03/04 SVT Cobra - what would 400 wheel hp be at the fly?

[matth76]

Hi

Most of the US use wheel horse power where as Europe use bhp (at the flywheel) for comparison. I have been told an SVT Cobra Mustang can achieve 400 wheel hp with just bolt on mods. However this doesn't mean much to me or how it compares to other cars as I'm used to bhp (at the fly). Please can anyone tell me (approximately) what 400 wheel hp would equate to at the flywheel on a 2003/04 SVT Cobra? I know it would only be approx as it relies on a calculation based on transmission losses etc.

Thank you for any help and info.

[steve.c]

just add 15%

[300bhp/ton]

matth76 said:

Hi

Most of the US use wheel horse power where as Europe use bhp (at the flywheel) for comparison.

All car makers the world over rate HP at the flywheel, in the USA this is typically done as bhp SAE Net.

However to get such a number you need to slap an engine onto a proper engine dyno. There fore taking it out of the car. Very few people want to do this, so rolling roads are the most common way of deriving HP, however a rolling road dyno measure torque at the driven wheels (you then use this to calculate HP).

In many ways it's quite good, on an individual car by car basis, becuase it tells you how much power you are putting to the pavement.

In the UK and Europe we use exactly the same rolling road dyno's and derive HP in exactly the saem way.

However - we tend to mess the numbers up more by pretending to calculate drive train loss to give a flywheel figure.

There are many ways of doing this. The age old formula is manual cars loss 15% via the drivetrain and auto's around 18%.

However this doesn't really make sense as there must be a proportion of fixed loss. So you could use 12% + 10bhp but this won't work for low or really high powered cars.

Some dyno's will measure the drag resistance of the drivetrain on run down and use that, however that too is flawed as it doesn't account for frictional losses under load.

If you have a vehicle with a "known" factory rating which is accepted to be fairly accurate you could do a base line run and this would tell you the minimal amount you'd lose. So if you increase HP you'd lose this amount + a percentage more for increased friction and other variables.

matth76 said:

I have been told an SVT Cobra Mustang can achieve 400 wheel hp with just bolt on mods. However this doesn't mean much to me or how it compares to other cars as I'm used to bhp (at the fly). Please can anyone tell me (approximately) what 400 wheel hp would equate to at the flywheel on a 2003/04 SVT Cobra? I know it would only be approx as it relies on a calculation based on transmission losses etc.

Thank you for any help and info.

So to the case in hand. Well if we take 400rwhp as being the number then you cuold do this:

400 / 0.85 = 471bhp flywheel (15% rule)

(400 / 0.88) + 10 = 465bhp flywheel (12% + 10hp rule)


HOWEVER, and sadly it is a BIG however, it's not that simple.

There are several key factors which can cause MASSIVE variance in numbers.

1. Dyno type - this is probably the most significant variance. There are in essence two main type of rolling road. You have an intertia based dyno which uses a static drag weight. These are very common State side in the form of Dynojets.

However you also have eddy current dyno's which have the ability to vary the amount of load (or weight). Such dyno's are often Mustang Dyno's, but there are many more.

Both dyno's do the same thing essentiall ( HP = torque x rpm / 5252 )

However they can produce different numbers due to how they measure torque. On a typical 300rwhp car this variance is around 20rwhp+

2. Correction factors.

A dyno can tell you exactly what the car is making at the time. These are often referred to as STD numbers. However with changes in temp, humidity or even ltitude these numbers will vary. The same car with no other changes than the weather might dyno 20rwh+- difference.

There are correction calculations which you can use to make a more level playing field. A typical correction used is SAE. Using this that same car that varied by 20rwhp due to different weather conditions might makes less than 0.5rwhp difference if rated in SAE.

Such correction factors can be used to skew numbers up or down and often without the owner of the car even realising.

3. Graph smoothing.

This is fairly simple, some dyno's produce a very peaky/spiky graph. These will then be miss read to give a higher PEAK power and torque rating. Graph smoothing literally smooths the chart. It can account on a powerful car for 5-10rwhp error in reading in the worst cases.

4. User error

And dyno that isn't calibrated correctly or is being used incorrectly can produce differing numbers.

5. Setup

Even typre pressures can alter a rwhp reading so always take rwhp with a pinch of salt. And due to most enthuiasts guessing flywheel from rwhp take them with an even bigger pinch of salt.

6. Lastly is comparing to HP in the UK or Europe watch for other things like metric HP, often listed as PS. They are similar units but not the same. A 425bhp imperial works out to be 419PS or there abouts. Also in Europe you'll see DIN more often than SAE as a set of standards and correction numbers.


As for the Mustang Cobra, well Ford rated it at 390bhp SAE Net. This is likely slightly underated, partly due to Ford being caught with their trousers down in the past and part due to even a small % variance would equal quite a few HP's.

On all the reading I've done, I personally think the Cobra was in stock trim making in the region of 410bhp SAE Net, although most owners claim much higher.

Now taking account of all the factors above about rwhp numbers. For the exact SAME car with no changes to it could have rwhp numbers 40rwhp apart from different dyno's.

Sorry being such a long post, but there simply isn't a straight forward answer, not one that means anything anyhow.

You should have enough info to make some judgement for yourself.

However with a Cobra, power is easy to attain. CAI, catback, pulley change and tune could easily see 460rwhp on most dyno's. So no matter how you crunch the numbers you are looking at a 500bhp+ (flywheel) motor.


Hope this helps

[300bhp/ton]

swerni said:

last time I dynoed mine it was 472 at the fly and 370 at the wheels.

That based on a 05GT with a Vortech HO charger

Kinda curious how you are getting the flywheel number.

Losing over 100bhp thru the drivetrain seems a lot to me.

In fact that would be over 20% of the engine power lost via the drivetrain.

Not saying its not right, but it seems a lot.

[LuS1fer]

Yes but doesn't a supercharger take something like 20% just to drive the supercharger so it wouldn't be just driveline and transmission losses?

[300bhp/ton]

LuS1fer said:

Yes but doesn't a supercharger take something like 20% just to drive the supercharger so it wouldn't be just driveline and transmission losses?

Yeah but you wouldn't quote flywheel minus the blower as it's driven by the crank. It'd be like having a turbo motor but using a 2nd engine to power the turbo's.

Although if the number was produced on run down on a rolling road as in the owners other post, then that explains it. However based on info I've seen elsewhere on the web I suspect if it's making ~370rwhp then It's more like 430-440hp flywheel.

But that's just IMO


EDIT:

Sorry just noticed it's a Roushe. Don't they rate them at something like 430bhp SAE Net anyhow?

Edited by 300bhp/ton on Tuesday 15th July 09:13

[LuS1fer]

Most of the non-intercooled are 410-420hp. The intercooled ones are 450-500.

[Sushi]

at this weekends mocgb dyno day at paramount performance an 03/04 cobra dyno'd 490 at the fly.