Predict Vacuum for Brakes
 

Hello all,

Is it possible to determine if a power assist device such as an electric pump or hydroboost like device will be required to support a power brake system?

I've installed wilwood discs on all 4 corners. I had initially considered going with a manual set up but I've had a change of heart. I would like to get away with a small power booster if possible.

Cam is a Comp Cams 248/254 @ .050, 618/638 lift 112+4 LSA . Displacement 522 and 340 heads.

When the motor was run in, it really had a pussy cat idle compared to what I had previously. Very smooth sounding but this is an area where I have no fundamental experience.

Thank you.

Is the engine in a situation where it can be started again? I assume it was on a run-in stand? Based on the camshaft, I am guessing you are right on the edge vacuum wise for vacuum power brake booster. I like 12" as an absolute minimum vacuum at idle. More comfortable with 14" HG. That cam on the wide lobe centers and the large displacement will be close, I think. But that's a guess. 5 seconds with the engine idling and a vacuum gauge, and you have an answer for sure!

I thank you. The motor was run up on a dyno. I'm deep in a bit of a complete car over-haul...brakes, trans, rear, TBI injection, motor...it's going to be a bit longer than expected before I can run it up again. While I have the motor out I was going to address replumbing of the brake lines and figured while at it would be easier to address any vacuum assist requirements while the motor is out. I really didn't think this through. I should have recorded or asked for this information when the motor was on the dyno. I thank you.

All is not lost. Look at your dyno sheets if you have them. A common item to monitor is manifold vacuum on the dyno. There may be a line of data showing the vacuum at idle possibly of they were recording at that time. Possibly the dyno operator may remember what it was while they had it running.

For what it is worth - even after redoing the OEM (disc front drum rear) I was not comfortable with the braking, both the system itself (just an opinion) and the low vacuum that was on the edge for power brakes. I did the cannister thing no luck, then a CVR vacuum pump which helped. Then I swapped to Wilwood discs at all four corners, their master and proportioning valve. The Wilwood tech highly recommended AGAINST using my vacuum pump with their brake system for safety reasons. Using this as an opportunity to get rid of my huge OEM style brake booster I took their advice on considering a hydroboost. I have had the hydroboost in since Jan 2018 and the braking is incredible and way better than ever before. And tons of room for the taller valve covers I have to use. If you go that route I highly recommend professionally finished hose connections. I learned the hard way.

It also all depends on the booster. The 79-81 Trans Am 4WD dual diaphragm on my 78 worked down to about 10". Big solid cam at 9" would be iffy after burnout at first.

Hydroboost is a very powerful and reliable system. Some plumbing, and fab issues to get it all in there but I really like the feel and results as well. I work on a fair number of trucks with hydroboost. I can't remember ever having to replace one.

Blueprint Engines runs nearly that identical cam in their 540 crate engines, which are 10:1 and pump gas friendly. Same 248/254 at .050 on a 112 LSA.

They claim it to be fine for power brakes. If I remember right it makes around 12-13 inches of vacuum.

Depending on what ICL you install that cam at will change that a bit. Your compression ratio will also play a big roll in vacuum production. Without knowing either of those it's a guessing game, but honestly a 248 cam in an engine of that size on a 112 LSA isn't overly aggressive anyway. It should idle pretty nice and be very street friendly.

I'm running a 254 @ .050 in a little 302 with 11:1 compression and it runs the power brakes fine. Although it's a stick which helps. I had that cam done on a 112 LSA and installed it at 110 LSA per Comps recommendation. It makes 10 inches of vacuum way up here at 5000 feet elevation, and 13 inches down at sea level. It's not bad at all, we daily drive it.

I also run a 242 @ .050 cam in a 454. A step down from the 248 cam but the engine is also nearly 70 cubic inches smaller. That cam is on a 110 LSA installed at 106 ICL with 10:1 compression. It idles with 8 inches of vacuum a mile up and just over 11 inches at sea level. It runs the power brakes at 5000 feet with the assistance of a vacuum can but down at sea level the brakes are fine without.
Elevation plays a roll. Your tuning abilities also play a pretty big roll. The amount of initial idle timing you dial in will affect vacuum production dramatically, along with the carb idle circuits.

I thank you all...a bunch of good stuff to ponder.

I don't currently have a booster of any kind. I do have a Wilwood master and I believe it's a 7/16" diameter piston which was said to be sized for manual brakes. I understand that this may not be reusable with a power system and I'm ok with that.

There were a number of questions asked here about the engine setup/dyno info so I'll just put the dyno sheet down below. There is a "PanVac" Hg which I don't think is what's being asked for here. I did call the shop yesterday and asked if vacuum was something that was recorded unfortunately it was not. In talking with the builder he said it's hard to tell strictly from specs but as some have you have noted, the cam for a motor of this displacement is rather mild. It really did idle surprisingly smoothly. He thought it might have enough vacuum but also said it could be marginal...couldn't tell until it was measured.

I know now that this is a stretch of a question to ask.

I'll just post the dyno sheet here. There's a few things on here I'm not sure what they mean to be honest.

I think 7/16" will be too small for any manual brakes, never mind power brakes. Manual four wheel drum brakes on first gen Fbirds were 1" and power brakes for disc were 1-1/8" bore. I used a 1" bore Wilwood master on my brother's car with GM D52 front disc which work well.

Looking like 11-13" is your range, I used to be 11" and it was horrible, what a mistake and made driving a bit more sketchy than it needed to be.

I'll never run factory vacuum for brakes again, I've got a 20" vacuum Hella Pump on my GTO and it stops incredible. There is a Leed vacuum pump going on my GN and that makes 24-25" and this makes an insane difference in booster psi.

2ft of stopping distance can be the difference between big bucks or thanking yourself for buying a killer vacuum setup with the saved money from the accident you just missed being in.

I thank you, I'm just going to suck it up. It's going to be much easier to work on now rather than later when the motor is in as I do need to re-run some brake lines anyways and having a vacuum assist is like an insurance policy at this point.

Thanks all, the push over the edge that I was needing.

Running a 246/252 @ 112 in a 517 but have manual brakes. In gear idling at 800 it only pulls around 9”. It will pull 11-12 in neutral but that doesn’t really matter on the road.
Probably enough on the open road for power brakes but being stuck in stop and go traffic would be tough.

I thank you. Even further validation I should just go the route of power assist.

If you bought another brake pedal and cut the pedal off one and sister the two together and lower the brake pedal 2". This really changes things with the manual set up. I like a 8:1 ratio.