What's the boost cut on MP stage 2 LM?

[Jackrabbit]

Anyone know off hand? Looking to raise the boost a little on my '87 CSX. Currently running a Mopar Stage 2 logic module.

[GLHSKEN]

14.7 psi

[Jackrabbit]

Sorry, I meant at what point does it go into overboost shutdown. :o

[BadAssPerformance]

'87 Stage II you can run just under 18psi.. all day long :thumb:

If you dont want to blow it up, use 42pph injectors to do that tho ;)

[Jackrabbit]

Roger that.
Thanks!

[TurboGLH]

'87 Stage II you can run just under 18psi.. all day long :thumb:

If you dont want to blow it up, use 42pph injectors to do that tho ;)

My understanding (both what I was told, and what happened to me on the dyno) is that it's time based. Once the map is maxed out, it has a 10sec timer until overboost. I heard of it, but never hit it until I was on the dyno at carlisle, after 10sec at 17psi it went into overboost.

[Clay]

Sorry, I meant at what point does it go into overboost shutdown. :o

14.7 psi.

The map sensor cant read any higher than that. My GLHS when into overboost shutdown at 14.7 psi.

[TurboGLH]

14.7 psi.

The map sensor cant read any higher than that. My GLHS when into overboost shutdown at 14.7 psi.

Stock computer? He's asking about Stage II that you could order from mopar.

[Clay]

I was pretty sure it was the Stage II LM. I had it in the GLHT the last time I ran it at the track in Cinci, and I was hitting overboost cutout like crazy because I adjusted the grainger to much between passes. I guess I could have REALLY gone over 14.7, but I was going up a little between each pass.

[TurboGLH]

I was pretty sure it was the Stage II LM. I had it in the GLHT the last time I ran it at the track in Cinci, and I was hitting overboost cutout like crazy because I adjusted the grainger to much between passes. I guess I could have REALLY gone over 14.7, but I was going up a little between each pass.

I'd be surprised if it was StgII. I ran one in my lancer and my GLH and they both had the same cutout, 14.7+ for 10sec (or so) and then cutout.

[Pat]

I'd be surprised if it was StgII. I ran one in my lancer and my GLH and they both had the same cutout, 14.7+ for 10sec (or so) and then cutout.


Same with two Stage II computers I've run.

[TurboJerry]

The boost cut is set to 13.8 psi. And it does take 10 seconds to get it to happen.

[Clay]

I'd be surprised if it was StgII. I ran one in my lancer and my GLH and they both had the same cutout, 14.7+ for 10sec (or so) and then cutout.

Ok, so your saying the cutout for a stage II is 14.7 psi, right?

thats exactly what Im saying.

[ShelbyZD]

The Stage 2 GLHS ecu my bro got from Chrysler has no apparent cutout. It's been up to about 18psi thus far.

[Jackrabbit]

Now I'm seriously confused.

It can't be 14.7 for 10 seconds, because it pegs 15ish (14.7 I'd guess?) all the time with the stage 2 MP LM. There's got to be a higher cutout I'd think???

[BadAssPerformance]

A 2-bar MAP sensor can only read to 14.7psi. If it is above range for more than 10 secons it will hit boost cutout.

I do not think that this typically occurs in a 1/4 mile run, because you typically drop below 14.7 reseting the timer during shifts.

There is also something about higher than 18psi. Although the MAP does not know what anything is past 14.7, I have run 18 psi for up to the 10 second limit, but can only run 20psi for 2 seconds before it cuts out. Not sure why tho.

[TurboGLH]

Ok, so your saying the cutout for a stage II is 14.7 psi, right?

thats exactly what Im saying.

Kind of. It's 14.7, but it doesn't hit right away. Instead, when you max the map sensor out the computer starts a 10 sec countdown, if you remain above 14.7 for 10sec then it cuts out.

