V-twin Tech: All About Torque – and how to get some
February 8, 2007
Filed under Uncategorized
Readers often ask me for guidance on getting more performance from their Harleys. Most are looking to improve top-gear acceleration from moderate speeds, like passing a truck or moving into a faster lane. Touring riders need the same sort of power to comfortably climb mountains. Most Harley owners refer to this midrange or mid-rpm power as torque. Almost everybody with a stock Harley wants more torque.
Few aftermarket performance suppliers offer parts, kits or services that achieve this aim. What they do offer, for the most part, is what I like to call barstool horsepower. You get barstool horsepower when your engine produces gobs (a technical term) of power at or near the very top of its operating range, but very little more than stock power in the more useful mid-rpm range. Barstool power is easier to get and initially more impressive than more torque, but it also tends to be expensive.
Torque is relatively easy to improve, and it need not be expensive to do so. And while it is possible to increase power across a V-twin’s rpm range, that isn’t what usually happens. Almost all hop-up kits and services are directed at the top third, which is 4,000 to 6,000 rpm for the Big Twins. In many cases, the resulting engine is sluggish in the middle third, where we do most of our street riding. Adding power in the 2,000- to 5,000-rpm range is the key. Below 2,000 rpm is for parking lots where good balance is more important than acceleration, and above 5,000 rpm is where few of us ride our Harleys anyway.
Size Equals Power. Yes, this equation is a bit simplified, yet it’s largely accurate; bigger motors make more power—especially in the midrange. An engine’s power output is determined by how much air/fuel mixture gets burned in a given time. A small-displacement engine at high rpm can match the power output of an engine twice its size turning half as fast.
The simplest and most effective way to get more power out of your Harley at a given rpm is to make it bigger. If you can find a way to stuff 10 percent more in, you’ll get 10 percent more out, everything else being equal.
You might bore your TC88 to 95 inches or your Sportster 883 to 1200cc. If you do nothing else, the mid-rpm power increase will be proportional. That would bring your stock TC88 to about 70 horsepower and the Sportster to 64—that’s 8 and 30-plus percent more, respectively. Those numbers may not seem all that impressive, but you’ll get them in a useful rpm range.
Size Plus Efficiency Equals More Power. If you can increase pressure in a combustion chamber at the time of ignition, pressure during combustion will also be higher. Higher pressure pushes the piston harder and more work gets done—your Harley accelerates quicker or climbs the hill more easily. Higher-compression pistons, thinner head gaskets, or anything else that make the combustion chambers smaller can achieve this result.
There are many limitations to both of these approaches. For instance, enlarging the engine quickly becomes expensive if one goes beyond simply boring the cylinders; and too much compression can make an engine overly sensitive to gasoline quality, which can lead to detonation and engine damage.
Heresies: High-performance cams, ported heads, larger carburetors and wide-open exhaust systems add little useful power below 4,500 rpm. They can and often reduce power below 4,000 rpm. This may go against most of what you may have heard about cams, pipes, air cleaners and so on. But what I say has been verified many times on dynamometers and on the street: Displacement and compression are your best friends when it comes to getting more power between 2,000 and 5,000 rpm from your Big Twin Harley engine.
Mild Cams Work Best. Useful gains are possible in mid-rpm power without changing the cams. So let’s look at what sort of cam design you might consider if you do choose to install new ones.
Harley cams are excellent in design and execution, bringing quiet, reliable operation and good performance in normal use. However, their design is otherwise compromised by the need to meet emission standards. These considerations, and more, result in cam designs that produce moderate power.
Only the smallest design changes are needed to meet our goal of increased mid-rpm power. Specifically, the intake valve needs to close sooner to seal the chamber early in the compression stroke and thus raise the pressure in the chamber at the time of ignition. The sooner the intake valve is closed, the lower the rpm at which useful power is produced.
Most stock Harley cams—and there have been many variations over the years—close the intake valves between 35 and 38 degrees after bottom dead center. This means that the engine begins to do its best work near 3,000 rpm (about 60 mph in top gear for most Big Twins). Testing and experience indicate that strong power as low as 2,500 and 50 mph is more desirable.
If the intake cam closes the valve at 30 degrees ABDC, then responsive, useful power can start as low as 2,400 rpm. There are few such cams. John Andrews and I developed one, the EV13, for the Evo Big Twin almost 20 years ago. Andrews Products makes similar cams for the Twin Cam engine. Other suitable cam designs are currently available, including Harley’s own 204 for the Twin Cam (provided it is advanced 4 degrees with an aftermarket cam sprocket).
