Gotta learn this stuff somewhere..
I know the difference between gross, net and rear wheel. I also know the difference in ratings for GM starting with the drops in advertised HP between 1970 and 1972. BUT....

How does the 355 HP rating on a 2002 ZZ4 crate motor compare to a 350 HP rating in 1970? It seems after reading, the numbers are more arbitrarily inflated by air stacks with turbines, use of the dyno fuel pump. etc, creating numbers that could not be duplicated in real world applications where the ratings in the late 60s and 70s are closer to "real world" performance.

Am I missing something?

 

well 1970 stock corvette did not run in the 13's with 370hp.
ZZ4's seem to bump into the mid 13's.
http://www.corvettearchive.com/image...e70/page34.jpg

Not that this answers your question, but at least adds some more information about the 1970 vette :cheers:


[Modified by flood, 6:24 PM 7/12/2002]


[Modified by flood, 6:25 PM 7/12/2002]

 

I have actually read that article. I personally could care less what mine will do on the track relative to anything built today. There will also be faster cars. I have read reports though, that the LT-1 would push high 12's in the 1/4 with 5.7 sec 0-60 times, and were more closely rated to 430 HP (though not really the point).

This just seems to illustrate the lack of any comparability between HP ratings, making them pointless (gonna catch heat from someone :yesnod: ). Since most seem to be concerned about the numbers, I am just curious.

 

I have over 15 years of experience running engine dyno's so I will try to give you a break down of how they work. This might get a little winded so quit reading whenever you feel like it.

Most reading taken on a dyno are to find maximum torque and HP readings. Along with this you chart the numbers and read the "area under the curve" to see if you changed the curves at a particular RPM or changed them throughout the RPM range. To only look at Peak HP and Peak torque is very misleading but give you bragging rights at the "bar". :cheers:

When you mount an engine to a dyno the only accessory that will be used in the car is the water pump because on a normal dyno pull you are not concerned with water flow.

You will use a velocity stack to channel cool air to the carb. The velocity stack has an air turbine built into it. The turbine is what measures the air flow and does not increase or decrease HP. But the velocity stack will give increase HP over any air cleaner or no air cleaner.The readings from the air turbine are used to calculate volumetric efficencies. An average performance street motor will have a volumetric efficency of about 85-88%. An all out race motor will have a volumetric efficency of about 105-105%.

You will use the fuel pump(s) that are on the dyno as they are plumbed through fuel turbines. Again these turbines are just to take readings and do not increase or decrease the HP. But the pumps on the dyno may increase HP as they will deliver any amount of fuel the motor needs. Also the motor does not loss any HP by having to run the stock pump. The number that the fuel turbines give you are use to calculate air/fuel ratios and also "pounds of fuel per HP per hour" both of which are used to tune the motor.

The motor will not have a fan blade on it as the dyno has a cooling tower that will keep the water temp of the motor at a steady temperature to take that variable out of the dyno runs.

When you make a dyno run, after bring up the water and oil temperature, you will make a pull, evaluate the data, make one change and start all over again. By doing this you will find the optimum timing, jetting, header size - length -step but it takes a considerable amount of time and resources. Many times I will have a race motor on the dyno for two or three days solid testing combinations to bring it up to maximum power across the RPM band not just peek power. It is not uncommon to step a race motor up 30 to 40HP from the time it is put on the dyno till it come off. But with dyno time reaching $150.00 an hour it is too expensive for a street motor.


The procedure I described above is the what all car manufacurer used until 1972. That is when they were required to dyno the motors as they were installed in the cars. This meant all accessories, air cleaners and timing and jetting set as they are when the car is built.


So the ratings on the ZZ4 (or any crate motor) would be comparable to any of the number from factory cars prior to 1971. A 350 HP LT-1 would be almost equal to the 355 HP ZZ4.

The bottom line is "You can't race a dyno." It is just a tool for tuning and measuring changes to a motor and rarely should number from one dyno be compared against another. If your motor is running good and you are satisfied with it don't be discouraged when the dyno reads 100 HP or a 1000 HP. It is how it runs down the road. :cheers:

 

Awesome and totally understandable response!! Thank you Pete.

Now a follow up question. Since I assume that the crate motors are comparative to pre 71 factory reports, what about HP ratings in current production models?

 

Good info Pete! Another poster mentioned that ZZ4 equipped cars seemed to run better times than similarly rated early engines and this has to do with the "area under the curve". Even though both engines may make the same peak hp, the ZZ4 is likely to make more power below peak. If the cars were launching at peak power and staying there throughout the run, they would probably run closer times. In reality though, they have to accelerate through lower rpms to reach their peaks and the ZZ4's better power below the curve gives it an advantage.

 

Then the ZZ4 should outrun similarly rated engines in shorter runs (better than 0-60 times), but those other engines should yield better 1/4 mile times - yes/no? Kinda like a sprinter and long distance runner.

 

What we don't need here is another NET - GROSS discussion but it seems that after hundreds of posts on this issue, many of us don't quite understand including me. However, any ZZ4 owners out there will tell you that the 355hp contained therein is potent power and will run away from most factory smallblock cars. The C5s with 345hp can run 12.8 - 13.2 in the 1/4 stock. Is there a formula for conversion? I believe that the horsepower drops of the 70s lowered ratings even on identical engines from one year to the next equipped with all the same accessories. Could it be that the old ratings did not take into account accessory load? If this is the case, then how much hp is taken up by engine accessories? Dyno results are pure numbers but what frame of reference did automobile manufacturers use to calculate the different hp ratings?

I don't have the answers but I'm sure someone else does :confused:

 

VetteNut72,

Part of the reason that the C5s can run what they do is aerodynamics. A C5 can out out (in many cases) and higher HP vehicle because of drag, etc.

I posed the question, not for net vs gross, but rather to ask how a 370 rated 1970 small block compares to a 355 rate ZZ4 to a ??345 rated C5.

I can tell you that some of the rating drops in the HP #s came from minor engine changes, especially in the LT-1s.

 

Quote:
______
The procedure I described above is the what all car manufacurer used until 1972. That is when they were required to dyno the motors as they were installed in the cars. This meant all accessories, air cleaners and timing and jetting set as they are when the car is built.
______

So, what you are saying is that the old ratings (pre 1972) were dyno based without accessories and with maximum tweaking for power and that the new ratings are based on full accessories and street tuning. This would seriously overrate pre 1972 hp ratings compared to the new numbers. This is exactly what I thought. Your information is excellent and most likely solves the question correctly. Does this make sense?

 

VetteNut72: Your asumption is correct. And that also applies to all crate motors because the builder has no idea what type or car or accessories the motor will be coupled with.


Wireless Guy: a 370 rate 70 small block would be about the same as a 355 rate ZZ4 AT PEAK because both rating are "gross" or tweak to maximum number. The 345 HP C5 would be considerable more because it is "net" or as it is installed in the vehicle.

Vetterodder: You are right on track with your statement.

One thing to keep in mind is that while both the ZZ4 and the LT-1 are about the same "at peak" I would asume (and I may be wrong) that the area under the curve on the ZZ4 would be greater because of advances in camshaft profile (flat tappet vs. roller), improved intake manifold design, and definitely improvements in cylinder head design. All those improvements should give the ZZ4 a flatter more useable torque curve. This is one of the reasons that I stated not to look at only peak numbers as they can be misleading.


One other variable to keep in mind when trying to compare old information about performance of early muscle car against times that modded C3's run today is the VAST improvments in tires. The early C3's could buzz those F-70's at will where as the tires avialable today hook these cars pretty tight.