I've heard a lot of stories about the old muscle cars having trouble delivering all the gross HP the engine produces to the wheels. The modern Hellcat has 707 gross and about 640 to the wheels, while I've heard of some old Challengers with built 440's that have 600-700 gross, and they only put down 400-450 to the wheels. Why is this and what can I do to prevent it?

My old pump gas 518 C.I. stroker motor made 600 + HP(probally closer to 680 later) to the flywheel, 545HP to the rear tires on a Mustang chassis dyno at 5000 RPM and losing HP with increased RPM above 5000 RPM I'm sure the converter and engine RPM has a lot to do with the rear tire HP

Thats all because of the drivetrain. It takes a good bit of power to run the trans and diff. All you can do is build the drivetrain to have the least amount of drag you can like rollerized bearings and so on. There will always be some drivetrain drag that will rob hp. I guess my 63 makes around 600 flywheel hp but I bet I am lucky if it gets 520 hp to the wheels. Ron

There are two factors... engine flywheel HP ratings are different plus drivetrain losses.

Up until the early '70's advertised engine ratings were "SAE gross". Power was measured on a blueprinted engine without accessories, air cleaner or exhaust system. Around 1974 (somebody will correct me!) ratings changed to "SAE net". The test engine was run with normal accessories which is more realistic. The Hellcat ratings are SAE net. Typically quoted dyno HP is similar to SAE gross. Difference can be 10-20%.

Also drivetrain losses factor in. An auto trans plus rear end can consume up to 20%.

Adding both rating differences and drivetrain losses can make a big difference between dyno HP and rear wheel HP.

How to minimize the difference? Good air cleaner system, preferably cool air and low restriction exhaust. Efficient drivetrain with manual trans being most efficient.

Also build enough "gross HP" to start with!

put a clutch in it, problem solved.

A loose converter and lower gears can eat up #'s on a chassis dyno. I had an old car of mine put on a local dyno. I was told the gear change I was considering would probably cost me 20 HP on his dyno. That's a perfect example of why you don't race dynos.

The other thing is that the hell cats seem to be way under rated at 707...

On the hellcat its called traction control... also
if a engine is dynoed at sea level and you run it
at 5000' it wont be the same power

Chassis set-up, the driver and plain old luck play a role too. My 410 flywheel horsepower '68 Barracuda with an automatic beat a manual tranmission Hellcat in the drag racing finals at Willow Springs to take home the prize money. He didn't smoke his tires either.

He sure was building a head of steam near the end, but it was too little too late.

make sure your comparing apples to apples. most engine dyno's use J607 for a correction factor, chassis dyno's use J1349. there is approx 4% difference between the 2.

some engine dyno's give you both plus uncorrected



Joe

I don't want a manual, I'm planning on an A-518 with lock-up converter.

If you are still in the planning stage, have you considered a 200R4? I went that route and am very satisfied.

The most efficient (low loss) auto trans are the most fragile.

First, the engines HP ratings are measured different.
New cars are tested with all the accessories and such installed, and the drivetrains are more efficient than the old cars.

Watching several of the "car" shows on TV, they test the engines with no accessories, usually even without a mechanical water pump, and open exhaust and no air filter.

Take that engine and add all the accessories, air filter, mufflers, and put it behind a 727 trans, and it will likely be down from the engine dyno power by 20% or more.
I have seen one case where a 700 HP engine was choked down with a really small air filter so the engine would fit under the stock hood.

With a manual Trans the main losses are friction through the gear mesh, the bearings and the rotational mass (Inertia) itself.

With a conventional torque converter automatic the engine is basically used to turn the hydraulic pump that is your automatic transmission and the transmission drives the wheels through that same downstream rotational mass. All that slip (from not being directly coupled) and the resultant HEAT (energy converted to heat rather than power to your wheels) consumes a lot of flywheel power verses a manual, efficiency has a lot to do with why so many MFG's are developing DSG (direct shift Gearbox) transmissions.

Do your own homework

Yeah I am still considering the 200R4 but I've heard they aren't quite as robust as the 518. The reason I wanted a lock-up converter was to take care of that slippage problem. As long as I'm only loosing 60-90 horsepower I can deal with that, just so long as it isn't 25-50% of my power that I'm losing. It'll require the right combo of rear gearing, tuning, and cam too.

The Cumulative losses would be the sum of the Transmission plusthe remaining drivetrain so a higher percentage loss at the transmission output would I think result in a proportional further loss (% slip) of net output for an automatic by the time it gets to tires. Remember the load itself is causing resistance (at the tailshaft and working backwards to the converter) within the transmission, we tend to think of the trans tailshaft losses as though it is just free-wheeling.....perhaps not fully considering all the accelerative and braking forces working against it. With a direct coupled transmission it's "all there" and absorbed moreso by the crankshaft but with an automatic (fluid coupling converter) much of the heat is in the pump (and to a lesser extent the motor) overcoming the downstream load though accelerating the turbine housing.

Visualize the same motor/same trans in a 4500 pound truck vs a 1300 pound dragster ve freewheeling....or that 4500 pound truck pulling a 6000 pound trailer, uphill, lol

Also with regard to friction, production based transmissions are seeing much higher RPM than conventional street transmissions so I would presume the heat factors rise somewhat proportionally over the 'Base tested' RPM.

just thinking out loud here a bit

In stock form, the 200R4 has several serious weak links. Once those issues are addressed and a few other modifications are made; they can handle an amazing amount of power. The low parasitic loss, small size and nearly perfect gear ratios combined with the option of a high-stall converter with a lock-up funtion controlled by a single 12V switch makes it an attractive option.

Another nice feature is that they can use an SFI rated bellhousing to adapt them to almost every Mopar engine, including modern Hemis.

That sounds pretty good...I was kind of hoping to have automatic lock-up though, without having to hit a switch. I also saw in your sig that you have an EFI 496 that has 715 HP, how much would you say you get to the wheels?