thoughts on rpm and speed limiters using fuel disable

[greggbruceauto]

I just found this thread and have wondered about this several times. In the 7427 PCM ($0D), I have my MPH to enter fuel cutoff set @ 255 MPH, and MPH to resume fueling set at 254 MPH. Basically I thought I would eliminate the MPH limiter and instead only go for a rev limiter.. I just noticed that my RPM to enter fuel cut off is 6375 RPM and RPM to resume fueling is also set at 6375 RPM. It sounds like in order to properly set this rev limiter the Resume fueling RPM should be set a little lower than the RPM to enable fuel cut off, is this correct????? Playing around with this one in TP, it looks like the next lowest setting would be 6350 RPM. So I'm thinking it should actually be set to 6375 RPM and 6350 RPM to enable fuel cutoff and resume fueling, respectively. Does this sound right to you guys?

if I remember correctly, this is close to what my stock bin was set, just a few rpm below the cut-off, but (LOL) it was at something like 10,500....
on the most recent bin I programmed, I set it to 750 rpm difference, haven't yet tried it though.

[RobertISaar]

there does need to be a bit of a hysteresis for on/off, otherwise you'll run into a "floating" condition, doesn't quite feel like a rev limiter.

[CDeeZ]

Hysteresis, there's that word again! So basically the parameters need a little space in between.... Just like when I was tinkering with my Efan ON/OFF settings, I remember I originally had set the parameters too tight, or with not enough 'hysteresis' for the fan to cycle on and off properly..

Bada bing!

750 RPM difference? Do you think that is a little much?

[Six_Shooter]

I have my rev limiter (no speed limiter in my code) set with a hysteresis of around 150 RPM, IIRC off at 6350 and on at 6200.

[RobertISaar]

i like the ~100 range, but i've rarely run into the limiter after modifying factory calibrations.

[1project2many]

One consideration with TBI that PFI folks likely won't know about: If you spend any significant amount of time against the limiter with fuel disabled may actually cause a stall. Bouncing between a two or three hundred rpm limit is not an issue but coasting down a hill at 75 with the speed limit set at 75 and fuel disabled for 6-8 seconds is likely to be an issue. BTDT

HTH

[CDeeZ]

I think a rev limiter is a good idea. Before I had a tach, I wound it up so tight in 1st one time that I blew up the oil pressure sending unit at the back of the block. (High volume pump)