brake booster selection

**STR8 2.8** · 26-09-2006, 02:58 PM

im looking at a new brake booster that says it needs 18" of vacuum to work properly.
now if my 7mge makes 17.32" at idle, the booster should work ok shouldn't it?

stupid question, but with an NA engine, vacuum increases with revs? or decreases? i cant get my head around it...

Cheers,
Chris

**ViPeR_NiPPleX** · 26-09-2006, 03:00 PM

just about any time the throttle is closed you are generating negative manifold pressure aka vacuum.... 17.32" close enough at a guess

**jezza323** · 26-09-2006, 03:02 PM

decreases as the throttle opens

if u have heaps of revs and snap the throttle shut u will have a whole lotta vacuum!

**STR8 2.8** · 26-09-2006, 03:02 PM

under decelleration, the vacuum should be greater than at idle shouldnt it, so it should be sweet...i think

ah i think i get it now... the engine is trying to suck in more air than the throttle allows hence you end up with a vacuum...right?

**bobob** · 26-09-2006, 03:05 PM

Vacuum is caused by pressure difference, between the manifold and the atmosphere. So at wide open throttle there would be very little difference and less vacuum.

No idea if 17.32 is enough, one could guess so, it gets held / stored doesnt it? What would be the effect of not enough vacuum, less assistance with the braking?

**STR8 2.8** · 26-09-2006, 03:08 PM

yeh less assistance.
but if the vacuum at idle is 17.32 then it should be ok, because im braking, then the throttle will be shut 99% of the time, with revs higher than idle, hence it should be making more vacuum than 17.32". am i on the right track here?

**jezza323** · 26-09-2006, 03:11 PM

you are on the right track!

**bobob** · 26-09-2006, 03:12 PM

Exactly, like if you were going down a hill in a low gear, throttle shut but the revs high. The engine would be demanding more air than at idle, but with the same 'restriction' of the throttle preventing it it would be a higher vacuum.

Where are you getting the booster from maybe they have an expert you can ask, I guess they will say 18" is the spec. How good is your braking foot?

**STR8 2.8** · 26-09-2006, 03:16 PM

Originally Posted by bobob

Where are you getting the booster from maybe they have an expert you can ask, I guess they will say 18" is the spec. How good is your braking foot?

yeh ill see if teh seller can help me out.
i was looking at this or something similar if i can find it locally.
http://cgi.ebay.com/ebaymotors/ws/eB...RK%3AMEWA%3AIT
the price for this seems very cheap for a new booster & master, i wonder if its dodgy.

lol my braking foot? id prefer not to get leg cramps after a trip to the shops.

**bobob** · 26-09-2006, 03:24 PM

They look nice!

From their site at http://www.piratejack.net/troublesho...oostersFAQ.htm

How much vacuum is needed to operate a booster properly?

For a power booster to function properly you will need at least 18" of vacuum. Anything lower will give you a hard pedal.

There are more questions / answers along the same line on that page, you can email them [email protected] and ask them I guess.

**takai** · 26-09-2006, 03:35 PM

Using miniscule amounts less of vacuum wont create a massively hard pedal. But 18inHg is a pretty normal vaccum reading.

**jonra23** · 26-09-2006, 03:38 PM

A booster should have a one way valve plumbed into the supply line, the 18" of vacuum does not have to be happening when you have your foot on the brake, if you are making around 17.32" at idle you will be making a heap more whenever you take your foot off the throttle.

Worst case scenario is you never see more than 17.3" of vacuum means your brake pedal will be 4% harder than if it had 18" of vacuum.

regards
jon

**STR8 2.8** · 26-09-2006, 04:11 PM

ok thanks heaps to everyone!! i guess if shipping isnt too painful i might get it.

**M.J.H** · 26-09-2006, 08:52 PM

Originally Posted by STR8 2.8

yeh ill see if teh seller can help me out.
i was looking at this or something similar if i can find it locally.
http://cgi.ebay.com/ebaymotors/ws/eB...RK%3AMEWA%3AIT
the price for this seems very cheap for a new booster & master, i wonder if its dodgy.

lol my braking foot? id prefer not to get leg cramps after a trip to the shops.

What sort of brakes are you looking to use that with?

That looks like a good deal BUT (and I could be wrong on this) but won't the size of the booster also affect the pedal effort?

For example you surely you couldn't run a small Celica bosster on say a 123 Crown and expect the same pedal feel could you?

EDIT If you want one that small the have a look at a 180B or 1600 Datto as they were similar in size from memory.

**mrshin** · 27-09-2006, 07:48 PM

Then again, many cars have a pedal that is too soft from factory (for my liking, anyway.) They are either over-boosted, or have an under sized master cylinder.