I have an LT1 with a Vortech S-Trim Supercharger. I have an Autometer Boost/Vacuum Mechanical Gauge. I have the vacuum line attached to the manifold on one of the stock vacuum ports. When my car cranks, it pulls vacuum but when I rev the motor. It increases the vacuum readout to near 20psi. Any Idea why my gauge isn't working right?

Thanks,
Brandon

 

did the gauge come with an orifice for somewhere in the line?

 

what port do u have it to?

 

When you turn on the key, does the needle do a full sweep up and then down? That's how it calibrates itself and if it doesn't do it, the gauge is bad.

 

Just to verify, I have my Autometer electric boost/vacuum gauge connected to a stock port on my intake manifold, and it works correctly. All the vacuum ports are pretty much the same on the LT1 manifold, just a couple of different sizes.

 

engine is running backwards Does it momentarily increase vacuum or does it hold at 20?

 

when the engine speeds up vacume will increase some, but at WOT you should lose the vac. and get into boost

 

Maybe you have your supercharger spining backward and its sucking the air out. lol jp

 

He is running a Mechanical gauge, not electrical. You will not make any boost just by revving the motor. You have to have a load placed on it to read boost.

 

"did the gauge come with an orifice for somewhere in the line?"
---The gauge had a threaded connection for the mechanical connection.
---From there it runs straight to the port on the intake.

"what port do u have it to?"
---The first small port on the passenger side.

"When you turn on the key, does the needle do a full sweep up and then down? That's how it calibrates itself and if it doesn't do it, the gauge is bad."
---Not sure about the sweep. I never noticed it so perhaps not...

"engine is running backwards Does it momentarily increase vacuum or does it hold at 20?"
---lol, when cranked it holds around 5 on vacuum. When reved, it goes to almost 20

"when the engine speeds up vacume will increase some, but at WOT you should lose the vac. and get into boost"
---I agree but it increases more than I thought it should. Haven'd had it at WOT too
---much yet cause its a brand new motor and want to make sure all is well.

"Maybe you have your supercharger spining backward and its sucking the air out. lol jp"
---Lol nope I checked. haha

Thanks,
Brandon

 

You need a load on the motor for a supercharger?

 

Yes, just sitting and revving the motor up, you will not see any boost. Same as on the street, just cruising you will see no boost, but when you go WOT and you start accelerating, you will see boost, which means when you are accelerating, you are putting a load on the engine.

 

His supercharger will not act the same as your turbo. He can build boost just by revving it

 

I know its not the same as a turbo car, but a supercharged car won't build boost just by revving. It has to have a load. Do this, take a supercharged car and put it on the dyno and make a run,it will build boost, then put a jack under the rear-end and make another pull on the dyno, but this time in the middle of the run, raise the rear-end off the rollers, what happens, you lose all your boost, because the load was taken off the motor! I don;t recommend doing this as it is very stupid, but you get my point.LOL

 

uhh, supercharged cars don't need load to see boost in the manifold...

 

No sir they do not need the load. They only need to be mechanically driven and engine load is irrelevant. The supercharger's boost is entirely dictated by the rpm at which it is being spun. You absolutely can make full boost on a supercharged car with no load.

 

With turbos the variable is exhaust, which is increased under load and hardly present in neutral even bouncing off the limiter. With superchargers, its belt driven so RPM is the variable. Therefore, supers can build boost anytime as long as RPM is adequate.

 

While in most cases you probably CAN see a pound or two of boost by simply revving the engine in your driveway, you're more likely to be able to see the boost when the engine is under a load.

 

i can get boost just by revin it, cruisin along i get a couple lbs-i have had about 30 supercharged cars over the years, most of the procharged ones was easy to wing it and see boost-i read my boost in the carb hat, now under the carb it takes a little more doing, but with the engine idleing its making boost, just pull the site plugs out on a Holley and watch the fuel squirt out, lol

 

Lol, I cant at all.