Power matters 402CI

[jojomiamia]

Hi guys,

Do you know how GMC handled the down power of their 402CI in respect of emission laws ?

In 1970, this engine had 330 hp, and in 1972, 210 to 240.

What has been changed ?

Thanks,

[geezer#99]

Only the way they rated them!
THe 1970 was hp at the flywheel.
The 1972 was hp at the rear wheels.

Typically there is about a 30-40% loss thru the trans and rearend.

[saskman02]

This thread discusses some of the differences...

http://67-72chevytrucks.com/vboard/s...d.php?t=390787

I maybe incorrect, but I don't think any North American manufacturer publishes rear wheel horsepower. They all publish flywheel horsepower but changes were made in 1971 with regards to publishing net vs gross horsepower and limited the variables of the tests...accessories, smog control, exhaust systems. Here's an article that explains the differences.

http://ateupwithmotor.com/terms-tech...et-horsepower/

[geezer#99]

Net ---flywheel.
Gross ---at the rear wheels.

Or vice versa!!

[saskman02]

I don't want to start a war with Geezer#99 but I think he is mistaken. A quick search of the internet makes no mention of wheel horsepower in the testing parameters. From the Wikipedia page for horsepower...

SAE gross power

Prior to the 1972 model year, American automakers rated and advertised their engines in brake horsepower, bhp, which was a version of brake horsepower called SAE gross horsepower because it was measured according to SAE standards (J245 and J1995) that call for a stock test engine without accessories (such as dynamo/alternator, radiator fan, water pump),[29] and sometimes fitted with long tube test headers in lieu of the OEM exhaust manifolds. The atmospheric correction standards for barometric pressure, humidity and temperature for testing were relatively idealistic.

SAE net power

In the United States, the term bhp fell into disuse in 1971–1972, as automakers began to quote power in terms of SAE net horsepower in accord with SAE standard J1349. Like SAE gross and other brake horsepower protocols, SAE Net hp is measured at the engine's crankshaft, and so does not account for transmission losses. However, the SAE net power testing protocol calls for standard production-type belt-driven accessories, air cleaner, emission controls, exhaust system, and other power-consuming accessories. This produces ratings in closer alignment with the power produced by the engine as it is actually configured and sold.


From the article I linked above...

Gross output, which in the U.S. is typically measured using the methodology laid out in SAE standards J245 and J1996, is the output of a ‘bare’ engine running on a test stand with no external engine-driven accessories (e.g., alternators or water pumps), free-flowing exhaust headers with no mufflers, and optimal ignition timing. Gross ratings are also mathematically corrected for standard atmospheric conditions. In other words, gross output represents a particular engine’s maximum output under ideal conditions.

For that reason, the SAE and similar bodies have also established standards for measuring net output. Net ratings, such as the ones defined by SAE standards J1349 and J2723, are still taken with the engine on a test stand, but reflect stock ignition timing, carburetion/fuel delivery, exhaust systems, and accessories. The specific methodology varies depending on the specific standard being used, but the gist is that a net rating is a closer approximation of an engine’s output as actually installed in a car.

[RenoKeene]

Originally Posted by saskman02

I don't want to start a war with Geezer#99 but I think he is mistaken. A quick search of the internet makes no mention of wheel horsepower in the testing parameters. From the Wikipedia page for horsepower...

SAE gross power

Prior to the 1972 model year, American automakers rated and advertised their engines in brake horsepower, bhp, which was a version of brake horsepower called SAE gross horsepower because it was measured according to SAE standards (J245 and J1995) that call for a stock test engine without accessories (such as dynamo/alternator, radiator fan, water pump),[29] and sometimes fitted with long tube test headers in lieu of the OEM exhaust manifolds. The atmospheric correction standards for barometric pressure, humidity and temperature for testing were relatively idealistic.

SAE net power

In the United States, the term bhp fell into disuse in 1971–1972, as automakers began to quote power in terms of SAE net horsepower in accord with SAE standard J1349. Like SAE gross and other brake horsepower protocols, SAE Net hp is measured at the engine's crankshaft, and so does not account for transmission losses. However, the SAE net power testing protocol calls for standard production-type belt-driven accessories, air cleaner, emission controls, exhaust system, and other power-consuming accessories. This produces ratings in closer alignment with the power produced by the engine as it is actually configured and sold.


From the article I linked above...

Gross output, which in the U.S. is typically measured using the methodology laid out in SAE standards J245 and J1996, is the output of a ‘bare’ engine running on a test stand with no external engine-driven accessories (e.g., alternators or water pumps), free-flowing exhaust headers with no mufflers, and optimal ignition timing. Gross ratings are also mathematically corrected for standard atmospheric conditions. In other words, gross output represents a particular engine’s maximum output under ideal conditions.

For that reason, the SAE and similar bodies have also established standards for measuring net output. Net ratings, such as the ones defined by SAE standards J1349 and J2723, are still taken with the engine on a test stand, but reflect stock ignition timing, carburetion/fuel delivery, exhaust systems, and accessories. The specific methodology varies depending on the specific standard being used, but the gist is that a net rating is a closer approximation of an engine’s output as actually installed in a car.

Excellent post.

[geezer#99]

You knuckledraggers always gotta be right!! LOL!!

[garyd1961]

I thought net hp was measured at the crank but was amended to include all accessories including transmission and differential. The trans wasn't hooked up but it was calculated into the net hp.

[davepl]

Actually in 1970 the engine was rated at 310. It's basically a 396/325 with a little less cam, I think.

As covered, the big reason is the change in the way the tests were done. It's not rear wheel as mentioned, but they added requirements for running accessories, actual exhaust systems, warmer test air, that sort of thing.

They used to be able to test (pre-72) with open headers and no alternator or water pump with really cold air.

If you dyno an engine at a shop on an engine dyno, you're largely doing gross. And my 1970 402 dynod at 323hp with the 325hp cam.

[jojomiamia]

Wow, so it's only words and methodology.

So, there is no "horsepower lock" installed on those engines like recuced headers diameter, carb downgrade or others tricks, if we want to power up, we just need to tune up engine.

[72 tigger]

When did compression ratios begin to drop? I've always thought that happened in the early 70's

[geezer#99]

Originally Posted by jojomiamia

Wow, so it's only words and methodology.

So, there is no "horsepower lock" installed on those engines like recuced headers diameter, carb downgrade or others tricks, if we want to power up, we just need to tune up engine.

Yup!!
Headers, intake, carb, new cam and lifters will wake it up.