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Hitting the Sweet Spot

By October 8, 2009Uncategorized

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 hat’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 advise: “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 star 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. 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.

“I would rather have a 2,200 – 2,400 cruising rpm and have the engine in its power band where it runs efficiently. The ’32 with that high rear gear needs the deep 3.06 first gear of the 700-R4. Depending on how aggressively it’ll be driven I would recommend our Level 2 4L60-700R4 with a 2,400 stall 12-inch lockup converter.” With the 700-R4 the ’32 is now going to be spinning 1,850 rpm at 75 mph. We figured this by multiplying 2,652 x .70 (the 700-R4’s overdrive ratio). We’re now looking at the possibility of making the engine work too hard. To get it back in the 2,200-2,400 rpm range we’re looking for a set of 3.73:1s and we’ll be cruising at 2,300 rpm (75 x 3.73 x 336 = 93,996 divided by 28.5 equals 3,298 rpm then multiplied by .70 (overdrive) equals 2,308 rpm. This combo will really make the ’32 come alive as the 700-R4’s lower first gear and the new rear gear will make the car come out of the hole a lot better than the Turbo 350’s 2.52 first gear and the 3.00:1 rearend gear.

With a 300hp 302 Ford-powered ’34 coupe with a C—4 trans, again running a 9-inch with 3.00:1 gears and with 32-inch-tall Firestone grooved rear tires it would be running at 2,362 rpm at 75 mph. The C—4’s first gear is a pretty high 2.46 so it might not leave the stoplight too hard. If you’d like to get pushed back in the seat a little harder and still keep the engine in the sweet spot while cruising, Zack recommends a Level 2 four-speed AOD with 12-inch lockup converter and a lower rearend gear. The Ford AOD’s first gear is even higher at 2.40, but with a 4.11:1 rear gear it’ll throw your head back and still cruise 75 mph at 2,168 rpm thanks to the .67 overdrive. It may be useful for you to know the final dive ratio of the Ford AOD is 0.67:1 with converter dampener equivalent of lockup, or lockup 0.60:1 without the dampener which is how Gearstar builds their Level 3 and 4 transmissions.

Finally, I had to ask him about my own present car, that ’46 Ford with a 305, Th350, 27-inch tires, and a 3.08:1 gear, which calculates out at 2,875 rpm at 75 mph. “A Level 1 4L60-700R4 would work best here. I would not recommend a 200-4R for this application as it works best with a low gear ratio,” Zack told me. This trans would put the ’46 just a tick over 2,000 rpm at 75 mph. If I wanted to bump it up a little I could go with a 3.50:1 rear gear and bring the rpm up to just under 2,300.

But what if you have a TH350 or any other transmission for that matter that you’re happy with? Well, there’s another option, and it’s a product that can not only increase performance but also reduce fuel consumption. Gear Vendors’ Under/Overdrive bolts to the rear of a regular transmission and allows the driver to shift to a gear with more mechanical advantage than the next stock gear in automatic transmissions or wide ratio manuals. It also splits the gears so you end up with double the number you started with. For instance a three-speed auto gains first-over, second-over, and final overdrive as well as first, second, and third gears. If you also have factory overdrive you will now have three overdrive gear choices. All this means six or eight forward ratios that you can shift to sequentially. Heavy, high-horsepower cars will benefit most from a Gear Vendors unit, though a lower-horsepower motor in something like a Deuce roadster will also benefit from the close-ratios that the unit provides and make it more fun to drive too.

With a ratio of 0.778:1 a Gear Vendors Under/Overdrive reduces engine rpm by the same rpm as fifth gear on a manual transmission and can be more fun to drive than a standard overdrive. Why? Because of that gear-splitting capability. As the unit is mounted on the back of the transmission it is behind the governor, so when it is engaged it reduces the governor speed by 28.6 percent, automatically providing kickdown to the second-over gear which gives headroom for acceleration, rather than second gear which might be too low. A common misconception also is that a low rpm stall converter is required with Gear Vendors unit, but this is not the case. Even with a high-stall converter such as 3,500 rpm, when cruising, nowhere near peak horsepower is being made, so the converter will still be hooked up. More detailed information can be found at the Gear Vendors website if the idea of over and underdrive appeals to you.

It should be obvious now why John Beck’s advice that everything has to work together is important. Once the final ratio at the driveshaft is determined, you can work out you differential ratio, remembering that rear tire diameter is also a major factor here. There are a number of websites that offer rearend ratio calculators which can be found by searching the Internet, requiring you to input data such as tire diameter, engine rpm, road speed, and such to determine the ideal ratio needed. Various ratio ring and pinion sets are available for all popular axels. For instance the 9-inch and 8-inch Ford can be fitted with everything from a 2.80:1 set right up to pretty much whatever you can think of, though it’s unlikely any road-going car will need anything above 4.88:1! The range of ratios available between these two is amazing, enabling you to dial in your requirements perfectly. The GM 12-bolt and Dana 44 and 60 axles are also well represented with many ratios available for each, though Zack Farah recommends that the ideal gear ratio with overdrive transmissions is 3.55, 3.73, or 3.90:1, saying you go with a taller differential ratio (lower numerically) with one of today’s performance engines you risk dropping the engine rpm below the camshaft’s powerband.” Again though, tire diameter is a factor too, and that’s where we come back to aesthetics again. Looks, economy, and performance, but nowadays you can have it all!

Classic TrucksChanging a trailer tire will affect cruising speed, performance, and economy. In extreme cases you may need to change the rear gear ratio just to get back to where you started.

Classic TrucksCurrie Enterprises can supply complete crate rearends, third members, or ring and pinion sets in many ratios for Ford 9-inch, 8-inch, 8.8-inch axles, as well as Dana 60, and GM 12-bolt axles. Be sure to work out your combo on paper once you have determined your tire diameter and transmission choice so you can get the right gears and save money by not having to replace a wrong set.

Classic TrucksEdelbrock’s AFB-style Performer carburetor is very tunable and can produce good mileage and is a favorite of John Beck at Pro Machine for a mild engine.

Classic TrucksAn overdrive transmission can have a big effect on reducing fuel consumption, such as this 200-4R from Gearstar, though choice of transmission depends on your engine and is closely associated with axle gear ratio and tire diameter.