As the term suggests, two-step rev limiters are just that; rev limiters that can limit engine speed in two steps, or more precisely, at two levels independently of each other. The practical application of this technology first appeared in a handful of exotic European supercars, in which the two-step rev limiter was a feature of the Launch Control system, which in its turn, was a feature of the Traction Control system. This article will briefly discuss the nature of two-step rev limiters, how they work, and what to look out for when you are asked to install a two-step rev limiting system on a modified street racer, or other vehicle intended for competition in motor sports, starting with this question -
On an exotic supercar, or for the purposes of this article, a modified vehicle, the launch control system uses one setting of the rev limiter to keep the engine speed at a preset limit during the prelaunch stage. The advantage of this is that while the first stage is active on a forced induction engine, the turbo charger is allowed to spool up to a preset boost pressure before the actual launch to eliminate turbo lag during the initial acceleration.
The second stage of the rev limiter comes into operation at the maximum allowable engine speed for that application. This is more commonly known as the “Red Line”, which can also be set by the user, or programmed into the engine management system to prevent damage to the engine.
On most of the vehicles the average Australian technician will see, there is only one rev limiter- the one that prevents the engine from revving beyond the maximum allowable engine speed. These can come in one of two “flavours”, a “soft cut” limiter, or a “hard cut” limiter. With soft cut limiters, the ECU retards the ignition timing progressively up to a point where the engine cannot rev any higher, which results in the engine losing power gradually.
With hard cut rev limiters, the ECU not only retards the ignition timing, but also abruptly cuts the fuel supply at the maximum allowable engine speed, which results in a sudden loss of power hence the term, “hard cut”.
While exotic supercars come with two-step rev limiters already pre-installed, it is possible to install the technology on an aftermarket basis on standard OEM applications, but it is more common to see two-step rev limiters on modified vehicles that are used for anything from rallying, to serious track racing, to illegal street racing.
For the purposes of this article, we will ignore two-step rev limiters as they are found on super cars, since the average Australian technician is unlikely to encounter supercars and the issues that come with their traction control systems on a regular basis. Instead, we will focus on two-step rev limiters as the technology applies to aftermarket installations, where the user is able to define the system’s operating parameters.
It must be noted though that operating parameters depend on both the application, and whether or not changes or modifications can be made to the ECU’s basic programming. However, most ECU’s allow aftermarket two-step rev limiting interfaces to modify their basic programming to alter the following settings for the first stage-
Assuming that the ECU allows all the required programming changes, the above settings will allow the engine to rev up to the speed set by the user when the system is activated, usually through a clutch pedal mounted switch. In practice, this means that the engine will rev up to, and maintain its preset, pre-launch speed until the preset boost pressure is achieved, which ensures consistently reliable launches. Once the vehicle is launched, the ECU will revert to standard programming with regard to ignition timing and fuel delivery strategies until it reaches the preset, second-stage rev limit.
NOTE: It should be noted that two-step rev limiters cannot be installed on vehicles with automatic transmissions, since the engine needs to be decoupled from the drive train to be able to rev freely during the pre-launch phase.
As a practical matter, most aftermarket two-step rev limiting systems maintain the preset, first stage engine speed by intercepting, and modifying or interrupting the primary ignition signal between the ECU and the ignition coils. This creates random, artificial misfire events that are designed to keep the engine at the selected speed, and while this is an efficient system, there can be serious consequences if the vehicle is fitted with catalytic converters.
NOTE: Most two-step rev limiting systems cause fuel to be combusted in the exhaust manifold to build drive pressure for the turbo charger(s), but keep in mind that this combustion process is relatively inefficient. Therefore, excessive amounts of uncombusted hydrocarbons can enter the catalytic converter and if the uncombusted fuel ignites in the catalytic converter, the converter’s core can melt in a matter of seconds. Also, keep in mind that removing or tampering with catalytic converters on vehicles intended for use on public roads is illegal in many jurisdictions, so keep this in mind when setting up a two-step rev limiting system on a customer’s vehicle.
On forced induction engines, the random misfires have the effect of increasing both the density and velocity of the exhaust gas in the exhaust manifold while the throttle is open during the prelaunch period. The increased density and speed of the exhaust gas is the mechanism that provides the additional drive pressure that allows the turbo charger to increase boost pressure during the prelaunch stage while the engine is not under load.
In addition to the above, some aftermarket two-step rev limiting systems include a controller to prevent over revving during manual gear changes. These systems use a clutch activated switch to interrupt the ignition and/or retard the ignition timing when the driver keeps the throttle open while depressing the clutch pedal momentarily to activate the second-stage rev limiter. Among engine turners, this feature is known as “no-lift-to-shift”, meaning that the engine will not over rev when the clutch is disengaged to perform gearshifts, even if the throttle is kept open during the clutch disengagement.
