Rev Limiter, Fuel or Spark?

[Mr Nick]

I have the FAST EZ-EFI and MSD 6530 ignition box, both of which have rev limiters. What would be the better choice to use for peak RPM rev limiter?

I'm leaning toward using the MSD box, for the following reason(s): Cutting fuel at high RPM may cause a lean condition and damage the motor. If I was running port injection, there would be NO fuel going into the cylinder in that particular revolution so there wouldn't be any risk of lean detonation because there was no fuel. However, being that the FAST system is throttle body injection, when it cuts fuel for rev limiter, it could cause a lean mixture into one or more cylinders. I don't know if the FAST ECU cuts fuel to all 4 injectors at once, or alternates two of them, or many of the other options. Add in dual vs single plane intakes, and well you get the picture.

Alternatively, using the MSD box as rev limiter would cause NONE of the fuel to burn in the cylinder at that time, and the O2 sensor would see a very rich mixture, and try to lean it out? Possibly having the same affect as above?

Or am I way over thinking this, especially since I don't plan on running on the rev limiter for extended periods of time!

This could be an interesting subject to discuss, actually. Any thoughts are welcome and appreciated!

[cornfedbill]

Cut fuel. Check with the manufacturer to make sure it cuts all 4 injectors. I believe you will find it does.

Actually the O2 sensor cannot read fuel, only oxygen. The system will actually think you are running lean and try to add fuel.

Cutting spark can fill your exhaust will a nice mixture of air and fuel which can ignite as soon as combustion starts. It can create a nice backfire. It will also send unburned gas out the tailpipes.

[Mr Nick]

Actually the O2 sensor cannot read fuel, only oxygen. The system will actually think you are running lean and try to add fuel.

Cutting spark can fill your exhaust will a nice mixture of air and fuel which can ignite as soon as combustion starts. It can create a nice backfire. It will also send unburned gas out the tailpipes.

Thanks Bill. After doing some more research it seems most OEM's use fuel cut for their rev limiters. And thanks for the reminder that the O2 sensor won't read the raw fuel, I knew that just but it didn't click while brainstorming this.

Looks like I'll use fuel cut for max RPM rev limiter, and spark cut for burnout and launch RPM limiter. (no choice there)

[cornfedbill]

Glad to help. Good luck.

[MonzaRacer]

Best way for rev limit on fuel is on port systems in firing order of 18436572 it would go this way
-8436572
1-436572
18-36572
184-6572
1843-572
and so on so you never actually run OUT of fuel on a cylinder it simply cuts one cylinder then lets it catch up, ignition rev limiters ar similar so as to not was down cylinders.
Several different ways really just depends, N/A can use either, NOS/Boosted cars need another.