hp numbers...confused

[Christopherrr]

okay... a TA is rated 305 at the flywheel. When you get it on a dyno, an percentage is lost, I know that part. Question is, when you modify engine components to increase horsepower, are you increasing the 305? or are you freeing up horsepower to gain as much as you can to get to 305? Let's not talk about power adders...

I just don't seem to understand when someone speaks of adding 10 extra hp to the wheels from an air lid, if it means you're adding more hp to the 305 hp you already rated at or just getting closer to 305...

[Joe K. 99 Zeee!!]

I'm not a certified expert, but here's the basics....

At the flywheel is just that....if you took the engine out of the car with all the accessories attatched, that's the point where the measurment is taken. Now when some one says RWHP or rear wheel horsepower they're talking about hooking the entire car up to the dyno and measuring HP and Torque at the rear wheels.

Now there are a number of things between the fly wheel and the wheels....transmission, read differential, etc.. The engine transfers it's force through these and in the process power is lost mainly due to friction. Therefore the power at the wheels will always be less than at the engine. The exact percentage that is lost varies, even from car to car. The generally accepted amount is 20% loss for auto cars and 15% for manual. Remember, auto transfers power through a viscous fluid while manual has a direct connection (clutch) which is more efficient in transferring power.

Make sense?

[Joe K. 99 Zeee!!]

Couple more things to add....

When you use a power adder mod to increase HP/Torque at the flywheel then you can assume an increase at the rear wheels, but not by the same amount. Remember a portion is always lost through the drive train.

In addition...when gears are changed, wheel size changed, or tire size is changed you will get a different measurement at the rear wheels. While it does not effect engine output you are changing the mechanical advantage of the drive train. Changing from a 2.73 rear to a 3.42 read differential gear ratio would generally increase peak torque and decrease peak HP yet result in much improved quarter mile times.

Just to comlicate matters more....Don't always trust the ratings used by the manufacturer. When the 98's came out (305 HP rating) owners would dyno their some of their cars in excess of 290 HP at the rear wheels. 2001-02 cars can dyno in excess of 300 HP. Each car varies, but if you do the math you'll find the LS1 F-bodies are putting out at least 330 HP at the flywheel. Looks like the General didn't detune it as much as they led us to believe.

[JL8Jeff]

If the car is rated at 305hp, then any modification done to increase power will add to that rating. But be careful when you set expectations on how much hp you will pick up from certain modifications. For example, let's say that a K&N FIPK has been tested to provide an increase of 15 hp on your particular vehicle. And a specific cat-back exhaust system adds 20 hp. When you combine those 2 modifications on your car, you won't see a 35 hp increase. More likely in the 25-30 range at the most. When you start to combine mods, their overall effect is diminished vs the manufacturers claims because you have already changed the layout of your car from stock with the first mod you did. Hope this helps.

[markd79ta]

Let me see if I can explain this in a easy way. Your engine generates a certain amount of HP and Torque through the RPM range at the crank so this is your 305 peak hp rating. Now, your drive-train and suspension places the power to Rear Wheel Horsepower (RWHP). There are a lot of factors of how this is placed to the rear wheels such as auto vs. manual, gears, 3link vs. 4link suspension types, control arms (location), torque arms, springs, shocks, etc. When you make any modification to your engine you change the engine power (curve) sometimes for the good and sometimes for the bad depending on what you are doing, and how well it works with the rest of the engine build. Now, depending on your suspension, and what mods you make to that, you will experience a fractional benefit or loss of that power gain or drop to your rear wheels. Now, see how this is a holistic approach. In other words, everything you do must have a rhyme or reason that works with everything else or you are wasting your time.

Additionally, don't pay any attention to advertised hp gains from products. Those do not include what all needs to be done to achieve that percentage of gain. Basically, do your homework, and then decide what parts best fit your planned goal.

[Christopherrr]

LoL...oh boy.

Okay, I understand the frictional lost throught the tranny and wheels, etc...I already knew about that.

I knew GM underrated LS1s too...read about it and everything.

I know the difference between HP rating from the crank and HP rating from the wheels...

I also know every single f-body is different...

Finally, I know that diffrent mod yield different outcomes...some bad some good...and that advertised claims shouldn't be any base line...

I just want to know, when you're increasing HP via mods, are you increasing the HP rating at the flywheel? Or are you just freeing up the HP that the engine was already making?

