mechanical or vacum secondary?

[67chevemall]

What is the pros and cons ?

Can't decide.

Looking at a 4150 holley in the summit book.

[67chevemall]

Also what is the straight leg booster and down leg booster?? never heard that term...

[69halfton]

ive always had mechanical secondaries...just becasue WHAT COULD GO WRONG?

its a very simple design... *eH*

[LONGHAIR]

It depeneds on what you are trying to get from it.
Vacuum secondaries should get better milage and be a little more streetable.
Mechanical secondaries (double pumper) is more of a racing carb.

This is very over-simplified of course, but that is the general idea.

The booster question is more a matter of tuning. "Straight leg" vs. "down leg" is just a reference to where the booster itself sits in the ventri (barrel). The height of the booster, in relation to the throttle blade, has an effect on the vacuum signal. There is another type of booster, called "annular discharge". These are thought to atomize the fuel better because it is dispersed through smaller holes, but there are more of them.
These are all very specialized tuning factors that probably shouldn't concern a street driven truck enough to determine which carb to buy though.

[Billla]

The really simple answer is that vacuum secondary carbs are designed to add capacity as the engine demand more air. Mechanical secondaries are a race-only carb that add capacity solely based on the position of the accelerator pedal

Vacuum secondaries will always suffer from a bit of "lag" when the throttle is opened all the the way, especially from a standing start. Careful tuning can minimize this, but for drag racing it's an issue. Note that many complaints about the lag with vacuum secondaries is due to incorrect tuning - ignition and carb.

Otherwise, if you're not drag racing then a vacuum seconary carb will give significantly better fuel economy and driveability.

Longhair nailed the booster question - the annual discharge has shown benefits on the dyno.

[71 super]

Also depends on what tranny you are using. Auto does better (unless you have mechanical advantage i.e deeper gears stall) with a vaccumn secondary- but you will have to tune the carb to your setup with different springs etc - trick kits are great tools to have as well as a couple of different power valves and jet sets. The mech. secondary carb works better for manual trans. or when you have mechanical advantage as stated before. The last big motor I had used a 750 cfm w/850 baseplate adjustable secondary metering plate (air adjusts) power valve block off in rear, 76 and 76 in front and 84 jets in the rear. It took some playing to get the thing right and you might have to stagger jet sizes depending on what your plugs tell you and depending on the intake you are running. In response to what Billa state about vac. secondaries is true, the lag will feel like a bog but is actually a lean spot rather than a bog- please buy a holley book and tune the carb right - no screws or other crap in the linkage- holley cabs will not and should not bellow or "kick in" like a quad if they do then it needs to be tuned. Also buy a unit that has the power valve blowout protection built, people are constantly complaining about holleys and most of the time it is because of power valve issues. One other thing buy a vaccumn guage to adjust your carb with, this makes holley carbs very easy to deal with. My brother used one on a 1970 LWB - carb was vac. secondary 600cfm- he had to clean the carb 1 time because he had a cruddy gas tank and did not take care and replace his filter early enough, no other problems at all in 60,xxx miles. If you have any questions about any of this pm me and I can help.

[crazy longhorn]

I have a 3310 (750 vac sec) on my 383. It runs great on a 350-400 inch engine. Ran the same carb on a 461 " Olds, & it didnt have near the "kick" as the 800 DP that I took off.....& the DP didnt use any more fuel!(unusuall?) IMO, on the smaller engines(street bound), the Vac secondiarys are the ticket. If I were to go DP on my 383, I think I would step down to a 650 cfm. As far as lag or bog, I dont notice any on the 3310. The carb is pretty stock, 2 # leaner on the primary jets, & a purple spring in the secondaries, & a little pump tuning. crazyL

[67chevemall]

Wow great info guys.

http://store.summitracing.com/partde...5&autoview=sku

Here is the baby I have been eye balln....

