Is 32-34 degrees on magnum heads also because of better quench ?The nice thing about the MP distributor is the ability to adjust the amount of mechanical advance, the rate at which it comes in and the ability to adjust when the vacuum advance starts to come in. Mallory sells a nice tuning kit (#29014) for this distributor.
As it is delivered it has a fast advance rate but otherwise is set up for a stock engine. If you have a bigger cam, more compression or aftermarket heads it is not optimal and your engine won't provide max power if you just install it.
FWIW, 38 total is a good ball park number for a BB but small blocks with open chamber heads typically make max power in the 34-36 range and closed chamber heads like the magnum in the 32-34 range. And as others have stated the vacuum advance will improve economy and minimize plug fowling at cruising speeds.
If your car is driven on the street it will be spending 80-90% of it's operational time at light throttle cruising so it only makes sense to dial the vacuum advance in too.
Is 32-34 degrees on magnum heads also because of better quench ?
AgreeIf your car is driven on the street it will be spending 80-90% of it's operational time at light throttle cruising so it only makes sense to dial the vacuum advance in too.
UTG swears by ported & 4secondsflat swears by manifold. someday when I have time I am going to try/tune both on a project & see what develops
I agree with what you said however I disagree with the statement that "ported vacuum is useless" ported vacuum functions the same as manifold vacuum once the throttle blades are opened. Which is most of the time you are driving your car. So if manifold vacuum is beneficial at times when the throttle is opened so to is ported vacuum. They are the same once the throttle is opened. So in those cases where manifold vacuum at idle is not beneficial ( including not having the proper canister for the application such as with engines that have low unsteady vacuum at idle) then ported vacuum may used to achieve nearly all the benefits of a vaccum advance.RR,
MVA has it's benefits...........................PVA is useless & we got it because of emissions [ like lower comp ratios that made engines inefficient ]; engines that used PVA had MVA in the background to come to the rescue & MVA do so until the end of the carb era.
There are some combos that will not benefit from MVA......buuuuuuuuuuut there are very few.
MVA is not a crutch, no more so than using higher ratio rockers to increase HP. The engine needs a certain amount of ign timing advance at idle for best power & MVA is one method of achieving it. Another method would be a locked dist. However, this method may cause detonation because the timing is no longer load sensitive; it is load sensitive with MVA.
The problem with MVA is that there is no procedure [ that I have seen ] to dial it in.
The instructions that come with the adj VA units such as Crane, Accel, Mr. Gasket etc are written by people who are clueless to what MVA was designed to do.
D. Vizard reviewed the then new Crane Elec Dist in Nov 04 of PHR magazine; note that Crane went to the expense of adding VA curves, not just centri curves.....
" At idle & low speed operation, the amount of advance reqd to most efficiently utilize the air & fuel entering the engine can be as much as 50-55*. This is handled by the vac adv: a function many hot rodders believe is not needed because their favorite drag racer does not use it. Now is the time to listen & listen up good. A functional VA is the single most effective camshaft tamer you can get...."
And closer to home, Mopar related, MM magazine Ovt 2015:
" The initial timing went from 15 to 26* while adding an inch of vacuum. Total timing didn't change, but a dramatic gain in low rpm throttle response was realized. If tweaked correctly, you'll see your idle vacuum creep up, as we did here."
UTG swears by ported & 4secondsflat swears by manifold. someday when I have time I am going to try/tune both on a project & see what develops
In many situations the reason manifold vacuum is helpful is the mechanical advance. Using manifold vac is a way to crutch it and much simpler and easier. It does require a stable and reasonable strong vacuum at idle.So in those cases where manifold vacuum at idle is not beneficial ( including not having the proper canister for the application such as with engines that have low unsteady vacuum at idle) then ported vacuum may used to achieve nearly all the benefits of a vaccum advance.
No. This belief was being promoted by some guru a few years back.Ported vacuum from my understanding is venturi signal based. So it should increase as velocity increases thru the carb.
You can go with his system, and for most people it will work out. However, his theory of why it works is not entirely correct. This why for many years his stuff didn't always work with vac advance. I've tried to discuss that with him here but he always disappears.Ported vacuum source, Constant manifold vacuum source explained, the difference between ported and manifold vacuum.
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
You can go with his system, and for most people it will work out. However, his theory of why it works is not entirely correct. This why for many years his stuff didn't always work with vac advance. I've tried to discuss that with him here but he always disappears.
The idea that GM did not use vacuum advance in the pre-emissions era is hogwash as can be seen by looking at some pre-68 high performance Chevy engines.
So maybe not as useful but not necessarily useless. Agree?92b,
PVA is useless in the sense that MVA does the same as PVA...and more. It adds timing at idle, which helps the engine run cooler & increases idle fuel efficiency. A win-win.
MVA is still with us today, in electronic form, with EFI. The LS engines idle at about 22* & their efficient, high compression, chamber only requires about 27* at WOT. When you extrapolate the numbers on our old dinosaurs that need 35+* at WOT, suddenly 28-30* at idle doesn't sound excessive.
So maybe not as useful but not necessarily useless. Agree?