Bypassing The Stock S2 Boost Limiter

[QWK32]

I thought i'd post up this little "how to" on bypassing the stock S2 boost limiter. There will be a few out there that will probably never need to worry about this, but for those of us chasing more power by installing aftermarket turbo's and winding up the boost, it will come in handy to know this little ~10min job. Of coarse this only applies to people running the stock ecu's that are either chipped e.g. Nistune, or running a piggy back setup e.g. SAFC or Emanage (although i do believe the EMU can "clamp" the sensors output to also get around this).

Now before anyone goes rattling on about there being no such this as a boost cut, and that its R&R, the RB25 NEO does indeed have a boost limit, and its all thanks to a little MAP sensor mounted off the firewall at the rear of the engine. It seems that the only function of this MAP sensor is purely to let the ecu know if a certain boost limit, which is ~14psi, has been reached or exceeded and plays no part in the general running of the engine or tune.

"How do I know i've hit the boost limit?", well for one you should know its a pretty good chance of being hit if running boost pressures around and above 1 BAR (~14psi). When you hit this limit it feels like you drive into a wall, the engine powers down hard until a set rpm and load point is reached before allowing you to accelerate again.

Just a little warning, if your S2 is hitting boost cut and you still have the stock turbo, don't just bypass it, either your running too much boost for the stock turbo, or something is wrong with the boost control in your car.

"How do I overcome this problem?", there are a couple of ways to get around it, one more technical than the other but both work to the same effect.

The first way is to remove the MAP sensor all together. This will through an error on the ecu and is overcome by feeding a voltage down the ecu's MAP sensor input to trick the ecu into thinking the sensor is still there and the limit hasn't bee reached. I don't know the voltage to feed the ecu but i know it has been successfully done.

The second way, the easier way, is the way i have done it. This involves just removing the vac line from the sensor so it see's no boost pressure, leaving the sensor connected to the ecu so no errors are thrown, and because the sensor is purely there to let the ecu know of an over boost, the engine will run fine without it.

What you need:

- A pair of Pliers or Side Cutters

- A clean M6 bolt, used to block the vacuum line

- A cable tie, to secure the blocking bolt in the vac line

The procedure is as follows, and its that simple anyone should be able to do it. First locate the MAP sensor, mounted off the firewall above the rear of the engine.

Remove the vacuum line from the sensor, i took it off just before the one way valve thing, the other end goes to the crossover pipe.

Block the end of the vacuum line off with the M6 Bolt, i used a cap head bolt (allen key bolt). I also had to trim back some of the hard plastic cover on the line. If you pull the vac line out of this cover and want to get it back in there just spray some silicone spay/lubex or similar on the line and it should push through without getting stuck. Once the bolt is pushed down the end of the vac line secure it on there with a cable tie.

Thats it. Just leave the MAP sensor side of things as is, blocked vac line just sitting there.

Just one thing left to do, and thats boost it up.

[kidafa]

Good post mate, Im sure it will be put in DIY and HOW TO section

[DSTROY]

I did this to my car today, I'm only running a stock turbo with exhaust and cooler and it makes 10psi with no boost controller but my car had really bad R&R. After this mod the car revs out strong with no signs of hitting that dead spot up in the revs I just want to get a dyno run to make sure the AFR's are still looking good.

This is the dead spot in the revs that seams to have totally gone away now

[32_Dave]

nissan and there f**ked up designs.

good one bro. !!

[SABBAi]

The factory computer throws in heaps of fuel, especially when you start pushing boost into it. This is done so that there is little to no damage done to the engine. If you keep boost below 11.5-12psi, you wont see that nast dip in power.

[QWK32]

I did this to my car today, I'm only running a stock turbo with exhaust and cooler and it makes 10psi with no boost controller but my car had really bad R&R. After this mod the car revs out strong with no signs of hitting that dead spot up in the revs I just want to get a dyno run to make sure the AFR's are still looking good.

This is the dead spot in the revs that seams to have totally gone away now

I find it strange that you were hitting it with a low amount of boost, i was running 11psi through my stock turbo, sensor connected, for a while without the boost cut issue. Only once i upgraded the turbo and boosted it up it started cutting out around 14psi, and from what i've seen, most people (R34's included) are hitting it the cut at around that same pressure. maybe your sensor is slightly off calibration? oh well, doesn't matter, just leave it disconnected and you wont have to worry about it

nissan and there f**ked up designs.

good one bro. !!

cheers mate, it kind of is for people chasing more power, its just one more limiting factory control to get around. but i also think its a good idea from nissan, who don't really intend for there cars to be modified. just say there is a problem with the wastegate flap not opening, with the boost limit control, the engine would cut hopefully before any serious damage is done, without it you'd probably hit 20psi and say fair well to the stock turbo.

