Figuring out what Carburetor I need

[rmorman]

Thanks gents!
LOTS of good info here.

Google the boosters and you might be persuaded by your use and driving.

I am trying to get a handle on the boosters. I have actually read articles that 100% contradict each other. I initially was leaning towards annular. However, with the stronger "signal" from a dual plane manifold, maybe down leg is better. On ANOTHER hand, annular seems to have better throttle response at "normal" driving RPMs......

Here are a couple of the best articles/forums I've read thus far concerning Annular vs Down leg Boosters:
http://www.corvetteforum.com/forums...ter-vs-down-leg-boosters-what-works-best.html

http://www.hotrod.com/articles/carburetor-showdown/

http://garage.grumpysperformance.com/index.php?threads/holley-annular-vs-down-leg-boosters.5229/

http://www.chevelles.com/forums/13-performance/219426-carb-annular-downleg.html

My plan, this Spring, is to take the car to the chassis dyno and compare my Holley 750 DP against whatever I pick. Wish I had $$$ to buy 4 different ones to compare CFM vs Booster differences.

Have you considered EFI?

Holley Sniper
Fitech

The price all in (considering on what fuel system you run) could be done for approx. $1500 or less.

I have considered it. In fact, I'm likely to do that in the future. However, for the next year or two, I've been watching a couple carbs on e-bay for around $350. Really curious as to the MPG of mechanical vs vacuum secondary. Seems like my car/engine/trans are exactly what a vacuum secondary was designed for. I pulled just over 17 MPG with the double pumper (normal driving). We tuned it on the chassis dyno according to the tailpipe fuel sensor. Fitech was the brand I looked at in EFI.

Dual plane intakes are more sensitive to carb sizing than open or singlee plenum versions. The large plenum fools the motor by allowing it to use the plenum volume where as the dual plane only operates on 180° of the manifold. I would take a long look at what crate engines come with in the 350 and 383 variety....most are 750 cfm or so. The intake puse of the 383 is much more sensitive to changes along the intake tract than a 350. Add good flowing heads and a cam and a 750 may not be enough.

The stronger signal on the dual plane intake is one of the plus's of going with the down leg booster as well. However, the size of the cam, according to the guy who designed the Demon carb, dictates annular boosters. Barry Grant recommends using the annular-discharge carburetor on any engine with a camshaft duration of 220 degrees at 0.050 or more. Mine is 234.

I have close to 400Hp at the crank give or take on my .040 over 350, using a 1406 Edelbrock (600 CFM) and no starvation issues. I think anything bigger would be way too much gas.... Im also running a dual plane Edelbrock Hi Rise intake with a transdapt 1 inch spacer.

Correct me if I'm wrong, as I'm REALLY trying to learn here. The bigger CFM of a carb does not necessarily mean it burns more fuel, correct? The down side if you go TOO much CFM in a carb is poor fuel atomization due to low signal, right? I THINK I'm ok in the 670 - 750 CFM if I jet it correctly. The currently 750 is also sitting on a 1 inch spacer with the 4 holes. It dyno'd 10 HP better with the spacer.

 

[rmorman]

So what issues are you having with the 750 DP that makes you want to change it?

Great question. Couple things. My car is 95% street driven. The vacuum secondary carburetor works best on mid-weight or heavyweight cars, (like my 3500 lb Monte) with an automatic transmission (mine has a 700R-4). They are more forgiving than a double pumper is because they work by sensing engine load. The mechanical secondary carburetor is best on a lighter car with radical camshaft and a lower gear and manual transmission or on a car that is going to be used for racing purposes. Plus, I'm curious as to if I can increase efficiency without giving up power. Truth told, the 750 DP has been GREAT. Just wanna see if I can do better. Ain't that why we do this crazy stuff?

I'm signed up for another Power Tour and would love to pick up a couple MPG's. However, I realize that won't pay for the carb (even a used one).

