1970 was the best engine year for that era...

In 71 the engines were toned down, I believe the insurance industry caused that... Then again in 74 (if memory serves correctly) when very crude emissions started taking hold...

 

because they lowered compression ratios and added engine choking emissions controls. if you look at the comp. ratio for the above mentioned early Gen 1 LT-1 put in corvettes (370 hp in 1970) and camaro Z/28's (360 hp in 1970) you will notice that the comp ratio is astronimical compared to any other cars built post-1970 (up until the late 80's to present). as far as gross hp is concerned, the engines were dyno'd with no acc., long tubes, deep sump oil pans and whatever else the manu. wanted to put on there. the newest sae net method requires that the engine dyno'd be exact to the specifications of the car it resides in.

 

So why did GM go with crankshaft HP on the LT1 and LT4's versus traditional RWHP? Was it because 300 hp sounds better than 270 hp?

 

All manufacturers go with crank hp with accesories connected = net hp. When we tune our cars it is much easier to do rwhp, who wants to have to pull the engine to see what the crank hp rating is? Just add 15-20% to get the net rating. RWHP is mainly used to tune so you can see before/after changes to mods.

 

No.

GM has ALWAYS rated its cars in terms of crank HP. It has never spec'd a car at RWHP.

It just changed the way it rated them at the crank in 72. One is as if it was just a race motor with headers, WP, and alternator....one is as if it was actually in the car with everything that goes into production.

 

As others have correctly noted here, manufacturers always list crankshaft (or flywheel) HP. This is the output of the engine as measured directly at the flywheel. Sometimes expressed as FWHP (FlyWheel HorsePower), this is measured with an engine dynamometer, and must be done with the engine out of the vehicle.

A chassis dynamometer is used to measure power output with the engine installed in a vehicle. It will produce measurements expressed in Rear Wheel HorsePower (RWHP) for a rear-wheel drive vehicle.

A chassis dyno cannot directly or accurately measure flywheel HP. A reasonable approximation of flywheel HP may be calculated based upon the RWHP, using derating values to subtract friction losses in the drivetrain (approximations sometimes used are 15% driveline loss for a manual transmission, and 18% for an automatic transmission).

The SAE (Society of Automotive Engineers) mandated that all manufacturer's engine output ratings would be expressed in Net HP (at the flywheel, of course), as opposed to the previously-used Gross HP, beginning in 1972. This was done to eliminate much of the chicanery that had been previously used by manufacturers to artificially inflate the power output for bragging rights during the horsepower wars of the '60s and '70s. SAE Net HP is a far more accurate measurement than was Gross HP of an engine's actual, useable power output.

The LT-1 of 1970, which was rated at 370 peak HP, actually produced an output very comparable to the '92 - '96 LT1 (one of the reasons for the decision to re-use the LT1 designation, albeit with the hyphen removed). In the current issue of Corvette Enthusiast magazine is an article by Hib Halverson, featuring a C3 LT-1 car. In that article, Hib notes that the LT-1 of 1970 would have generated a SAE Net HP rating somewhere in the range of 290 to 310, which is somewhere in the range of 80% of the 370 Gross HP rating claimed for this engine in 1970.

The 1971 LT-1 was rated at 330 peak HP (expressed in Gross HP, as was the case in 1970). The 40 HP reduction was a result of a lowered compression ratio (a change dictated by GM management in preparation for the impending elimination of high-octane leaded fuel). If I recall correctly, the LT-1 compression ratio was 11:1 in 1970 and 9:1 in 1971.

The LT-1 was rated at 255 SAE Net HP in 1972. Compared to the 370 HP rating of the 1970 LT-1, this reflects a relatively small actual ouput reduction as a result of the reduced compression ratio, with the remainder of the apparent reduction being a result of the change to expressing output in SAE Net HP, rather than Gross HP as was previously permitted.

A stock C4 LT1 car would probably turn 1/4 mile ETs that are slightly better than could be turned by a stock 1970 LT-1 car, with traction accounting for much of the advantage to the C4 LT1.

Does this help?

Be well,

SJW

 

The 92-96 LT1 is called LT1 cause it is the first engine to go into a Corvette that is more powerful than the old C3 LT-1 or so i am lead to believe....

 

Thats the way I heard it. The ratings were based on a fully acessorized engine with air, etc.

 

I think the gross/net/rwhp explanations make sense. I appreciate all who chimed in.

Thanks,

90Indy

 

Strictly speaking, this isn't exactly correct, as the LT5 produced more power than the LT1 (and it appeared before the LT1). But the LT5 also wasn't, strictly speaking, a Small Block Chevy engine, so it's not necessarily incorrect, either.

As I understand it, the LT1 designation was selected for use with the new 300 HP (SAE Net) second-generation SBC because its output and performance were very comparable to the old high-compression, solid-lifter LT-1 engine that debuted in 1970.

Be well,

SJW

 

Yeah i left out the traditional smallblock bit. The LT5 doesnt count.

 

What is NET power what is Gross... is gross before all the accessories are added (exhaust powerstearing alternator, etc.) and net is a measure of the FW HP when the motor is in the car hooked up to everything

 

We just told you.

The motor is not in the car to measure net, its on an engine dyno but with the production tune, exhaust, and all accessories that could put a drag on it.

 

Exactly

Others, please stop

...and now we have (voluntary) SAE ratings

 

RPO B2k = 345hp - 403hp
465 lb-ft trqe - 575 lb-ft trqe 1987-1991

 

Nope.

The old method allowed the engine to be run on the dyno and rated without alternator, without air filter, without mufflers, without power steering pump, without AC compressor.

The new method required that all accessories and exhaust be functioning, just like on the actual car.

At one time, GM said that the LT4 was "the most powerful small-block ever made for production." If this is true, then it would have had to out pull the 375hp small blocks of the sixties/early seventies.
The question now is this: was that statement true?? Who knows.

Larry
code5coupe

 

Gross HP ratings were based upon engine dyno measurements taken using blueprinted engines that had no emissions control devices, air cleaners, exhaust systems, or accessories installed. They did a better job of representing a theoretical maximum output of a particular engine's configuration, than of representing an actual real-life output of an as-installed engine.

Net HP ratings are based upon an SAE standard, and are based upon engine dyno measurements taken using an engine that has all emissions control equipment, air cleaner, etc installed. These are engine dyno measurements, so this is FlyWheel HorsePower, not RWHP. Manufacturers' engine output specs are FWHP, as they speak to the engine's output, not the output to the driven wheels of a vehicle (which would vary depending upon transmission installed -- manual vs automatic -- etc).

Be well,

SJW

 

True mate yeah, but it WASNT a factory fitted option was it!!

 

RPO is a factory option, period.

 

The place i am sure i have seen it stated FACTORY FITTED ENGINES. Not RPO. Which means it doenst count, cause the car never left the factory in BG with the RPO-B2K engine(which i know youll know since youve got a few of them!! ). It was near the start of the 1994 owner's video tape.