I'm asking because, 305 HP for the LS1 (using numbers as a reference) is the maximum HP that engine can make in it's stock form because the HP rating is measured without any frictional lost. Am I right or wrong? If I'm right then (not including force induction or power adders) when you mod your car aren't you just freeing up HP that engine was already making??? Theoritically, if you didn't add and power adders, it would be impossible to gain a maximum of 305 HP at the wheels...

Regards...

[Joe K. 99 Zeee!!]

Ok....I'm not sure if I follow you completely, but I'm bored so here's another shot at your question.

You go to buy a 98 Camaro Z28 Automatic. It's engine is rated at 305 HP. The engine is making 305 at the flywheel (ignoring the under rating that GM used with the engine). When measured at the wheels it makes 244 HP.

You go buy a Cold Air Induction and throw it on. Hooked up to the dyno it makes 254 HP at the wheels. You can't measure the HP at the flywheel without taking the engine out, but you can estimate it: 254/.8=317.

So, you aren't "freeing up" horsepower per se. What you're doing is allowing the engine to take in more Oxygen and add more fuel to make more power than it was making stock (305HP). The power is measured at the wheels so the drivetrain friction, etc. is taken into account and you record 254 HP. The engine wasn't making 317 before, but it is now.


Did I do better this time?

[FRMLAV8]

No, It works like this!

305hp at the flywheel = 260hp at the rear wheel

you add an air lid and exhaust which gives you 25 more HP at the rear wheels.

that means your flywheel HP went up to about 345 HP.

** These numbers aren't actual, I am just using them to illistrate a point***

If RWHP increases, then your FLYWHEEL HP has increased as well -> in general!!

[markd79ta]

If I understand what you are saying, and from what I said in my convoluted answer from before.

I would say yes to your question "...are you increasing the HP rating at the flywheel?" ...if you are modifying the motor. This will in turn give you more RWHP minus the frictional loss of the driveline and suspension. And, I would say yes to your question "...are you just freeing up the HP that the engine was already making?" ...if you are modifying the drive-line or the suspension. Or, in the event of modifying both, then both statements apply relative to these two answers.

[Injuneer]

Now, your drive-train and suspension places the power to Rear Wheel Horsepower (RWHP). There are a lot of factors of how this is placed to the rear wheels such as auto vs. manual, gears, 3link vs. 4link suspension types, control arms (location), torque arms, springs, shocks, etc. When you make any modification to your engine you change the engine power (curve) sometimes for the good and sometimes for the bad depending on what you are doing, and how well it works with the rest of the engine build. Now, depending on your suspension, and what mods you make to that, you will experience a fractional benefit or loss of that power gain or drop to your rear wheels.

While I agree that "suspension" determines whether you can transmit the power to the ground, I'm not aware of any significant influence the suspension has on the difference between flywheel HP and rear wheel HP. You could stretch things to infer that how you manage the driveshaft angles will affect the friction losses in the drivetrain, but that is really a "stretch" and not really relevant to the question.

LimTeng seems to feel that "305 HP for the LS1 (using numbers as a reference) is the maximum HP that engine can make in it's stock form because the HP rating is measured without any frictional lost. Am I right or wrong?" And that is wrong. The engine makes 305HP at the flywheel in the configuration it left the factory. If you make any changes to the engine that improve the "volumetric efficiency", or "breathing", you increase the amount of air in the cylinders, you can add more fuel to match that increased amount of air, and you make more power. There is no inherent limit of 305HP. Port the heads, and it "breaths" better.... same with a cam, an airbox, headers, etc. All of these things increase the power produced by the cylinders, and hence the power produced at the flywheel.

[markd79ta]

Originally posted by Injuneer
While I agree that "suspension" determines whether you can transmit the power to the ground, I'm not aware of any significant influence the suspension has on the difference between flywheel HP and rear wheel HP. You could stretch things to infer that how you manage the driveshaft angles will affect the friction losses in the drivetrain, but that is really a "stretch" and not really relevant to the question.

I was always under the instruction that energy loss occurred with the twisting and flexing of the power delivery mechanism (rear-end). However, this may be covered in your statement regarding "...transmitting power to the ground"; in which case I am a little off on what constitutes RWHP.

[Jay 02 TA ws6]

Originally posted by LimTeng99TransAm
LoL...oh boy.

Okay, I understand the frictional lost throught the tranny and wheels, etc...I already knew about that.

I knew GM underrated LS1s too...read about it and everything.

I know the difference between HP rating from the crank and HP rating from the wheels...

I also know every single f-body is different...

Finally, I know that diffrent mod yield different outcomes...some bad some good...and that advertised claims shouldn't be any base line...

