So I was looking at VMS and ARH's Dyno numbers and it raised a question...

I noticed that their "before" dyno numbers reflected a 20-25% Hp loss stock (around 55-70 Hp). Assuming that the real driveline loss is about 60 and the variation is based on different dynos,cars correction factors etc, would the correct way of working out the crank Hp be:

(manufacturers stated HP- stock rwhp)+(modded HP)= flywheel HP

or would it be

(manufacturers rated hp/stock rwhp)*(modded HP)= flywheel HP???


The first formula will give you a lower number... the difference between the first and the secon will have about a 20% variance (about 10 HP). This diffence comes down to assuming a tranny always takes the same amount to turn at a given speed (which makes more sense to me) as opposed to a tranny always being assumed to use a cetain percentage of engine power (which makes no ense but is how it is usually stated.

In other words... if I have two otherwise identical cars, why are we assuming that the one with the higher HP engine takes more hp to turn the same tranny?

Wow, looking forward to the answer, however it won't come from me. Sorry to say but while reading your post all my feeble mind kept hearing was.........a train leaves chicago traveling at 100 mph @ noon while a bus leaves los angelas traveling @60 mph the same day at 6:00am, both are headed to Denver. The train stops for 4 hours in Kansas city. Which means of transportation hits Denver first and at what time does it arrive??:)................LOL

Assuming that we use as the crow flies distances, the bus will take 17 hours and 10 mins and get there at 2310 on the same day. The train has a total travel time of 15 hours and 30 mins including the break and will get there a 0130 the next morning.

Lol!!!!

Seriously ninety views and noone can answer this?

Seriously ninety views and noone can answer this?
Seriously, I'm beginning to think they all use a dart board.....a different one each time. :p

Six. Do I get a cookie?:gr_jest::gr_jest::gr_jest::gr_jest:

Six. Do I get a cookie?:gr_jest::gr_jest::gr_jest::gr_jest:

That is quite possibly te worst non sequiter I have ever seen.

OK, someone start baking the cookies.

First off, take the "modded hp" out of the equation. Now, here's the equation you want to use:

CHP = crankshaft hp (don't use flywheel, as that's technically part of the clutch and counts as a loss of hp)
RWHP = rear wheel hp
DTLF = drivetrain loss factor (my acronym, not official)

CHP - (CHP*DTLF) = RWHP

So, let's calculate:
CHP = 361 (factory #)
DTLF = .25 (use the higher # since it's an auto)
RWHP = ??

361 - (361*.25) = 270.75rwHp

So, I'd expect to see a rwhp around 270 on a dyno.

Now, you want modded stuff. You go to a dyno and they tell you your rwhp is 315. To find what your new CHP is use the following:

RWHP + (RWHP*.3333) = CHP

So:

315 + (315*.3333) = 419.9991

Why .3333? Well, when you are reducing, you start with 100%. 25% is 1/4 of 100. But, when you're going up, you're starting with 75%. 25% is 1/3 of 75%.

OK, someone start baking the cookies.

First off, take the "modded hp" out of the equation. Now, here's the equation you want to use:

CHP = crankshaft hp (don't use flywheel, as that's technically part of the clutch and counts as a loss of hp)
RWHP = rear wheel hp
DTLF = drivetrain loss factor (my acronym, not official)

CHP - (CHP*DTLF) = RWHP

So, let's calculate:
CHP = 361 (factory #)
DTLF = .25 (use the higher # since it's an auto)
RWHP = ??

361 - (361*.25) = 270.75rwHp

So, I'd expect to see a rwhp around 270 on a dyno.

Now, you want modded stuff. You go to a dyno and they tell you your rwhp is 315. To find what your new CHP is use the following:

RWHP + (RWHP*.3333) = CHP

So:

315 + (315*.3333) = 360.991

Why .3333? Well, when you are reducing, you start with 100%. 25% is 1/4 of 100. But, when you're going up, you're starting with 75%. 25% is 1/3 of 75%.

thanks for the answer but two things...

