Horsepower - Fact or Fiction?
By Andy Thomas
How much horsepower does a 440 Magnum produce? Several years ago, a less enlightened Mopar magazine dyno'ed a blueprinted '70 vintage Magnum and were "surprised" to find that it only produced 350 horsepower. 25 short of the advertised 375 HP. Where did they go wrong? I was a Chrysler dyno engineer during the late '60s and early '70s. From 1968 through 1970, I worked especially on the 440 engines. Believe me, a 1970 440 Magnum will make 375 HP. You just have to know how to do it!
What is horsepower anyway? First, we better look at torque, since power is a function of engine torque. Torque is a measure of an engine’s ability to do work. Work, like moving a car from Point A to Point B, for instance. A dynamometer is used to measure the amount of torque an engine produces.
A dyno works like this. The engine is mounted in a test fixture and the crankshaft is attached to the rotor of the dyno. See Figure 1. The dyno stator is mounted concentric to the rotor in low friction bearings and balanced so that the scale attached at the end of the arm or “beam” reads zero pounds when no force is acting on the stator. With the engine running, the rotor is turning at engine speed. If no restraining load is applied to the rotor and the throttle is held wide open, the engine speed would simply increase until valve bounce occurs. (I saw them do this with a 440 6-bbl engine. It ran up to six-grand or so for a minute or two before it stopped. When the engine was disassembled, they found that the only damage was bent valves and nicked pistons. A new set of valves and it was ready to go again!) To measure the engine’s torque, the rotor is loaded by the stator just ' enough to hold the engine at the desired RPM. The earliest dynos were simply mechanical brakes. The stator would clamp down on the rotor, allowing it to slip at the test RPM, much like you use the brakes on your car to maintain a certain speed going down a hill. Dynos these days are usually hydraulically operated “water brakes” or electrically operated.
The load applied through the stator to the rotor, causes the stator to want to turn in its bearings, in the same direction as the rotor. The beam of the stator is constrained by a spring scale (25 years ago we used a Toledo scale) or a load cell (used on modern comput-er-controlled dynos). The constraining force or “beam reading” is read off the scale in pounds. If we multiply the beam reading times the length of the beam (i.e., from the center of the stator/rotor to the point where the scale is attached) in feet, we obtain the torque in lb-ft at that RPM. Once we know the torque, we can calculate the horsepower at that RPM by the following formula.
Torque(lb-ft) x RPM
As stated.above, torque is a measure of an engine’s ability to do work. If we move our car from Point A to Point B, the same amount of work is done, whether it takes ten seconds or ten hours! Power, on the other hand, is a measure of the rate at which work is done. The higher the rate at which we want to perform the work, the higher the power must be to do it. Or to look at it another way, the quicker (acceleration) or faster (steady state speed) we want to move our car, the more power we will need to do it. That’s why horsepower numbers are important to us Mo’ rodders.
The word horsepower, by itself, is somewhat meaningless, though. The 440 Magnum puts out 375 HP, right? Sure, I’ve seen it myself. It also puts out 350 HP, as the other magazine found out. It also puts out 332 HP or even less! So what gives?
If you ever look at engineering textbooks or technical reports, you’ll find a number of adjectives associated with the word horsepower. Some of these include brake, indicated, friction, gross, net, observed and corrected. Let’s sift through these adjectives for horsepower and see if we can make any sense out of all of this.
Brake horsepower (BHP) – The brake HP is the power of an engine as measured at the flywheel by a dyno. As noted above, the early dynos were simply mechanical brakes. The term is still used.
Friction horsepower (FHP) – The friction HP is the power required to turn or motor a warm engine over, after the fuel and ignition are turned off. Electric dynos will do this. It takes about 120 horsepower to turn a 440 at 5000 RPM. This includes the friction of the pistons/rings and valvetrain, etc. It also includes the power used to pump air through the intake and exhaust systems. Additionally, it includes the power needed to pump oil and water through the engine, plus drive any accessories such as an alternator and fan, etc.
Indicated horsepower (IHP) – By definition, indicated HP is the sum of the brake HP and the friction HP. This is the power actually produced at the pistons. If an engine produces 350 BHP and consumes 100 FHP, then it’s producing 450 IHP, at the pistons.
Obviously, BHP is what we are interested in. That’s what the input shaft of our transmission sees. It is interesting to note, though, that if we increase the IHP and/or decrease the FHP we get an increase in the BHP. Most hop-up mods do just that!