I just want to know, when you're increasing HP via mods, are you increasing the HP rating at the flywheel? Or are you just freeing up the HP that the engine was already making?

I'm asking because, 305 HP for the LS1 (using numbers as a reference) is the maximum HP that engine can make in it's stock form because the HP rating is measured without any frictional lost. Am I right or wrong? If I'm right then (not including force induction or power adders) when you mod your car aren't you just freeing up HP that engine was already making??? Theoritically, if you didn't add and power adders, it would be impossible to gain a maximum of 305 HP at the wheels...

Regards...

No, you are not "freeing up horsepower the engine is already making." The 305 HP (let's assume that is the correct number, just to simplify this...) is what the engine makes the way it is stock. That means, with the stock pulleys, stock arilid, stock exhuast, etc...

When people say "freeing up" what they mean is they are getting power that the stock motor COULD have been making if it didn't have the restrictive stock parts on it.

If a car has 305 HP at the flywheel, and you put a pulley on it, it will then have 320 HP at the flywheel. (Assuming pulleys are good for a 15 HP gain).

The 305 number is the way it is bone stock... not an "ideal" that you approach as you modify it.

[Christopherrr]

auh, I see...thanks guys...lots of information to read and lots of new knowledge...

[Joe 1320]

Looks like most everything has been covered except:

Not all mods that increase HP to the rear wheels also increase HP at the crank. A lighter weight driveshaft, lighter weight wheels and tires will increase HP to the rear wheels by reducing parasitic HP loss through the drivetrain, it will not add HP to the crank.

Claims by aftermarket manufacturers are often too optimistic. If they claim 20HP added to the rear wheels, that is most often what you'll get at the crank. Reduce it by 25% and that is what you'll get at the wheels if you're lucky. The mods may increase power throught the rpm range, but not add anything to the overall peak rating. For example, a manufacturer claims their part adds 10 HP. That doesn't mean that your 305 HP is now 315 HP. The 10 HP may have been added at 4700 RPM, but there is no gain at the 5500 RPM peak that the 305 HP was measured at. Dyno graphs really show the true colors what power is actually gained.

[Injuneer]

Originally posted by markd79ta
I was always under the instruction that energy loss occurred with the twisting and flexing of the power delivery mechanism (rear-end). However, this may be covered in your statement regarding "...transmitting power to the ground"; in which case I am a little off on what constitutes RWHP.

Rear wheel HP is exactly what you measure on a chassis dyno, and suspension "flex" isn't playing a factor. The losses in the drivetrain are made of of two components. One of them - INERTIA - is a function of the mass of the rotating pieces.... gears, bearings, clutch or TC, drivshaft, axles, wheels, tires, etc. The HP absorbed in bringing these components from a stop to a max road speed is HP that can not propel the car forward. This number is a CONSTANT for any setup, and does not vary with HP transmitted. You will get the same inertia losses for a given drivetrain, whether it is carrying 100HP or 1000HP, except for the fact that the more powerful car is probably going to accelerate the mass faster, using a little more HP.

The second component of loss it FRICTION. Obviously, any gears or bearings, as good as the design might be, consume HP. And this is by far the largest component of losses in the drivetrain. This loss is also proportional to the HP (torque) transmitted. If you double the power transmitted, you double the frictional losses.

Throw in a non-locking torque convertor, and it gets a little more complex, because there are fluid frictional losses that can be altered (roughly) in porportion the square of the power being transmitted.

This is why you can only APPROXIMATE drivetrain losses by using a "percentage". I had the advantage of running my engine on an engine dyno, with full intake and exhaust installed. That gave me true flywheel HP. Then the engine was installed in the chassis, and chassis dyno testing done. By measuring HP at three different levels I was able to determine the drivetrain losses for each level, as a percent. This was a drivetrain consisting of a McLeod Street Twin, stock T56, 3" chrome moly DS, Strange 12-bolt with 3.73 gears and street wheels/tires.

Losses were:

486flywheelHP - 425rwHP - 12.6% loss
633flywheelHP - 555rwHP - 12.3% loss
762flywheelHP - 670rwHP - 12.1% loss.

By solving using simultaneous equations, you can see the the FIXED component of loss - INERTIA - is less the 20HP, and the VARIABLE component - FRICTION is the larger component of loss.

Going to a TH400 tranny, with loose, non-locking convertor, I lost a ton of HP in the tranny.... that 762flywheel only produced 600rwHP, a loss of an additional 70HP with the automatic trans, for a 21.3% loss. Losses at 486flywheelHP were only 19.8%.