boost vacum ????? mpg??????

91scoobiesubie · Post by **91scoobiesubie** » Sun Jun 15, 2003 12:48 am

i put an obx racing boost gauge on today right after the bypass valve im reading 12vac at neutral and 5psi at 3300 and it flutters really quick between 5 and 7 psi at 5000. is this thing working correctly??? are these numbers correct everything stock so far beside hyper ground system and blitz sus filter i also get between 14 and 15 mpg what do you guys get??????

91White-T · Post by **91White-T** » Sun Jun 15, 2003 1:21 am

The 12mm sounds kinda high.. I usually have between 15-20mm at idle. And the mileage is really low, I get 16-17mpg and I'm running 17psi... Other than that everything else sounds good.

91scoobiesubie · Post by **91scoobiesubie** » Sun Jun 15, 2003 2:28 am

so what could be causing the low milage? and vacum? does your boost flutter really fast at peak is that normal

vrg3 · Post by **vrg3** » Sun Jun 15, 2003 4:11 pm

Let's be clear about what units we're using to measure vacuum.

You should get around 17 to 19 inches of mercury (inHg) of vacuum at idle. That's roughly 8 or 9 psi of vacuum.

Is your idle smooth? It's weird for an engine with low vacuum to idle properly.

The boost should not be fluttering. In fact, I don't even think it's possible for boost to fluctuate rapidly by 2 psi.

That makes me suspect that you have some kind of leak in the tubing going to the gauge. If some vacuum is leaking then it'll read low. And, when it's pressurized, whatever is causing the leak probably flaps open and closed to cause that flutter.

Try listening along the full length of the tubing; you should hear a hiss where the leak is. You could also try spraying starter fluid or carburetor cleaner around the hose while the engine idles. If you spray around a vacuum leak it should increase the idle speed slightly.

91scoobiesubie · Post by **91scoobiesubie** » Sun Jun 15, 2003 5:32 pm

idle is smooth. when ac is on it is about 10 psi. what kinda hose should i get and where could i pick it up. i think i want to replace all the hose. i just got the car. every thing seems old. the hoses are stiff. you guys dont think there is any thing wrong with bypass valve or actorator. what could be the cause of the bad milage.??????

vrg3 · Post by **vrg3** » Sun Jun 15, 2003 6:33 pm

What kind of fitting is on the back of the boost gauge? You'll need to make sure your hose properly mates to the gauge itself. Sometimes people have gauges with BSPT fittings but they try to install with NPT attachments to the hose. You can kind of screw them together but it won't thread properly and certainly won't seal.

Some people actually just use 3/8" vacuum line and a small hose clamp around the fittings on their boost gauges, and find that that seals well. The standard vacuum lines in your car are probably 1/8" or maybe 1/4". You may as well get a bunch of it and run a new line to the boost gauge with something appropriate to fit it to the back of the gauge itself.

There are two types of vacuum line -- the rigid nylon line and the flexible rubber line. Our cars come from the factory only with flexible line but most boost gauges come with rigid line. If any of your flexible line has hardened it should be replaced because small cracks in it can leak.

14-15 miles per gallon isn't terrible, especially if you're driving hard and using boost a lot. When was your car's last tuneup? Worn out plugs or wires can bring your mileage down significantly. Several of us have found that improving the ignition system further with a better coil pack and an MSD ignition booster improves mileage further.

91scoobiesubie · Post by **91scoobiesubie** » Mon Jun 16, 2003 12:22 am

what kinda rubber hose silicon? i want the best! i put in denso iridium plugs and ngk wires. maybe one month ago. i also did the hyper ground system.where can i get the coil pack and booster.what name coil pack? thanks for all the help.

vrg3 · Post by **vrg3** » Mon Jun 16, 2003 12:34 am

Normal rubber vacuum lines last about 10 to 15 years before you really ought to replace them; silicone hose should last much longer. Silicone's a bit more flexible, so the hose usually has to have thicker walls to keep it from collapsing. That ends up making the hose thicker overall, so it can be harder to get through the firewall. Silicone is also much more expensive than rubber. But, you can usually get it in pretty colors. It's your call.

As for the ignition, the booster I speak of is the MSD DIS-2. Do some searches on this board to learn more about it...

Denso iridium plugs are decent, but people seem to sense that the copper ones actually perform a little better. Usually "iridium" or "platinum" spark plugs just last longer than copper ones; they don't actually perform better. NGK's plugs in particular are favored by Subaru turbo people in general.

NGK wires are quite good; they're probably the second best you could use. Magnecor wires in my opinion are the best in design and performance. I don't know if anyone has used MSD wires on Legacy Turbos; those might also be good but you would have to construct them yourself I think.

The coil pack on stock Legacy Turbos isn't quite as strong as the Diamond pack that many other Subarus got. Naturally aspirated 5-speed BC/BFs got the Diamond pack, as did most (all?) 1995+ model year Subarus. Again, you can do a search to find out more about these. If you'd like to buy a Diamond pack, PM me with an offer; I have an extra one.

ciper · Post by **ciper** » Mon Jun 16, 2003 10:16 pm

VRG3 knows his stuff.

The only correction I would make is to make sure you dont get a coil with built in ignitor!

Brat4by4 · Post by **Brat4by4** » Mon Jun 16, 2003 11:26 pm

You might want to change the oil, also. If the oil is something like 20W-50 this will knock down the mileage quite a bit (don't ask me how I know

). And vacuum line isn't the best for a boost gauge, under boost the line that is meant for vacuum might swell and cause a slightly inaccurate reading at the gauge. Plugs/Wires really helps if they are quite old, and a diamond coilpack with that is even better.

And the fluttering boost problem is hopefully in the gauge or the lines. It sounds like it is running solely off of the wastegate, checking your engine codes should tell you why it would do that.

91scoobiesubie · Post by **91scoobiesubie** » Tue Jun 17, 2003 1:56 am

what is the part number for the best plugs and where can i get the coil pack i have the vac boost gauge tap off after the by pass valve

vrg3 · Post by **vrg3** » Tue Jun 17, 2003 2:34 am

Just go to an auto parts store that carries NGK plugs and ask for the ones for your car. The box should say "NGK V-Power" on it.

Like ciper said, make sure the coil isn't the kind with the integrated ignitor. I think some time around 1998 is when they started integrating them. Like I said, I have a coil pack you could buy off me if you want to PM me.

By "after the bypass valve" do you mean you tapped it into the small vacuum line going to the bypass valve? That's a good place. If you mean you tapped into either of the wide hoses coming off the bypass valve though you might reconsider, as that's not quite what the intake manifold sees.

91scoobiesubie · Post by **91scoobiesubie** » Tue Jun 17, 2003 3:10 am

the one that goes back to the manifold smaller one

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boost vacum ????? mpg??????

boost vacum ????? mpg??????