When the driver and engines are both evenly matched, the right transmission and differential in your strip burner can mean the difference between a winning pass and an almost winning pass. At the extreme, it can also mean the difference between a winning pass and your crew having to sweep up the mess after it all grenades on you.
Trust me, that’s not something you want to have happen. It wasn’t fun.
What Is a Transmission and Why Do I Need One?
The transmission in your car transmits the torque from your engine, through the driveshaft to the differential, where the rotational energy (torque) has its direction changed from longitudinal to axial rotation, in other words the direction of the torque is changed from along the front to rear centerline (perpendicular to the direction the vehicle is moving) of the vehicle to along the axis of vehicular movement.
A transmission’s second job is to give you more control over both engine and vehicle speed. Without a transmission, your engine has to work doubly or triply hard, both getting up to speed and then staying there. Your engine runs most efficiently when it’s operating around the middle of the RPM range. Horsepower and torque output are at their lowest at low RPM, and they start dropping off quickly at the higher end of the RPM range. The gearing in the transmission allows us to keep our engines working in the optimum RPM band.
What Are Your Options When It Comes to Transmissions?
Some people might say that this is an easy question; you either choose a manual transmission or an automatic transmission. Even back when I first started racing, this wasn’t an easy question. Yes, narrowed down to its most simplified equation, the choice is between a manual and an automatic.
However, there’s much more. If you go with a manual trans, do you want a two speed-hi/lo trans, or a three or four speed? What gearing do you want it to have? The same questions are pertinent when choosing an automatic. There’s also the question of branding. The easiest part of the whole transmission choosing equation is this: You can’t use a stock transmission in a race car. Period. Unless you want your crew to have to clean up the remnants in your lane and shut down the track cleaning up all your spilled oil, that is.
A Stock Transmission Can’t Handle the Torque Your Strip Burner Makes
Stock transmissions have two things going against them when considering them for our purposes. The first, and most important is that they aren’t designed to handle the amount of torque and horsepower our drag racing engines produce. They’ll just fall apart.
The second problem is that the gearing isn’t right. Most stock transmissions are made to sedately accelerate to either street speeds or highway speeds to save gas. When we’re trying to make a sub-ten second pass on the strip, fuel economy is the last of our worries. We want our engines to rev to the max torque output as quickly as possible, hold that max torque output for a few seconds, shift gears, and do it again.
But My Car Came With ‘This Type’ of Transmission
If the car you’re building up originally came with an automatic transmission, it won’t have a clutch pedal. This doesn’t mean you can’t use a manual transmission. This is especially true if you own an older model car, as most of these gave buyers the option of either manual or automatic at the time of purchase and the automakers made them so either type of transmission could be installed. This was an economic decision on their part that benefits us.
For example, if your car came with an automatic, installing a manual is a simple matter of going to the junkyard or a parts supplier and buying the triple pedal pack-clutch, brake, and gas pedal and removing the old set, removing a few grommet plugs in the bulkhead and running a clutch cable. Switching from manual to automatic is just the opposite: Remove the triple pedals and clutch cable/linkage, plug the holes, and install the double pedals.
Some newer cars might be a little more involved. You may have to buy a pedal mounting kit from an aftermarket supplier, since manual transmissions in passenger cars have become somewhat passé. Even then, it isn’t difficult to do and usually only requires installing a mounting kit for the clutch pedal. The process of swapping out to a manual might take a couple hundred more dollars, and maybe another day’s worth of labor, but I think the end result is definitely worth it.
The Pros and Cons of Manual and Automatic Transmissions
If you spend any time watching NHRA events, you’ve seen one of the main arguments against manual transmissions-missed shifts and blown shifts. If you’ve spent any time at all driving a manual, you’ve probably blown or missed a shift or two in your time, also. I know I have. Another argument against manuals is that you’ve got to lift in order to shift, unless you pop for a full-on racing transmission, in which case, just mash it and shift it!
