Carb question

[72step]

I am looking at replacing my challenged 650 AFB with a Speed Demon 650. On the speed demon website, it recommends vacuum secondaries for an auto. My question is, what happens if you opt for the mechanical secondaries although I have an auto. I've heard that mechanical secondaries provide a more direct feel of power.. I'd appreciate you all's views/advice. Thanks

[Elsie72]

I would also recommend the vacuum secondaries for your application. It just seems more driveable to me to have the secondaries opening when the engine is asking for more fuel/air vs. my right foot telling the engine that it's getting more air/fuel. Seems far easier to tune the vacuum version, too.

If it's not a drag-only car, you're probably better off with vac. over mechanical.

[piecesparts]

Let's see; you are running an AFB, that has the mechanical secondaries. How does it handle for you? Holley, Edelbrock, and Demon---all agree that vacuum secondaries is the "Cats Meow" for daily driving and it is the best for all around use in a city. I have had both and have had no real issue with the use of either. I presently am running the Edelbrock AVS Thunder Carb and it is hooked up to an auto tranny. It is agressive when I want it and I love it. I would consider the Demon carb next in line, because Mr. Holley Carb and I have had some wild experinces in the past.

[72step]

The Carter AFB I have is just okay, and has been since new. When setting the idle/mixture, it doesn't do anything, and has always ran way fat. Also I have never been that impressed with the response, and power. I have a strong feeling that I will see a severe difference with a speed demon 650. I am running a hyped 327 with a 280H Comp cams, 9.5:1 comp ratio, roller rockers, gear drive, much more. It really screams, but it's the off the line that really sucks with this carb.
Elsie72,
It's not a race vehicle, but it is a toy and is only driven when I wanna have fun. It's not used for transportation at all so that's why I'm considering the mechanical secondaries. In past cars I have had the mechanical secondaries, and it seems like you really feel the power when the additional barrels open up almost like a mini turbo boost. Plus I would think the off the line would be better when you mash on the gas.

[Psycho71]

Originally Posted by 72step

It's not a race vehicle, but it is a toy and is only driven when I wanna have fun. It's not used for transportation at all so that's why I'm considering the mechanical secondaries.

If it were mine, I'd put mech secondaries on it. I have a vac sec Holley Street Avenger 670 on my 350 now, but use it as transpo. and towing the boat so I stayed w/the vac model. But even with the vac sec, I can change the springs to bring them in at different rates. Of course I still have the softy in there,hehe. Come to think of it, it is the only spring mine has ever seen. But if it's a toy, put the mech sec. on it.

I'm not familiar with the AFB carbs, but can't you re-jet it? You should be able to, I'd think. It seems like that would be alot cheaper than replacing the whole thing. Thats why I like holley stuff, infinately tunable. And a tech-line with all the info you could want/need.


Jay

[piecesparts]

The Carter and the Edelbrock AFB design carbs are BOTH the same units. The bodies are built be "Magnetti Marelli USA" and are capable of using the same parts. I have done some time looking at this and the parts that were in my Carter AFB were Edelbrock numbered parts. I did this because the parts were priced cheaper by the suppliers, from Edelbrock. The performance of an AFB can be enhanced seriously by changing jets, needles or springs---all of which is easy to do. Carter has a great book on the tuning of the carbs. Most of the AFBs on the market is rated at 600 CFM and appear to be limiting in air flow. I upgraded to the Edelbrock AVS Thunder series carb, since it is rated at 650 CFM and has the adjustable secondaries plate. I found this carb to work well with my 383 stroker motor and the 700 R4 tranny.

I spent a lot of time trying out different carbs and looking for a good all around performance unit for my type of driving. I even had a 750 CFM AFB on m y engine for awhile and that was a BAD thing. The 750 gave me a strong "TOP END " on my motor and opened up the performance curve to well above 5000 RPM. However, the carb and my stall converter slobbered around the city like a lazy tank and my mileage sucked BIG TIME. I talked to many distributor and they all centered on the fact that a truck of about 4000 lbs. needs to keep the air/fuel mixture flow up and the smaller carbs will do just that. If I had it geared with a 4.10 rear end, then the 750 might have worked better, but it would still be a mess. I went back to the 600 CFM units and fined tuned them and then upgraded to the 650 unit. I run a Comp Cam that has some lope to it and with the carb and stall, i have a good combo that will push my truck hard.
You will gain some soft drivablitiy with the pring loaded secondaries, but the mechanical will NOT hurt you in any way on your weekend cruizer.

[71chevylowrider]

I went from a Edelbrock 1406 (600cfm electric choke) to a Demon "Road Demon" 625cfm electric choke vacume secondaries, & it made a huge difference power wise, now when the secondaries kick in on the Demon, people that have ridden in my truck before now reach out to grab on to something, & have a little bit of a scared look in their face when I punch it.

[72step]

There's a 99.9 chance I'm going with the Mech. Secondaries. You gotta have better response off the line when you mash that gas because there's no vacuum buildup delay. Also in the future, i would consider converting it over to a stick if my TH 400 was to blow up again. But that's another story.