Vacuum bypass mod

[IggDawg]

Here's the link to the mod in "the library"

http://www.xmission.com/~dac/library/shik_bypass.htm

I tried this with weird results. I have no boost gauge yet, but I could tell my boost was all over the place. maybe the ECU was just learning. But when I'd let off the throttle, it sounded like the turbo spun up significantly breifly before letting down again. It was just overall weird. Here's some "educated guesses" on the functionality of hte mod:

1) It sees manifold pressure, not intake pressure. This means that it will not open the wastegate till there's 8.7 lbs in the manifold. If there's any pressure loss after the turbo due to plumbing or an intercooler, it will be eliminated by this mod. If you would normally loose a couple pounds through an intercooler due to turbulence, this mod would cure that. The solenoid would be blind to what's going on before the manifold. It would want 8.7 lbs at the point of contact, which is the front left intake runner.

2) When the throttle slams shut, normally the solenoid sees a pressure spike and presumably opens the wastegate. After this mod, it would instead see a vacuum spike, and keep the wastegate clsoed. I'm not entirely sure what this would cause, but I don't think it's a good thing.

3) The pressure it would see at the turbo would be constant. However the pressure it sees at the intake runner would varry. It would rise slightly when the intake ports for that runner are closed and drop slightly when they're open. Perhaps this is why my boost felt like it was all over the place.

So that's what I got. what do you guys that have done this feel about the mod?

-IggDawg

[vrg3]

I have not done this mod, and do not ever plan to. I think it isn't a good idea, at least with an otherwise stock setup.

The compressor outlet is the right place for the wastegate actuator to read boost.

The ECU measures manifold pressure when deciding how much air to bleed away from the wastegate actuator anyway, so in the end it always is your manifold pressure which is being controlled. It's just controlled by software rather than by mechanical linkages.

The ECU closed-loop boost control and the wastegate control solenoid had to be designed together with the setup of the entire air pressure circuit in mind. The solenoid is operated by continually opening and closing at a constant rate with a varying duty cycle. The relation between this duty cycle and the actual amount of pressure bled off is something that had to be preprogrammed into the computer (this is why you couldn't expect stock behavior if you just used any old 3-way solenoid to replace your stock one). You can confuse the ECU quite badly when this behavior changes so drastically.

Finally, it's not good to allow the turbo to produce high manifold boost at part throttle. A partially closed butterfly valve makes boost very inefficient, heat-soaking the whole system. The ECU code was written with this in mind; that's why at part throttle it won't give you high manifold pressure. If the wastegate actuator reads off the manifold rather than from the compressor outlet, then the pressure at the compressor outlet could rise extremely high.

It's important to note the tradeoff between response and heat. Our underhood temperatures are really, really high.

Wow, I wrote a lot more than I meant to. I'll wait for responses before writing more.

[Aaron's ej22t]

Well, i had this mod for about 1 month. today, i took it off and put everything back to stock. the only mods on my car are removed snorkel, k&n filter, ngk plugs/wires. anyway, i have been experiencing some bad hesitation at about 4k rpm. until today, i noticed it really bad. now, it isn't as bad as it was after removing this mod.

with this mod, boost does come on quicker and hit harder. but the downside is it spikes. before, at about 3700 rpm, the car would surge (which i assume was a boost spike) and then start hesitating around 4k.
now, it spools a little slower, but no spike and alot smoother.

I know when i had my gauge hooked up, when i hit full boost, the needle was bouncing all over the place from 7.5-9.5psi. i had the boost gauge tapped right at the manifold where this mod was as well.

over on nasioc.com , this mod is called "3/16 mod" .....I have heard conflicting reviews on the effects of this. i am with vrg3, and don't really recommend it. i would think the effects of this mod would be even more profound if your car was modded with full exhaust, intake, etc.....

my 2cents,

-Aaron

[JasonGrahn]

BUY A BOOST GAUGE.

(i have one for ya. Greddy gauge that near-perfectly matches the stock lighting, great condition, PM me if you're interested)

-Jason

[Aaron's ej22t]

UPDATE:
After removing this mod, and resetting the ecu, the car has been driven all day. to be honest, it is not as fast, not as loud, and not as fun to drive. the hesitation is still there and it spools significantly slower.

i honestly don't know about this mod, it does make a difference, but is it good?

[vrg3]

I have a hypothesis: I'd bet that exhaust gas temperatures would go up with this mod.

Can anyone verify or refute this?

The reason I'm thinking this, Aaron, is that you say the car isn't as fast, loud, and spools slower. You could easily remedy all of these by installing a ball-and-spring manual boost controller plumbed from the intake manifold to the wastegate actuator. That would give you better performance but less efficiently, and the danger that everyone notices with an MBC is high EGTs.

[Aaron's ej22t]

Well, i forgot i had to let the ECU re-learn after i did the reset. once the car is warmed up, it is just as quick, and is a little smoother. it is not as loud, though, i think i was running slightly more boost pressure with the other mod than now. i don't care though.

[IggDawg]

My only real worry is that the turbo sounds like it actually speeds up when I let off on the throttle. jsut for a second before it goes back down. has anybody else noticed this? It seems that would be a bad thing.

[vrg3]

The turbo probably does speed up just as you release the throttle. The pressure on the wastegate actuator line would drop suddenly, so the wastegate would slam shut. This would only be transient because with a mostly closed throttle the turbine wouldn't receive enough energy to sustain the speed.

This probably isn't good because it puts more stress on the turbo, since it accelerates rapidly. The stress of rapid deceleration caused by a throttle slamming shut is relieved by the blowoff valve, but there's no such protection for rapid acceleration. I can't say whether or not the acceleration here is on par with the deceleration of a closing throttle, but it seems reasonable that it would be.

So, according to my possibly misguided logic, this mod could potentially be as harmful to your turbo as running without a blowoff valve.

[whitey]

Just a thought but moving the pipe closer to the turbo (away from the intake manafold) means when you back off and the intake pressure (in the intake manafold) decreases it takes a while for this information (the decrease in pressure) to reach the solinoid (as the bleed point is away from the intake). So the solinoid will hesitate to open when you back off causing the turbo to overboost (spin to fast and create to much pressure) which is very bad.

However this will work the opposite when you are on boost i.e. as the bleed is closer to the turbo when it spools up the solinoid is more sensitive (faster acting) and should provide better response (what you see as a more positive pick up or a performance gain)

any way this is just a theory from a bpred engineering student who should be working

[whitey]

sorry for the first bit it should be when the intake pressure increases not decreases