Most aftermarket cams close the intake valves later, in most cases much later, than stock. They are almost all designed to increase power above 3,500 rpm and their timing reflects that. For instance, a cam that closes the intake valves at, say, 40 degrees will not start doing its best work until 3,500 to 3,600 rpm. No matter what claims their makers may make … well, you just can’t fool Mother Nature. Stay away from any cam that closes the valves later than stock—if you’re after torque, that is.
Other factors include air/fuel mixtures. Stock Harleys run lean at part throttle to meet EPA regulations. This leanness inhibits roll-on performance; however, it is simple to correct. A simple needle change and re-tune of the stock carburetor or slight re-tune of an EFI system are required to correct the emission-related air/fuel mixture problem.
Beware, a number of aftermarket kits are available for the stock carburetor that mostly claim your money. Some of them reduce mileage substantially and get so rich with altitude that the engine may misfire. Some also begin to wear and perform even more poorly within a few thousand miles. You only need to change one part in your stock carburetor to correct the lean mixture problem; use Part No. 27094-88, which is the standard needle for 1988 and ’89 Harley Sportsters.
High-flow air cleaners are not as important as you might think. While the single greatest peak power gain comes from fitting a high-flow air cleaner to an otherwise stock engine, that power gain does not begin until about 4,500 rpm and nets only a couple of horsepower at 5,000. All it will do is make more noise at 3,000 and might drip oil on your leg, too. The stock filter assembly does well in the range we are concerned with here.
As for exhaust systems, in general, the louder they are the less good they do in the midrange. In fact, most of the power you may think you feel you’re getting from aftermarket mufflers comes from the psychological effect of the added noise. The easiest, cheapest and most effective way to truly improve acceleration is to simply downshift. That’s right, downshift. In a fraction of a second you can greatly increase power to the ground. If you downshift twice, you might even double it.
Let’s say you are stuck in traffic and have been putting along in fourth gear at 2,000 rpm (40 mph or so). You finally get an opening and want to get up to 65 quickly, so you can be on your way and dry some of the sweat you’ve been collecting under your leathers. At 2,000 rpm your engine might be able to make, say, 20 horsepower. If you downshift twice and raise its rpm to 4,000, you’ll have at least twice as much power and will get up to 65 much more quickly. That was easy, wasn’t it?
This is what Harley tells us to do. It is what we know works. It is obvious. But few of us, including myself, do this simple thing. Instead we strain our poor engines at a too-low rpm and rail at the Harley gods: We want more torque!
Well, the first step is to correct the lean air/fuel mixture in the 15- to 40-percent throttle range. Enriching the mixture by 10 percent greatly improves throttle response and acceleration, and the simple needle change along with some idle mixture adjusting can accomplish this. In the case of EFI systems, only a slight recalibration is necessary.
When you bore the cylinders for larger displacement, be sure you get more compression with your selection of pistons for the bore job. You want to end up with a cranking pressure between 180 and 190 psi. This is a good range for the Evo Big Twin, TC88/95 and Sportsters. (Stock engines measure only about 160 psi.) Cranking pressure is the number you get when you do a standard compression test. It is related to the compression ratio but is affected by head-gasket thickness and cam selection. In general, the higher this number the better—until detonation becomes a problem, which is likely to occur in contemporary Harley engines from 195 psi and higher.
For example, if you fit 95-inch 9.5:1 pistons with a 30-degree ABDC cam and otherwise stock combustion chambers to your TC88, the cranking pressure will fall close to the recommended numbers. An example of such a combination is Wiseco 9.5:1 pistons, a 0.040-inch-thick head gasket (Cometic or Harley SE) and the Harley 204 cam with a 4-degree-advance cam sprocket.
An engine with the above components can produce 90 pounds-feet of torque at 2,500 rpm. The stock engine delivers about 60 pounds-feet at that rpm—that’s right, 50 percent more power. This engine smokes tires with the throttle only, in second gear, at that same rpm. It is also more responsive and has a wider useful powerband than stock. With correct air/fuel mixtures, it delivers better mileage than stock, due to the greater thermal efficiency of the higher cranking pressure. Hundreds of these engines have been built and continue to plate their owners’ teeth with bugs.
Is there a downside, you ask? Sure, there always is. Because of its higher cranking pressure, this engine is more sensitive to fuel quality and often cannot be given full throttle below 2,000 rpm or so without detonating. However, this is seldom a problem and the cure is to shift down a gear.
There’s nothing like cubic inches and compression for making torque. Harley is certainly aware of this. Forty years ago, the Sportster engine displaced 883cc, and the Big Twin was 1200cc (74 cubic inches). Today most Sportsters are 1200cc, and we can buy 96, 103 or even 110-cubic-inch Big Twins. So bore away, recarburete, remap, up the compression pressure, and bring on the decibels. There’s a mountain out there to climb, and we wanna do it in top gear!