Why do rev limiters "bounce"?
03-05-2011, 10:10 AM
#21
It sounds like a lot of people don't understand the concept of the rev limiter built into the stock PCM. f bodies have only a one rpm difference between cutoff and resume. By doing it this way you have max power up to the cutoff, and max power at the resume.
Not all vehicles have the cutoff and resume so close, try driving a ford van at wot. It cuts out at 96 mph and cuts back in at maybe 90. It feels like the gas tank is almost empty. And when you rev it it cuts on and off with such a wide gap it feels like the key is being turned off and on.
In theory, and if your rev limiter is set properly, you can bounce off it all day long. What people get confused about is valve float and rev limiter. If your rev limiter is set too high and you float the valves you're going to have engine damage. And when you downshift too far with a manual (or even an automatic) and the engine revs real high the rev limiter isn't going to save your engine, the valves are going to float.
Not all vehicles have the cutoff and resume so close, try driving a ford van at wot. It cuts out at 96 mph and cuts back in at maybe 90. It feels like the gas tank is almost empty. And when you rev it it cuts on and off with such a wide gap it feels like the key is being turned off and on.
In theory, and if your rev limiter is set properly, you can bounce off it all day long. What people get confused about is valve float and rev limiter. If your rev limiter is set too high and you float the valves you're going to have engine damage. And when you downshift too far with a manual (or even an automatic) and the engine revs real high the rev limiter isn't going to save your engine, the valves are going to float.
03-06-2011, 08:24 AM
#24
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It sounds like a lot of people don't understand the concept of the rev limiter built into the stock PCM. f bodies have only a one rpm difference between cutoff and resume. By doing it this way you have max power up to the cutoff, and max power at the resume.
In theory, and if your rev limiter is set properly, you can bounce off it all day long. What people get confused about is valve float and rev limiter. If your rev limiter is set too high and you float the valves you're going to have engine damage. And when you downshift too far with a manual (or even an automatic) and the engine revs real high the rev limiter isn't going to save your engine, the valves are going to float.
In theory, and if your rev limiter is set properly, you can bounce off it all day long. What people get confused about is valve float and rev limiter. If your rev limiter is set too high and you float the valves you're going to have engine damage. And when you downshift too far with a manual (or even an automatic) and the engine revs real high the rev limiter isn't going to save your engine, the valves are going to float.
03-06-2011, 10:04 AM
#25
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The normal setup has some deadband in it. That's why it
bounces.
As to why the deadband, I think it may have to do with
using fuel to cut power. All or nothing is pretty OK as
far as the motor goes. If there's no fuel then you won't
fire. Full fuel, that's normal. Try to starve "just enough",
you'll be WOT and lean, still firing, pop goes the piston.
bounces.
As to why the deadband, I think it may have to do with
using fuel to cut power. All or nothing is pretty OK as
far as the motor goes. If there's no fuel then you won't
fire. Full fuel, that's normal. Try to starve "just enough",
you'll be WOT and lean, still firing, pop goes the piston.
05-08-2016, 07:30 PM
#27
With my megasquirt the engine would go into a cool stutter at the limiter, and truely hold a set rpm within 100, but you have to understand what is going on. The Megasquirt falls into a timing retard and alternate firing sequence to work it's deal.
The stock limiter can't modulate the power, it can only turn it on and off. So you have a hysteresis built in of a few hundred rpm, hence the bounce. ON off ON off ON off.
The stock limiter can't modulate the power, it can only turn it on and off. So you have a hysteresis built in of a few hundred rpm, hence the bounce. ON off ON off ON off.
05-13-2016, 01:32 PM
#28
Wow a 5 year revival.... Might as well chime in.
By now you guys should know that rev limiters have changed with DBW setups. Most of the newer cars have a "soft" rev limiter. That meaning, the throttle blade closes and keeps the RPMs at the limit with no bouncing. If you hit the limiter hard enough, it will cut fuel but most of the time, the throttle blade will close before it happens (excludes manual over revs). Cable throttle body cars, sorry but you are still stuck with the normal limiter.
By now you guys should know that rev limiters have changed with DBW setups. Most of the newer cars have a "soft" rev limiter. That meaning, the throttle blade closes and keeps the RPMs at the limit with no bouncing. If you hit the limiter hard enough, it will cut fuel but most of the time, the throttle blade will close before it happens (excludes manual over revs). Cable throttle body cars, sorry but you are still stuck with the normal limiter.
05-14-2016, 08:33 PM
#29
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You don't have adequate mechanical knowledge to understand this. And as to WHY, well that's already been explained on this very page, and you didn't seem to catch it then did ya?
