hi i'am working on a 1951 panel with a 235 from a 1957 car. sm 420 trans.
the motor has been rebuilt using a mild comp cams, dual zenith 28s with a edlebrock intake, and fenton duals. i want to install the PCV system described on deve's tech page. and i have a couple questions regarding vacuum. first, i need to connect the distrubutor to a vacuum sorce. i have read various opinions on where this can/should be done. the problem i'm having is the zeniths don't have a vacuum port. so can i pull vacuum off the manifold for the advance? also, can i split that same vacuum for the new PCV or should i stick with the old style crank tube?
please be advised, this is new territory for me and thanks to forums like this, i'm finding my way but, if i've described something that sounds like i don't know what i'm doing well, i don't!
i did get a reply from deve which was very helpful and informative. i thought i'd just paste it here for anyone who may be asking the same questions
Doob... the cool thing about this hobby is the different configurations that are possible. I had no idea the Zenith Carbs have no vacuum port. I didnt know ANY carb of this vintage was without a vac port. That being said, Air goes in the throat of the carb, due to the Venturi effect, suction happens and off you go. I do not THINK (someone please correct me if I am wrong) it makes any difference if it comes from the upper intake(Carb) or the lower intake (Vac NPT port on the intake manifold). So, I would run the Distributors Vac Advance AND the PCV from the intake manifolds port. FYI, out of the STOCK 20 inches of mercury available in vacuum, the PCV takes only 2 using the V-237. That is really an insignificant number and should be no problem.
All that being said, I am not a fan of dual carbs and dual exhaust because it has been proven over and over that they do not give you even ONE horsepower more performance due to the way the stock 235/261 Heads are designed. Until someone makes a performance Head, its all show and ZERO go. Just being honest. I dislike them from the perspective that they cause you to have to change the way your shifter works, the way you add oil filter kits, and the way you deal with trailer hitches and other under the vehicle items. BUT, we deal with it because people LIKE them. I did not want to be the first person to respond, was hoping a real expert would chime in first, but wait a few days and hopefully someone like Dave will respond. Hope this helps, but wait for someone who really knows! thanks again to all who replied.
Ported vacuum advance does not take effect until the throttle disc has passed over the port in the carburetor throat, which is above its normal at rest idle position. Full time vacuum advance is always exposed to engine vacuum since the source is below the throttle disc, so it's always present. These differences change the idle characteristics depending on the construction of the vacuum advance canister ("can"): how far it advances the spark and how much vacuum is needed. For many engines different cans are available or can be modified to work. Engines with low compression or large overlap may have a shaky idle with full time vacuum, you really have to try both to make an informed choice.
doobspanel - From what I have found the OEM specs for 1950-1951 Trucks and Cars, 216 or 235 is as follows:
Vacuum Spark Control: Delco-Remy No. 1116043 Plunger Travel: 17/64" Eng. Deg. 0° at Vacuum (" of HG) 7-8.5" Eng. Deg. 20° at Vacuum 16.5-18.5"
There is practically no difference in compression, combustion chamber design, and cam specs to later engines. I would shoot for these numbers to start with. I believe all of these specs are PORTED vacuum.