I define a missed shift as one where you skip a gear or two, bogging the engine, which can cause serious top and bottom end problems. For instance, you’re trying to shift into second, but you grab fourth instead. A blown shift is just the opposite-you’re looking for a higher gear, but hit the gear below your current gear. For example, you’re trying to hit fourth but grab second instead. Your engine won’t like that.
Personally, the best thing I can say about an automatic transmission is that you really can’t miss or blow a shift with them; they always go into the correct gear. However, unless you’ve got a manual automatic or have a modified trans, the shift points are probably going to be just below the beginning of the major part of the torque curve of your engine or just inside that “sweet spot” in the RPM/torque band. Either way, you end up missing out on the major part of the torque your engine can produce.
Another problem I have with automatic transmissions is that they waste far too much torque through slippage. Manual transmissions only experience slip in the clutch and a racing clutch can eliminate almost all of it. Automatic transmissions, even with lockup converters, have multiple points where slippage and torque wastage can be introduced. Running an automatic behind an engine that produces 600 and more foot-pounds of torque only exacerbates that problem.
Automatic transmissions have more failure points, too. With a manual transmission, you’ve got the clutch assembly and the main transmission-the gears inside. With an automatic you’ve got a torque converter, clutches, band packs, gears, and a whole cooling system that can fail. When a manual fails, it’s a pretty straightforward repair, but when a manual fails, you’re more than likely going to need to replace it.
Now I’m Going to Walk All That Back
To be completely honest with you dear reader, and with myself, what kind of transmission you install will depend on two things: Your expected slip times and most especially, how much money you have to spend on the transmission. Manual transmissions are lighter. Automatic transmissions that are specially designed for strip use shift quicker when tenths of a second count.
If you’re on a budget and building a car that will run in the mid- to high-ten second range, a manual transmission is going to be the best option. You’re not going to be going fast enough for tenths and thousandths of a second lost in shifting to matter much.
On the other hand, if you’re building a car that you expect to run sub-ten second passes in and you have the money, you definitely want one of the automatic transmissions that are specifically built for high speed, high torque applications. It’s going to be heavier, but that is offset by the fact that it will shift faster and more properly than a manual.
The long and short of choosing the right transmission is making sure you get one that is geared so that your engine is constantly making max torque throughout the whole pass. I spoke with a number of friends that take their cars to Bonneville and to the drags and I got multiple answers. The long and the short of it is what can you afford? Unless you’re able to spend a few thousand dollars on a transmission, you’re going to want a manual.
Gearing Your Transmission for the Strip
Gearing is the ratio of input revolutions to output revolutions. Ideally, you want your low gear ratio to be around three or four to one. This allows you a quick start from the line. As a final high gear ratio, you want one to one or lower. This makes use of every bit of power the engine is producing at max RPM.
Determining the exact gear ratio you need for your transmission isn’t really that easy though. You have to take a number of factors under consideration if you’re going to be serious about your strip burner. The John Maher Racing site has an awesome gear ratio calculator that lets you experiment with some of these factors, although it leaves engine horsepower out of the equation.
Who Makes Racing Transmissions?
If you’re building an engine that puts out less than 500 HP, your transmission options are varied. B&M, Richmond, Centerforce, ATI, TCI, Quarter Master, RAM, and McLeod are all well-known makers of transmissions that can handle the kind of engine power output.
However, we’re building a strip burner, right? One that makes more than 600 HP. We need a transmission that can handle gobs of power. Now we’re looking at makers like Richmond Performance, Lenco, Jerico Racing Transmissions, G-Force, and the Tremec T-56 Magnum to put power to the road.
Every single one of the ultra-high performance transmission makers are going to ask you for several pieces of information when you call to order your box:
- Horsepower output of the engine
- Max RPM of the engine
- Tire size-This will usually be diameter
- Vehicle weight
With this information, they’ll be able to give you a couple of gearing options for your transmission. Some of them may ask you about the gearing of your differential, also. In my next installment in what I’m calling my two-piece drivetrain issue, I’m going to describe what a differential is, how it works, the different types of rear-ends, and the optimal gear ratios.