We dig deep into our bag of tricks to show you how gear and converter selection can beat big horsepower (almost) every time.
Words and Images by Tim Stockwell
Let’s face it, to build a successful street/strip car you have to decide on your honest intentions and build accordingly. If you want a quarter mile car you gear it and set up the suspension and torque converter to that particular goal, while the set up for a car you plan on having some realistic street battles with may be completely different. In my 30+ years, I’ve come to realize that 95% of my street races weren’t ¼ mile, or even 1/8 mile bursts. But basically short little light-to-light runs or competitions where as soon as one car gets a few lengths the other backs off and it’s over. Unfortunately we don’t have a local “Street Outlaws” match every week.
While pondering this I came to see that most cars in this area are set up for quarter mile racing, meaning not so steep gear ratios, and set up to hook only on a well prepped sticky track. I wondered if you could overcome brute power with smart gear ratio, torque converter, and suspension choices and thought we’d give it a shot with our stock (seriously) Mustang LX.
Our little test subject is a 1989 Ford Mustang LX 5.0 with 74k original miles and completely stock motor. When I say stock, I mean it has never been opened up at all, never had a valve cover or intake off, and has obviously never been out of the car. It does have the simple free mods like removing the intake baffle from the fender and timing adjustments, but the only aftermarket parts are a C&L MAF meter, a set of E-bay underdrive pulleys, and a pair of Strange Engineering single adjustable front struts at their loosest setting just in case this goes how we want it to. This model also came with the AOD automatic transmission that has never been apart other than to install a shift kit and to do the epoxy mod to correct the shift pattern that Ford got so wrong. With this as a starting point we decided to try a few different gear ratios, and finally a torque converter to test our version of local street racing, which is a simple 0-60 mph test with a G-tech meter that I have found to be a great tool to compare apples to apples when modifying.
Before we started we wanted all tests to be equal, to the timing was set at 14* and nothing was changed during any of these tests other than rear end gears and torque converter, and all runs on M/T 275/60/15 Drag Radials to keep it fair.
Our first test point was pure agony, being the typical stock Mustang with a stock AOD, a very low stall factory torque converter, 2.73 gears, and making it even worse we used 28” tall tires. While running at least 10 timed passes the best we could do was a painfully slow 7.38 seconds. That’s as slow as most stick shift 4-cylinder cars and just may get embarrassed by that little Civic with the fart can muffler and 17 year old high school student driving it.
Our second attempt included the swap from a 2.73 gear to a 3.55 gear. While most people that build what others tell them to, this gear ratio used to be the favorite for 5 speed cars, and people would also then put them in AOD equipped models not realizing the huge difference between a 5-speed’s acceleration versus an AOD equipped car as a starting point. Big mistake for the money in my opinion, while it does help seat of the pants feel and drop 0-60 times, it still isn’t enough for a stock converter AOD car. Our Mustang did feel a little quicker, and the 6.88 0-60 times showed that.
Next up on our list was a set of 4.30 gears. This is the point where we saw serious potential and that we were getting somewhere. This feels like the gear that Ford should have put in this car, accelerating much quicker the car actually felt right, shifting at a reasonable mph and not going an eighth mile before shifting into second gear. With overdrive being a mechanical lock up, rpm at 65 mph was still ridiculously low and now actually usable without shifting in and out. Gas mileage doesn’t seem to be effected at all, but I didn’t drive it 1000 miles to verify. The addition of a 4.30 gearset gave us the start of what we were looking for, and a best of 6.02 in our 0-60 runs … chopping well over a full second off of our original times.
The last gear ratio tried was what most people consider to be nothing that should be run on the street, a 5.13 gear, and was the steepest we could find, period. This is where the car suddenly became fun, and really started to show its true potential. The 5.13 gears and a lock up AOD is COMPLETELY drivable, so much fun to drive, and is about as quick as most anything on the street, bar running up against anything that has the horsepower edge, dead hooks, and has a driver that knows what they are doing. Loving the newfound power, I must have done 25 runs over a period of a week or so, and shifting at 5200 rpm like all the other tests it proved to be (in my opinion) the only gear to put in an AOD car if you do more driving around than highway trips. At a 65 mph cruising speed it was still only 2800 rpm in overdrive, and completely acceptable for the purpose of this car. Launching was now actually fairly impressive, hitting second gear and chirping the tires and still pulling on our way to a best of 5.21 seconds and consistent runs under 5.40 or so. If you look up 0-60 times for some pretty fast street cars you’ll notice our little car is right there, and consider that most of those times are exaggerated and/or corrected to sea level and weather conditions for advertising and promotional use we are definitely in that basic area. For a big number of short street races this would look very impressive compared to most any stick shift car that would either bog or spin on the street, and the cars that are set up too aggressively for honest street action and lose traction.
Last up was our change to a Precision Industries 9.5” lock up converter. While it is a much smaller converter, it is fairly tight for its size, with a true stall speed of only about 3000-3200 rpm… high enough to work with our stock motor and keep shift extensions in our range but high enough to leave as hard as we wanted and still dead hook with our Mickeys. At this point the car is so much fun. It feels like it has so much more power than it does and is a real hoot to drive. With our stock suspension and our struts still at their loosest setting, this car leaves hard enough to lift the left front wheel on the street … not much, but considering it has only 225hp as rated by Ford, and a realistic 185 or so rear wheel horsepower, this took our Mustang all the way down to an impressive 4.76 seconds (later bettered to 4.69) and to a point that the vast majority of regularly street driven cars couldn’t touch in the real world. Unless your competition is set up for this purpose like this one, I don’t care what you have or how fast it is, in a quick little run from light to light we would be well out in front. If they spin the tires whatsoever, or sleep at the light at all, it’s too late and we have bus lengths on them. Anything with regular street tires is fair game, anything with a manual transmission is game, and unless it’s all wheel drive and the driver is quick off the light, I wouldn’t be afraid to line up against most anything in an actual real world jaunt in street mode.
What did all of this tell us? It told me that horsepower doesn’t need to be in your first few rounds of mods; that dyno numbers don’t mean squat in the real world; and that making a car that even most Mustang guys make fun of a contender in the local street action is easy if you choose well. We don’t race from a roll, and we don’t want to race much further than a few hundred feet, but as a daily driver that encounters an occasional Civic at the light, we are light years ahead of our starting point with only a gear and torque converter change.
This is it, that’s all we are changing. Notice the difference in pinion size between 2.73 and 5.13 ratios.
The mighty 5.0, 25 years since Ford assembled it.
Here’s where our little car ended up after only two driveline mods, pretty impressive for 185 rear wheel horsepower and 2,970 pounds including me inside.