Boost gauge readings ... vac

[ranj]

I used to run my boost gauge at carb top but now i have moved it into the vacum line that runs into renix ignition ...... my dump valve also runs in this line. Just wanted to check if the gauge is reading ok or if i have plumbed it into the wrong place ..... At idle its reading about 20vac , then when driving and coming on boost its peaking at 11psi , so thats the boost, but when i back off it goes to 30vac and infact its bouncing on its limiter at the end there ........ does all this sound correct ???

I used to run 20psi carb top before on a Grp A carb , so im guessing thats probaly about 17/18 psi at manifold. Im now back to standard carb and gonna start increasing the boost soon once i have started modding the carb , just wondered what manifold boost pressure you guys were running ???? Deciding my max boost is my first decision i feel.

[RussellT]

mine is around -18 according to the reading on the Blitz boost controller

[Jay]

New poster! (less than 10 posts)

I'm running 14p.s.i @ the manifold on a standard carb and when i let off it goes to -10 at the most. Im not running a dump valve though. Maybe that makes a difference as when you let off and the dump valve opens the pipe what you take your reading off is open to atmosphere ?

[Mart]

It's physically impossible to reach perfect vacuum, so I'd say your gauge/needle is out of true.

[ranj]

OK , i have the gauge plumbed in right then. I have a old boost gauge , ill try that and see what says .......... the boost gauge on now is brand new ...... and seems ok at idle , could something in the pipework cause a -30 reading ????

[Brigsy]

The gauge sounds like its working ok, My boost gauge on the aei line shows -0.6 idle, -0.9-1 on overrun with closed throttle, As seen in this pic.

[Mart]

I'll post it again, incase my previous post was invisible, but it's physically impossible to obtain a perfect vacuum...

[ranj]

NO it was not invisible mart and i take your point and am not doubting it...... i just cant figure out how a brand new gauge can be so wrong unless i have plumbed it in wrong or an air leak can affect it readings i just dont know .... just want to know why ...

[Andrew Cooke]

I'll post it again, incase my previous post was invisible, but it's physically impossible to obtain a perfect vacuum...

where do you see talk of a perfect vacuum?

without units all measurement is meaningless

[Mart]

where do you see talk of a perfect vacuum?

Space.

[Andrew Cooke]

a friend of mine worked on the biggest vacuum outside of space, not once did she offer to clean my flat with it

[Adey aka Ewok]

Space.

wrong, space isnt even a perfect vacum

[THE MASTER]

I'll post it again, incase my previous post was invisible, but it's physically impossible to obtain a perfect vacuum...

Mart: sound does not travel in a vacum. maybe mart was shouting in a vacum and was not heard

but i have to add that the more vacum one gets on overrun the better the engine
as in the valves are seating and sealing piston rings are sealing . basicaly if one shuts of the air going into an engine and if everthing is sealed propperly the stronger the vacum should be.
there is also a second advatage to a strong vacum
ones servo works better.

if you dont beleve the servo vac thingy try this... PLEASE BE VERY CARFULL WHEN TRYING THIS OUT. please only try this out on an open road with no one around

with your right foot hold the throtle at about half more if your brave (dont move your throttle foot). with your left foot apply brakes the car should hold back on the brakes . witout moving the throttle foot lift off the brakes and apply again . youl notice you have to use more brake to hold it back . now try it again . remebering from the first your not moving your throttle foot just your left braking foot . about third or fourth try appling the brakes with your left you will have almost no brakes .
next bit is a laugh
once you found the point of hard almost no brakes . keep your left foot on the brakes and lift off the throttle. wahhhooo the servo gets vacum and wolla brake sudenly work nearly puttin one through the w/screen

[Mart]

wrong, space isnt even a perfect vacum

I was waiting for one of the 'fan boys' to post Ok, Stephen fecking Hawking, it's about as good as ya gonna get to perfect vacuum though.

[Alex]

I suspect it's possibly due to your new engine build Ranj - everything is sealing very well hence the vacuum readings you're getting...

[ranj]

Well i hope so , so just gonna leave things as they are for now ..... maybe check the other gauge if i get time , at moment im just running the engine in before i change the turbo to a bigger one ........

Still i wanta know what kinda boost people are running at manifold ...... may help me decide what max boost to run ...

[Ian S]

It could be that I'm forgetting something, but isn't the max possible -ve figure relative to normal atmospheric pressure -14.7psi?

So therefore the lowest figure a correctly reading boost / vac gauge could show is -14.7psi?

