I see my name tossed around so I’ll give my thoughts on the ported versus direct vacuum hookups to the distributor. I’ll also take this opportunity to start a new thread with this as this is a different topic than the thread this was started in besides the fact that the other thread is now several pages long. Hopefully this is as wound up as I get so try to bear with me on this.
I’ve been both ways on the vacuum supply to the distributor and how the engine combination is initially setup will dictate which is best. Compression ratios, camshafts, carburetors, etc. all play into this. There is not a set rule which is best which is why it’s always a controversial subject. If the engine has been modified, then experimenting with both ported and direct vacuum signals to the distributor can be tried to determine which is preferred. I’ll also add that the use of vacuum at the distributor is purely a drivability related system and engines being used in a racing environment will typically refrain from using a vacuum assisted igntion curve.
Many of the replacement carbs are still calibrated for ported vacuum and as such, the idle mixtures are on the rich side which prefers ported vacuum for a smoother idle versus using direct vacuum for these particular carbs. The universal line of Holley carbs would appear to be the less friendly with direct vacuum while the Demon carbs are more direct vacuum friendly. If the carb is calibrated for lean idle fuel mixtures such as the Demon line of street carbs, then you’ll find that the distributor will like either direct vacuum to the distributor or considerably more intial advance on a ported vacuum or non-vacuum (mechanical advance only) distributor. In almost all instances, there are a multitude of options in which to obtain the optimum or desirable advance curve. As a general rule, the mechanical ignition curve attributes of the distributor must be tailored to suit which vacuum signal is being used to supplement the distributors vacuum advance mechanism.
I’ve run my ’55 Customline with its Road Demon Jr. 525 with both vacuum scenarios to the distributor and although fuel mileage was marginally better with a direct vacuum hookup to the distributor, overall drivability was better with the ported vacuum signal. At this point, I’ve modified the Demon with a new and improved ported vacuum signal versus the one supplied with the carb and it gives the best of both worlds using a ported vacuum signal. To go along with the carb modification, the mechanical curve in the distributor was shortened so that more intial timing can be supplied without having an excess amount of mechanical total advance. This along with very strong ported vacuum signal gives maximum fuel mileage under cruise situations while low end performance with the 272 allows tire boiling performance. Drivibility with this particular setup is flawless but it wasn’t accomplished without some trial and error work.
There are two areas where the ported and direct vacuum scenarios differ significantly. One is at idle where ported vacuum allows a true initial timing setting that will be totally controlled by engine rpm and no other factors such as varying manifold vacuum or carburetor quirkiness being entered into the mix. The other is on deceleration where and when using ported vacuum and upon the throttle being fully lifted, the vacuum advance timing simply cuts off and the mechanical advance then slowly retracts as the engine rpms come down. With direct vacuum to the distributor and without any recurving of the distributor taking place, an extreme amount of ignition timing is in place on deceleration and especially if the distributor has not been retailored to optimize the timing curve. In this scenario and even though the engine is in a no-load situation, this creates a situation where detonation takes place depending upon the fuel mixture and heat within the engine and thus puts the head gaskets and bearings under stress. Because of the incomplete burn taking place in this situation, then reengaging the throttle can also make for a short burst of detonation so you get a double hit. Most detonation is not audible so just because you don’t hear it doesn’t mean it’s not there. If using direct vacuum on a distributor originally intended to be used with ported vacuum, then the mechanical portion of the curve must be retailored as the potential for too much ignition timing during cruise and deceleration can be taking place. If the engine has been modified and this includes simply changing the carb, then the ignition curve must be re-evaluated to determine what the new ignition curve attributes are to be.
There is no right and wrong on the distributor vacuum hookup. It’s simply finding the setup that works best for your particular combination. Ported vacuum hookups are definitely more operator friendly and safe while being less prone to engine problems that are a result of misapplied direct vacuum hookups. The saving grace for the Y engines has been the overly low compression ratios which makes them ‘get by’ with a misapplied direct vacuum hookup to the distributor. If applied to an engine with higher compression ratio and without the other suitable modifications to go with it, detonation would have been obvious from the onset. Over the years and with upgrades or replacements of carburetor and/or distributors taking place on these engines, many of these originally supplied ported vacuum signal engines have been ‘converted’ to direct vacuum signal for the distributor and this is where drivability and starting complaints come to the forefront. This is simply because the rest of the system (both carburetion and ignition curve) was not addressed appropriately. In many instances these engines were switched to direct vacuum simply because of the misconception that the engines were suppose to be direct vacuum or the original ignition systems were not fully understood.
Direct vacuum was the answer for emissions related fixes by the oem’s starting in the late Sixties, not the other way around. These systems ended up being complicated with temperature sensors and various electronic relays to make them workable. But for those vehicles originally designed to use a ported vacuum signal at the distributor but have been inappropriately converted to direct vacuum, these are simply restored back to to an original ported vacuum signal for the distributor which cures all problems. Engines start well (just a flick of the key), off idle response is good, fuel mileage is good, but most importantly drivability is restored. But I’ll add that recurving the distributor is still not unusual depending upon the other modifications that have been performed such as camshaft, head work, and carburetion. In these instances direct vacuum hookup for the distributor then becomes a viable option once again. Each combination must be examined in detail for what is optimal.
I trust this didn’t cloud the issue. And additional comments are always welcome.