Guestion about dyno power ratings

My car was dyno-tested and had 296HP and 313 FTLBs torque at the rear wheels 4.56 gear 28" tall tires 4th gear 1to1. I broke the crankshaft last week and am looking at a motor that is rated at 420HP and 430 FTLBs torque at the flywheel. My question is how will this motor compare to the power of the old motor?
Thanks.

 

Depends a lot on the transmission & axle choices. If I had to make a guess, with a typical TH-350 or C4 & Ford 9", I'd factor out about 50-60 hp....and it's only a guess.

A lot of sources say that the Ford C6 is good for a 50-75 hp loss in and of itself due to rotating weight & friction. I dunno that I buy into it being that much, having run a few....but I also know that if you completely rollerize one, it makes a HUGE difference (along with adding some reliability). What I'm getting at is that you'll see a lot of opinions on this one from the Net & elsewhere, but it comes down to your parts choices & setup.....

 

50 is a reasonably safe number to subtract. Assuming you're making rated power.

Post your trans/driveshaft/rearend/wheel combo and somebody will probably make an extremely educated guess.

 

the way i was always told was that you lose about 30% of the horsepower from the flywheel to the tires so if you want 420 hp at the rear wheels plan on building around a 600 hp motor depending on what kind of motor yuo run id say plan on spending $2500-$5000 if you put the motor together yourself or a crate motor might be a better way to go.

 

One other thing to think about is that dynos vary by a lot of factors....type of dyno, weather conditions, the operator....sure, there are compensation factors, but that's up to the operator....and it's difficult to directly compare a chassis dyno to an engine dyno.

To test the C6 theory (actual loss), for example, is very difficult in an actual chassis, although you could, say, compare a C6, C4, & Powerglide (built, & with a JW bell or something) to each other in the same car. Get the right adapters & you could compare Torqueflites & so on as well.

 

Thanks for the info so far. The car has a T-5 standard trans and the rear wheel dyno was done in 4th gear 1 to 1 ratio.

 

The number I have heard repeatedly used for manual trans setups is 18-20% loss. I am sure it depends on the rate of acceleration and a few other factors, but you won't be too far off with that estimate.

 

I have owned a chassis dyno for many years and have many other shops in our neighborhood with dynos, chassis as well as engine.
First of all there doesnt seem to be a horsepower meter as horsepower is a calculation based off torque. Torque then is measured many different ways.
Example : If i test a vehicle and the car in high gear loaded to a 2inch manifold vacuum with 326 horsepower , i can retest the same car in second gear and get a different reading every time.
The real utilization is using the same load and gear every time we test then compare to the readings before and after a change.
A engine dyno is somewhat different as most are programed to gather the peak horse power and shut er down.
Alway use the same dyno for testing each time.

 

The actual difference between flywheel and rear tire performance is figured on percentages. A lot of variables as I'm sure that yoyu are aware and your best bet is to talk to a dyno guy that knows his shit.