1) You did not answer the question I asked
2) Your explanation gives the same hp readings for two different RWHP... both give a crank hp of 361 but have very different hp (off by 45)

I assume these discrepancies are due to the made up numbers but still...

again thanks for the response.

hahaha - you're right - I didn't answer your question! Sorry, I didn't read the last line in your first post.

A transmission always takes a percentage, not an absolute number.

Well, that's not quite true. It takes an absolute number, but that number changes for every mph increase. Especially acceleration, which is what we're talking about.

Think about this - it's the engine's job to turn a few gears. Let's simplify it and assume that you're the engine and the gear is just a big round-about on a playground (you know, the things kids ride on that goes around?). Now, let's say you want to turn that thing so that it makes 12rpm (1 revolution every 5 seconds). You can do that pretty easily, so it requires relatively little force. But, let's say you want to turn it to 60 rpm (1 revolution every second). This may take all the power you have in your entire body(not to mention it's gonna throw thsoe kids across the park!).

An engine is the same way. The more power it has, the faster it's going to want to turn the transmission gears. And, the faster it tries to turn the gears, the more power is taken up by the gears. This is why it's a percentage. It'll always take 25% of the engine's power to accellerate the gears.

#2 - Dang, I typed the wrong number. 315+(315*.33333)= 420. Sorry, I'll fix the original post)

hahaha - you're right - I didn't answer your question! Sorry, I didn't read the last line in your first post.

A transmission always takes a percentage, not an absolute number.

Well, that's not quite true. It takes an absolute number, but that number changes for every mph increase. Especially acceleration, which is what we're talking about.

Think about this - it's the engine's job to turn a few gears. Let's simplify it and assume that you're the engine and the gear is just a big round-about on a playground (you know, the things kids ride on that goes around?). Now, let's say you want to turn that thing so that it makes 12rpm (1 revolution every 5 seconds). You can do that pretty easily, so it requires relatively little force. But, let's say you want to turn it to 60 rpm (1 revolution every second). This may take all the power you have in your entire body(not to mention it's gonna throw thsoe kids across the park!).

An engine is the same way. The more power it has, the faster it's going to want to turn the transmission gears. And, the faster it tries to turn the gears, the more power is taken up by the gears. This is why it's a percentage. It'll always take 25% of the engine's power to accellerate the gears.

#2 - Dang, I typed the wrong number. 315+(315*.33333)= 420. Sorry, I'll fix the original post)

Aight... so here is where I am going to hve to argue with your conclusion...

Let us say you have a 2004 pontiac gto a-4. Crank HP is 350. Let us say wheel HP is 295 at 5000 rpm. That means drivetrain loss is 55 HP or 16%.

Let us say you now have a 2005 pontiac gto a-4. Crank HP is 400... logically wheel HP would be 345 at 5000 (400-55 assuming idnetically shaped HP curves)... because well why would it now take 64 HP (16% of 400) to turn the same tranny at the same speed. Doesn't make any sense does it?

I mean seriously... how does the tranny know that the engine is making 50 more horsepower so it should "make" 9 more hp drag?

Answer is it doesn't

In that case... if you take that 04 and add all the mods to make the LS1 into an LS6, why would the tranny put that extra 9 hp worth of drag... again it wouldn't.

So... if you gained 50 at the crank, why wouldn't it be the same at the wheel.

Where did I go wrong?

Think of it like this. When you go to a dyno, you are trying to turn those transmission gears as fast as possible. But, it's not good enough to just get them going fast, you need to get them going fast in the shortest time possible. Therefore, you are going to need to accelerate them.

Because they have a constant weight, the harder you try to accelerate them, the harder it is to get them to turn faster. You're right in that it only take a constant amount of energy to turn them at a constant speed. But, when you try to accelerate them, it takes more energy to accelerate them.

Let's answer your question about the two GTOs. They have the same tranny, etc. At 55mph, the do take the exact same amt of hp to turn. However, when you go to a dyno, you're talking about acceleration.

55mph is a single velocity (miles per HOUR). Acelleration is change of velocity (miles per hour SQUARED - for instance, the force of gravity is meters per second squared). Because a higher hp car has the ability to accelerate thsoe gears at a higher rate, it uses up more absolute hp, even though it's the same percentage.