1956 322 vacuum

[Guest imported_NikeAjax]

How many inches of vacuum should I be pulling on a '56 322? I just rebuilt the entire engine and am getting between 16 and 17-inches. Shouldn't I be getting more like 21?!?!?!?!?!?!?

Thanks,

Jaybird

[Judd]

You are right on spec. the Manual says 15.5 to 16.5 at 450RPM.

Judd

[Guest Mr. Solutions]

And while we're on this subject, but relating to a different car...

How many inches of vacuum is normal for a '51 Buick Special 263? It is being rebuilt starting this weekend...

Thanks

[NTX5467]

There are some engines which, by design and related issues, might not achieve the "ideal" 18" of vacuum at idle. Usually, if you can get it to a "maximum" value for your engine, that's all it will do (at idle). That value can vary due to several outside influences too, like altitude. Certain levels might be related to particular engine families too.

If you find some of the Motor Repair Manuals (or similar) from the 1950s or 1960s, it'll probably have a section on using a vacuum gauge for diagnostics and also give you a range of values for particular operational situations. In reality, a vacuum gauge is more of a diagnostic tool than a generator of specific or absolute values (as a timing light or tachometer would be).

Usually, if you aim for the maximum idle vaccum level for your engine, it should be at or slightly below 18" of Mercury. If you then move the throttle to obtain a higher rpm, say about 2000 rpm or so, and then snap the throttle closed, that's when you'll see the 21" of Mercury reading on the gauge or during periods of closed throttle coast down.

As for the maximum idle vacuum situation, first make sure the spark timing is at the baseline specification and specified rpm. Then adjust the idle mixture screws to their "best lean" setting, usually maximum rpm but just as the idle vaccuum starts to decrease from the highest level. Then fine tune the idle speed and mixture to the final specified values and that should also still be the highest vacuum level too, or extremely close to it. For good measure, you can also step to the back of the car and smell the exhaust for excessive hydrocarbon emissions--it should be pretty neutral in smell if things are "right" also.

Usually, the stock configuration engines that will produce the highest manifold vacuums at idle will also have the most "low speed oriented" camshafts and smallest carburetors (with respect to engine size). As I understand, the earlier Buick V-8s used a little more camshaft duration and then used a little smaller than normal intake valve to build an engine with better torque characteristics and performance over a broader rpm range than might have been "usual" for engines of their sizes.

So, use the manifold vacuum specs as a guide for values to aim for. As that reading is the result of a "pressure bias" between atmospheric pressure and what's in the intake manifold (higher and lower, respectively), altitude and probably barometric pressure can affect the readings at a particular location. Figure production differences into the mix and it could result in possibly a lower or higher value than the factory spec that was mentioned.

As simple as it is, a vacuum gauge can be a really good diagnostic tool for tuning an engine and seeing if everything's in good condition internally. It worked for decades before electronics came into play.

Hope this might help,

NTX5467

[Guest imported_NikeAjax]

So the lazier the engine, the more vacuum; theoretically. The reason I ask is, I have after market power-brake system on my 1956 Century, and I need all the vacuum I can get. The guy who set it up for me told me I need 18-inches of vacuum to operate it. My Century is one of the rare, so I've been told, cars that came with the manual "brakes of death!" I've seen first hand how much these brakes truly-suck, and heard of the demise of more than a few state troopers who couldn't stop or slow down fast enough. I have put a reservoir between the manifold and power-brake unit to act as a vacuum-reserve, are there any more tricks I could use to ooch out more vacuum?!?!?!?!?!?

Thanks,

Jaybird

[NTX5467]

Maybe I'm incorrrect, but I suspect the "no stop" condition might have been more related to brake lining material/lining size issues that just the power brakes themselves. Basic braking performance is more related to the interaction of the brake lining with the brake drum than whether or not it has power brakes--unless the lining is so hard or has too much metallic content to work well when "cold". Maybe a brake remanufacturer could be of help in getting some different linings on the car that might work better?

