Brake vacuum

[forestb]

I have front disks, rear drums with power brakes and an automatic transmission I recently installed a new motor with a more lumpy cam. I checked my vacuum at the hose that comes out of the back of the carb and at idle it fluctuates between 10 and 12. When I drive around normal it works fine but my parents house has a vary steep driveway and when I am backing out I am not able to bring it to a full stop until I hit the bottom. Is that enough vacuum. If I remember correctly 12 is a good amount.

[Steeveedee]

You mean at the bottom of the driveway, not the pedal goes to the floor (bottoming), right? Is your foot constantly on the brake pedal, or are you on and off of it? With low vacuum, the engine may not be able to pull it fast enough and you can lose the assist if you are pumping it, even a little.

[forestb]

Should I assume that you feel that 10 to 12 is not enough vacuum?

I meant when I hit the bottom of the driveway. I am holding it constant not pumping it. The break peddle is bottoming out at this point but not on the floor it just hits a hard spot.

[forestb]

So I just saw on the summit website that it should read around 16-18.

[Steeveedee]

So I guess your vacuum is too low for that booster. You may have to install a vacuum pump. That must be one rumpty cam. Is the booster single or double diaphragm?

[geezer#99]

14hg of vacuum should be enough.
You need to increase your vacuum.
A couple ways to do it.
A vacuum pump or fine tuning your carb and timing.
If you close your throttle completely your vacuum will increase.
Then to get it to idle you’ll need to increase your initial timing.
What carb?
What’s your initial timing at?
Where’s your vac advance connected to?

[forestb]

My engine is currently detuned for the breaking in process. I need to go another 200 miles before I can adjust the timing for better performance. It sounds like that might help increase vacuum.

[geezer#99]

Originally Posted by forestb

My engine is currently detuned for the breaking in process. I need to go another 200 miles before I can adjust the timing for better performance. It sounds like that might help increase vacuum.

Have you finished the breaking in and fine tuned your motor with more initial timing?

[Gconroy]

You can use manual brakes just fine. I was running disk in the front and drums in the back with 15 inch rims but changed to 20 all the way around and now it will not stop at all in a panic stop. I have an aggressive cam with 8 inches of vacuum and a vacuum power brake booster calls for 14 inches. So I purchased a right stuff hydro boost kit to cure the stopping problem. Few more days until it gets here.

[forestb]

Originally Posted by geezer#99

Have you finished the breaking in and fine tuned your motor with more initial timing?

Yes and it didn’t improve the vacuum.

[forestb]

I checked my timing today but made the mistake of forgetting to plug the vacuum advance.

16 degrees of initial timing.

33 at 1500
Around 55 at 2500
Blueprint recommends 32-34 at 3500 (or at the point the mechanical timing stops advancing) for their engines. I didn’t feel comfortable taking it up to 3500 rpm

It was running at 600 rpm at idle which seems a little low so I bumped it up to 7.

The brakes are so close to working good. I think the semi metallic brake pads and boots with a vacuum canister will solve the problem.

[geezer#99]

Originally Posted by forestb

I checked my timing today but made the mistake of forgetting to plug the vacuum advance.

16 degrees of initial timing.

33 at 1500
Around 55 at 2500
Blueprint recommends 32-34 at 3500 (or at the point the mechanical timing stops advancing) for their engines. I didn’t feel comfortable taking it up to 3500 rpm

It was running at 600 rpm at idle which seems a little low so I bumped it up to 7.

The brakes are so close to working good. I think the semi metallic brake pads and boots with a vacuum canister will solve the problem.

So you basically have low initial timing combined with a higher idle setting both of which reduce your idle vacuum.

New pads and brake shoes won’t change how your brakes function.You still won’t have enough vacuum for your booster.

[geezer#99]

How much vacuum do you have at 1000 rpm?

[MT65]

According to the people at Master Power Brakes, 18” at idle is the number you need, can you get by with less…sometimes…..

[Steeveedee]

Originally Posted by geezer#99

How much vacuum do you have at 1000 rpm?

Right? Maybe leave it in neutral and coast down that steep driveway? I've never run a big enough cam that I didn't have vacuum-assisted power brakes work properly. Though now that I think on it a bit, those cars didn't have power brakes.

I know! Spin a doughnut and head down hill going forward! What could go wrong? My dad used to get so ticked off if one of us burned rubber in the driveway or the street.

[geezer#99]

In neutral or just hot foot it.
Hot foot it still gives you control if you need it.

[forestb]

Going down in neutral sounds like a good idea. It should help. It is a little dangerous. I will check to see what the vacuum is at 1000 rpm next weekend. My tools are not in the same place that I live.

[rbruno68]

What motor are you running? What are some of the specs of the cam and motor?

[forestb]

Originally Posted by rbruno68

What motor are you running? What are some of the specs of the cam and motor?

This is the motor that I am running.

https://blueprintengines.com/product...ads-roller-cam

[Cohi]

Originally Posted by forestb

This is the motor that I am running.

https://blueprintengines.com/product...ads-roller-cam

Im currently running the same engine at 5280 ft. I couldn't get more than 11 out of it when running a carb and ended up adding a pump.

[forestb]

Originally Posted by Cohi

Im currently running the same engine at 5280 ft. I couldn't get more than 11 out of it when running a carb and ended up adding a pump.

How much does elevation make a difference. I am at around 607’

[Steeveedee]

Elevation variation will yield less boost when the atmospheric pressure is lower, so at your elevation, you're pretty much getting the full benefit of that. Something else must be going on with your braking system.

[Cohi]

Originally Posted by forestb

How much does elevation make a difference. I am at around 607’

Not sure on the vac. I was getting 11inches with a 600cfm holley (from blueprint) on the same motor you have.

I believe I lose a 16% loss in HP due to elevation.

Best, Jon

[vin63]

This is the meme I used to send to my friends who insisted I run a vacuum booster on my build, since I'm still thinking of putting a 6-71 supercharger on mine:

In all seriousness, I would probably do away with the vacuum booster if you plan to keep your engine. It's those low speed, in-traffic situations that you'll need the immediate brake response the most. I run a Wilwood MC (no booster) and their 6-piston caliper up front with 4-piston Mark Williams disc brakes out back. Driving in our SoCal traffic, I'm thankful for the instantaneous brake response.

[forestb]

"do away with the vacuum booster if you plan to keep your engine".

Do you mean that using a booster will damage the engine or do you mean that I will have to make the decision between my brakes working (while using a booster) or changing the engine?

I think my next step is to wait and see how much vacuum I end up having after the engine is properly tuned and then start changing things if I am still not getting enough vacuum.