Flat shift rev limiters and shift lights
contributed by Ian Simpson
A small bit about them.
What do they do?
A rev limiter prevents the rpm from going much or any higher than a pre-selected point. After market ones usually allow the user to define the point.
A full throttle or flat shift gear changer holds the rpm at the point that the clutch pedal pressed and the clutch starts to be disengaged. The switching point is usually user definable. The purpose is to allow the throttle to be held fully open during a gear change and the engine's rpm to be prevented from soaring upwards during that moment. It doubles as a useful safety feature if the gear is missed.
How do they do it?
They usually consist of a small sealed electronic device that, when activated, conducts electricity from the ignition coil to chassis, therefore reducing the size of sparks or preventing them. A 'soft cut' is where some sparks are affected, eg, one in four sparks or alternate sparks cut and the engine keeps running but on reduced power and may sound 'rough'. The rpm may still rise but more slowly than normal. A 'hard cut' is more severe and removed power is removed so the rpm cannot rise. There will be a graduation between the two extremes within different makes and designs and across different operational parameters.
Some traction controls work by using the above methods when detectors on the wheels indicate a wheel or wheels is or are turning faster than it or they ought.
Omex or Armtech. Which do I choose?
Omex own Armtech and told me "Neither brand is superior, they are complementary".
The features offered by the different brands vary quite a bit though.
The Omex has a definite cut, and you will not get past it. The Armtech has a gradual cut, which allows you to rev quite a bit higher than when it starts to cut.
The OMEX limiters are set by programming the maximum speed into the unit itself by pressing a button which steps the cut point up by 100rpm increments, where as ARMTECH rev limiters are set by running the engine at half the intended maximum rpm and releasing a button. The former therefore means you can achieve a known rpm for limiting whereas the latter depends on how accurate your tacho is and how carefully you set it.
"Some people think the Omex hard cutting may 'assist' in head gasket poppage!!! (perhaps through a double dose of boost and fuel in the cyclinder....who knows)
the choice is yours. Too much time on that rev limiter ain't going to do any engine much good." Sparkie Turbo Wizard
Per Omex setting instructions; it has an option for soft cut starting 200rpm below the limit.
My Armtech starts to soft cut at maybe 500rpm below where I set it then eventually holds the rpm at about 500 above the chosen limit point. Or something like that. Never really looked closely. But I now set it to 8000 so it's right out of my way. But what good would that be if I miss a gear?
Full throttle shift limiter
The point of this gadget is to allow the throttle to be kept wide open throughout the gear change process without the revs going too high.
Integrated into some of the Armtec rev limiters. And probably available separately.
They employ a switch that closes when the clutch pedal is pressed. The revs are then held at the rpm the engine was at when the pedal was pressed. With the Armtech the it only functions when the rpm is above the setting point for the rev limit, ie, half the limit.
"The clutch switches are crap best to use a different switch"
There doesn't seem to be an alternative switch available. If there is please let me know. A brake light switch won't set the full throttle shift limiter soon enough, if at all as the revs need to be held from as soon as the clutch pedal starts to release the clutch. Then the pedal moves a long way further. So a long travel is needed in the switch after the point of switching. Brake light switches do not allow this. If you close a switch to hold the revs at the bottom of the pedal travel then they will have shot up before the limiting starts.
Personally I find it useful for road use as I don't change at max speed every time but I do flat shift quite often. And the limiter does come into effect, even if for only 0.5 seconds. It helps, especially in lower gears.
For the strip or when making a point on the road then the flat shift limiter does not come into play as the shift is so quick anyway and the rpm doesn't rise much in maybe 0.1 seconds or probably less.
Without one and with a slightly slower change then there can be a slight advantage when the engines momentum is dumped into the wheels and a brief acceleration surge takes place, but this put a severe load through the transmission and can cause brake of traction.
Shift lights
These vary from one to four or more. Their purpose is to warn the driver that the rev limit has been or is about to be reached. The multiple light arrays act as a secondary and narrow band tacho and can tell the driver with plenty or warning that a gear change is about to be needed. This may sound pointless but in the heat of a race, or overtaking in a tight space, this can be useful.
Some rev limiters or tachos come with a light built in or attached. But the arrays are usually separate. The most reasonable I've recently seen is
http://www.autospeed.com/cms/A_0798/article.html
http://www.autospeed.com/cms/A_0097/article.html
http://www.demon-tweeks.co.uk/catalo...ODE=CDSSFL-401
Omex also do a 4 LED shift light. http://www.omextechnology.co.uk/Revl...nloads/SLS.PDF
Armtech shift light, check out the sequential steering wheel mounted one. £125 from Demon Tweeks or, for comparison, about £85 from a US on-line vendor I just found through Google search. https://secure.vbcomm.net/store/bresults.asp?plg=1
SPA also do some sequential shift lights.
Fitting tips
The clutch switch. There is often a hole already present in the flat peice of metal just above and in front of the clutch pedal. A hole will need to be drilled there otherwise. When the pedal is pressed it allows the spring on the switch to move the rod out so the other end touches and conducts.
To set the Armtech you press the button on the unit until the LED flashes then rev to half the desired limit, hold the revs and release the button.
http://www.omextechnology.co.uk/page4.html
http://www.armtech.co.uk/product/rl.htm
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