Moroso makes an auxilary vacuum reservoir for applications where there is low manifold vacuum at idle. It's more for later model cars that someone has put a much bigger cam in the motor in and they still want to have operable power brakes. I'm not sure if that's what you already have or not.

On middle 1980s Oldsmobile 307s, they had a belt driven vacuum pump as those motors apparently ran such low manifold vacuum that they needed extra vacuum just to operate the a/c vents and such. Not sure if it could be adapted to a Buick V-8 application, though, without some major modifications. I know there are electric-driven air pumps for the emissions systems on late model Camaros and Olds Intrigues, but I don't know (or have looked about) similar electric-driven vacuum pumps. Got to be something as 4 cylinder motors used to not have very much manifold vacuum at all.

Back then, Power Brakes were more of an "effort reducer" than a performance enhancer. True, they might react quicker than manual brakes, but I doubt they would make great differences in stopping distances or capabilities per se. Also, the internal components in the booster might need some upgrading (reaction valving and such?) for potentially better performance (i.e., boost levels per inch of manifold vacuum). There are several remanufacturers of power brake boosters, but they usually come with a master cylinder attached for warranty purposes.

I suspect the suggestion of 18" of vacuum at idle to run the power brakes on that car was more of a guideline than anything else. Remember, too, that when you put the car in gear (if it's an automatic), then the vacuum will drop to about 15" or less, but when running down the road it should be back to the 18" region, which is where the reservoirs come into play. Might need to check the check valves in the vacuum hose connections too.

Which brings up another issue--hydraulic pressure multiplication. If there's a similar application with a larger master cylinder piston bore size than what you currently have, that might be an option too. There is a particular relationship between the master cylinder bore size and that of the wheel cylinders. Don't forget that some of the earlier cars also had two pivot points for the master cylinder push rod on the brake pedal. Not sure if these issues might apply to your vehicle, but they might be things to think about and then let others know what your findings were.

As for the vacuum issues, you might try advancing the basic timing to see if that might increase it some, but still keep the same idle rpm level. On some motors, just a few extra degrees will make a difference.

I really suspect that if you address the brake lining issue and the possibility of maybe using larger brakes (with wider brake shoes) from a later model to increase the basic stopping capability of the vehicle, that might yield better results as that's where the "action" takes place more than on the pressure side of things. Then maybe the master cylinder bore size issue next. Of course, maximizing the manifold vacuum would be a helpful side issue.

As a word of caution, whenever you start changing brake system components to something other than the factory production items, you do so at your own risk. Although the intent is to make it better, it might take some trial and error before that happens. Always use OEM approved service procedures too.

Just some thoughts . . .

NTX5467

[brad54]

On my '54 Special, I've replaced the single reservoir with a dual reservoir master cylinder from Master Power Brakes. It's still manual, not power. The new master bolts in place of the original, and works great. I also opted for a remote reservoir, mounted under the hood. Very happy with it.

I also replaced the brake lines with new stainless lines, and bought new wheel cylinders.

I went with new brake shoes in the stock drums. HOWEVER, I replace the narrower rear drums (Special) with wider RoadMaster drums, and the corresponding wider shoes. With the drums well adjusted, the brakes worked fine. It wasn't a hot rod by any means, and when I drove it I drove it knowing I had to stop earlier, etc., but it worked very well. There was no problem with the manual brakes from an effort stand point, even for my wife.

When I put the car back on the road (some day), it will have four wheel finned aluminum drums to fight brake fade, which was a problem with the iron drums (that might have been the patrolman's biggest concern) I'll also convert to self-adjusting brakes, because I hate adjusting them, and don't care for the period of time right before they get adjusted. I also don't like one side pulling harder than the other, etc.

Between the new master cylinder, aluminum drums, self adjust and decent brake shoes (wider at least in the back), I'll be totally confident driving the car anywhere.

Just my .02

-Brad