Author Topic: Information on N75 valves and MBC setups  (Read 1777 times)

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Information on N75 valves and MBC setups
« on: January 05, 2005, 02:18:20 PM »
Good information from Rusty:

MBC=Manual Boost Controller
DBC=Drive By Cable

IIRC correctly, in comparison to the stock N75, the "J" valve shifts the boost response curve to the "left", i.e. it gives you more boost sooner (earlier in the rpm band), but less boost later; whereas the "H" valve gives either a moderate increase across-the-board or is shifted "right", i.e. later in the rpm curve.

This boost shift from your "J" valve is happening so early in the rpm band that your ECU doesn't like it, hence the overboost codes. It probably doesn't even happen at max boost - even if you have a 15 psi chip, if the chip expects 7 pounds at 2000 rpm and it's seeing 10-12, that's what will throw the code. I'll call this "early-curve limp."

Now for the MBC setups:

Overboost/Parallel - the MBC is connected in parallel with the N75. In this configuration, the MBC is working as a "cap" on the max boost. The N75 still completely controls the boost curve, both shape and amount. The MBC only chops the top off the curve. Using the above numbers as a theoretical example, you will still get early-curve limp unless you dial the MBC all the way down to 7 psi. But then 7 psi will be the max boost you'll ever get.

Inline - This is how I run my MBC, connected in-series between the turbo nipple and the N75. In this configuration, the N75 only controls the shape of the curve. The MBC controls the value. The setting of the MBC modifies the whole curve of the N75 by a percentage amount. This setup has the best hope of a solution - just keep adjusting the MBC down until the limp goes away. In the example, this would be 7 psi at 2000 rpm, but boost would still increase vs. rpm (unlike the overboost setup, which would be a flat 7psi accross-the-board). However, this might mean that your max possible boost is less than 15 psi.

Bypass - The N75 is bypassed. Both shape of curve and amount is entirely controlled by the MBC. This is the worst of possible configurations unless you have standalone engine/fuel management. For a given rpm, the MBC is going to always give as much boost as the turbo is capable of putting out. So you get immediate early-curve limp, because without the N75 to moderate the boost increase, that turbo is going to spin up FAST!!! This is actually very fun to drive, and works great to a max boost of 12 psi. I ran bypassed 12 psi in my DBC car for a long time without any overboost or pinging. The throttle was like a lightswitch - it was either on or off. As soon as I tried to go even a mere pound above 12 psi, it was too much at lower rpms and I'd get overboost. Now that I'm running 15 psi I need the holey MAF to keep away overboost, and I need the N75 in-the-loop to moderate boost increase and avoid pinging.

Another advantage to the bypass mode is that you can build boost on the dragstrip - with a Tip, you can stand on the brake and the throttle and build 3-5 lbs boost before the light turns green. With the N75 in the loop, the ECU refuses to give the car any boost because it's not moving.

My best advice to you is to dump the "J" valve. It appears that its boost curve is not compatible with your chip's programming. Using your Boostvalve in-line with the stock N75, and increasing the boost by 1-2 lbs, should give you a nice overall improvement w/o causing limp. You could just keep increasing by 1 lb until you hit limp, then back off by a pound or two. But keep an eye on your fueling - if you pump the boost too much higher than the chip's max, you could run lean. Ouch.
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