[crazy longhorn]

Street driven? if so, you could get the run of the mill 650 DP, for about 1/2 the coin. My butt squeaks when I walk, but I dont mind tuning a bit on a carb(nice to have a choke also). If you are racing, you may be happier with the one shown on Summits site....crazyL

[67chevemall]

no choke! good call. Didn't see that.

back to the search stage.....

there is this one
HLY-082651

HLY - 082751

Which brings my next question

How much difference is a 650 and 750 cfm
What is the best way to know what size carb to run....?

The 600 Edel I was running wasn't enough.
Going back to a Holley.

[crazy longhorn]

its hard to make a reccomendation on carbs.....need to know the spec of the eng, & intended rpm useage. I think I would try this on on my 383, if I went with a DP http://store.summitracing.com/partde...5&autoview=sku keep in mind that I dont plan to run past about 5700 rpms , & are not racing the truck. 650 cfm doesnt sound a bunch bigger than what you have, but....that DP holley will run a bunch "fatter" than your 600 edel. crazyL

[crazy longhorn]

http://www.slowgt.com/Calc2.htm#MinCross air flow requirements @ the bottom of the page...

[bryanw1968]

The basic rule of thumb is this;

Heavy car/truck with an automatic, use a vacuum secondary carb and a dual plane intake. Light/race type stuff you can use a mechanical secondary and a single plane intake.

[chevy72blu]

That is a lot of coin that you dont neccesarily need to spend with a little patience. Grab a used 4150 with a similar cfm on ebay, buy a rebuild kit (25-30 bucks), and a walkthrough book (15). 4150's are a snap to rebuild.

[67chevemall]

383 67 Chevell Block. Cammel Back Heads w/ACdelco R 43
Wieland sqaure bore.
headers

3500lbs truck
4speed tranny M21 1:1 FD

1st carb 4150 DP holley 70's carb. too old had problems
2nd carb 4160 600 holley bought cheap replacement
3rd carb 600 edelbrock. never ran right detonation problems.

4th carb I like the sounds of the 650 Holley

HLY -04777C

HLY-082651

HLY - 082751

[Bowtie001]

you didn't have detonation issues because of a carb.

If you don't know what to do definitely get a vaccuum secondary. It will take a while to set the mechanical so you don't get huge bogging and the gain will be tiny.

[67chevemall]

Maybe I will take the Edel to the shop and see if they can get it running good.
I tryied everything I could.

I had a bunch of Holleys that I sold as a box buy! Doe!!

I thought the Edel was going to save the day.

I know the Holleys inside and out, and should have stuck with them.

I will look over ebay! Good tip.

[crazy longhorn]

What are you running for cam, distibutor & Ign curve on that 383? crazyL

[67chevemall]

not sure! lol

I bought it built.
It was done at a pro race shop in the city back in the 70's.
The truck has SAT a long time.
He had it in build stag for 20 something years and I have had it in the build stag for 11 years now.
I had it down to Florida and back and that was it.

Someday......


I wish I knew.
The distibutor is HEI OEM
I am looking at a new distibutor and
Carb for it buy Spring.

[hotrod 80]

I run a 650 demon DP on my 355 and it runs rich on the big end due to air flow . I would say 750 on a 383 . Any carb you buy will have to be tuned to your application . Even the edelbrock , which is very tunable , sounds like it needs adjustments . Holley , Demon & Edelbrock (Carter AFB) are all good carbs . I have had Carters and like them for the street . I run Holley style on the trucks that end up on the strip . Even if you don't plan on racing the truck the drag strip is a safe place to tune a carb . Chassis dyno with broadband 02 is the ultimate . I think vacuum secondary for better milage , but with a 4 speed mechanicals will work good .

[sickchevy68cst]

http://67-72chevytrucks.com/vboard/a...t.php?f=3&a=17

[67chevemall]

Well I sold the Edel Still New for $220 CDN
I think I am going with
Part # 1282010
650 Speed Demon
Take it in for setup and tuning.
The Motor came with a old Holley 4150 double pumper,
so this should be a great replacement