409/409

I've read multiple threads on this forum about gross vs net horsepower and torque. I dynoed my '66 427 today and it pulled 323 HP and 430 ft. lbs. of torque at the rear wheels. The formula used according to one post based on the LS6 425 gross HP 325 net HP (GM's numbers) was .07647. According to this mine calculates to 421 gross HP. If I use this .07647 to calculate the torque it comes to 562 ft. lbs. torque. This number seems awfully high. How do you figure gross torque if this is not the correct way?

Easy Rhino

Completely stock as built by the factory 427, or a modified 427. If any mods, what are they?

409/409

It's bored .30 over, 454 crank, mild hydraulic Comp Cam, '66 L36 oval port heads w/ a little pocked porting (10 1/2 to 1 ???), '66 L72 intake, '66 L72 780 holley. I was told on this forum by numerous posts I could use the L72 intake with no issues. It started life as an L36. I think the horsepower is OK, but is it possible to have that much torque?

DansYellow66

The torque sounds a little high but not greatly and is certainly plausible. I built a Ford 428 to 459 CI (your's must not be too far from this size) and it made 535 ft lbs torque on the engine dyno. I believe chassis dynos actually are measuring torque and the HP is calculated from that (unless I need more coffee). So I would think the same calibration factor would relate to both. Some engine combinations just crank out the torque.

409/409

It works out to about 462 cu. in. I think the horsepower is within range, but I like to figure the torque. What was your horsepower?

DansYellow66

My HP was 470 @ 5400. A fairly mild motor (cam - 231/236 and compression - 9:1) but it has heavily reworked heads, a 427 dual quad intake and was blueprinted/tweaked by a well known Ford specialist. I understand chassis dyno results are a little iffy - different manufacturer's dynos come up with different results on the same car. And then the losses through the drivetrain will vary from car to car.

You might play around with a HP/torque calculator and see what you come up with. http://www.1728.org/mtrtrq.htm

409/409

Was that at the rear wheel or calculated?

Easy Rhino

With those mods, I think that your results may be within the accuracy of the dyno.

DansYellow66

Sorry for omission - this was on the engine dyno in the shop.

Powershift

Here is a CF link (from archives) that will help you understand/answer your question: https://www.corvetteforum.com/forums...o-numbers.html

You need to consider what transmission gear you were in and what the axle ratio was, in addition to the engine rpm and model dyno used for the test. The link I provided should help you (maybe ).

Larry

409/409

254 Muncie in 4th gear. 3.36 posi rear. Typically HP and torque numbers are within 50 to 100 apart. Is it reasonable or unreasonable to get a number (562-421=141) with that big of a spread? Is the formula given to calculate horsepower the same or different to calculate torque? A stock L36 is rated at 390 HP and 460 ft lbs of torque. It seems 31 HP increase is reasonable, but I'm not sure 141 increase in torque is. Even a stock 454/390 HP doesn't have that much advertised torque.

Powershift

Torque is measured on/by the dyno as resistance to rotation. The formula between the two for engine dyno testing is:

HP = (Torque x RPM) / 5252. Horsepower and torque are equal at 5252 RPM.

A chassis dyno measuring at the rear wheels has some other inputs/factors that can effect the reading and its interpretation. Friction power losses in the transmission and rear axle are two of these. They are typically 10-15 percent of engine power as losses.

Did you read the info I linked?? Some of these differences were discussed in the link.

Larry

EDIT: You need to get and post a copy of the dyno report, RPM where your numbers were taken, and ID the dyno model used. That will help. Probably a good idea for you to discuss the results with the operator as well to get his input/take on the results.

Also, SAE net horsepower is typically 80% (or so) of GROSS horsepower. The 427/390HP gross HP was usually around 320 SAE net HP which at the rear wheels might be around 290 HP. Many games were played with these numbers and RPM where they were taken for corporate marketing and sales and insurance purposes.

409/409

Larry, unless I'm doing it wrong, 430 ft lbs (@3500 rpm) x 3500 divided by 5252=286.6 HP. The dyno says 323. What I really want to know is:
If 323 rear wheel HP factors out to 421 gross HP, What gross torque would 430 at the wheels be??? I'm looking for a logical gross torque number based on the dyno net numbers.

DansYellow66

Did they provide your with a print out of the results or a graft? If the torque and horsepower are not identical at 5252 rpm that is a good indicator that something was wrong with their testing or program.

