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manifold vacuum on a turbo ???


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10 replies to this topic

#1 ruparts

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Posted 08 August 2004 - 12:55 PM

Hi Guys,
I need to ask if the intake manifold has vacuum (as on a N/A engine )at lower speed , I know it is pressurized when you are on it, but where is the vacuum for options and stuff .
Mainly interested in the selective 4wd on my 86 xt turbo , I see the vac source tube on the manifold to the selonoids and to the selector diaphram, but how can it operate if the manifold is full of pressure instead of a vacuum ?
Is there something else to it ?
Is there vacuum at lower RPM ??
Thanks for any help

#2 rallyruss

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Posted 08 August 2004 - 01:01 PM

the manifold is only under pressure during boost conditions.

most of the time it operates in a vacume just like any other motor.

all you need is a check valve to a vacume canister to get a good constaint vacume source.

#3 archemitis

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Posted 08 August 2004 - 01:03 PM

no, theres nothin different about hookin up the vac. the switches in the car, hold vac, so the stuff even works when the car has sat for months. and your brake booster, is fine gettin boosted=].
you dont use brakes when your uner boost, so thats all the same. and you never shift into 4x when under boost.
its got vac just like a regular car, cept when your boostin.

#4 TheMeatWagon

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Posted 08 August 2004 - 04:22 PM

Yeah ive got an 86 gl turbo coupe. When im on the gas the boost guage reads +6-7 but when i let off it creats about 20psi of vacuum.

#5 calebz

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Posted 08 August 2004 - 10:08 PM

Yeah ive got an 86 gl turbo coupe. When im on the gas the boost guage reads +6-7 but when i let off it creats about 20psi of vacuum.



Holy ****!! 20 psi of vacuum?!!>>


to the original poster. on the EA82 cars, there is a little white canister on the passenger side behind the strut tower. Thats what th eheater controls and the 4wd(or difflock) controls run off of. Not only does it keep a small reserve built up in the canister, it has a check valve built into the feed line to stop it from going in to positive pressure.

#6 TheMeatWagon

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Posted 08 August 2004 - 10:16 PM

aye my little needles flips around to the 20 on the vac side when i let off the gas and coast.

#7 calebz

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Posted 08 August 2004 - 10:20 PM

ahh.. so maybe 20 in/hg?

#8 myossfeece

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Posted 09 August 2004 - 01:31 AM

yeah I have a vacuum gauge in my loy-a but before I had the loy-a I picked it up on the raod trip to rescue trashwagon 5, and installed it in the rx. when I let of the gas it would make its way to 20-22pounds of vacuum and when at highway speed pull a downshift to 3rd it would peg out at 25 pounds of vacuum.

the only engines I know of that don't create intake manifold vacuum are diesel engines being as they don't have a throttle plate. Or a forced injection engine(turbo, supercharged) under boost.

#9 calebz

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Posted 09 August 2004 - 01:56 AM

Repeat after me

In/Hg

In/Hg

In/Hg




in-Hg

Inches of mercury. One inch Hg is the pressure exerted by a column of mercury one inch high.

#10 iluvdrt

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Posted 09 August 2004 - 07:49 AM

A little o/t here, but does a N/A car's vacume zero out when applying the gas like a turbo? Or does it pull more vacume, or stay the same? I was just wondering because my sube sits at 25 in/hg, at idle, and under boost it reaches 7 psi. Now if it didn't have the turbo and the vacume stayed the same, than wouldn't the turbo be creating 31 psi in order to bring it from 25in/hg to 7psi boost?Does this make sence to anyone?

#11 calebz

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Posted 09 August 2004 - 08:18 AM

Now if it didn't have the turbo and the vacume stayed the same, than wouldn't the turbo be creating 31 psi in order to bring it from 25in/hg to 7psi boost?Does this make sence to anyone?

No. first off, in/hg does not equal psi

Second, on an N/A ca, when accelerating, manifold vacuum reaches ~0 in/hg. Thus to reach 7psi, your turbo must put out ~7psi (not accounting for boost leaks or boost drop due to IC or anything like that)




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