[TurboJerry]

Kind of. It's 14.7, but it doesn't hit right away. Instead, when you max the map sensor out the computer starts a 10 sec countdown, if you remain above 14.7 for 10sec then it cuts out.

boost cut is not 14.7 psi. The map sensor is maxed out at 14.7 psi, Which is 5 volts. The timer is what seems to throw people off here. If you run 14.0 psi boost, it *will* hit boost cut in 10 seconds. The reason it happens in 2 seconds at a higher boost level is knock. The timer is bypassed if there is constant light knock, or heavy knock. The sensor is stuck at 4.9 to 5 volts at 14.7 psi no matter what happens boost wise. (18 or 20 psi, the sensor doesn't care or know) And also the timer is slowly incremented back up to 10 seconds. So the shifts will only get you back 1 or 2 seconds, which may be enough to get a full 1/4 mile pass in.

[GLHSKEN]

Thanks Jerry... Great explanations....

[TurboJerry]

Thanks Jerry... Great explanations....

Sorry, I tend to get a little too detailed in trying to figure out the hows, and whys of everything. I figure this may help people with a weird boost cut problem realize that it's actually normal. The funny part is the fuel is scaled all the way to 14.7 psi on everything stock even though the boost cut happens at 13.8 psi or less.

[BadAssPerformance]

+1 Good explanation :thumb:

Anything else it could be at 20psi other than knock? I know for sure that there was enough fuel.

[TurboJerry]

Put a scanner on it, you may be surprised! It takes 1 deg of retard to make it so.....

[amcpacer]

That is odd. I used to run a GLHS Stage II computer in my Turbo-Z and it never hit any kind of cutout. I ran as high as 18psi and had many long runs up steep hills in the mountains that lasted much longer than 10 seconds. Maybe I throttled down momentarily or something that started the timer over. Interesting.

[BadAssPerformance]

Put a scanner on it, you may be surprised! It takes 1 deg of retard to make it so.....

The same fuel/timing was good to 27psi with a MAP bleed and then with a Stage 2 cal w/o boost limit...

[johnl]

:clap2:
boost cut is not 14.7 psi. The map sensor is maxed out at 14.7 psi, Which is 5 volts. The timer is what seems to throw people off here. If you run 14.0 psi boost, it *will* hit boost cut in 10 seconds. The reason it happens in 2 seconds at a higher boost level is knock. The timer is bypassed if there is constant light knock, or heavy knock. The sensor is stuck at 4.9 to 5 volts at 14.7 psi no matter what happens boost wise. (18 or 20 psi, the sensor doesn't care or know) And also the timer is slowly incremented back up to 10 seconds. So the shifts will only get you back 1 or 2 seconds, which may be enough to get a full 1/4 mile pass in.:clap2: :rockon: :rockon: :wow1: :wow1: :thumb:

[TurboJerry]

I've found that the map sensor could get "tired" and only get 4.4 to 4.5 volts even though you may have 30 psi going to it. That's why they set the boost cut to a lower psi. This way if the sensor can't reach 5 volts, you'll still get cut out. But if it can't reach 4.54 volts, then you won't get cutout anyway. BTW the timer only increments when the map drops below the cutout voltage, so throttle has nothing to do with it, except when the boost actually drops.

[BadAssPerformance]

I was refering to no knock at 27psi now but I had some cutout at 20psi before... same '87 LM S2 2-bar cal just w/o boost cutout.

[TurboJerry]

The same fuel/timing was good to 27psi with a MAP bleed and then with a Stage 2 cal w/o boost limit...

Huh? You may have an ignition problem. I had this a while ago and had to run a .022" spark plug gap to get it to stop that. If the boost cut was eliminated, then the cal can't be causing the problem........ Maybe too rich? I do find the cal has too much timing for alot of boost, unless you have 116 in the tank, or other alternate fuels going in there too.....

[TurboJerry]

:clap2: :clap2: :rockon: :rockon: :wow1: :wow1: :thumb:

I just try to figure out what happens.:cool:

[BadAssPerformance]

OK, my previous posts are not written too well, I'll try again here:

Old Cal: Stock T2 S2 LM would run 18psi all day long (as long as that day was within the 10 seconds) but if it hit 19 or 20 in much less than 10 seconds it would cutout. With a MAP bleed I could get it to 27psi before cutout on the highway. It was really wierd w/o the MAP bleed as soon as it passed 18psi, boom, cutout!

New Cal: T2 S2 LM w/o boost cutout (only mod) will run up to 27psi (turbo wont make more) all day long w/o any cutout.

