Vacuum assist and big cams

[pureadrenalin]

Sorry, this turned out longer than I expected.

I’ve only ever given this a small amount of attention, so I thought it was about time to ask others opinion/thoughts on the matter.

I know that when installing bigger duration cams, the vacuum they produce at idle is less than standard cams. Hence this getting put to the back of my head as I’m still running standard cams, but, it came rushing back to me on my warm up lap of race one at SS2000.
Scenario goes like this; I’m on the loud pedal going down the back strait, and I step on the brakes to warm them up a little (while still giving it the gas), let off the brakes, step on the brakes, let off the brakes, step back on, and I have a very hard pedal, and I hardly slowed down at all.
Instant thought is “ahh, crap, what’s happened to the brakes” as the adrenaline kicks in as you are thinking of how to slow the car down.
I take my foot off the accelerator and cruise down the strait (I still had a few hundred metres to go), getting nearer the corner, I step back on the brakes, and wallah, there they are again.
Great, time to go racing, so I didn’t give it another thought till after the race, when it hit me, “ahh, dumbass, you ran out of vacuum assist”

So, lesson for all you left foot brake’rs who still trail on the accelerator pedal, you have two, maybe three stops (in an SW20 turbo) in which time you better get back into some intake manifold vacuum, or your going to get a shock at the third/fourth corner!

I’ve seen a bigger “reservoir” that someone ran in an MR2 in the states (jeklyandhyde?) as one solution, but does anyone really have a problem with this while racing?

Guys with bigger cams, do you find that there is a lack of vacuum assist to start with?
If so, how have you solved this?

I personally don’t have a problem with the lack of assist at the moment (still on stock cams) or the duration of stops (two/three) because I’m always in vacuum while braking, so “recharge” the system for the next corner each time.

Opinion/thoughts?

[AE92blktop]

would it be possible to use a diesel alternator with a vac pump on the back of it? or some other driven pump to assist the vaccum from the manifold? I figure it could work but have no idea how much effort would be involved

[pc]

I would assume that the one way valve on the brake booster wasn't working correctly, leaking etc.

[rollaholic]

the one way valve only helps vac created by throttle closed running from being replaced with pressure when the throttle is opened, it cant magically create vac when the inlet manifold is full of boost

[RedMist]

Large older american 8's had the same problem. They have an electronic vacume assist pump. I've actually got two of them sitting here that I imported for use as scavenge pumps.
The other alternative is to do some gym work. I hear real men can pull up a 1100kg car with 6 pot rears and 4 pot fronts with no assist... at all... ever.

[matt dunn]

I wouldn't think that you will have problems at the track as you wont run anti-lag,

and you should be high in the rev's and come right off the throttle for the corners, so the sheer rev's with the throttle closed will create a large amount of vacuum, even with aftermarket cams,

[pureadrenalin]

ahh, I found the thread on MR2OC I was looking for.
This is the vacuum bottle some of the guys run



from summit racing

http://store.summitracing.com/partdetail.asp?autofilter=1&Ntt=crane+cams+vacuum&N=4294922533&part=CRN%2D99590%2D1&autoview=sku&Ntk=KeywordSearch

[Bazda]

I dont find I have this issue at all. As I dont left foot brake. Not good for the brakes.

I dont see why you would need to left foot brake unless you have a sequential gear box. Or your foot is really slow

[matt dunn]

I dont find I have this issue at all. As I dont left foot brake. Not good for the brakes.

I dont see why you would need to left foot brake unless you have a sequential gear box. Or your foot is really slow

I agree, and dont even see a use for it with sequential's.
Left foot braking is for rally cars and rolling starts,
Dont really see a point of it on the circuit under normal circumstances.

[85AW20v]

Yep - I don't have a problem with no vacuum assist either as I don't left foot brake when racing and my right foot is hard on the throttle or hard on the brake. I do know what you're talking about though as during warmup I'm left foot braking to get some heat into the brakes and tyres and experience the same thing.

[QikStarlie]

running 310deg cams in my 4age with quad throttles, has plenty enough vac for the brake booster