Optimum Performance Combinations for Drivetrains - Hitting The Sweet Spot

We've all read car features in magazines and noted the major components used in each one's construction, but how many builders actually build to a plan and carefully select those components? Sure, they may know what engine they want and realize the benefits of an overdrive transmission if they do a lot of driving, but will the two be ideally matched? Why pick a 700-R4 over a 200-4R for instance? And what about rearend ratio? Is this even considered when having those 32-inch-tall 8.20/18 Firestones mounted on the rims to replace the old 28-inch-tall 15-inch radials? And how many people choose a carburetor that's way too big for their engine, maybe losing performance as well as gas mileage in the process? See, there's always an optimum combination for any given drivetrain, for both performance and economy. This is R&C after all, so we're mainly concerned with performance, but in the current climate, economy is never far from our mind either!

But where should you start if you want to optimize your selected choice of drivetrain components? The rear tire diameter would seem a logical choice, and as our hobby cars are as much about looks as performance, you're going to choose what looks good and work around that, surely?

Maybe, but being able to select the correct rearend ratio to combine with tire sizes and your transmission choice means this isn't really as important an issue as you might think, and perhaps we should start at the engine and work our way back. Zack Farah at Gearstar offered this advice: "Before thinking about the transmission, you need to decide what kind of engine you will be running and whether it'll be a daily-driver, a cruiser, a race motor, or a combination of these. The application is important, as only then can you choose the correct transmission."

In these times of multiple choice crate engines available from Ford, GM, and Mopar, we know many of you opt for a certain performance level and install your motor as it came from the factory and won't ever want to get inside it and start swapping cams for instance, but for many building their own motor is still part of the fun. Having a better understanding of the rpm range where most normal driving is undertaken is going to be important to coming up with the right combination. Modifications that make horsepower and torque in the lower rpm range between 1,500 and 3,000 rpm are way more useful than a high-revving race motor, unless that's what you intend to use it for. One of the biggest mistakes people make when building an engine for their hot rod is to look at a list of cams and find one that looks like it might have the right specs and then they drop down the list and get a cam two sizes bigger because the catalog said it would make more noise. The cam selection is very important so it's best to leave it to the pros. Comp Cams has a wide variety of cams and their help line can guide you toward the best selection.

Intake manifolds and exhaust headers come into their own here, with something like Edelbrock's Performer dual plane low-rise intake manifold offering great torque in the lower (idle to 5,500 rpm) rpm range (not to be confused with the Performer RPM which is a slightly higher range manifold in the 1,500-6,500 rpm range, similar to Holley's Street Dominator) or one of the ready-to-run aftermarket EFI systems such as the FAST XFI system or Edelbrock's Pro-Flo system.

So you really need to decide on your engine and what you'll be using it most for before considering the rest of the drivetrain. John Beck of Pro Machine offered this advice: "Everything has to work together. One of the mistakes people make is to fit an overdrive transmission and something like a 2.90:1 rear gear and then find they are doing 100 mph before they reach their cruising rpm of 2,200 rpm. The other variables have to be figured out for the engine to work efficiently, and the key is to install the right cam." Talking to John, it's evident that he rates a good vacuum gauge as an invaluable tool when it comes to analyzing what an engine needs or is doing. "You can tell from the vacuum gauge how much load your engine is under and get a good idea of fuel efficiency. You can also see when you have your foot in it too much! 12 inches of vacuum at 2,200 rpm on the freeway is a good start, then the carb can be tuned. I like the Edelbrock AFB-style carburetor as it's extremely tunable. I've seen as much as 25-27 mpg from a 327 with one of these carbs. They make a 500-cfm version but this is really for dual setups. The 600-cfm Edelbrock is best for economy. There is a mathematical formula to work out what size carburetor an engine will require (find this by searching the Internet for 'carburetor size'), but this tells you what it'll take to feed that engine, not for peak horsepower. When we do dyno runs we're measuring full throttle, not partial."

John is also a fan of smaller V-8s for fuel efficiency, recommending the 289 and 302 Fords, along with 305, 307, and 327 Chevy motors. Harking back to the fuel crisis of the '70s, he even waxed lyrical about the then-available SP2P Edelbrock manifold, which actually had ports smaller than a stocker and offered better low-end torque, improved throttle response at the lower end of the range, and improved economy. Asked about aftermarket aluminum heads, he didn't recommend them for cars that are mainly used for attending shows or cruises or for fuel efficiency, as they're intended as performance additions, though certain versions could be used to improve mileage by raising the compression ratio of a low compression engine, such as the 8:1 305 small-block in this author's '46 Ford. Adding a pair of World Products heads, for example, to bump the compression to 9:1 would reap mileage benefits, according to John, as it already cruises at 2,300 rpm at 80 mph and is fed by an Edelbrock 600-cfm carb on a Performer intake, but is otherwise stock.

Whatever you do to your engine though, and to what state of tune you build it or buy it with, you want it to cruise on the freeway at around 2,200 rpm, not just for economy's sake, but for ease of stress on parts and a more relaxed driving experience for the driver. Which brings us neatly to transmission choice.

Gearstar's Zack Farah is a firm believer in overdrive transmissions and manufactures them to withstand some serious horsepower, offering a 700-R4 treated up to 520hp (the Level 4, 502 package) and a 200-4R that can take 800hp (the Level 4 572 package). Incidentally, we're using Chevy transmissions here as examples as they're the most common.

With a few basic starting points and figures you can calculate the rpm your combination will be spinning at a given speed. The formula is as follows: (miles per hour x gear ratio x 336) / tire height = rpm. We asked Zack to recommend transmissions for a few specific cars to give some idea of how to make a choice.

Let's say you have a '32 roadster with a 400hp 350 Chevy, a TH350, 9-inch with 3.00:1 gears, and BFG 285/70R15 radials measuring 28.5 inches in diameter. Using our formula above we can calculate that with this combo you'd be cruising at 2,650 rpm at 75 mph. Here's how it works: (75 mph x 3.00 x 336) = 75,600 which is then divided by 28.5 and the result is 2,652.