Well I hear a lot about vaccum, especially when talking/reading about cams. From what I've gathered I'm thinking it's how much air/fuel the piston can suck in when it goes down on the Intake stroke. Now obviously I don't know what I'm talking about so possibly one of you motor whiz's could explain this to me. Thanks

 

Im not exactly a whiz yet but il do my best,

The engine vammum is the suction in the intake manifold, not primarly how hard the piston is sucking. The reason why the cam specs will affect the vaccum is because if there is alot of overlap from excessive duration and/or a narrow lobe seperation. The overlap means the exhaust will be open as the intake is opening. This allows some exhaust gas to enter your intake maifold at low rpm before the incoming air is drawn into the cylnder and that is wat reduces the vaccum. This will also reduce idle quality because the engine will be rebreathing exhaust so the idle will be increased. The main reason why people wory about vaccum is because you need it if you have power brakes and not enough vaccum will make the brakes hard. Hope I helped.

Eric Myers

 

how would exhaust gases gettin back into the intake manifold relate to youre brakes?
and narrow lobe separtation...is 112 LSA narrower than 114? so the lower the number the narrower?

 

Think of vacuum more as "air that DOESN'T get into the cylinder".

There is a huge column of air pushing down on everything on the face of the earth. It is equivalent to a pressure of 14.7psi (absolute), or about 30"Hg. When the piston drops, that pressure forces air into the cylinder. If there was nothing between the cylinder and the outside atmosphere, and if there were no "leaks" in the cylinder, the air pressure in the cylinder would be exactly equal to the pressure outside the cylinder.... or 14.7psi (absolute)

But, it doesn't work that way. Things get in the way... air filter, inlet duct, MAF sensor, throttle body, intake runners, valves, etc. Each of those items causes a pressure loss, so that when the piston is filled, it is at a pressure LESS than the 14.7psi (absolute) that was trying to push air into the cylinder. Less pressure means less density = less air to mix fuel with. The difference between the pressure of the air inside the cylinder and the pressure of the outside air that was trying to fill the cylinder is the vacuum you see in the intake manifold (roughly, there would still be a little pressure loss in the runners).

The objective is to get the cylinder pressure as close to the atmospheric pressure as possible. (Yes, you can actually get the pressure in the cylinder above atmospheric pressure with inlet wave tuning, but let's not go there). That's why you use a better air filter, make the inlet duct larger and smoother, open up throttle body and intake runners, etc.

You get maximum idle at vacuum, because you purposely stick a large obstruction - the throttle blades - in the path of the incoming air to "lose" some pressure, and reduce the density of the air in the cylinder.... lower pressure = less air to burn less fuel. 'cause that's all the air and fuel that is needed to keep the engine icling.

All of the above has avoided a discussion of the part the cam plays in vacuum. When the intake and exhaust valves are open at the same time, you have the outside air pressure trying to push the air into the cylinder and the exhaust manifold back pressure (which is generally greater than atospheric pressure) trying to force the exhaust gasses back through the open exhaust valve. Obviously, with any overlap at all, there is the possibility that incoming air can pass through the cylinder due to its momentum and out the exhaust, or that exhaust gasses can be forced into the cylinder to mix with the incoming air, and raising the pressure in the cylinder = less vacuum.

I'm really at my limits here, but I would expect it would take an extremly large amount of overlap to actually force exhaust gasses back through the intake runner and mix with the gasses in the plenum.... but I'll leave that up to the guys that actually build engines with these radical cams. In any case, the vacuum in the intake is going to be less because "pressure" is leaking into the cylinders through the exhaust system. The dropping piston is pulling some of the "fill" from the exhaust and less from the intake runner.

Losing vacuum in the intake manifold reduces braking "assist" from the power booster, because the pressure is lower, not because there is exhaust in the intake.

Enough.... time to go back to basting the Easter ham with my favorite Jim Beam bourbon, coffee and brown sugar glaze.....

 

How do you know when both valves will be open at the same time? LSA? Duration? A mix? Thanks Injuneer you cleared a lot of things up. So when you have overlap, exhaust gases are being pushed BACK in the cylinder because of backpressure in your exhaust system? wow not too difficult. Brakes though, how are they affected by vacuum?

 

If you have vacuum assited brakes and the vacuum coming from the intake is low due to an aggresive cam it can't help in braking, thus making your pedal harder, but if you have manual brakes you don't have to worry about that. I'm no expert, but I'm pretty sure thats the jist of it.
bob

 

It's a mix. The time when both valves are open at the same time is called overlap. I betcha there's a lot of information in this forum if you Search for that word