what is the purpose of a vacuum canister

[BW Jones]

i've noticed that some vacuum brake conversions use a vacuum canister (reserve). what is its purpose and why don't most conversions use it?
thanks in advance for your input.

bw jones

 

[BlackMetal]

Stores vacuum for cars with big cams running low vacuum (under 14"). Supposedly they actually amplify the effect of the vacuum, giving you more assist than just having your booster hooked up to manifold vacuum alone.

 

[cool 84]

I think it should always be used. It stores vaccum which is important since our turbo cars don't always have vacuum available. If your check valve leaks and you're under boost and you have to suddenly hit the brakes, you may end up with a hard pedal and no assist.

With the small cams we usually run, idle vacuum is not an issue. With a 212-212 cam mine pulls 18".

I've heard stories of locals around here going up the grapevine and the cruise control (which is vacuum operated) quit working when it ran out of vacuum because the cars were in boost for so long.

 

[T Spool]

I used to use them in my V8 cars with big cams. Honestly, I never really noticed the difference with or without them. I doubt they are of any benefit on a mild Turbo Buick, but that's just my opinion

 

[turbofish38]

I think it should always be used. It stores vaccum which is important since our turbo cars don't always have vacuum available. If your check valve leaks and you're under boost and you have to suddenly hit the brakes, you may end up with a hard pedal and no assist.


I've heard stories of locals around here going up the grapevine and the cruise control (which is vacuum operated) quit working when it ran out of vacuum because the cars were in boost for so long.

I never understand statements like this. WhoTheHeck is cruising around under boost? Reguardless of the cam once the throttle blade is closed you have vacuum. It may not be 15 inches but you'll have enough. Especially if the motor is buzzing around 3-4K. It takes a split second for the vacuum to replinish itself. If you get in a panic situation just take your foot off the F'n gas pedal! I'm going to have to try the cruise under boost. That should be a scarey ride. Did you ever hit resume to pass a car? Same thing.

 

[Silver Fox]

I never understand statements like this. WhoTheHeck is cruising around under boost? Reguardless of the cam once the throttle blade is closed you have vacuum. It may not be 15 inches but you'll have enough. Especially if the motor is buzzing around 3-4K. It takes a split second for the vacuum to replinish itself. If you get in a panic situation just take your foot off the F'n gas pedal! I'm going to have to try the cruise under boost. That should be a scarey ride. Did you ever hit resume to pass a car? Same thing.

On some of the long grades we have out west (at 8000-9000 feet plus elevation), my TR has actually held about seven pounds of boost to maintain a steady speed. The grades are long and somewhat steep. Pretty neat watching the boost gauge go up while the speed remains fairly constant. I imagine at lower elevations with more dense air the TR would toss you back in the seat. I'm moving back to closer to sea level when my sentence with the Air Force is done!

 

[TURBOV8R]

S.Fox,no doubt on the boost on grade,Plus the noise the turbo makes!Never gets old!!

 

[cool 84]

I never understand statements like this. WhoTheHeck is cruising around under boost? Reguardless of the cam once the throttle blade is closed you have vacuum. It may not be 15 inches but you'll have enough. Especially if the motor is buzzing around 3-4K. It takes a split second for the vacuum to replinish itself. If you get in a panic situation just take your foot off the F'n gas pedal! I'm going to have to try the cruise under boost. That should be a scarey ride. Did you ever hit resume to pass a car? Same thing.

10 miles of a decent grade plus a stock GN in OD will pull boost the whole way. I really don't need to defend myself on this, it does happen. You need to try the Grapevine or Tehachapi on the 58. It's possible to be under boost for nearly 10 minutes at a time.

If you're under boost and your check vavle is leaking, you're pressurizing the booster so you have no brakes. You may get vacuum quickly as soon as you let off the gas but going from boost and hitting the brakes as quickly as you can, you're not going to have enough vacuum in the booster to do anything.

 

[cool 84]

I used to use them in my V8 cars with big cams. Honestly, I never really noticed the difference with or without them. I doubt they are of any benefit on a mild Turbo Buick, but that's just my opinion

Mild or modified, big cam, small cam is not the issue.