At the risk of getting flamed, I did do a search, with all the keywords I could think of relevant to this topic. If this has been asked and explained before, I apologize, just post the link.

I just bought a Powerdyne S/C Z28, and it runs pretty well - a hell of a lot faster than my RS. The only thing I have to complain about is the boost gauge - It's an Autometer gauge, and it's a boost/vac gauge. It seems like it may be backwards, though - it never reads boost, but the vaccuum goes from like 20hg to 10 when I get into the throttle some, so it looks like it's running the right direction... It zeros out when the key's in the ignition, then runs over to the vaccuum side...

A gearhead friend said that it's most likely backwards, but I don't want to do anything until I get some second opinions. It's going in for a dyno/tune in a couple of weeks (payday) but I'm curious about it anyway.

Thanks.

 

where do you have it hooked up to, and are you sure it has a good sealed connection? It should have vacuum at idle, cruising, and decelleration. An NA car would go to 0 at WOT, you should see boost. It should be T'd into one of the vacuum lines coming out of the intake manifold, just make sure there are no check vavles on the vacuum lines before the guage. The line for my autometer guage is a clear/white hard plastic tube. Pull the line for the guage and blow/suck on it to make sure it is working properly. If it is not working, remove the guage and check the connection on the back.

 

I have a gauge that is not working properly and does this. It makes a very quiet vibrating sound every once in a while when it should be in boost. Don't know if that is your problem, but it is a thought. I know mine is my gauge because it is in my GTP and I have a factory boost gauge also. I can't tell what boost is by it, but when it reads full boost and my auto meter says it is still in vacuum I know the car is making boost. Check your connections and then see if you can borrow another gauge from someone just to test it.

Good luck

 

From that statement, I'll assume it's an electric gauge, not a manual one. Did you make sure to hook the power line up to one that keeps power when starting?

The autometer Electric boost gauges zero themselves out when starting by calibrating itself with the MAP sensor when power is first applied to it. If you hooked the power line of the gauge up to a line that loses power, it will recalibrate while the engine is running and the gauge will be way off.

 

Are you're reading about 20 inHg vacum at idle? If so then I would guess you have the power hooked up correctly. You may not see boost until you get in the higher RPM's. What do you read at 5000 RPM?

 

I went out today with my brother and we looked at it - it's definitely a mechanical boost gauge - the hose gave it away - and it looks like it's hooked up correctly. There's a "T" from the manifold, and then it "T"s again to somewhere else (not a whole lot of room to look around) and then runs right through the firewall to the gauge... So I'm going to take it out tomorrow and romp on it a bit and see what happens at WOT... Hopefully I'll get into the boost range.

It just seems odd to me to be reading about 20hg of vaccuum at idle or 15~18 at cruising speed... Seems like with the engine running there'd be a positive pressure on the hose...

Since I'm going to the speed shop tomorrow, I might see if they have a spare gauge laying around that I can try to switch it out with... We'll see what happens there.

 

20hg at idle and 15-18 at cruise sounds right to me. you won't see boost till you go WOT at ~3k RPM while driving the car.

 

The line for your boost gauge is T'd off the manifold, NOT the supercharger ductwork....and that is correct. That's why you're still seeing vaccuum at idle. To understand it think of it like this: The throttle body completely closed when you're at idle, only a small amount of air is passing through the IAC valve in the throttle body. Yes, your supercharger is still spinning and pushing air ....but most all of it is staying upstream of the throttle body and being compressed in the discharge tube of the supercharger. But you're not taking the reading from that tube, you're taking the reading from the intake manifold (downstream of the throttle body) where the engine is sucking more air (vaccuum) than it's receiving from the supercharger and atmosphere due to the throttle body blocking most of it off and no load being present. Despite being connected to the crankshaft and always spinning, superchargers are not always making boost ...they are passive. Only when enough load is placed on the motor will the supercharger begin to push more air into the engine than it is consuming on it's own (boost). You can be cruising at 4,500 rpm and still be making plenty of vaccuum, because again .....the throttle body is almost closed, very little load is being placed on the motor since it's cruising at a steady speed. So, again ... the nearly closed throttle body is blocking most of the air from the supercharger and there is more suction at the vaccuum port on the intake than pressure (vaccuum). It sounds like your boost gauge is working properly to me as long as you see boost on it when you really get on it. I have a Powerdyne and have to get on it pretty hard above 2,500 rpm before I see any boost at all....and mine is overpullied for 8 PSI on a stock motor. It made 384 rwhp and 402 rwtq on stock motor with headers and 1.6 RR's FWIW.

 

THANK YOU!

That's the sort of response I've been looking for all along - a detailed reason "why" things work. Not to disrespect anyone who has posted, trying to help, but that sort of post is what works best for me.

As a matter of fact, I caught one of the guys from my speed shop online last night, and he said that it sounded pretty normal to him, too.

I'll have it out today romping a bit, I'll try to get the boost gauge showing positive.

 

It sure is... Today, though, I got on it coming up the interstate - there was a jack**s in the merge lane that felt he *had* to get over in front of me - and when I romped on it, though I didn't get to look at the boost gauge (staying on the road and all that) I felt it really kick - if that gauge wasn't in boost, I'd be really surprised - felt like the front wheels almost came off the ground.



I'll take the girlfriend along and try that again next time, with her watching my gauges.