Flywheel to Rear wheel HP conversion

[SKINNEDR]

Is there a way to estimate the Horsepower at the rear wheel of your Cobra if you only know the Flywheel HP (Manufacture's Dyno spec.)?
Dex.

[Bernica]

You might do a search on this site, but I think most folks are using 15-18% loss from flywheel to rear tires. If I'm wrong, I'm sure I'll hear about it here soon...

[Ron61]

When I had mine I used 17% to figure it and I wasn't that far off. When I finally got a chance to put it on a rear wheel dyno, it was about a 14% loss.

Ron

[CSXDreamer]

Conventional wisdom from other forums is RWHP = FWHP*.85 for manual transmissions and RWHP = FWHP*.80 for automatic trans. Most data came from chassis dyno operators comparing RWHP with the manufacturer's engine rating. Maybe a couple tuners would run an engine on an engine dyno, then a chassis dyno. Various brands of chassis dynos would give different HP numbers for the same vehicle. Chassis dynos calculate torque and HP by measuring incremental angular accelerations of the big drum. Its inertia "loads" the engine and goes into the calculations. Unknown and variable quantities of inertia come from the wheels, engine flywheel, transmission etc that contibute some inertia to the whole system aft of the engine. If one really wanted to see large chassis dyno numbers, he'd use an aluminum flywheel and the lightest wheels and tires he could find. The engine would make the same torque and HP, but the chassis numbers would be higher. I believe that's how it works.

[Bernica]

CSXDreamer is right. However, don't expect HUGE gains by just changing to an al-um-i-num flywheel and lighter wheels. You may see something, but you need to consider what you gain to be a benefit. Read the threads and look on the web about using which flywheel for your application.

[dlampe]

There are also variables on the engine dyno as well. For the best numbers I have seen builders use electric water pumps, carb spacers with the shops carb and stepped headers. There is also parasitic loss due to accessories like water pump and alternator, not to mention the differences in cobra headers and the short little air cleaners we use. This all happens before the drive train loss!

I have always resolved myself to the 17% number but it doesn't matter. What really matters is how much are you able to put to the ground and how fast can that peak power be delivered.

[Bernica]

These cars weigh between 2,300 and 2,600 pounds. When you think about it, even 300 HP at the wheels is pretty damn good. Beyond that, like in the 400+ range. is just plain sick and off the charts for a car as small as we all drive. I think chasing a big RWH number on these cars is more vanity than common sense. And, yes, I have this sickness. Next car will be the "common sense" car, I promise.

[Tommy]

I saw the point made by CSXDreamer illustrated on one of those half hour car shows where they hop up cars. They took a showroom stock import sedan and prepped it for road racing. They started by taking dead stock dyno readings and lap times. Next they upgraded the wheels, tires and brakes, and did some intake and exhaust improvements before putting it back on the dyno. They were surprised and befuddled when the modified HP at the wheels was lower than the baseline. They later figured out that the upgraded wheels, tires and brakes had more angular inertia than the stock equipment. Ultimately, all the improvements they did to the engine served only to restore the HP at the wheels to what it had been when the car was stock.

[CobraDan]

Rear Wheel Horsepower to Flywheel Horsepower Conversion
Convert rear wheel figures to estimate flywheel figures.

Flywheel Horsepower = FHP; Rear wheel Horsepower = RHP

FHP = RHP + 17% (auto trans)

FHP = RHP + 25% (man trans)

RHP = FHP - 15% (auto trans)

RHP = FHP - 20% (man trans)

Example: 312.25 FHP = 249.8 RHP + 62.45 HP*

* Manual trans: +25%

http://www.cobranet.com/calcs.htm

[Dwight]

Keith Craft told me 20% for a Cobra. He has dyno'ed motors and then installed them in Cobras. Dyno'ed the Cobra.

Dwight

[danc30]

Also depends on how loose your converter is in an auto. Ask me how I know.....

[Cashburn]

15% loss on a BDR with T-56. Dyno chart:



Video:

[madmaxx]

That engine is so UGLY. I would be embarassed to open the hood!!!