How much vacuum do you need?

[mungus]

Just wanting a few views on how much idle vacuum one needs on a 68' with a rebuilt Midland brake Booster? Not factory AC, manual.

Would 10" Hg be enough, or 12"? etc. From my reading it seems every car is different. Some folks saying their cars are OK with as little as 8" and others making general statements saying you need 14. So it seems the practical figure varies somewhat.

I know there are factors like how hard a pedal can you cop and how often do you use the brakes at idle etc. But practically speaking what sort of vacuum have folks here found is required for a decent, reliable pedal on a 67-70?

I am asking because I will soon start testing my engine on the stand, and once tuned I'll see what idle vacuum I get. But as many of us here will have warmer cams, whats the vacuum experience?

Cam is a COMP XR280HR roller. So mid range: 110 LSA, 65 degrees OL and noticeable idle. Some folks on the web searches say that 11-13" Hg at idle is typical. (but even then what is their "idle", 750,800,900?). Others say its lower and one guy says its higher.

I see COMP CAMS sells a vacuum canister claiming its for anything with less than 14"...
So to 67-69 Mustang folks is that a fair figure or sales BS?

Thanks in advance.

[nassi]

will your non factory A/C still require vacuum?

[mungus]

Don't think so. Its a new "Perfect Fit" system from Classic Auto Air. The controls are electronic. Only vacuum service is the brakes.

Over on the FORD FE tech forum I asked how much vacuum my cam will likely give, and a professional / known FE builder reckoned my cam makes 15" of vacuum. That surprised me given that another COMP cam with 110 LSA and 46 OL showed about 14" at 850 RPM, and some videos with the XR282HR (same specs as the 280), showed barely 10”. The 280HR is not a big toe cam but its not a smooth idling RV/4x4 cam either, with 65 degrees of OL and a very noticeable idle. I suppose he could be right? Lucky me if so. But my hunch is he's wrong on this one. I'm thinking 10-11" is all I'll get.

All a bit frustrating as I sit in my BNE hotel room, unable to go home and finish my engine test stand, thanks to the COVID lockdowns, etc.

[ozbilt]

Non factory a/c will not need a vacuum supply.

[Shaunp]

worst case you put pump on them.

[mungus]

Sure thing, or maybe a canister, being simpler and cheaper. Big variety in prices and models of pumps. From 2nd hand euro car pumps to new US made, $100 up to $500+. I'm aiming for a period correct 68' engine bay, so if I have to add something I'll have to hide it somewhere, up under the LHS fender I guess.

But I'd rather not fit either if I have enough vacuum to run the brakes. That's why I'm asking folks with 67-70 cars what they found was needed to make it work well enough. Plenty of us have warm cams, and vacuum gauges are common enough, so some folks must know?

Perhaps being newly rebuilt, my booster can hold a bit of vacuum for my country driving. But we'll see.

[nassi]

if you do need a canister you could use the factory A/C one perhaps.

[mungus]

No idea of their specs. The brake ones are set to around 18-20”.

[Shaunp]

I have factory AC, when I did the RHD conversion I put the canister up in under the LH front guard, you cant see it.

[mungus]

Thanks Shaun. If needed I'll compare them. As I don't have factory AC, originality isn't a big issue. New aftermarket brake vacuum cans are pretty cheap, half the price of a little OEM AC can. All the aftermarket cans look the same, as far as I can see the only difference is whether or not it comes with a cam makers sticker on it. (doubles the price).

Just checking the US concourse site, the 67-70 OEM AC vacuum cans, mounted on their side on the RHS inner fender, look odd anyway and I trust the brake canister fittings and non return valve more than the AC cans one.

Some of the OEM AC cans I've seen do make me laugh a bit though. They remind me of a big tin of peaches, but with a non return valve plonked on top...

[nassi]

Some of the OEM AC cans I've seen do make me laugh a bit though. They remind me of a big tin of peaches, but with a non return valve plonked on top...

Yep, ugly as sin but if you want the factory look.......

[ozbilt]

Am I missing something here? The only thing that this car will need vacuum for is the brakes. The aftermarket a/c does not use a vacuum feed.

So why all the discussion about a canister? It will not hold enough to be a backup for the booster.

[mungus]

You are correct in that the only heavy vacuum service is the brakes. The vacuum reservoir was being discussed as an option vs. a pump. I have no interest in using a little factory AC can. Someone suggested that for aesthetics...

I saw web comments elsewhere, saying cans don't work because they don't produce vacuum. Logic says they wouldn't sell if that was true, (being specifically marketed for up-cammed cars). But as I thought, the can's non return valve holds its volume at the max vac it experiences, (well above idle vac). And as the can holds more volume than the booster, AND it drags up the boosters own stored vacuum, it lasts a few brake applications. There's a few web threads and YouTube video's showing folks with low idle vacuum / lumpy cams where a canister fixed their pedals.

I'm guessing a pump is typically used because space is limited. Or if you do so much stop-start city driving that even adding a can isn't enough. Amazon has electric pump kits that aren't too expensive, perhaps they are a better option?
Anyone have a view on that?

https://www.amazon.com.au/Dracarys-Elec ... 9493&psc=1

[Nuts]

Try it first before you spend. I'm running a stock bendix booster. These normally don't retain a vacuum with engine off. I have 10in of vacuum and was running out of brakes before the engine warmed up. Fitted one of those el cheapo vacuum tanks under the fender and my brakes are fine. Every car is different. Wait until you get it running then see what you need, if anything..

[mungus]

Try it first before you spend. I'm running a stock bendix booster. These normally don't retain a vacuum with engine off. I have 10in of vacuum and was running out of brakes before the engine warmed up. Fitted one of those el cheapo vacuum tanks under the fender and my brakes are fine. Every car is different. Wait until you get it running then see what you need, if anything..

Thanks Nuts, I'll probably do that. As I live in country VIC I likely won't be doing much low speed / stop-start driving, so who knows? With my booster being fully reconditioned, and a Midland (provides more boost they say), it might be OK as is...

Going on what posters on the FE forum have said, 10-12" is about what my cam will make. I really wish I could go home and finish my test stand, run the motor and see what vacuum it shows. (damn corona virus work roster). If needed, I prefer the idea of a can. No pump noise or wiring, and cheaper. And it seems they can be hidden under the fender OK. Any chance of a photo of your install?

Just wondering about yours though. I watched a video from West Coast Cougars, where he visits a specialist classic brake booster restorer in Portland. He had a pile of old Mustang Bendix and Midland boosters, makes new "unavailable" Bakelite booster parts and even supplies those Midland rebuild kits, sold around the world. So I guess he knows a bit about our boosters... I remember he said that a good booster can hold vacuum for weeks, even months. Those that didn't possibly had a dud diaphragm, NR valve grommet or NR valve. Just a thought. Its at about 14:50 into the video.

https://youtu.be/PGO0aWJkrmo