Dyno testing can deliver some deceiving numbers if you're not well versed on how dynos wor
Dyno Rat Tricks
Brady Turley, via Car Craft e-mail: I am planning on building my motor soon and would like some advice. I want something that will rev to 6,500 rpm without a problem, and I'm also looking for around 700 hp. It would be something similar to your 700hp-for-$6,720 big-block (May '07) since I already have a 454 block that I plan on putting a four-bolt main kit into, stroking it to a 496 with all forged internals and running good heads. I want it to be fairly streetable, but I am running a five-speed and hydroboost brakes, so a big cam is OK. I also want to reuse my Edelbrock RPM intake if possible and stay with oval port heads that have the exhaust ports in the same location so I can reuse my current headers. Is there a way to make a nice, normally aspirated street package out of this? I will also be running full exhaust and converters to boot, just because I can. I am expecting that 700 hp to turn into around 660. If it helps, it will be dropped into a '71 Nova with 3.42 gears, hopefully weighing in around 3,300 pounds.
Jeff Smith: Let's start with a reality check, Brady. If you want to run oval portheads and an Edelbrock RPM dual-plane intake manifold, you can forget about 700 or 660 fully streetable horsepower. Let's first deal with a concept I'll call optimized horsepower. This is a number that only exists on an engine dynamometer. If you believe the Internet forum critics (and there are a lot of 'em!), all the dynos that Car Craft uses are optimistic by at least 5 percent (which would be a ridiculous 35 hp at 700 hp). The reality is that the 707hp number we made with our 496 was created with the benefit of idealized dyno headers and 4-inch-diameter mufflers that are sound dampers in name only and present very little restriction to power. Next, for quick setup, we used an electric water pump rather than an engine-driven pump. The engine had no accessory drive and no air cleaner and was operating at 165 or 170 degrees of water temperature with an oil temperature of 220-plus degrees. Also, any engine dyno number you read is corrected to 60 degrees' inlet air temperature, zero humidity, and an atmospheric pressure equal to sea level at 29.92 inches of mercury. Those are hardly typical at your local dragstrip. Now put that same engine in a car with a muffled 211/42-inch exhaust system, an engine-driven water pump and fan, and all the other stuff that makes up a truly streetable engine package; test it on a chassis dyno and remove the parasitic losses from the trans and rearend, and you're looking at perhaps 625 hp. Rear-wheel horsepower with this engine in a car with a TH400 trans and loose converter we'd estimate at roughly 525 hp.
Our ultimate job is to sell magazines. One way to do that is based on big horsepower numbers. They are honest, but not necessarily real-world. This is the story behind the story that nobody wants to talk about. Why? Because most people who read this magazine want to believe that their engine makes 700 hp. It makes for great bragging rights. But the truth is that once you get that 700hp engine in the car, it is barely making 650 at the flywheel. What we haven't even talked about are the custom headers at $1,500 a set necessary to fit those raised exhaust ports. There's the truth.
'Cc Quickies Maybe this Pirates of the Caribbean thing has gone too far.
But all is not negative, Brady. You might make 600 hp from a dual-plane 496ci Rat motor with oval port heads if the moon and stars are perfectly aligned and your name is David Reher, Warren Johnson, or Ken Duttweiler, to name a few. We simulated a 10.0:1 compression 496 with a set of Brodix Race-Rite oval port Rat heads (PN 2061001 from summitracing.com, $1,929 a pair) and a Comp hydraulic roller 270 cam spec'ing 218/224 at 0.050 with 0.510 lift, small headers, and a 750 carburetor, and the simulation said 590 hp at 5,500 and 626 lb-ft of torque at 4,000. Yes, that's a very conservative camshaft. We chose that duration figure in order to keep the peak rpm below 6,000. That would be an outstanding combination, but again, these numbers are closer to optimized horsepower than real-world power numbers. Subtract 50 hp from that and you're close to what you could make in the car on an average day. These are not optimized horsepower numbers, but they are excellent and reliable levels for an extremely streetable Rat motor.
If you want to make more peak horsepower, you will need to run a camshaft with more duration and lift. The problem then becomes the restriction of the dual-plane intake manifold and the small carburetor. A single-plane intake and a bigger carb will make more horsepower, but you have to spin the engine up over 6,000 to achieve that goal. This is still a safe rpm, but anytime you start revving above 6,000 on a consistent basis you'd better have really good parts, because Rat motors are famous for abusing valvetrains due to both heavy parts and angled pushrods.
We're not sure why you would want to run catalytic converters in an early car like this. If low emissions is your real goal, this is the wrong combination for that. The problem with even "high-flow" catalytic converters is the restriction of power in the upper horsepower range. As a wild guess, we'd tag the loss at around 20 hp, perhaps more. That's a great reason not to run cats, especially on a big-inch big-block.
As far as the rest of the car is concerned, plan on using sticky tires that are the largest you can stuff under the stock '71 Nova wheelwell, because this motor will make gobs of torque. That equates to lots of tire spin at lower speeds even on the dragstrip. With a five-speed it will be difficult to bang gears without spinning the tires. But if that's your goal, then this will deliver the goods. It would be fun to drive! We talked to one of the tech guys from World Products about a 598ci Rat motor customer. The guy complained that he had already shredded one pair of rear tires because of the massive torque the engine made and wanted to know what to do about it. Our advice is the same as theirs-buy bigger tires. This actually sounds like a great engine package if you can hook it up. Just don't put catalytic converters on it.