MP SBEC overboost

[dwh4784]

I've been chasing this problem for as long as I can remember. I'm hitting overboost (code 45 verified) way too early. Like 12psi without any delay at all like I thought the MP computer was supposed to give. I just came across this post from back in the day and am wondering if this is the problem. Is this a common variance with MP computers? It's the first I've ever read on the subject and I've used them before with different cars graingered to 14psi without an issue.

The MP computers all are designed to work with the stock injectors (except the S-60 computer). The 85 will up the boost from the stock 9-10psi to around 11-12 psi and it's got a more aggressive spark curve from what I remember. Putting one in my Shelby Charger was a noticeable improvement from stock. I'm pretty sure they still have overboost cutout enabled, so it may not let you run the 14psi your looking for depending upon how picky it is. I've seen some cars cut out at 12-13psi, and others run 15psi all day, so different combos of computers and Map sensors yield different results.

So, all this time and my computer might be setup to cut out like that? Which means if I use a cutout raiser I'll have to back it down to <12psi and it's going to lean out if I turn the boost up to 14-15. I have a walbro 255, stock fuel system otherwise and I'm deep in the 10's (AFR) at WOT at 11psi so I know there's room but this stinks if it's the case. I had hoped to set it and forget it at 14psi with this stockish setup and leave it be.

[Dr. Johny Dodge]

when I built my turismo / turbo conversion I used the 85 d-c computer but I also had the bleed that was supposed to be used with it in that car

I can't say I ever had any overboost issue with that

the m-p smec in my 86 turbo z (88 harness) never had an overboost issue either with just the stock vacume lines and wastegate solonid but I don't think it was supposed to increase the boost level beyond stock TII


edit - afterthought
in the day when they were selling the m-p smec they were also selling the super 60 kit
the m-p smec might not be able to take extra boost , reason being they could sell you something "bigger" if you wanted more boost

it was however severial hundread dollars cheeper than the stock smec which is why I bought mine (3-4 hun less)

[dwh4784]

I wish I could find it but when I bought this SBEC the guy included the original owners manual. It stated that it was meant for a stock engine, no intercooler and would boost to 11psi without any delay waiting for RPM's to climb. So I wonder if they really did program overboost to 12psi thinking you still had a non-intercooled mitsu turbo 2.5 I guess it would have kept engines alive with the aggressive timing they use.

[sdac guy]

I've been familiar with a number of the MP/DC computers. They all have a pre-programmed boost target of 11-12 psi (or close to that). They all have overboost limit enabled. So if using one of those (or a stock computer) and some kind of manual boost controller (bleed, grainger valve, etc) that is set for more than the limit in the calibration, overboost shutdown and the setting of code 45 is normal operation.

For those that have experienced higher cut off points using the MP/DC computers, it is probably due to a faulty MAP sensor (or a leak in the vacuum line to the MAP), or an inaccurate boost gauge. The 2 bar MAP has a limit of 14.7 psi and MP/DC never programmed any of their 2 bar cals anywhere near that limit.

In fact, the Super 60 SMEC, which needs a 3 bar MAP sensor to operate, is the only MP computer that ever had a setting that high. It had 2 settings and was switchable between 14 and 18 psi.


Barry

[GLHNSLHT2]

My 85 T1 MP puter would run 16psi all day long with an Autometer phantom gauge and new nylon factory vac lines everywhere. My friends 86 GLHS would run 18psi for about 13 seconds or so then hit overboost.

[cordes]

My 85 T1 MP puter would run 16psi all day long with an Autometer phantom gauge and new nylon factory vac lines everywhere. My friends 86 GLHS would run 18psi for about 13 seconds or so then hit overboost.

Some of the MP cals have a real long time delay before cutout. IIRC the GLHS MP stgII cal is crazy long so that makes a lot of sense. Most all of these are available in the stock cal repository so you can download them and look at them with MPtune.

[dwh4784]

Well I definitely learned something here. I had always been under the assumption that overboost cutout was initiated when the MAP saw 14.7psi at the end of it's range. It's been so long since I had the stock SBEC in it I can't really remember if I ever set the boost high enough to know if the MP actually cuts out lower. I just know from previous vehicles and friends with stock computers that we were always able to run just under 15psi without overboost.

I ordered a Stage 3 from Cindy that will hopefully set things right. I always thought it was kind of neat to use an original MP but 11psi just won't cut it.

[sdac guy]

Ya know, your response kindled something in my thoughts. You are right that overboost shutdown only occurs when the MAP sees more than 14.7 psi. I don't know if that is true with all MP computers, but thinking back, my experience with the MP logic module was like that. I believe code 45 is set whenever boost is more than the boost goal for a period of time. I have the documentation for that here somewhere, but it has been some time since I have seen it.

Barry