gross vs. net horsepower ratings, 472/500

[Scot Minesinger]

Questions:

We all know that compression ratios impact power and engine efficiency.  The higher the compression ration, the more powerful and more efficient an internal combustion becomes, and this is limited by gasoline octane, metalurgey and the like.

1.  In 1970, the 472 gross horsepower was rated at 375 and with a 10.0 to 1 compression ratio, no air pump.
2.  In 1971, the 472 gross horsepower was reduced to 335 along with compression ratio (about a 10% reduction), air pump was installed by factory.
3.  In 1973, the net horsepower was rated at 235 hp, and that was also with the lower compression ratio and air pump, but it enjoyed a hotter cam.

When people ask me about this, my general rule of thumb is that a lower compression ratio, generally decreases power by about 10%.  And in this horsepower range, I generally say the net horsepower is about 30% less than the net.

I read recently that the stock 472 engine 1970 Cadillac gross horsepower is 227.  However, a 1973 Cadillac 472 with lower compression and an air pump nets 235.  How could that be - is the cam?  A 1970 Cadillac 472 engine is going to be more powerful than a 1973 whether rated gross or net.  Anyway, I usually tell onlookers that the 375 hp is about 250-260 net, in lieu of 227.

Also, the 1968 and 1969 Cadillacs were equipped with 10.5 to 1 compression ratio and 375 hp gross?

[Fred Pennington 25635]

You can not compare 71 - 72 Hp and Torque ratings to 73 up.
Previous to 1973 manufactures could give you any HP rating they really wanted to.
They could dyno an engine without any limits or controls what so ever.
They would run motors with electric water pumps, without any accessories and external dry sump oil systems so the engine was not even driving the oil pump. They could run light weight oils and just about anything you can imagine to gain HP.
Starting in 1973 the federal government required the manufactures to use a standardized test called SAE Net.
Now the HP and Torque ratings required the use of all required accessories and the motor to be configured as if it was installed in a standard equipped car   Most of the loses in rating were not loses. They did not really exist any way.
If you look only at the numbers it is easy to believe huge changes happen between 72 and 73 and it is easy to blame emission controls. The reality is, yes there is some loss due to emission laws but the big loss, at least on paper, was loss you already had when you added the water pump, oil pump, alternator, etcetera.

["Cadillac Kid" Greg Surfas 15364]

As Fred says, to a great extent the difference between the engine dynamometer and the Marketing Executives pencil is what caused the discrepancy in HP "ratings".  Just changing from 10:1 compression ratio to 8.5:1 reduces the Net HP by about 14. The rest is smoke.
Greg Surfas

[hornetball]

I think Scot was pretty careful to differentiate between net and gross.  When it comes to Caddy's, I prefer to talk about torque.  Looking up the MVMA specifications (download from the GM heritage site), my current '74 has 380ft-lbs net at 2000RPM.  That's pretty nice.  Honestly, doesn't feel much different from the 1970 I used to have in normal driving.  Plus, I can use 87AKI fuel!

500CID specs from the heritage site (note that 1971 was dual-rated and I couldn't find a 1973 document for the 500):

1970  400HP GRS@4400  550ft-lbs GRS@3000

1971  365HP GRS@4400  535ft-lbs GRS@2800
         235HP NET@3800  410ft-lbs NET@2400

1972  235HP NET@3800  385ft-lbs NET@2400

1974  210HP NET@3600  380ft-lbs NET@2000

Interesting that the 1972 dual-rating peaks happen at different RPMs.  Goes to the "wild-west" gross testing methods mentioned earlier.