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02-25-2008, 04:06 PM | #1 |
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mechanical or vacum secondary?
What is the pros and cons ?
Can't decide. Looking at a 4150 holley in the summit book.
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1968 Chevy C10 307 3ott fleetside 1967 Chevy C/10 V8, 3spd, fleetside lwb.Sold 1967 Chev C/10 step, 383, M21. SOLD |
02-25-2008, 04:10 PM | #2 |
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Re: mechanical or vacum secondary?
Also what is the straight leg booster and down leg booster?? never heard that term...
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02-25-2008, 06:22 PM | #3 |
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Re: mechanical or vacum secondary?
ive always had mechanical secondaries...just becasue WHAT COULD GO WRONG?
its a very simple design... *eH*
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- Jason If you can turn, Youre not going fast enough. Favorite caption: first car to 200 mph in the 1/4 mile, no parachute, drum brakes all the way around.. ..back when men were men. Last edited by 69halfton; 02-25-2008 at 06:23 PM. |
02-25-2008, 08:40 PM | #4 |
just can't cover up my redneck
Join Date: May 2004
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Re: mechanical or vacum secondary?
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. |
02-25-2008, 10:06 PM | #5 |
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Re: mechanical or vacum secondary?
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. |
02-25-2008, 10:30 PM | #6 |
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Re: mechanical or vacum secondary?
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.
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02-26-2008, 01:48 AM | #7 |
Fabricate till you "puke"
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Re: mechanical or vacum secondary?
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
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69 longhorn,4" chop,3/5 drop, 1/2 ton suspension/disc brakes,1 1/2" body drop,steel tilt clip, 5.3/Edelbrock rpm intake/600 carb, Hooker streetrod shorties,2 1/2" exhaust/ H pipe/50's Flows , 6 spd Richmond trans,12 bolt/ 3.40 gears.... |
02-26-2008, 03:15 AM | #8 |
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Re: mechanical or vacum secondary?
Wow great info guys.
http://store.summitracing.com/partde...5&autoview=sku Here is the baby I have been eye balln....
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1968 Chevy C10 307 3ott fleetside 1967 Chevy C/10 V8, 3spd, fleetside lwb.Sold 1967 Chev C/10 step, 383, M21. SOLD |
02-26-2008, 06:52 AM | #9 |
Fabricate till you "puke"
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Re: mechanical or vacum secondary?
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
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69 longhorn,4" chop,3/5 drop, 1/2 ton suspension/disc brakes,1 1/2" body drop,steel tilt clip, 5.3/Edelbrock rpm intake/600 carb, Hooker streetrod shorties,2 1/2" exhaust/ H pipe/50's Flows , 6 spd Richmond trans,12 bolt/ 3.40 gears.... |
02-26-2008, 07:22 PM | #10 |
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Re: mechanical or vacum secondary?
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.
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1968 Chevy C10 307 3ott fleetside 1967 Chevy C/10 V8, 3spd, fleetside lwb.Sold 1967 Chev C/10 step, 383, M21. SOLD |
02-26-2008, 09:45 PM | #11 |
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Re: mechanical or vacum secondary?
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.
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RIP EastSideLowLife '72 C20 SWB convert. 5 lug, LS1/4l60e |
02-26-2008, 11:44 PM | #12 |
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Re: mechanical or vacum secondary?
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. |
02-27-2008, 03:06 AM | #13 |
Fabricate till you "puke"
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Re: mechanical or vacum secondary?
What are you running for cam, distibutor & Ign curve on that 383? crazyL
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69 longhorn,4" chop,3/5 drop, 1/2 ton suspension/disc brakes,1 1/2" body drop,steel tilt clip, 5.3/Edelbrock rpm intake/600 carb, Hooker streetrod shorties,2 1/2" exhaust/ H pipe/50's Flows , 6 spd Richmond trans,12 bolt/ 3.40 gears.... Last edited by crazy longhorn; 02-27-2008 at 03:07 AM. |
02-27-2008, 06:46 PM | #14 |
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Re: mechanical or vacum secondary?
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.
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1968 Chevy C10 307 3ott fleetside 1967 Chevy C/10 V8, 3spd, fleetside lwb.Sold 1967 Chev C/10 step, 383, M21. SOLD |
02-27-2008, 07:43 PM | #15 |
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Re: mechanical or vacum secondary?
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 .
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02-28-2008, 10:06 AM | #16 |
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Re: mechanical or vacum secondary?
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distrubin the peace since i got the thing and broke the origianl radio on 95.5 classic rock Ride: 1968 c20 lwb 327(orig motor)/np435/eaton 4.10 w/posi puttin in a 496 rebuilding trans need to rebuild diff :| need to start takin pics and start a project thread Last edited by LONGHAIR; 03-03-2008 at 02:34 AM. Reason: WTB and hi-jack |
03-02-2008, 08:13 PM | #17 |
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Re: mechanical or vacum secondary?
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
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1968 Chevy C10 307 3ott fleetside 1967 Chevy C/10 V8, 3spd, fleetside lwb.Sold 1967 Chev C/10 step, 383, M21. SOLD Last edited by 67chevemall; 03-02-2008 at 08:14 PM. Reason: sp |
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