash - Tech Review - Turbo Magazine

Here's another, dynojet numbers vs dynodynamics:

Dyno Comparison - Dynojet & DynoDynamics (Apples to Apples) - NASIOC

There are hundreds and hundreds of more examples.

Anyhow, while it is tempting to just compare peak chassis dyno numbers across different dynos, different cars, different situations, I hope we can all at least agree that this is using dynos improperly and the results of which do not actually mean anything.<!-- google_ad_section_end -->
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Well thats what i want to do. I already placed an order for a full RPI Ram Air Kit. So when i get it installed i will go back to the same place. After that i will get the pulley kit and so on. But how do i guess? i want to know my overall hp too.
RWD cars generally demonstrate a 10% to 12% driveterrain loss on DynoJets. Again, this is just a guesstimate based on experience. You can work out your crank HP from this factor.

The driveterrain loss on Mustang and Dybo Dynamics is higher because the wheels run between 2 rollers and some other factors as well.

All the information provided by NobleM400 is true and fair.

I have used the RPI products and have achieved great results. If you are going for the Ram induction kit, I would suggest that you get the block off plates as well. They make a huge difference to the package.

When the time comes for an exhaust, you must consider the Tubi Exhaust. Good Luck! :thumbsup:
 

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We can also go back to the drivetrain loss issue, where this loss is a fixed figure, not a percentage. Provided your drivetrain components remain unchanged after modifications. Right?

We dynoed stock on a dynojet and got 391whp. SAE Correction Factor Smoothing 5.
After installing all mods only, we got 438whp
After mapping the ECU we got 473whp

My question is, should we always take the manufacturers 507hp as a base figure for crank? Just out of trust? ANd work our way around that figure?
 

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We can also go back to the drivetrain loss issue, where this loss is a fixed figure, not a percentage. Provided your drivetrain components remain unchanged after modifications. Right?

We dynoed stock on a dynojet and got 391whp. SAE Correction Factor Smoothing 5.
After installing all mods only, we got 438whp
After mapping the ECU we got 473whp

My question is, should we always take the manufacturers 507hp as a base figure for crank? Just out of trust? ANd work our way around that figure?
Dear Ahmed! You are the expert on these things and I am sure you know a lot better than myself.

I believe that the manufacturer's HP is calculated on an engine dyno or a crank dyno and the numbers published are average numbers under ideal circumstances. Also, keep in mind that the HP calculation formula used by European manufacturers is different than that used by US manufacturers.

I believe it is always best to dyno the stock car, as you have stated, and work around that number. We just have to make sure that we use the same dyno brand and the same correction and smoothing factors. Working around crank HP is pur guesstimate and not a really reliable source of comparison.
 

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MY car dyno'd 410 stock. (Dj)
That ~20% drivetrain loss.

Probably a good figure.

While its fair to say 'driveline losses may vary".
Drivetrain losses vary less than different dyno's, different dyno runs on the same day, and in many cases consecutive dyno runs.

We are asked to pay a lot of money for products that produce HP gains smaller than the difference between serial dyno runs.
No doubt I could gain 2 or 3 HP by peeing on the airbox during a dyno run.
Except for headers and a real quality ECU tune there is a large degree on snake-oil and placebo effect in many of the HP adders.
 

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My question is, should we always take the manufacturers 507hp as a base figure for crank? Just out of trust? ANd work our way around that figure?

Nope, we shouldn't if we want accuracy. Engines vary quite a bit, straight from the manufacturer, and frankly, so do drivetrains. That's why trying to get a crank number from a chassis dyno is always a guessing game.

If BMW is precise enough with their manufacturing tolerances that engine outputs only vary by an astoundingly small 3%, we're still talking about 15 horsepower on a 500hp engine.
 
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