ITB and Vacuum

Magpie · Postby **Magpie** » Fri Jul 25, 2014 6:50 pm

Data has now been collected regarding how much vacuum is being produced by the ITB's and is it sufficient for the brake booster or is there a vacuum tank needed.

For a NA6 with stock master/booster 44 lbs of force on the pedal and no engine vacuum, line pressure should be between 156-171 psi. With 19.7 in of vacuum and the same pedal force, the line pressure should be 754-796.

Whilst the cams being used will effect the amount of vacuum, at least for me on the road the ITB's are producing more than enough for the brake booster. I will collect some track data but it should not be any different, in fact I suspect that it will be better due to the changing RPM's

At idle (yellow = TPMS, green = brake pressure, black = speed, purple = RPM, pink at the top is vacuum)

ITB Vacuum (1) on Flickr

Acceleration

ITB Vacuum (2), on Flickr

At cruise

ITB Vacuum (3) on Flickr

Light braking

ITB Vacuum (4) on Flickr

sailaholic · Postby **sailaholic** » Fri Jul 25, 2014 9:35 pm

Nothing like some good hard data.

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gslender · Postby **gslender** » Sat Jul 26, 2014 8:29 am

Then why did you say before you didn't? Or did I misread that?

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Magpie · Postby **Magpie** » Sat Jul 26, 2014 8:40 am

I thought that I did not have sufficient vacuum and hence the discussion on booster delete. If there was no sufficient vacuum from the ITB's then the booster will not function as designed. However my foot was confirming that the booster was functioning as designed. The data just confirms what I thought, no need for a booster delete or a vacuum box!

I prefer to backup my statements with facts and then put the facts out there to let others make their own decision. Nothing like in this instance proving that a proposed course of action was wrong and I saved myself some headaches as well as money!

I'll post some data after the next time attack round. Plan will be next to monitor the vacuum between/at the booster and how quickly the vacuum is replenished/used.

gslender · Postby **gslender** » Sat Jul 26, 2014 10:02 am

No problems. When I installed the ITBs and got the engine started, the first thing I looked at was if I was getting less than 50kpa at the vacuum port, and I was, so I was fairly confident it should be fine. The stock car idles with around 35-40 kpa so anything close to that should be fine.

Normally, it is cams that impacts idle vacuum (and therefore brakes).

To test, I adjusted my cams to have significant overlap, and the kpa rose to 65kpa, so I'm thinking depending on cams you can have some issues.... but even with that low vacuum it still seems to be assisted enough (so volume is probably more a factor than vacuum pressure).

I believe the issue with ITBs is more often where or how to grab a combined vacuum line from 4 manifolds, and with the simple approach being to drill/tap into a single throat and therefore you get 1/4 of the available volume, which is probably where the reputation comes from.

I've made sure I've got decent sized 1/8" ID air ports from all four low in the manifold as close to the valves as I can. Therefore I think I've got good volume and so far the feel under the pedal is the same as stock (when compared to the other na6 I have).

G

Magpie · Postby **Magpie** » Sat Jul 26, 2014 11:19 am

Units of measure are important here

35-40 kpa = 19 in mercury = 10 in merc absolute = 70% vacuum (all approx)

gslender · Postby **gslender** » Sat Jul 26, 2014 12:36 pm

Magpie wrote:Units of measure are important here

35-40 kpa = 19 in mercury = 10 in merc absolute = 70% vacuum (all approx)

I think 70% is a bit generous... in the range above it is exactly 61-66 %

Vacuum is relative the atmos pressure, and at sea level, it is 101 kpa. So 35 kpa would be 66 kpa of atmosphere pressure trying to rush air into the vessel that contains a vacuum, but yeah I'm probably splitting hairs....

G