 

[vanrah]

Greetings guys; Ask a question like this & you'll get 101 opinions from a 99 guys, plus you get some of us old guys to toss in $.02 cents worth! I had called Holley-Quick Fuel again for the tenth time just last week. I'm always picking their brains as time & tech marches on. We have similar engines, mine is a 406" 10.2 to 1 with AFR 195 heads, a 230-236 @.050 cam & same RPM Air Gap intake. Long ago I started & stayed with the vacuum secondary Holley's. They are easer to tune for us novices (no metering rod spring or rods). My first Holley came on my L-78 (that's a 396" -375 HP solid lifter rat @ 11.25 to 1 comp). Also at that time GM was putting the same carb on the Little 302 Z-28's. (also 11.25 to 1 comp). Anyway I've seen so many guys put a DPer on their street car only to have a Big Flat BOG when the hammer goes down. If you use a DPer it's Very easy to over carb, but with vacuum secondaries the carb will only give the engine what it wants when it wants it. And I don't think anyone here will dispute that Holley's under full ON distribute fuel & air better that all others. That said under part throttle they won't meter fuel as efficiently! Hench Q-Jets usually get better millage. Holley's fuel distribution can be made to be equal to your front & back cylinders & you can get or make them idle on all 4 corners. NO other carb that I know of can do these things? Take a good look at mine & note the idle adjuster on the secondary metering block. This carb works VERY Well & you can make one just like it using some of your parts keeping your costs to a minimum? Hope I helped, Ole' Bob.

 

[GreenRegal87]

Thanks gents!
LOTS of good info here.


I am trying to get a handle on the boosters. I have actually read articles that 100% contradict each other. I initially was leaning towards annular. However, with the stronger "signal" from a dual plane manifold, maybe down leg is better. On ANOTHER hand, annular seems to have better throttle response at "normal" driving RPMs......

Here are a couple of the best articles/forums I've read thus far concerning Annular vs Down leg Boosters:
http://www.corvetteforum.com/forums...ter-vs-down-leg-boosters-what-works-best.html

http://www.hotrod.com/articles/carburetor-showdown/

http://garage.grumpysperformance.com/index.php?threads/holley-annular-vs-down-leg-boosters.5229/

http://www.chevelles.com/forums/13-performance/219426-carb-annular-downleg.html

My plan, this Spring, is to take the car to the chassis dyno and compare my Holley 750 DP against whatever I pick. Wish I had $$$ to buy 4 different ones to compare CFM vs Booster differences.




I have considered it. In fact, I'm likely to do that in the future. However, for the next year or two, I've been watching a couple carbs on e-bay for around $350. Really curious as to the MPG of mechanical vs vacuum secondary. Seems like my car/engine/trans are exactly what a vacuum secondary was designed for. I pulled just over 17 MPG with the double pumper (normal driving). We tuned it on the chassis dyno according to the tailpipe fuel sensor. Fitech was the brand I looked at in EFI.




The stronger signal on the dual plane intake is one of the plus's of going with the down leg booster as well. However, the size of the cam, according to the guy who designed the Demon carb, dictates annular boosters. Barry Grant recommends using the annular-discharge carburetor on any engine with a camshaft duration of 220 degrees at 0.050 or more. Mine is 234.




Correct me if I'm wrong, as I'm REALLY trying to learn here. The bigger CFM of a carb does not necessarily mean it burns more fuel, correct? The down side if you go TOO much CFM in a carb is poor fuel atomization due to low signal, right? I THINK I'm ok in the 670 - 750 CFM if I jet it correctly. The currently 750 is also sitting on a 1 inch spacer with the 4 holes. It dyno'd 10 HP better with the spacer.

As far as Edelbrocks are concerned, it depends on compression ratios..... Im running 9.5:1 in mine, and I called Edelbrock to see what they said about my setup. They said that the 750 and even the 650 would be way too much fuel for the optimum air/fuel ratio with the CR I have... Mine is going to be 99% street driven, and I have a .444/.466 lift (290) flat tappet cam as well.Think of the air/fuel ratio as a oil/gas ratio in a 2 stroke engine... the more oil you run vs gas will make the engine run really really rich, therefore fouling plugs and running sluggish. Its the same with a carb that has a larger cfm, Itll do the same thing run really sluggish and foul plugs. So 600 is optimal for my application, according to the man himself. I prefer a Edelbrock over a Holley all day long, but that's me.

 

[rmorman]

Greetings guys; Ask a question like this & you'll get 101 opinions from a 99 guys, plus you get some of us old guys to toss in $.02 cents worth!

