Vacuum Secondary Conversion
1. Additional vacuum port for brakes. I didn’t want to drill out the manifold.
2. Would help in overall tuning given the issues of a non ideal motor for this set up.
3. Has lower height which aids in clearance for my hood.
4. Had to be pulled to drill anyway if I went that route.
I am in the process of final restoration and the engine has not run for the last 4 years. So I have a lot to do. But this is my restating point. I just wanted to investigate going to VS to help my understanding if and when I wanted to convert. I’ll try and tune the carb and recurve the distributor as best I can as I shake down the car. This discussion is very insightful as you guys have a lot of experience here.
Whoa. So you're still putting things together.
OK. Put whatever intake on it you think will make you happy. Seriously. If it fits, has the ports you want, go for it. Street contender/dominator single plane if you want short, or RPM for taller (although a drop base cleaner may be needed and check the stud hight before slamming the hood), or whatever.
Here's my 2cents. If you're making all these changes thne pull the heads and get the cc. With pop top pistons there's a couple ways to estimate the volume they will take up in the chamber at TDC. I think that is well worth it if you think the pistons are putting it close to 12:1. If so, you've different ball of wax trying to run a street car on pump fuel that an untouched factory 10:1
If its going to be octane limited, then that will have to get built into the advance curve.
MSD Billet 8534 non vacuum dizzy
I did plot my curve and recall it being very gradual to soften the detonation. I think I was all in like at 3500. This along with a slight retard did stop the detonation. But the cost trade off is performance.
The MSD type advance can only do a straight line, and without vac advance, yes its limited in what you can do.
If its going to run without vac advance, then a
MP Tach Drive distributor will take care of the high rpm retard, and give it easy starting. With vac advance, then a modified Chrysler distributer - but there will be considerable time tuning. How high of an rpm does this need to turn for your purposes and to hit its hp peak ? if the advance stops at 3500 to 4000, then its only going to retard a couple degrees by 6000 rpm. But if going to 8000 rpm, then stopping advance at 3500 will show retard from the slew rate of most electronic ignitions.
As far as the carb, I'd probably stick with what you have. More so if its an older one. If the car is geared high/is fairly heavy then a vac secondary could have some advantage, but on the street, both mostly work off of the primary side. The difference you may note on some Holley's is the sales department 'size them' differently. Compare the venturi and throttle sizes of the a 4779 (double pumper) and a 3310 (vac secondary) and IIIC you will find them different. But they are both called/rated 750. Its an approximation.