The factory computer throws in heaps of fuel, especially when you start pushing boost into it. This is done so that there is little to no damage done to the engine. If you keep boost below 11.5-12psi, you wont see that nast dip in power.

the stock maps are quite rich under high load, from my experience with a stock turbo setup, you will still get roughly the same AFR at 10psi as you do at 14psi on them. but thats not what i am trying to overcome by bypassing the boost sensor, the boost limit can't be managed with a good tune, it needs to be disconnected/bypassed. easy if your going to a full aftermarket ecu, but if your say, running an auto and need to retain the standard ecu for its auto controls, a chipped stock ecu or a piggy back setup are the only options, which does leave the factory controls/limits in place.

[DaveB]

Thanks for the article Josh, always wondered what that sensor was for. Just thought I'd mention, that I have an auto and a Nistune in my series 2, and the tuner told me he bypassed the boost cut with the Nistune software. I've been running up to 18psi without issue (now I've just gotta solve that pinging...)

[QWK32]

Thanks for the article Josh, always wondered what that sensor was for. Just thought I'd mention, that I have an auto and a Nistune in my series 2, and the tuner told me he bypassed the boost cut with the Nistune software. I've been running up to 18psi without issue (now I've just gotta solve that pinging...)

thanks for mentioning that Dave, you just reminded me of the Feedback/DTC section in the Nistune software where you can turn on/off certain ecu inputs, i just had a look in the software and found a flag for "Boost Pressure Sensor". This might be what you tuner is talking about, i will test it out this weekend. If it does disable the sensor its another easy option for people running Nistune.

[amnash]

so ive actually misplaced the sensor. and the car is definitely running weird now. has anyone actually fixed this so the sensor doesnt have to be there at all?

Edited March 27, 2013 by amnash

[coparts41]

well , sorry to put a dampening on things but I have been building and racing Nissans for the last 40 years and I can tell you that the boost cut isn't there to look at.

What say the hose to the waste gate actuator fails(splits) on an aftermarket turbo that can boost to 42lbs and you don`t have any boost control ,What happens THINK no control, boost pressure hits 20/25 psi running 91/95 octane fuel because the servo you went to is out of 98oct(does happen because it happened to my son and luckily the engine survived because he knew what was happening).

So how to cure your problem, well there is a inline pressure switch on Ebay for under 30 bucks and with the same small amount of installation you can have a boost cut to say 15lb and bingo safe engine , seems a shit load safer to me.

[QWK32]

so ive actually misplaced the sensor. and the car is definitely running weird now. has anyone actually fixed this so the sensor doesnt have to be there at all?

if you are using a stock standard ECU then yes, it needs to be at least be plugged in or it will throw a fault code on the ECU. If your using a complete aftermarket ECU than you shouldn't need it, if your using Nistune than yes you can disable fault code feedback from it in the DTC filter section.

[amnash]

if you are using a stock standard ECU then yes, it needs to be at least be plugged in or it will throw a fault code on the ECU. If your using a complete aftermarket ECU than you shouldn't need it, if your using Nistune than yes you can disable fault code feedback from it in the DTC filter section.

well i dont have the sensor anymore.. lol i placed it on my workshop table and i have check EVERYWHERE its like a little fairy came in and walked out with it or something.. anyways i dont have one anymore. ive been driving it without it installed and i only notice the difference when at idle and its a little lumpy unresponsive. no way to trick it to think its there with a resistor or something?

[linluv84]

well i dont have the sensor anymore.. lol i placed it on my workshop table and i have check EVERYWHERE its like a little fairy came in and walked out with it or something.. anyways i dont have one anymore. ive been driving it without it installed and i only notice the difference when at idle and its a little lumpy unresponsive. no way to trick it to think its there with a resistor or something?

My swap never came with the sensor and it seems near impossible to find a replacement over here. It hasn't really been a problem since I've just been running wastegate, but now that I want to run 10psi the ecu is freaking out. If somebody was kind enough to measure the voltage coming out of the sensor at atmosphere, I could be off and running with a cheap resistor...

Thanks

Edit: Measuring the resistance of the sensor at atmosphere would probably be more useful.

Edited June 2, 2013 by linluv84

[amnash]

anyone? if Linluv figures something out and shares my car might get fixed

[Ben C34]

anyone? if Linluv figures something out and shares my car might get fixed

Wow. Why haven't you just bought a sh one by now?

[Tumble]

Does this also work for the M35?