Next, let’s look at the terms Net and Gross. By definition, per the Society of Automotive Engineers (SAE) J245 engine rating code. (This code was superseded in the early ’80s by a somewhat similar J1349 code.) Net HP is that obtained with a “fully equipped” engine as it is installed in a vehicle. This includes the air cleaner, a complete
exhaust system, a radiator and fan, the standard oil pan, normal spark advance, an alternator and a fuel pump. The J245 rating code came out in 1971 and Uncle Sam mandated that the SAE Net BHP rating be used as the advertised HP for the 1972 model year. The SAE Gross HP is obtained with the engine as above except without the exhaust system, radiator and fan, or the alternator. Deleting these items will increase BHP.
Previous to the SAE J245 rating code, Chrysler used a version of the ancient SAE J606 code. Chrysler’s Laboratory Net was virtually the same as the SAE Net. Chrysler’s Laboratory Gross was similar to the SAE Gross except for the following; a deep lab oil pan that lowered the oil level about six or eight inches (eliminating crank windage losses), and the spark advance was optimized at each RPM point. Both of these will increase BHP. Another configuration that Chrysler used, the one that produced the BIG numbers, was called Cold Bare Gross. This was like Lab Gross except the intake manifold heat crossover was blocked and the air cleaner removed. Again, both of these will increase BHP.
So, what does all this mean for the mechanically challenged? Well, if you start with an engine in the SAE Net configuration and go to the SAE Gross, then to Chrysler’s Lab Gross, and then to the Cold Bare configuration, you will get four different, progressively higher horsepower numbers from the same engine! So, which one is the right one? Let me ask you this. Which one would YOU use to advertise YOUR performance car?
You don’t have to be capable of programming a VCR to figure out why Uncle Sam mandated the SAE Net HP ratings for 1972 and beyond. The “Net Brake HP” is what your Mopar actually has available at the flywheel to stomp Fords and Chevys, since you don’t run your car out on the streets without an exhaust system, alternator, etc. But, how many horses does a GTX or R/T really make “as delivered”? Well, there are two more horsepower adjectives we need to look at first. They’re called observed and corrected.
If we dyno an engine at Atlantic City on a cold winter day, we will obtain a certain BHP. This is referred to as the “Observed” BHP. Now, let’s say we ship that engine to Denver and retest it on a hot summer day. Will the Observed BHP be the same? No! Even if we go to the trouble of optimizing the jetting at both venues, the engine will make more BHP in colder air and/or a higher atmospher-ic pressure (or lower altitude). But why?
A basic fundamental of engine performance is that the more air we can flow through our engine, the more power it can make. It’s the oxygen in the air that combines with the fuel during combustion, that produces the heat, which causes the pressure, which pushes the pistons, that turn the crankshaft. Most of our performance modifications are made to increase airflow through the engine. The problem we rodders have with airflow, though, is that we generally think in terms of volume per unit time, i.e., cubic feet per minute (CFM). We say, this carb flows 750 CFM or that port flows 250 CFM, for instance. The problem with this way of thinking is that it is the weight of air that matters, and the weight of a cubic foot of air varies with temperature and pressure. The higher the pressure and/or the lower the temperature, the heavier, or more dense, the air is, i.e., the more oxygen molecules there are in a cubic foot. The denser the air going into our engine, the more power it will make.
This is, of course, a dilemma for anyone conducting an engine develop-ment program. How can you compare data between dyno power runs when the atmospheric conditions are different, day to day, run to run? Engine guys solved this years ago. They adjust or “correct” the observed power data to a “standard” barometric pressure and temperature. Since humidity also has an effect, an adjustment is made for that too. This corrected power data can then be compared with reasonable assur-ance that the difference between power runs was due to something other than the atmospheric changes. The SAE rating codes specify a “standard” to be used. Before 1972, everyone used the J606 standard. The J245 standard of 1972 gave a lower corrected HP. The J1349 standard, now in use, will give a corrected power about halfway between the older codes.
The dyno data that you get from your local dyno shop will more than likely be corrected. I called Superf low, who is a major dyno producer, and asked what standard their customers use. The guy I talked to said that most use the J606 standard because, and I quote, “it gives more horsepower.” Just because your engine was dyno’ed at 300 HP doesn’t mean it’s making that much on any given day, though. Remember, the atmospheric conditions are constantly changing and your engines “observed” BHP, which is what makes your car go on any given day, changes with the conditions. That’s why it runs better on a cold, clear day.
