How much vacuum needed?

[we4Mateo]

How much vacuum is needed to run vacuum brakes? My current 208/208 FT cam seems to work well yet, but I'm a little worried about my next build. Anyone running a 212/212 roller cam with vacuum brakes? 9:1 compression should help draw a little more.

 

[CGASTON]

I am running 224/224 with 10:1 and have a lot of brakes!

 

[jpwalt1987]

I have run a 224/224 and now a 218/218 cam on vacuum brakes and they work great.

 

[we4Mateo]

Thanks guys.

 

[bison]

The overlap is what determines the vacuum on a given engine.

 

[we4Mateo]

That makes sense. That's determined by lsa? What other factors?

 

[bison]

That makes sense. That's determined by lsa? What other factors?

Displacement and rpm

 

[we4Mateo]

Displacement and rpm

What other cam factors affect vacuum? Or is it just lsa? If so, does a higher lsa produce more vacuum?

 

[we4Mateo]

I apologize about by lack of knowledge in this area. Cams are like converters for me; lots of voodoo inside.

 

[tonka]

FWIW, my 214/214 with 9.5 to 1, idles with 17 inches in gear. From searching in the past, 15 inches is considered by some to be a minimum for vacuum brakes, but have had them work fine with 13 inches idling in gear. Your results may vary....

 

[bison]

What other cam factors affect vacuum? Or is it just lsa? If so, does a higher lsa produce more vacuum?

With the same lobes it will. There are a lot of influencing factors.

 

[we4Mateo]

With the same lobes it will. There are a lot of influencing factors.

I'll just ask... I obviously want good vacuum, roughly a 210ish duration with stock rockers per previous information I've gleemed from you in other threads , roller, hoping to trap 130mph with a 6266 and I'm not afraid of 30psi, ported stock irons, like to stay around 5600rpm max with converter locked in 3rd and a tall tire, but other than that I'm stuck. What lobe lift and lsa would you recommend for a street car w/ vacuum brakes at this duration and expectations? Even possible? TIA

 

[we4Mateo]

ttt

 

[V6UnderPressure]

I was actually wondering the same thing about this. I keep reading about all these other hot rods and how they need some SSBC vacuum pump etc. to run vacuum brakes. Then you see all sorts of threads on curing that problem with a hydroboost. I always wondered what is the limit before vacuum brakes on TR's become useless. I see all sorts of back and forth arguments on which is better (vacuum supposedly can't hold boost). That portion I am not worried about as I will be setup for for autox/pro-touring. But it also would be nice to know if I would require one of those cheesy vacuum pumps. If I did, I would just recoup the money and buy a Hydroboost. Can anyone shed some light on this?

 

[earlbrown]

Keep in mind normally before braking there's a throttle chop before hitting the brakes. Under those conditions vacuum goes through the roof. As long as the check valve at the vacuum diaphram is working, you should be fine. (that's why you can get one or two working pumps out of brakes when the engine is off)

 

[we4Mateo]

Keep in mind normally before braking there's a throttle chop before hitting the brakes. Under those conditions vacuum goes through the roof. As long as the check valve at the vacuum diaphram is working, you should be fine. (that's why you can get one or two working pumps out of brakes when the engine is off)

I use two checkballs for this reason; vacuum in, boost out, gtg.

 

[Nick Micale]

I was actually wondering the same thing about this. I keep reading about all these other hot rods and how they need some SSBC vacuum pump etc. to run vacuum brakes. Then you see all sorts of threads on curing that problem with a hydroboost. I always wondered what is the limit before vacuum brakes on TR's become useless. I see all sorts of back and forth arguments on which is better (vacuum supposedly can't hold boost). ..........

First of all, vacuum does NOT hold against boost, the master cylinder, wheels cylinders, tires, road surface all hold against boost, the vacuum only provides an assist to your leg to push the pedal.

Sorry to be off topic on Matt's thread, but most people are off the gas pedal and in vacuum, not boost, when applying the brakes, either just stopping or just before building boost.

I know of many 9 sec. TR's that use a vacuum brake booster, but more cars that use a manual master cylinder with no assist for simplicity reasons.

 

[Joeld]

I've read 15 inches as well as min. After reading much about Manual as I was thinking of going that way, there are many variables that are said to have to come together for manual to be effective, including swapping out calipers and the like to balance master cyl piston and volume with caliper piston and volume to get enough breaking power out of manual. I went with the vacuum as I did not want to mess with calipers. But my motor is stock so I don't expect any issues with building vacuum.
Joel

 

[V6UnderPressure]

I completely agree that you are off the throttle when braking and that it shouldnt be an issue. Now one thing I am not is a cam guru. Does vacuum shoot through the roof when off the throttle with a big cam?

 

[earlbrown]

Vacuum always shoots through the roof during engine braking. I'ts a byproduct of the throttle blade being closed and the engine is spinning fast.