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Compression Ratio - Ask Anything

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460HP, 5.3L Engine Combo

Tucker Ryals; Gainesville, FL: I'd like to pose a question regarding an article Jeff Smith wrote on the Chevy 5.3L motor "Bolt On a Cam and Heads and Add 120+ HP" (April '08). I read the article with great interest, as I am planning on a budget motor build, utilizing a Gen III or IV Chevy V8. I plan to install the motor and a T-56 into a second-gen RX-7 and use the car for open track days, HPDEs, and time trials. I would like to piece together a package using various off-the-shelf parts to create a motor that is willing to spin pretty high but still have a nice, wide powerband. I'd be willing to sacrifice some low-end torque to pick up some higher-rpm power.

While I don't have a stated financial limit, I would really like to put together the package as inexpensively as possible, mixing and matching parts across the engine family if necessary. For instance, some folks claim a 4.8L crank in a 6.0L block yields a spin-happy motor while others pooh-pooh the idea, claiming driveability issues. Given those stated goals, I would welcome your recommendations on a workable recipe.

Jeff Smith: Wow, we could go on for days on this subject, Tucker. Let's try and narrow it down to a few essential truths. I like the idea of a strong V8 in a lightweight car, and I hear this is a popular swap for the RX-7. Before we get into the specific details, let's hit a couple of basic engine ideas that may help color your decision. The first thing we should discuss is displacement. The easiest way to make power is with displacement. Given a relatively non-restrictive induction and exhaust system and camshaft, a bigger engine will make more torque and more horsepower than a smaller engine. It's that simple. So starting with a 6.0L engine might make more sense than swapping parts around to build a 5.3L or 5.7L engine. But it appears you are looking to spin this engine a little higher in the rpm range for a road-race-style application, so we need to address the factor of rpm as well. High rpm and long-stroke engines are generally not compatible unless you spend big money on quality parts. So let's assume we will be using a shorter-stroke engine. As we all know, we can calculate horsepower based on the torque created by the engine at a given rpm. The formula is very simple: (torque x rpm) / 5,252 = HP. From this, it's simple to see that if an engine makes 400 lb-ft of torque at 5,252 rpm, then it makes 400 hp. But by modifying the engine design, we could make that same 400 lb-ft of torque at 6,000 rpm. Plug that into the equation, and now this engine makes 457 hp. We're making the same torque, but because it occurs at a higher engine speed, the engine makes more horsepower. Good-flowing heads and a big camshaft are required to make this kind of torque. Big-displacement engines often rely on a long stroke to create that larger engine size, which means the piston now has to travel a greater distance at these higher speeds. This creates very high g-force loads on the piston and connecting rod. This is the main reason short-stroke engines are considered "rpm" engines. The pistons travel a shorter distance and therefore place less stress on the rotating assembly, which includes the crankshaft. Engine speed also affects the valvetrain, as those valves, springs, and lifters also have to travel at greater speeds and are impacted by increasingly greater loads directly caused by rpm. So if we decide to build a road-race-style engine that will see 6,500 to 7,000 rpm, we have to decide how big the engine will be and how much money we are willing to spend to ensure its durability.

Let's go a little deeper into the 5.3L engine story you mentioned. That combination was a stock short-block 5.3L engine with a Comp Thumper camshaft (227/241 degrees at 0.050 with 0.563/0.546-inch lift with a 109-degree LSA), a carbureted, single-plane intake manifold, and a set of West Coast Racing Cylinder Heads–ported Edelbrock heads. The cylinder heads increased intake- and exhaust-port airflow (267-cfm intake flow at 0.500 lift), and this, combined with the longer-duration cam, pushed the peak-torque rpm point up to 5,600 rpm (from the baseline engine's 4,200 rpm). Generally speaking, most engines will create a powerband of around 1,500 rpm. We define the powerband as that rpm spread between peak torque and peak horsepower. In this 5.3L engine's case, peak torque was at 5,600 rpm and peak horsepower occurred at 6,800 rpm for a powerband of 1,200 rpm. The engine did make a strong 460 hp, but we had to spin it to 6,800 rpm to achieve that number. We didn't get into engine durability in that story, but the reality is that cast pistons and those spindly, stock connecting rod bolts would probably fail within a couple of hours (perhaps minutes) of running this engine at 6,800 rpm. The g-forces on those components increase geometrically as engine speed increases. While those stock parts might live a relatively long time (at 6,500, for example), they will fail within a very short time at 6,800 rpm—a mere increase of 300 rpm. The 5.3L engine and compression ratio was designed as a truck engine, and it's fair to say that the GM engineers never intended this engine to spin anywhere near 6,800 rpm for long periods of time.

Compression Ratio That means we have to change some compression ratio components to ensure our little engine will spin that high and survive. The first change should be a switch to a high-quality forged piston. For the sake of discussion, let's look at the idea of adding the 4.8L engine crankshaft to a 6.0L iron-block engine. The idea behind this plan has some merit. The factory 5.3L (325ci) engine uses a relatively small 3.78-inch-bore diameter combined with a 3.62-inch stroke length. You can create almost the exact same displacement by switching to a larger, 4.00-inch bore and the shorter 4.8L engine's 3.26-inch stroke. This creates a 327.7ci engine that, ironically, is almost an exact replicate of the original small-block Chevy 327 with a 4.00-inch bore and 3.25-inch stroke. You could add a few inches by boring the cylinders 0.030-inch oversize, which would create a 332ci engine. There are a couple of advantages to this setup over a stock bore and stroke 5.3L engine. The first is a shorter stroke, which reduces piston friction, and the second is the slight cylinder-head-flow improvement with the larger bore's 4.030-inch diameter. The problem with this approach is finding a 4.030-inch piston and the proper combination of compression height and connecting-rod length that will put the piston at the deck height. Our initial search for a 4.030 piston with 6.100-inch connecting rod and the proper compression height wasn't successful. You could have custom pistons made, but that is expensive. A slightly better idea would be to go with the 5.3L crank stroke of 3.62 with a 4.030-inch bore that will add displacement (369 ci) and also piston speed, and therefore add friction. This combination can still make decent power, since the stroke is not that much longer than that of a typical 350ci small-block Chevy (4.00-inch bore and 3.48-inch stroke), and Lord knows there are tons of these high-rpm road-race applications running. The piston was also difficult to find, mainly because it appears that most of the off-the-shelf pistons are aimed at larger-displacement engines. You could fall back to a 0.020-over 5.3L engine with the stock stroke and good rods. Wiseco offers a forged, flat-top piston for a 6.125-inch rod. While this piston is more than $700, it, along with a good 4340 steel connecting rod, will deliver the durability. Add in the cam and heads we used in the story you referenced, and you would have a 460hp package that would not only live but also deliver excellent power, especially for a relatively lightweight car such as yours. This might be the least expensive way to go, but there will still be significant cost involved with building the engine. It sounds like it will be fun to drive!

More Compression Ratio Info

Comp Cams; 800/999-0853; CompCams.com
West Coast Racing Cylinder Heads; 818/705-5454; Proheads.com
Wiseco Pistons; 800/321-1364; Wiseco.com

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1 comments
alkycarssincethe70s
alkycarssincethe70s

Another SUPERB Car Craft article that successfully makes me understand a subject by teaching DIFFERENT aspects of an issue from a DIFFERENT approach. I've read hundreds of articles and textbooks on compression ratio. Yours is better. Thank you.

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