Let’s see what effect this correcting process has. I put a thermocouple at the air cleaner on my D-150 the other day and measured the temperature of the air going into it, at a steady 65 MPH. Also, I checked the barometer and humidity at the local weather station. The data are shown in Figure 2, along with the SAE J245 and J606 standards. Now, let’s assume that the “Observed Net” BHP of the engine was 300 HP that day. Using this data, we find that the engine was making 302 or 314 “Corrected Net” BHP, depending on which standard is used. Notice that the pre-1972, J606 standard, does give a much higher “corrected” HP! No wonder the dyno shops like it!
So, did the ’70 440 Magnum make 375 HP “as advertised”? Sure it did, every day, I saw it myself on a number of 440 engines over a two-year period, 25 years ago, 375 “Corrected (J606 standard) Lab Gross” BHP.
I recently came across a copy of an engine performance development report for the 1968 440 engines. In the report are the Lab Gross and Net ratings on the engines, using the J606 standard for correcting the data. Before we go any further, we have to understand that before 1972 (and to a certain extent afterward) horsepower ratings were a marketing thing. During the horsepower wars of the ’50s and ’60s, big HP numbers sold cars. Us dyno guys would do “rating runs” and send the results to the marketing guys. They would then decide what the advertised rating should be, based partly on the results and partly on marketing strategies. The Feds wised up to this, and mandated the use of the SAE Net HP rating for 1972, which, assuming no one cheated much, made comparing horsepower between vehicles actually meaningful.
Now, let’s look at Figure 3. Notice that the peak HP number for the Magnum (HP 440) under Lab Gross conditions pretty much matches the advertised number, but the peak torque is lower. Although the report does not show results under the Cold Bare Gross configuration, I can assure you that blocking the heat crossover will increase the peak torque at least 15-20 lb-ft (as well as increase peak power by 8-10 HP, assuming the jetting is right). So, the Magnum can match its advertised rating, if run (and corrected) under the right conditions.
Yeah, but how much power did the engine in your GTX make when you picked it up from the dealer? Well then, since the Lab Net rating is with the engine “fully equipped,” like it’s installed in a vehicle, then it must have been 332 HP, right? Well...there are a couple of things I forgot to tell you. When we did rating iuns, we always used a “good” engine. The engines we used for dyno development were usually production engines. We would give them a 13 1/2 hour break-in before we used them. After maybe 1500 hours of dyno time, an engine would loosen up to the point that it would produce maybe five or ten more horsepower than the other 440s we were running. Unfortunately, once it became “good,” it wouldn’t last very long. The crank would break. If we had a “good” engine and knew that we would be doing rating runs soon, we’d remove it from the dyno and save it for that purpose. Another thing we did was mill the heads to bring the compression ratio up to the advertised value. Production engines were typically a half-ratio low. Also, the valves and seats were touched up, just before the ratings started. Did we cheat? Well, it was still stock, right?
So, since your GTX didn’t have a “good” engine in it and it was a half-point low on compression, then it didn’t make 332 HP. Maybe more like 310-320 “Corrected (J606 standard) Net “ BHP. Of course, the “actual” HP or “Observed Net” BHP would have been more or less than this, based on temperature and pressure. Probably less. There’s not much difference in the pressure and humidity between the outside ambient air and the air going into the air cleaner. But, the temperature can be quite a bit different. Notice in Figure 2 that my truck heated the outside air from 54ºF to 88ºF, that’s a 34 degree increase, enough to reduce the observed BHP by about 14 HP. And this was in a breezy truck engine compartment. In a tighter engine compartment, like on a GTX, the air would be heated even more, further reducing the observed BHP, unless of course, the Air Grabber is open. The real bottom line, though, is that the state of tune and internal condition of your engine, the atmospheric conditions, and the amount of heat the air picks up before it finds its way into the carburetor, all combine to cause the actual “observed” HP on any given day!
So, the next time you see “486 HP Street Small-block” on the cover of one of the generic rod magazines, take a look inside to see how they ran it. Usually, they’ll have a picture of the engine on a dyno. Does it have an exhaust system or open headers? How about an air cleaner? Does it have a race oil pan that hangs way down? How’s the water pump driven; by the engine or by an external electric motor? Does it have an alternator? Has the power been corrected? To what stan-dard? What kind of chassis is it going into? Will it use underhood air? Etc, etc.
Performance enthusiasts have been duped so long with inappropriate horsepower numbers, initially by the auto companies and then by the aftermarket, that you’d be sneered at if you claimed your hot street small-block made 325 HP! Or your 440 Magnum put
out 290 HP!