Hopefully I can take a little from everybody and figure out what is right for me! 🙂 Old guys have more experience in my experience!!! LOL!

I've seen so many guys put a DPer on their street car only to have a Big Flat BOG when the hammer goes down. If you use a DPer it's Very easy to over carb, but with vacuum secondaries the carb will only give the engine what it wants when it wants it.

That is part of the reason to try a vacuum secondary. I like making changes to see if I can increase performance without sacrificing efficiency. Plus, I generally get bored if I don't have a project! LOL! For all the reasons I listed above, I would like to go to a vacuum secondary unit.

Holley's fuel distribution can be made to be equal to your front & back cylinders & you can get or make them idle on all 4 corners. NO other carb that I know of can do these things? Take a good look at mine & note the idle adjuster on the secondary metering block. This carb works VERY Well & you can make one just like it using some of your parts keeping your costs to a minimum? Hope I helped, Ole' Bob.

All the carbs I am looking at (Speed Demon & Holley Ultra Street Avenger) have 4 corner idle. Several of the 750 Speed Demons have come up on e-bay for about 1/2 price of new. I am concerned the 650 Demon might not be big enough at high RPM. So maybe I need to decide between the 750 Demon and 670 Holley?? However, the Holley only has "Straight" boosters. I think I at least want down leg if not annular boosters?? My 750 DP has down leg.

Looking at your carb pics, do you have "Straight" boosters on your primary and "down leg" on your secondaries? Can't tell for sure in the pic.

 

[vanrah]

Greetings Rmorman & all; I just noticed that you are only a couple hundred miles West of me. My boosters are all down leg. If you can get a 650 vacuum secondary Demon with 4 corner idle circuits reasonable consider jumping on it. Are you coming to the Car Craft Nat's? Ole' Bob.

 

[gbodytoys]

Thanks gents!

I have considered it. In fact, I'm likely to do that in the future. However, for the next year or two, I've been watching a couple carbs on e-bay for around $350. Really curious as to the MPG of mechanical vs vacuum secondary. Seems like my car/engine/trans are exactly what a vacuum secondary was designed for. I pulled just over 17 MPG with the double pumper (normal driving). We tuned it on the chassis dyno according to the tailpipe fuel sensor. Fitech was the brand I looked at in EFI.

Cool, seems well thought out. $350 vs $1300 are very different, so I get it. I'm most likely going Fitech, but I must get my trans and converter first. I'm in the same HP class at you at the crank...dyno'd at 430.

 

[rmorman]

Greetings Rmorman & all; I just noticed that you are only a couple hundred miles West of me. My boosters are all down leg. If you can get a 650 vacuum secondary Demon with 4 corner idle circuits reasonable consider jumping on it. Are you coming to the Car Craft Nat's? Ole' Bob.

I added a search for a 650 to my current e-bay search for a 750. You don't think the 650 will choke at top end? I figure that and what boosters to run and I've got the world by the nads!

I was at the Car Craft Nats about 5 years ago. Might be about time to go back. Hot Rod Power Tour is the only "National" thing on the list thus far for 2017.

Cool, seems well thought out. $350 vs $1300 are very different, so I get it. I'm most likely going Fitech, but I must get my trans and converter first. I'm in the same HP class at you at the crank...dyno'd at 430.

Good call. Honestly, a good part of me just wants to compare the DP VS. Vacuum Secondary. Then maybe VS. EFI on the same motor! 🙂

Thanks!
Russ

 

[lilbowtie]

With the results you are getting out of your 750, I would be more than happy and leave well enough alone but you appear to be a prefectonist always trying for better. I will tell you I ran a 750 double pumper and ran a 750 vac sec (vac spring set for car) back to back and saw no difference. Surprised me

 

[rmorman]

With the results you are getting out of your 750, I would be more than happy and leave well enough alone but you appear to be a projectionist always trying for better.

Truth is, I doubt I'll gain any peak horsepower/torque. But enough folks have told me that my setup is best suited for a vacuum secondary that I'd at least like to try. I just need to figure out a CFM and booster style best suited for me. And yea..... I'm always looking for the next project! 🙂

I will tell you I ran a 750 double pumper and ran a 750 vac sec (vac spring set for car) back to back and saw no difference. Surprised me

Did you see a MPG difference under normal driving?