NOTE: The above should NOT be confused with Anti-lag systems, which is an entirely different method of limiting engine speeds. An anti-lag system creates additional drive pressure within the turbo charger itself by means of combusting fuel inside the turbo charger, even when the throttle is closed, or the vehicle is stationary. In practice, this eliminates turbo lag, hence the term “anti-lag” but this method places extreme demands on the turbo-charger, and premature, catastrophic turbo charger failure is a common feature of anti-lag systems.
While it would be fair to say that the average, suburban runabout would never need a two-step rev limiting system, there are many people who derive a certain amount of pleasure and satisfaction from making modifications to standard vehicles, albeit for reasons that are not always clear to many of us. We need not delve into what might possibly motivate people to do this, but there is always a chance that you might be called upon to install a two-step rev limiting system on a modified vehicle, or perhaps to diagnose (divine?) the reasons why an existing installation does not work as expected.
Below is a quick rundown of the problems you are most likely to encounter with two-step rev limiting systems that were not installed by a professional technician-
Compatibility issues
Not all two-step rev limiting systems are compatible with all ECU’s and the single most common incompatibly issue involves the ignition system on some vehicles. On some vehicles, an “intelligent” igniter communicates directly with the ECU, as opposed to just generating a signal, and since the rev limiting system’s interface intercepts this signal, the rev limiting system may not work as expected, or in some cases, may not work at all.
In addition, since two-step rev limiting systems create random misfires, some misfire detection systems may set the misfires as hard faults, which could, and often does, cause the ECU to initiate a limp mode. Before you install a twp-step rev limiting system, do some research into whether or not the system is fully compatible with the vehicle; it could save you many hours of trouble later on.
Excessive wheel spin when the first stage deactivates
A common complaint about aftermarket two-step rev limiting systems is that excessive wheel spin occurs when the first stage rev limiter deactivates. The remedy is relatively simple; since the VSS (Vehicle Speed Sensor) is recalibrated during the initial setup, the initial setting may be too low. Increasing this setting will allow the vehicle to gain more momentum while keeping the engine speed, and hence, power, restricted to the maximum first stage limit until the vehicle’s speed matches the speed dictated by the recalibrated VSS. However, the trick in getting this setting right involves never setting the VSS’s lower limit higher than the speed value the vehicle would normally have made in first gear; exceeding this value guarantees that the first stage rev limiter will never deactivate.
Excessive wheel spin at launch
On forced induction engines, the most common reason for this is excessive boost pressure. Remedy this by reducing the maximum first stage engine speed to reduce boost pressure. On naturally aspirated engines increase the first stage engine speed at which ignition timing retardation kicks in to a value that is closer to the maximum first stage engine speed, to reduce engine power at launch. While this might appear to be counter-intuitive, the practical effect of increasing this setting is that the ignition timing will be retarded just before the engine develops its maximum power, which should (in theory), reduce, or eliminate wheel spin.
Engine shuts off unexpectedly
This is most commonly caused by an incorrect fuel cut-off setting on systems that cut the fuel supply to limit engine speed. If needs be, reset the first stage rev limit to where the engine develops peak torque, and increase the fuel cut-off RPM to a value that is higher than the engine can possibly rev (10 000 RPM or so would be good) to ensure that the fuel cut-off function does not engage.
Repeated exhaust manifold/turbo charger failure
Since two-step rev limiting systems retard the ignition timing at a set engine speed, combustion occurs later in the cycle, with most of the combustion at this point occurring in the exhaust manifold. As might be expected, this can raise the exhaust temperature to unacceptably high levels: in fact, cracked, warped, and even partially melted exhaust manifolds is a common feature of incorrectly set up rev limiting systems.
Moreover, turbo chargers were never intended or designed to cope with combustion that happens in the exhaust manifold, and melted turbine wheels, damaged waste gates, and cracked turbine casings are therefore a common feature of some two-step rev limiting systems. In addition, melted, warped, and dropped engine valves are sometimes a natural consequence of setting up a two-step rev limiting incorrectly.
There is no single solution that will cure all cases of premature failures of exhaust manifold and/or turbo chargers in all cases on all applications. While reducing ignition timing retardation might solve the issue in some cases, the best course of action in these cases is to review all the parameters of the rev limiting system, and to reset all parameters based on the engine manufacturers’ recommend settings. For example, when programming new parameters, start off by not exceeding the maximum allowable ignition retardation by more than few degrees, not exceeding recommended maximum boost levels by excessive amounts, and perhaps most importantly, not exceeding the maximum allowable fuel pressure at the injectors by more than a few percent.
There is no magic formula to follow to ensure that any given two-step rev limiting system will work as expected on any given application. Much of how well (or otherwise) the system works depends on to what degree the ECU can be reprogrammed, and how well (or otherwise) individual engine and fuel management sub-systems remain integrated after the reprogramming.
Bear in mind that while installing and fine-tuning a two-step rev limiting system on a vehicle that was not designed to cope with the increased loads, stresses, and strains might represent an interesting exercise in automotive programming, getting it wrong can result in serious and sometimes fatal damage to the drive train. Thus, apart from anything else, always consider the type of clutch fitted to the vehicle, and pay careful attention to the gear ratios of both the transmission and the final drive to obtain the best results.
