The Mystery of the Vacuum Can?

Ported or Constant?

Let me try and give you an explanation in layman's terms.... Generally, emissions equipped  vehicles use Ported and they are connected to a computer,  OBD1 or 2 to retard the timing based on information gather from a multitude of sensors in the vehicle.

Non-Emissions Hot Rods and Street Muscle Cars Pre-emissions use Constant. Many vehicles were built with specs that designated Ported, that was fine in 1975 when we used real gas and we could set the total advance to 25 with 5* initial and have the vac can pull the timing to 34 under load and fuel economy, formulations and costs were not an issue. These cars ran horrible and got terrible fuel economy, more gas ran out the tailpipe than got burned in the combustion chamber.

GM always used Constant Vacuum and built over 300 different set-ups for all there product lines until computers were introduced and everything changed to meet EPA emissions regulations.

So Your Hot Rod gets connected to CONSTANT manifold vacuum and the distributor needs to be set up properly to burn today's fuel formulations.  The Vacuum Can needs to be adjusted to pull enough timing in the motor to allow it to burn the today's fuel and limiters installed to set the part throttle cruise timing numbers so they don't pull the timing too high and cause a lean miss at cruise, it's a delicate balancing act to get it all correct.

Your carb has 4 Venturi's, as the air speed or velocity increases with throttle position a venturi creates high pressure in the center to fracture the fuel into small particles and "Opposite and Equal Reaction" is negative wall pressure.

The negative wall pressure is what draws the fuel through the booster and feeds the engine based on throttle position or air velocity. A carburetor is a simple metering device that delivers fuel and mixes it with the air to create clean combustion, the higher the velocity the more negative pressure the more it delivers, it's a pretty simple device.

Ported vacuum: With that thought in mind consider if the distributor hooked to Ported Vac, as air speed increases the ported vac activates and starts to pull more and more timing in the motor as velocity increases. So if you set you total timing to 34* at 3000RPM that's when the ported starts to do it's job and advances the the timing to the total stroke of the vacuum can arm, usually 12-18* (without Limiters) so now have your total of 34* PLUS the stroke of the Canister arm of say 16* net result=50* of total timing under hard acceleration and your motor WILL "Detonate".  If you had a OBD1 computer it would pull that timing back to the 34* as all the sensors feed information to it.... and no detonation.

Now to Constant:  At idle/part throttle cruise you have high vacuum, the carb is nearly closed causing a restriction which creates the high vacuum level. Under this light load condition and lean AF ratios the motor needs more timing to burn the fuel (Lean Mixtures take Longer to Burn than rich mixtures) so you need more timing at idle and cruise to burn the fuel correctly and completely. When you stomp the throttle you have NO manifold Vacuum so you have NO vacuum timing and at NO time under high load will it ever advance more than the mechanical "All In" numbers.  Stop pointing your finger at the carb for rich idle and top end lean conditions.. It's In Your Distributor Tuning!  If your Buddy tells you to hook your Hot Rod distributor to Ported Vacuum, find a new friend because that guys advice is going to blow your crankshaft through the oil pan.  Same goes for the guy who say's to disconnect it, they obviously have no idea of how it works or what it does or why we use it.

This not hear-say or an opinion its engineering, based on physics formulas, calculated by people who are a lot smarter than me and I know that so I do what they tell me.  They come up with the numbers, we set them up accordingly.

Rule of Thumb: If the Motor Makes 8" of vacuum at Idle with 30* of timing in it then it needs a properly calibrated Vacuum Can, we have Mopar and GM vacuum cans that will read 7" of vacuum. most stock or aftermarket Vac cans will only read down to about 15", the odd one (1 out of 100) will read to 12"

We manufacture  Mopar vac cans that will read to 7", our Lane Choice Gold Plated VC-1 BB or VC-1 SB

There's all sorts of unqualified guy's on the "Internet" and so called "engine builders" who will try and argue this point with me, I have given up trying to argue with these folks.  They have no understanding how a vac can works or what it does, if they did they would realize how ridiculous they sound to us, Jim and I account for over 100 years experience with well over 10,000 happy customers.

Your "Engine builder" may be the best in the world but he's an Engine Builder and very seldom do they know anything about the fine tuning of the vacuum can or distributor mechanical advance system