Rev Limiters & 2-Step; Fuel cut, Spark cut, or soft limit, pros and cons?

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ELS
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Rev Limiters & 2-Step; Fuel cut, Spark cut, or soft limit, pros and cons?

Post by ELS »

If we put aside the fact that a spark cut, or a soft limiter will make a lot of noise.
And also the fact that those limiters would destroy a cat or a weak exhaust quite quickly. what are the other pros and cons of each limiter?

The ones I can think of:
Fuel cut - reduced chance of oil dilution; no wasted fuel.
Spark cut - the fuel will cool the engine slightly when it evaporates; higher chance of oil dilution; higher chance of fouling the spark plug.
Soft limit - increased chance of burning exhaust valves, seats; ability to spin a turbo - anti-lag.

What do you think?
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Re: Rev Limiters & 2-Step; Fuel cut, Spark cut, or soft limit, pros and cons?

Post by BLSTIC »

A fuel cut limiter will result in at least one fireable cycle running at some lean mixture, especially if you're at full power and not on sequential injection. It's also relatively violent in terms of how far the engine accelerates and decelerates between on and off.

An ignition cut is instant, does not result in partially lean mixtures, and can result in near constant rpm.

I tried it once in a manual trans car with EFI, set the limiter to 3000 on a supercharged windsor pure street car. Fuel cut you could hardly focus on the road even with the hysteresis set low, ignition cut in comparison could put a full teacup on the dashboard, and it blew pretty fire out the exhaust. I'm sure my cats wouldn't have liked the pretty fire for very long though.

It was a pretty early ecu though, no option of a soft limit.

A soft limit has racing uses too, it's an audible gear change alert where you can get "just a bit more" if you know you'll need to change back down in a second anyway, and even if you don't use it like that it's going to upset the car a ton less than going from full power to negative power in the middle of a high speed corner.
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Re: Rev Limiters & 2-Step; Fuel cut, Spark cut, or soft limit, pros and cons?

Post by ELS »

BLSTIC wrote: Sun May 28, 2023 3:16 pm A fuel cut limiter will result in at least one fireable cycle running at some lean mixture, especially if you're at full power and not on sequential injection. It's also relatively violent in terms of how far the engine accelerates and decelerates between on and off.

An ignition cut is instant, does not result in partially lean mixtures, and can result in near constant rpm.

I tried it once in a manual trans car with EFI, set the limiter to 3000 on a supercharged windsor pure street car. Fuel cut you could hardly focus on the road even with the hysteresis set low, ignition cut in comparison could put a full teacup on the dashboard, and it blew pretty fire out the exhaust. I'm sure my cats wouldn't have liked the pretty fire for very long though.

It was a pretty early ecu though, no option of a soft limit.

A soft limit has racing uses too, it's an audible gear change alert where you can get "just a bit more" if you know you'll need to change back down in a second anyway, and even if you don't use it like that it's going to upset the car a ton less than going from full power to negative power in the middle of a high speed corner.
I'm pretty sure most factory limiters have a progressive fuel cut, so it isn't very abrupt.

I've read that old honda ecus had an ignition cut at the rpm limit, but also a hard fuel cut 200rpm above that limit, maybe in case of severe overheating when the fuel mixture could ignite from hot spots? it's not a bad idea to add a hard fuel cut slightly above the normal rpm as a last measure to slow the engine down.

"
A soft limit has racing uses too, it's an audible gear change alert where you can get "just a bit more" if you know you'll need to change back down in a second anyway, and even if you don't use it like that it's going to upset the car a ton less than going from full power to negative power in the middle of a high speed corner.
"
That explains why all the GP car racing videos I've seen, had soft limiters.
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Re: Rev Limiters & 2-Step; Fuel cut, Spark cut, or soft limit, pros and cons?

Post by Alaskaracer »

Any rev limiter is hard on an engine no matter the type, so pick your poison. Most common aftermarket is ignition, and usually sequential, which is the smoothest......If you're going to use one, best to keep the engine off of it as much as possible......
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Re: Rev Limiters & 2-Step; Fuel cut, Spark cut, or soft limit, pros and cons?

Post by digger »

Ignition cut is harder on the valve train than fuel cut
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