Could someone explain the diff so I can choose the best for me. Does it matter if I have a vac or mech dis? Thanx.

 

Vaccum secondaries are opened according to the load on the engine, mechanical secondaries are opened according to the position the throttle linkage is at. Mechanical secondaries would be better for a drag/bracket car, and vaccum would be better for a daily driver. Vaccum secondaries generally get better fuel mileage.

 

Unless you have some sort of electronic timing advance control, your distributor will have both vacuum and mechanical advance. Well, I suppose you could get a race-only distributor with only mechanical advance, but for a street-driven car, you should have both mechanical and vacuum advance.

The basic difference between the carbs is how the secondary flow is controlled. In a mechanical secondary carb, the secondaries are opened by a mechanical link after the primaries are about 1/2 way open, reaching full open the same time the primaries do. In a vacuum secondary carb, a vacuum signal is generated within the primaries, which is used to open up the secondaries. The more flow there is through the primaries, the farther the secondaries open.

All you need to know about choosing a mechanical secondary carb are your engine displacement and what the lowest the RPMs will be that you go WOT. If you have an automatic transmission, you simply need to know your converter stall speed, and pick a carb per the chart on Holley's website ( http://www.holley.com/data/TechServi...Carburetor.pdf , 2nd page) that stays above the line drawn. If you have a manual transmission, you need to choose the carb size to keep the launch RPMs above the line to avoid bog.

Vacuum secondary carbs don't allow the secondaries to open any more than the engine "needs". If, for instance, you have a 383 that maxes out at 5500 RPMs, you don't "need" a carb bigger than about 600 CFM. If you put on an 850 vacuum secondary carb, what will happen is the secondaries will never fully open. However, it won't "bog" when you go WOT because the secondaries won't be dumping in more air than the engine can suck in effectively. So, vacuum secondary carbs are more "forgiving" with regard to putting on too big of a carb. But, mechanical secondary carbs tend to provide better performance in racing applications than do vacuum secondary carbs (although you can always find someone who says it doesn't make any difference if you know how to tune them).

Mechanical secondary carbs tend to be a little better for manual transmission applications because with a vacuum secondary carb the secondaries can stay open when you let off the throttle (like between gears) - that can't happen with a mechanical secondary carb.

 

Mechanical secondaries also require more 'finesse' to drive with a stock-stall automatic. They work great with a higher stall or with a manual transmission though. Also, with a vacuum secondary carb, you can go a little bit bigger because the secondaries will only open up as the engine needs the air flow. Manual secondaries will be yanked open when you romp on the thottle, so you typically have to go smaller to keep the driveability.

Since you have a manual trans and an engine that looks fairly hot, I'd suggest going with a manual secondary carb, like a 750cfm Holley double pumper. I think you'd be very happy with that carb.

 

Mech secondary 750 cfm 4150 or demon it is. Thanks for clearing up the vac secondary question but I still need to know if i should get the msd with vac or mech only advance.

I know the mech uses springs only and the vac uses a vac source to make advance happen but what is best. My old mech only was easy I just changed the springs till it felt right. Would I get better performance from a vac model?

 

Stick with mechancial advance, as you've found it's very easy to tune and one less hose/vacuum port to leak. KISS principal, especially with the engine in your sig.

 

The MSD dissy (pro billet) has optional vacuum advance. I would get that one. If you decide you dont want to use the vacuum advance, you just unhook it and cap it, and it works just like the non-vacuum advance version. It only costs like $2 extra for vacuum advance equipped pro billet.

 

If the car is for race-only use, get a mechanical-only advance distributor. KISS works at the track.

Street duty, get a vacuum/mechanical distributor.

 

Can you explain why the vac is better for the street? My car is street but its no daily driver.....heck I barely drive it at all. LOL But what are the pro's and cons is what i'm looking for?

 

Vacuum advance gets you better part throttle performance and mileage, and keeps the plugs from fouling.

 

Vacuum advance advances the timing quite a bit at part throttle, so the gas mileage goes up a bit, driveability is improved, and (if hooked up to manifold vacuum) the idle quality is better as well.

The MSD non-vacuum advance works OK on the street, but you have to kick the base timing WAY up to drive it on the street.. like around 15* or so.

MSD's vacuum advance Pro Billet works much better on the street... you can keep the base timing lower, but still have all the timing advance under normal driving conditions because of the vacuum advance, so you will get better gas mileage and part-throttle driveability/performance.

The vacuum advance version is a MUCH better one to use on the street IMO. But like I said before, if you pay the extra $2 or whatever for the vacuum advance version over the regular Pro Billet, and you don't like it for whatever reason, just disconnect the vacuum advance by capping it off, and it will work EXACTLY the same as the normal non-vacuum advance Pro Billet.