Author: Rob USM
Moving Torque, part 1.
When people think of Corvettes they naturally think of performance. Generally, this means engine performance, or acceleration. For most folks, engine performance is directly associated with horsepower. So, if they decide they want to increase the performance of the engine, it is horsepower they focus on. This frequently leads to disappointment. Why? They overlooked a vital aspect of engine performance…….torque. To undestand the role of torque, lets examine different engine performance scenarios.
Generally, I find that customers fall into one of three types of engine modifiers. The first are usually daily drivers who are just looking for a bit more off the line acceleration without reducing reliability or economy. The second are performance enthusiasts who are interested in getting considerably more out of the current engine configuration yet still want streetability, highway fuel economy and reliability. The third are focused on all-out performance, usually drag racing, and have little concern for street or highway manners.
Often times it is this first group of customers that come to my shop the first time complaining about engine work that was supposed to make the car quicker, but did the opposite. After a little investigating I usually find that the customer was talked into “MORE HORSEPOWER!!”. The problem is that HP is based on torque and RPM. (I am trying to keep this as simple as possible.) Most daily drivers (which are usually C4 & newer Vettes), especially Vette drivers with A/T, almost never turn more than 3000rpm during their daily commute. Therefore, HP is not the factor, it is torque.
So, what happened with the upgrade? Most of the time customers are talked into too large an exhaust system. I have had L98 cars (L98 engines were designed specifically for low to midrange torque, and with stock intakes, useless over 4500rpm.) come into my shop that were intended daily drivers, with no other modifications except for full length 1 ¾” primary headers with a 3” exhaust. Three important factors were lost sight of: The intended operating range of the vehicle, the engine characteristics in that operating range, and the effects of the planned modifications on those engine characteristics. The intended operating range is rather easy. As a daily commuter, this engine will be spending 99% of its time between idle & 3000rpm. What are the engine characteristics at that range? For the L98, peak torque (340) is reached at 3200rpm. Peak HP (250) at 4000rpm. Therefore, we are operating the engine before the peak torque value. The proper modification would have emphasized more peak torque (Gen I and Gen II engines), not more HP.
When in the pursuit of more torque, and not trying to move the peak torque rpm point, you are actually trying to increase an engine system’s operating efficiency. Generally, this means maintaining intake/exhaust velocities and keeping both sides (intake and exhaust) balanced. So, when we say the L98 torque peaks at 3200rpm, what we are also saying is that this is the rpm at which the engine system is at peak efficiency, the engine achieves maximum cylinder pressure(can fill the cylinder the most completely), and the engine system is balanced. In a very simplistic way, we can say that before the torque peak the engine system (this includes the intake manifold, cylinder heads, camshaft profile, engine displacement, and exhaust) is too big, or the intake/exhaust gas velocities are too low, and the engine can not completely fill the cylinder. Above the torque peak and the system is too small. The engine system can not flow enough gases (volume) in the time it has to fill the combustion chamber (remember that as rpms go up, the time an engine has to fill and expel goes down). In the sense of balance, the engine has to work the same amount filling the cylinder (intake side) as emptying it (exhaust side).
So, why did the 3” exhaust hurt the performance? Loss of backpressure. Proper backpressure in the exhaust helps maintain exhaust gas velocity. Since we are focusing on efficiency (system velocity) and not power (system volume), a level of backpressure is necessary to keep the system in balance. The engine would have to operate at a higher rpm range (or have a larger displacement) to develop the velocity needed. Problem is, this rpm is outside our intended rpm range, and the intake side is now outside it’s peak efficiency range (too small, or too restrictive).
This issue can also occur on the intake side, though this is less frequently a bolt-on issue with daily drivers because it usually involves a cam that is too big or cylinder heads that are too big, which means engine work, and most daily drivers are not interested in this level of modification.
For an L98 daily driver interested in better bottom end and mid-range torque, offline acceleration:
Improve the ignition system. I prefer the MSD 6AL, with a set of good wires, coil and plugs.
Improve the exhaust. Do not go bigger than 1 5/8” primaries. If you keep the Y-pipe system go to a 2 ½” pipe, low restriction catalytic converter and a decent set of mufflers that flow better but do maintain some backpressure. If you prefer a dual exhaust, stay at 2 ¼” pipe. Either way, with the L98, I wouldn’t use shorty headers. Long tube headers are better for torque.
Change the T-stat to 180*. DO NOT GO TO 160*.
Improve the intake system. Low restriction air filter. Remove the REAR screen from the MAF, if you have one. Do NOT remove the front screen as this laminates the air flow and helps the MAF provide an accurate reading. For a daily driver I DO NOT recommend relocating the Intake Air temp sensor. Throttle body coolant bypass. Unless you live in Minnesota, considering bypassing the T.B. to help keep intake air charge cooler.
If you have a 6 spd., consider upgrading to a 4.11 rear. Seems high, right? With the O/D of the 6 spd. your overall ratio (.5 x 4.11) is still only 2.055. This is still lower than the 84-87 4+3 ratio of 2.08 (.68 x 3.07). True, you will be shifting sooner, but with the L98 torque and the rear ratio multiplication, & normal driving, you can skip 2nd & 4th.
Remember, these are some ways to increase torque in the operating range mentioned earlier without getting into the engine. |