Also, did they do the actual pull in 4th gear?

409/409

4th gear pull on a Dyno-Jet 224 XLC. Pulled to 5400. Over 400 ft lbs from 2500 to 4500 and peak was 430 @ 3250 rpm. Horsepower and torque crossed at 5252. I'm trying to figure gross torque as they figured back in the day.

GOSFAST

If the dyno numbers are good, you have 390 HP at the flywheel. This is a "nominal" number using a 16% driveline loss factor, if you use 18% you are at 398 flywheel.

Thanks, Gary in N.Y.

P.S. Without all the details about the build it's difficult to know whether or not there are issues?? We deliver most 396's (pump-gas deals) between 425 and 525 HP. These are all at or under a 10.5:1 C.R. Depends on the platform.

SWCDuke

That thread goes on for six pages, but no one understands the basic physics that underlie the operation of an inertia chassis dyno. I put in a response, but the thread did not come up to the top of the list because it's probably too old. You can click on the thread to see my post, but I've copied it here, too.

It's very simple. The drum has known inertia and the amount of power applied at any instant is directly proportional to the instantaneous angular acceleration of the drum, which has a high response accelerometer that measures angular acceleration multiple times per second.

The pickup attached to a plug wire relates this applied power at any instant to engine RPM, rather than drum RPM, so the output is RWHP vesus engine speed. Without that RPM pickup the dyno will only show power relative to MPH. The engine speed signal also computes equivalent engine torque at the drum. All this is done by the software and no vehicle parameters need to be input other than the number of cylinders, so the single ignition wire pickup computes the proper RPM.

Now, applying a typical driveline efficiency factor - say 0.85 for a typical front engine, rear hypoid axle with a manual transmission in direct drive allows a reasonably accurate estimate of flywheel torque and power.

You should always choose "SAE net" correction, but other correction factors can be applied. Get the test files on a disk and then download the Winprep software from the Dynojet Web site and you can choose the test data correction factor and output parameters to suite your purpose on your own computer.

It takes power to accelerate the internal engine components, driveline, and tires, and the shorter the gear the more quickly everything gains speed, so short gears absorb more power spinning up all the inertia than tall gears, however, this is offset somewhat by higher tire loss at high speed.

So if the axle is 4.11, speed at 6500 with OE equivalent revs/miles tires is about 130, but 170 with a 3.08 axle, and because these factors offset each other to some degree, the difference is relatively minor. Bias ply tires will absorb more power than radials, and high tire pressure will reduce tire loss of either type.

One should always take into account tire speed rating when running on a chassis dyno and low speed rated tires may make it prudent to run in third gear, which reduces transmission efficiency from about 98-99 percent in direct drive to 95-96 percent in an intermediate gear, which means power goes through the countershaft, so a driveline efficiency factor of 0.82 is more appropriate.

I further explained and gave test results in the "...Tale of Two Camshafts" article that I posted here not long ago.

Duke

409/409

I have read and reread all of these threads and posts and have seen formulas "HP equals (torque X rpm) /5252". What does the slash mean, divide? Also torque X what rpm? Rpm times peak torque or torque at 5252? As you can see I'm not good understanding this or at math. I'm looking for a formula that if the horsepower and rpm are known, will calculate the torque. Again, 430 RW torque is what gross torque at the crank???

bcwaller

Yes, it is a mathematical formula where HP is equal to torque times RPM divided by 5252. You can input the Torque at any RPM and get the HP. Torque is equal to HP times 5252 divided by RPM.

Nobody knows for sure the exact numbers at the crank. Most people estimate a 15% loss for manual transmission cars and 18% for automatics, but YMMV.

SWCDuke

Geez, didn't everyone have to take a basic algebra course in junior high?

/ means DIVIDED BY:

Working from the basic physics:

HP = torque x RPM/5252, which includes the definition by James Watt in the late 18th century that one HP = 33,000 ft-lb/sec.

If you don't understand, but are interested do a Google search for a detailed explanation.

So the basic formula has three "variables", and if you know any two you can calculate the third using junior high algebraic manipulation.

If the torque and horsepower scales are identical, the torque and power curves will cross at 5252 RPM, but the torque and power scales may be different on a low revving engine that has much more torque than horsepower, so don't get fooled . Look at the scales.

Duke