Horsepower

[Kevin Whiteley]

For some of you, this may be an easy question. As a non-engineer, I've read that horsepower is measured differently today than it was in the C1 - C3 era. I think one of the methods is bhp, but I'm not sure if this is the modern or older method.

So, the question is: all other things being equal, would a 300hp engine from the 60's have more horsepower than a modern 300hp engine? Or vice versa?

[William C.]

Re: Horsepower

To oversimplify, 300 hp today is a lot bigger horses than 300 hp in '65

[Ed Jennings]

Re: Horsepower

Through 1971 the measurement was expressed in "gross HP". Beginning in 1972, the ratings were expressed in "net HP". The difference being that the net HP ratings tested the engine with all accessories and exhaust restrictions, air cleaner, smog pumps, water pumps, alternators, etc. in place and operating. The old method tested an essentialy bare engine with no parasitic drag. There is no direct conversion formula, since the differences were not the same with all engines. If you knock off something like 30% of the gross rating, you may be pretty close to the net.

[Joe C.]

Re: Horsepower

Ed:

You are right. But I think the difference is closer to 15%-20%.

Joe

[Wayne K.]

Re: Horsepower

Kevin,

Read an article on this once and as I recall the rule of thumb was to take 70% of the gross horsepower ( older cars ) to compare to the advertised horsepower of todays cars.

[Brian Monticello]

Re: Horsepower

Was there any mechanical difference between the 1971 LT1 350/330 and the 1972 LT1 350/255?

brian

[Duke W.]

Re: Horsepower

In 1971 GM quoted both SAE gross and SAE net ratings, but quoted only net ratings in 1972, and most engines were carryover, so 255/330 = .773

The delta will vary from engine to engine, and two of the biggest consumers of gross power once the engine is installed in the vehicle are an engine driven fan and the exhaust system. With the Corvette's clutch fan and relatively low restriction exhaust system my rule of thumb is about 80 percent, but a V8 with a single exhaust and fixed fan is probably closer to 70 percent.

With SAE gross ratings the exhaust manifolds are connected to generous size piping that routes to an evacuation pump to positively clear exhaust gas from the dyno cell. The engine is devoid of any accessories other than the coolant pump, and the fuel and spark advance are set to optain maximum torque at all speeds, which may not represent the production calibration. The other big difference is correction factor for observed data. SAE gross is corrected to standard scientific sea level conditions - 29.92", 59F dryi air, while SAE net is something on the order of 29.38" at 80 or 85 F with about 50 percent relative humidity. The difference in correction conditions is at least three percent.

Duke

[Ed Jennings]

Re: Horsepower

Using this correlation, the 345 net horsepower in my LS1 C5 should equate to 446 hp in terms of gross HP? (345/.77) Sounds a bit strong, but maybe not.

[Duke W.]

Re: Horsepower

The difference between gross and net HP of a modern engine is probably less than a vintage engine. Consider that most modern EFI engines have cold air induction, very free flowing inlet and exhaust systems, no engine driven fan, and they tend to have optimum spark advance and fuel flow maps at WOT as long as the fuel is of sufficient octane to keep the knock sensor(s) and electronics from pulling back the spark advance.

Duke

[Ed Jennings]

Re: Horsepower

Probably as far as seat-of-the-pants comparisons go, would it not be more realistic to compare torque specs? Is there a difference in torque ratrings as well, and assuming there is, how would that difference calculate?

[Kevin Whiteley]

$1 Bill Test

Echoing Ed's seat-of-the-pants comparison, would the modern cars pass the $1 bill test ($20, if you consider inflation).

[Duke W.]

Re: Horsepower

Peak torque ratings can be factored with the same value, but the key to engine performance is torque bandwidth. Our vintage SHP engines have an 80 percent torque bandwidth from about 2000 on the bottom end to 6000/6200 on the top end.

The modern LS6 has NINETY percent torque bandwidth from about 1500 to 6500. This improved torque bandwidth and more gears is why a modern LS6 feels more like a vintage big block than a vintage small block.

Duke

[Ed Jennings]

Re: Horsepower

I guess all you really have to do is look at the 0-60 and 1/4 mile times advertised for the modern cars vs the comparable times posted by the vintage cars. Tires and transmissions play a significant role, but there's also a lot more grunt in smaller packages today than there was in yesteryear.

[Jack H.]

Blast Index...

I remember a rather complex formula (a number of items in the numerator and more in the denominator) that was refered to as the blast index. In addition to peak torque, other considerations were factored in such as rear end ratio, wheel base, steering ratio, Etc. When you did a dimensional analysis of the items in the blast index, they cancelled leaving a pure number.

Cars like '67 L88 Corvette gave a computed blast index on the order of 220 while modern performance cars like Dodge Viper came in somewhere in the 150 range. Basically, the blast index tried to assess the factors that can slam up back/move you around in the seat and produce the tactile feedback associated with motion/acceleration. Haven't seen mention of this measurement in some time....

[Doug Flaten]

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