I've been doing a lot of research lately trying to pick a carb for my swap. Let me know if any of this is wrong.

Mechanical secondary carbs will produce more of a "kick in the pants" when floored, at the expense of gas mileage. And also are more sensitive to having the correct size carb for the application

Vacuum secondary carbs dont have the same feeling when floored, but get better gas mileage and you can get away with using more carb then you need.


If this is correct, im thinking im gonna use a 650 Holley DP on my new motor. It is a stock bore 350 with a xe268 cam, stock heads for now but probably a set of aluminum edelbrocks soon, performer rpm intake, with a t-5 behind it and 3.08 gears (for now). Does this sound like a good choice in carb? The car is mostly a street vehicle with the occasional test-n-tune night once a month or so. Im not worried about mileage since its not daily driven but driveablility is somewhat of a concern. Thanks

 

Speaking very generally, yes you are basically right.

 

Of course its very general, i didn't really think i could sum up everything about carbs in 2 short paragraphs, lol.

My knowledge about carbs is VERY limited, my new motor will be my first carbed motor ever so its a huge learning experience. I just want to make sure i have the basics right so i can make a good decision on what to buy.

 

SOTP is a very deceptive indicator. I'd say the VS gave me more of a SOTP kick, but the DP produced better 60' and ET numbers.

I don't know where the stuff about DPs producing poorer MPG comes from. There isn't any technical basis for it, and I haven't seen it in comparable conditions.

DPs are more sensitive to sizing on single plane manifolds. The RPM is a dual plane, and with that cam, a good choice. You won't have any driveability problems with a 650 DP on that type of engine.

I haven't heard anything but praise for the Air Gap style RPM. The SBC guys I've talked to and heard talk about them here are unanimous, and the initial results on my BBC have impressed me both on the track & on the street.

 

You can use a somewhat larger carb when you use vacuum secondaries. Instead of say a 600 you can run a 700 or 750. That will help with the extreme top end power.

 

The issue with sizing a DP is the RPMs at which you go WOT. This is dependant upon CID, and a 355 can go WOT with a 750 DP at 1500 RPMs without bog.

That being the case, there's no reason to sacrifice the benefits of the DP with a manual tranny at the altar of some silly notion of overcarbing with a DP.

There is absolutely no "extreme top end power" advantage with a VS vs. DP. The DP will win there every time, including "over-carb" situations. Why? Because the DP will be wide open at WOT, while the VS will only open as much as the carb thinks the engine can "use".

 

This guy knows what he's talking about, listen to him

 

thanks for the help guys, im ordering my 650 DP today.