CALL911

Anyone know the % of HP that is lost between the motor and the rear wheels for our cars?

Basically I am trying to see how much HP I have at the motor, if I am at 614 at the rear wheels.

rickjames343

I think its lik 15 % for an auto, and i think 10 for a manual.

Stephen 87 IROC

No such thing as a percentage. Wild guess is 20% but that isn't accurate.

If you have a 250hp engine that loses 20% through the driveline, only 200hp reaches the wheels. You do an engine swap to 500hp. Nothing else is changed in the driveline but using a 20% conversion, that means now 400hp is reaching the wheels. How can you go from losing 50hp to losing 100hp through the same driveline with just an engine swap?

Each component uses up a specific amount of HP. For example, in stock form a TH400 uses up 44hp. A TH350 uses 36hp and a lightweight powerglide uses only 18hp. These can all be modified to use even less hp. Differentials also use a specific amount of power.

Thinking of drivetrain loss as a percentage is only a wild guess at a number.

Stephen 87 IROC

We don't race engines, we race cars. How much power an engine makes is only good when you're doing an engine dyno to determine a converter stall speed etc. How much gets to the wheels is the important number. It can be modified by increasing the engine output or decreasing the driveline losses.

A chassis dyno number doesn't mean much because it doesn't account for current air conditions, vehicle aerodynamics, rolling resistance etc. Just because a chassis dyno says you're getting 600hp to the wheels doesn't mean you'll get to use 600hp at the track.

Find out the vehicles race weight. Car, driver, fuel etc. Everything in the car sitting on the starting line ready to go. From a 1/4 mile run, get the mph. MPH is dependent on HP. ET depends too much on traction. Knowing the race weight and the MPH, rear wheel HP can be easily calculated. It takes so much power to move so much weight in a specific distance at a certain speed. Simple math.

matLT1

We usually use 18% drivetrain loss for automatic transmission vehicles when making calculations at work.

CALL911

Mine is a 6-speed. I understand the MPH in the 1/4 helps determine RWHP. Thats not really what I am trying to find out though (especially since I already know my RWHP off of my dyno sheet).

Trying to find a somewhat close WAG of what my 614 RWHP manual transmission Z28 is making HP wise at the motor.

Dave89IROC

right around 700 hp

Dave89IROC

thing is , it IS a percentage, frictional looses will build up , even with higher HP(granted, there will be a decrease in the percentage as the HP goes up), but 10% manual and 15% auto is not out of the question for his app.

Injuneer

The drivetrain losses are a combination of fixed values (inertia) and variable/percentage values (friction). Even the "fixed" values are variable to some extent, increasing with HP. They consist of the energy absorbed by the rotating components as they are accelerated up to max RPM. The more powerful your engine, the faster the rotating components accelerate, and the more energy they abbsorb (= greater drivetrain losses). The frictional losses in bearings and on gear faces are directly proportional to the amount of torque being transmitted - more torque (which computes to more HP for any given RPM) means more losses.

We've been through all this before... I'm surprised it keeps coming up.

I've measured them on my setup, using an engine dyno with the full intake and exhaust, then putting the engine in the car and running it on a chassis dyno. Drivetrain consisted of: steel flywheel Street Twin, stock T56, 3" chrome moly driveshaft, Strange 12-bolt with 3.73 gears, 17x9.5" wheels with 275/40-17 tires.

Losses ranged from 12.6% at 486 flywheel HP to 12.1% at 762 flywheel HP.

Flywheel - Rear Wheel - Loss - % of FW HP

486FWHP - 425rwHP - 61HP - 12.6%
633FWHP - 555rwHP - 76HP - 12.3%
763FWHP - 670rwHP - 92HP - 12.1%

After the TH400 conversion (non-locking 5000rpm flash stall converter, TH400, 3" C-M DS, 4.11's in the Strange, 15x10 Pro-Stars, 28" slicks), losses ranged from 19.8% at 486 flywheel HP to 21.3% at 762 flywheel HP.

486FWHP - 390rwHP - 96HP - 19.8%
633FWHP - 505rwHP - 128HP - 20.2%
763FWHP - 600rwHP - 162HP - 21.3%

I think everyone understands that 600rwHP on a chassis dyno does not reflect real world track performance. Using dynos is simply a way to measure RELATIVE HP, so that you can evaluate the impact of changes and HOPE that they will work out to real world gains. You will use that 600rwHP at the track... some of it is used up overcoming the aerodynamic forces, the rest is used to accelerate the car down the track. Introducing the concept of aerodynamics to the discussion adds nothing at all.