Everything else below was the same between both these two setups:

Stock fuel pressure
42 #/hr injectors
100 Octane Cam2/Sunoco
RN9YC plugs gapped at .035"
MSD 6-AL / Blaster 3
14° ignition timing
stock cam straight up

[TurboJerry]

OK, my previous posts are not written too well, I'll try again here:

Old Cal: Stock T2 S2 LM would run 18psi all day long (as long as that day was within the 10 seconds) but if it hit 19 or 20 in much less than 10 seconds it would cutout. With a MAP bleed I could get it to 27psi before cutout on the highway. It was really wierd w/o the MAP bleed as soon as it passed 18psi, boom, cutout!

New Cal: T2 S2 LM w/o boost cutout (only mod) will run up to 27psi (turbo wont make more) all day long w/o any cutout.

Everything else below was the same between both these two setups:

Stock fuel pressure
42 #/hr injectors
100 Octane Cam2/Sunoco
RN9YC plugs gapped at .035"
MSD 6-AL / Blaster 3
14° ignition timing
stock cam straight up

I see......... I still think it was knock, but there's one more thing. It's called "map delta cut out". If the voltage gets to 5 volts too fast, then that can cause early cutout also. Which could make the difference I guess????? I don't know? Of course I'm assuming that boost hits faster with it set to 27 psi. BTW 10 seconds is a short day! The other weird thing is I would think 42's would be "on the edge" with 27 psi. I have 53's in a friends Acclaim and they hit 100 duty cycle at 22 psi! This is with an 11.7:1 A/F ratio. It has a stock short block with a stock 287 head.

[BadAssPerformance]

"map delta cut out" might be it, cuz bled past 18psi it does come on quick!

It is a stock '87 Shelby Langer T1.5 motor in the car. 42#/hr and 100 Octane seem to be ok till 27psi. Still full rich on my Halmeter AF-30 NB A/F meter. BTW, it only ruins this much boost in 5th gear on the highway. Same Grainger setting (very tight) only runs ~19psi on the dragstrip.

[TurboJerry]

I actually never had problems with the LW rod motors sofar. 12's seem to be cake and they run for years that way if the knock is kept down to a quiet roar. Sounds like the gear with the most load gets the most boost as always. I wonder if a G head 2.5 is just more thirsty for fuel? That one gets 26 psi in 1st gear. It has a stock Garret turbo and is an automatic. (maybe the difference?)

[BadAssPerformance]

2.5L is def more thirsty... automatic should have more load in 1st too.

[TurboJerry]

It all makes sense like the pieces of the puzzle.....

[BadAssPerformance]

the funny part... with the tight Grainger, I think all my exhaust goes thru the turbine all the time :eek:

[WickedShelby88]

Hey JT, is that no boost cut-out mod something you did to it or just a mopar variation? Wouldn't you have to socket the LM then if you change something and modify the chip? I'd like to know this as I plan on leaving well enough alone with the LM in my SC being as I will be running an 86 long block with a mitsu until I get a CB motor in there. I'm thinking with an IC, AFPR, blue top DSM injectors, and possibly alky injection I should be fine. I can also get E85 around here for 2.50 a gallon. That may help quell the spark knock. CAM2 around here is 5.00/gallon. Too rich for my blood unless I plan on going to the track.

[BadAssPerformance]

Socketed LM and a hacked cal.

[TurboGLH]

boost cut is not 14.7 psi. The map sensor is maxed out at 14.7 psi, Which is 5 volts. The timer is what seems to throw people off here. If you run 14.0 psi boost, it *will* hit boost cut in 10 seconds. The reason it happens in 2 seconds at a higher boost level is knock. The timer is bypassed if there is constant light knock, or heavy knock. The sensor is stuck at 4.9 to 5 volts at 14.7 psi no matter what happens boost wise. (18 or 20 psi, the sensor doesn't care or know) And also the timer is slowly incremented back up to 10 seconds. So the shifts will only get you back 1 or 2 seconds, which may be enough to get a full 1/4 mile pass in.

I knew about the timer, and the time out of boost counter to add seconds back. I did not however know that it was 13.8+ rather than 14.7, I stand corrected.