boost gauge and understanding vacuum reading

dirtyhandssubaru555 · Post by **dirtyhandssubaru555** » Sun Dec 09, 2007 8:11 pm

what kind of vacuum reading should i see at an idle? if you have a vacuum leak would you see more or less vacuum? i was told use a vacuum gauge to tell what kind of gas milage you are getting or help you to have better gas milage.

vrg3 · Post by **vrg3** » Sun Dec 09, 2007 11:08 pm

You should see about 19 inHg of vacuum at idle. A vacuum leak on our cars typically results in an unstable idle and so an unstable -- but low -- vacuum reading at idle.

A vacuum gauge used to be called an "econometer" and was used under the presumption that higher vacuum translates to higher fuel economy. That's kind of true, but there's a lot more involved in fuel economy than manifold pressure.

Post by **Legacy777** » Mon Dec 10, 2007 7:02 pm

It is however a good indicator of engine load.

vrg3 · Post by **vrg3** » Mon Dec 10, 2007 7:14 pm

Yes. In fact, manifold pressure is typically the numerical quantity that people refer to when they talk about "load."

But generally speaking, a naturally aspirated engine actually gets its best fuel economy when it's lugging -- engine speed is very low and load is very high.

ciper · Post by **ciper** » Tue Dec 11, 2007 4:27 am

What VRG3 said is pretty true. When I was in China it drove me up the fricking wall that ALL of the cab drivers do this. We would literally be in 4th gear at like 25 miles an hour moving through traffic. I bet the engines have not seen over 2500 rpm for at least 100,000 miles!

555BCTurbo · Post by **555BCTurbo** » Tue Dec 11, 2007 5:44 am

Legacy777 wrote:It is however a good indicator of engine load.

Agreed...

It is so much easier to achieve decent fuel economy in the turbo cars with a boost/vacuum gauge...because you know right when you are going to start using a bunch of fuel (right around the 1 psi mark)

dirtyhandssubaru555 · Post by **dirtyhandssubaru555** » Wed Dec 12, 2007 1:08 am

well when my car does idle i see about 15inhg. so that would meen i have a leak some were right?

Fkyx · Post by **Fkyx** » Wed Dec 12, 2007 1:24 am

vrg3 wrote:But generally speaking, a naturally aspirated engine actually gets its best fuel economy when it's lugging -- engine speed is very low and load is very high.

Wait... noob question... is it opposite for turbocharged engines?

vrg3 · Post by **vrg3** » Wed Dec 12, 2007 2:51 am

dirtyhandssubaru555 - If the engine is idling smoothly, then you probably don't have a leak. Most likely is that your gauge is miscalibrated, but it's also possible that for some reason your engine needs more air than usual to idle and so the IAC valve is open kind of far.

Matt - No, that's a good question... Hm, well, I guess actually it would apply to turbocharged engines too, since you don't make any boost when you're lugging.

dirtyhandssubaru555 · Post by **dirtyhandssubaru555** » Wed Dec 12, 2007 3:48 am

i wouldnt use idle and smooth in the same sentence when im talking about my car. would a dead injector cause low vacuum? since the car would be running on 3 cyclenders or very poor on one any way

dirtyhandssubaru555 · Post by **dirtyhandssubaru555** » Wed Dec 12, 2007 3:51 am

if i should be seeing 20inhg that would meen 5inhg for each cyclender, 15inhg=one bad cyclender

vrg3 · Post by **vrg3** » Wed Dec 12, 2007 5:22 am

I don't know if the arithmetic works that way, but, yes, anything that causes a rough idle can cause a low vacuum reading.

You can check if an injector's dead by unplugging it and starting the car. If the car runs the same without the injector as with it, that injector's not doing its job.

93Leg-c · Post by **93Leg-c** » Wed Dec 12, 2007 7:55 am

vrg3 wrote:But generally speaking, a naturally aspirated engine actually gets its best fuel economy when it's lugging -- engine speed is very low and load is very high.