IIRC, my 5GTT used to idle at about -7 to -9 psi and suck about -13psi on the over run. According to both my Think Automotive gauges.

i wanta know what kinda boost people are running at manifold

24psi

[Andrew Cooke]

2.036254 inHg = 1 psi

so 20inHg is 9.8219573785981513111822002559602 psi

[Trevhib]

2.036254 inHg = 1 psi

so 20inHg is 9.8219573785981513111822002559602 psi

In which case 14.7psi is 29.9329338inHg. Is that a perfect vacuum then? And is that why most vac gauges go to 30inHg?.... Having just read up on google it would appear both of those statements are correct... therefore it would seem if Ranj's gauge is reading anywhere near 30inHg then there's defintely something wrong. Otherwise his GTT is in danger of becoming a black hole and sucking in the world

It would also be rather cold in there

[Ian S]

Right, so that was the something I was forgetting. Some gauges have inHg for the vacuum instead of psi.

[ranj]

Just had a check on my gauge and it is indeed measuring in hg ....

[Mart]

I wish I'd never mentioned it now...

[Trevhib]

I wish I'd never mentioned it now...

You love it

[CHUNK The Goonies]

New poster! (less than 10 posts)

my boost gauge runs on tick over at 20 vac and on the road on let off it vacs to 30. its fine, it means your 5 is sealed like a frsh packet of walker crisps. if your gauge needle shakes on tick over all that is ,is the boost pipe your using is bit to strechy, expands to much, just use a heat resistand 4mm boost piping, works a treat, had that prob myself... nowt to worry about.

IF IN DOUBT RIP IT OUT.....

[Trevhib]

New poster! (less than 10 posts)

my boost gauge runs on tick over at 20 vac and on the road on let off it vacs to 30. its fine, it means your 5 is sealed like a frsh packet of walker crisps.

The gauge might show that but it aint happening in real life.

[Mart]

[Trevhib]

Is that a shake of the head for me?

I'm only quoting physics. 29.9329338inHg is a perfect vacuum. The gauge might flick to an indicated 30 inHg but it's not physically possible, lol.

20 at idle seems plausible though...

[Mart]

No mate, it's a shake of the head at people who keep posting they're seeing -30inHg on their gauges. Given it's been mentioned enough times now that it's not physically possible to achieve that, it obviously points at their gauges being about as accurate as a Daily Sport 'world exclusive' (sic) headline.

[Andrew Cooke]

Is that a shake of the head for me?

I'm only quoting physics. 29.9329338inHg is a perfect vacuum. The gauge might flick to an indicated 30 inHg but it's not physically possible, lol.

20 at idle seems plausible though...

you're assuming it's calibrated absolute not gauge...

anyway, all gauges are crap and read wrong, if measurement was easy we wouldn't need the National Physical Laboratory http://www.npl.co.uk/

[Trevhib]

you're assuming it's calibrated absolute not gauge...

anyway, all gauges are crap and read wrong, if measurement was easy we wouldn't need the National Physical Laboratory http://www.npl.co.uk/

absolutely exactly what I'm saying guys

[Big Jim]

First post!

No mate, it's a shake of the head at people who keep posting they're seeing -30inHg on their gauges. Given it's been mentioned enough times now that it's not physically possible to achieve that, it obviously points at their gauges being about as accurate as a Daily Sport 'world exclusive' (sic) headline.

A gauge reading 30 when its actually 29 (which is entirely possible) is only 3(ish)% out. Your average 52mm gauge is not going to be that accurate. Why is everyone getting hung up on vac readings on over-run? Who cares how high it gets? Better that than having it too low and your servo not working...

[Scoff]

First post!
A gauge reading 30 when its actually 29 (which is entirely possible) is only 3(ish)% out. Your average 52mm gauge is not going to be that accurate. Why is everyone getting hung up on vac readings on over-run? Who cares how high it gets? Better that than having it too low and your servo not working...

ah ha, some common sence has arrived welcome to the boards jim.

[Mart]

First post!
A gauge reading 30 when its actually 29 (which is entirely possible) is only 3(ish)% out. Your average 52mm gauge is not going to be that accurate.

Pretty sure that's what I've been saying all along, but thanks for your clarification...

[ranj]

I was just asking to see if my gauge was working correctly as its the first gauge i have ever run plumbed to the manifold ........i now know it is.

Also learned a few other facts too ........ which is what these boards are all about

[Ian S]

Deciding my max boost is my first decision i feel.

I didn't decide my max boost, I found it.

Obviously, taking care to not be running it lean.

I turned the boost up bit by bit until it pinked then turned it down a little until it didn't.

Then added water inj and repeated the procedure, adjusting the water jet as appropriate. Possibly also adjusting fuelling.

Next step would have be adding a bigger inter cooler and repeating the procedure.

[Trevhib]

First post!



A gauge reading 30 when its actually 29 (which is entirely possible) is only 3(ish)% out. Your average 52mm gauge is not going to be that accurate. Why is everyone getting hung up on vac readings on over-run? Who cares how high it gets? Better that than having it too low and your servo not working...

So what we're saying is that the gauge conveys some vac on idle and a bit more on over run and thus renders itself proportionately pointless. Tongue in cheek there...