Wouldn't that be very hard on the piston rings, cylinder walls, and spark plugs which would then make it uneconomical in the long run?

Post by **Legacy777** » Wed Dec 12, 2007 4:26 pm

ciper wrote:What VRG3 said is pretty true. When I was in China it drove me up the fricking wall that ALL of the cab drivers do this. We would literally be in 4th gear at like 25 miles an hour moving through traffic. I bet the engines have not seen over 2500 rpm for at least 100,000 miles!

Yeah, Japanese taxi drivers were the same way....lugging the engine everywhere.

vrg3 · Post by **vrg3** » Wed Dec 12, 2007 5:23 pm

93Leg-c - Yeah, it's harder on the engine, and in many cars that may make it a false savings on the long haul... But our cars kinda last forever anyway.

dirtyhandssubaru555 · Post by **dirtyhandssubaru555** » Thu Dec 13, 2007 2:18 am

heres what im see on my guage. 2000rpm=20inhg,1750=about 18inhg, 750 or so i see about 14inhg or less, this is all while car is parked

vrg3 · Post by **vrg3** » Thu Dec 13, 2007 7:05 am

Is that with the engine warmed up and you using the throttle to hold those engine speeds?

dirtyhandssubaru555 · Post by **dirtyhandssubaru555** » Fri Dec 14, 2007 1:01 am

yes thats with the engine warm, no im not using the throttle, 2k is about "normal" idiol for my car but it comes down slow.

Post by **Legacy777** » Fri Dec 14, 2007 4:32 pm

Your idle must be 750-800 rpm range, or you won't get the same vacuum reading as what everyone else is stating.

vrg3 · Post by **vrg3** » Fri Dec 14, 2007 4:45 pm

What do you mean by "it comes down slow?"

Trace · Post by **Trace** » Fri Dec 14, 2007 8:10 pm

sorry, Legacy777....a side question. when you state idle, is it the same as in "P" and "N" for an auto transmission? When my car is warmed up and in P position the car idles at lower than 500. it even dips down to 300 sometimes. as for N it stays at 500 rpm. is that a major problem?

dirtyhandssubaru555 · Fri Dec 14, 2007 10:28 pm

when ever i start my car and step on the gas it sticks to 2k or so and comes down to 750 or so but it takes its sweat ass time. around 750-800 is see 14-13.5inhg

ciper · Post by **ciper** » Sat Dec 15, 2007 12:30 am

Trace wrote:sorry, Legacy777....a side question. when you state idle, is it the same as in "P" and "N" for an auto transmission? When my car is warmed up and in P position the car idles at lower than 500. it even dips down to 300 sometimes. as for N it stays at 500 rpm. is that a major problem?

P and N both are idle.
I doubt its idling at 3-500 hundred. Most likely you are reading the tach wrong. Look closely and you will see that the lowest line above zero actually corresponds to 500, and the second line is 750!

Fkyx · Post by **Fkyx** » Sat Dec 15, 2007 12:58 am

dirtyhandssubaru555 wrote:when ever i start my car and step on the gas it sticks to 2k or so and comes down to 750 or so but it takes its sweat ass time. around 750-800 is see 14-13.5inhg

Mine does the same thing. If I don't gas it a bit, it'll stick at 2k until it reaches about half running temp, then it'll drop to 750.

dirtyhandssubaru555 · Post by **dirtyhandssubaru555** » Sat Dec 15, 2007 2:12 am

mine will do it at operating temp. might have found one source of a leek, in the 90 degree elbow were all the pcv lines connect. but they want a 100 f-ing dollars for one, i got a tube of ultra grey that cost me like 7.00, i don't know how well that stuff sets up in 30 degree and below so i have to wait till next weekend so i can use my garage cause my bro blew up his sled

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boost gauge and understanding vacuum reading

boost gauge and understanding vacuum reading