Carb formula, anyone?

[78SilvAnniv]

Is there a standard formula for this, or can someone direct me to a site that will figure your best carb size when you plug in standard numbers? Also, what numbers are required for plugging in to figure this out?


If someone has a program at home, here are my numbers to work with...
350ci Chevy crate, rebuilt
1.94/1.50 heads w/ 120# springs
67cc combustion
170cc intake
70cc exhaust

110 lobe cam
216 duration for Intake & Exhaust, advertised at 272 duration
454 lift

3.08:1 rear gears

Thank you for any help. Hubby and I are having a discussion about this.
Heidi

 

[Sensei]

Your stock q-jet is a great street/performance carb, better than anything aftermarket unless you are building a race only car (even then, it is better than most). It is a variable cfm design that is capable of flowing up to 750 cfm (way more than you could ever use) with just a simple 5 minute mod. The formula you are looking for is:
(350, your displacement) X (max RPMs, probably less than 6000) X (Volumetric efficiency, this will be about 85 at best, trust me). Divide this # by 3456 and you have it, but I can save you some time. A 600-650 cfm is what you want. Anything bigger (except q-jet) will give bad low end (normal driving) performance. God bless, Sensei

 

[Bob Chadwick]

There was a similiar, if not the same, formula in that carb book I let Kenny borrow. Let me know and I can scan the pages and email them to you.

Bob

 

[78SilvAnniv]

I love my original carb, we sent it out to a premiere carb man who rebuilt it a few years ago, but the choke linkage is just plain wore OUT! It keeps sticking and it is driving me crazy.
I want a new carb. One without problems.
Kenny and I were discussing what size to get.
Thanks for all the help. Looks like we'll stick with 600-650, and probably purchase an Edelbrock w/ intake manifold.
Heidi

 

[blackandred78]

Edlebrock makes a great Q-Jet replica that has great machining, and very high quality parts. I had an electric choke put on it and it works perfectly - it is a 795 cfm and I mated it to a Weiland aluminum intake - lost tons of high placed weight too. Also put the open element K&N extreme air cleaner - the thing absolutely howls now...have fun!

 

[Jack]

Edlebrock makes a great Q-Jet replica that has great machining, and very high quality parts. I had an electric choke put on it and it works perfectly - it is a 795 cfm and I mated it to a Weiland aluminum intake - lost tons of high placed weight too. Also put the open element K&N extreme air cleaner - the thing absolutely howls now...have fun!

The spreadbore edelbrock qjet is a great carb ... a clone of the spreadbore rochester qjet. Unfortunately, edelbrock ceased qjet clone production a year or two back. Perhaps there are some qjet clones left in the pipeline? AFAIK, the only eb carbs produced now are squarebore clones of Carter AFB.

The qjet (&clone) are spreadbore ... has small primaries & large secondaries.

The afb (&clone & most holley) are squarebore ... has larger primaries & smaller secondaries.

As the afb has larger primaries (where you spend nearly all your time) ... a 600 cfm squarebore would be about right for a mild 350 with good street manners ... go much bigger & vacuum will drop along with low & mid range performance.
JACK:gap

 

[blackandred78]

I would check Jeg's - if anyone would still have pipeline inventory - they would. www.jegs.com.

Works much better than the original - but I kept my old and am going to restore it to have on the shelf just in case!

 

[Art Jett]

I love my original carb, we sent it out to a premiere carb man who rebuilt it a few years ago, but the choke linkage is just plain wore OUT! It keeps sticking and it is driving me crazy.
I want a new carb. One without problems.
Kenny and I were discussing what size to get.
Thanks for all the help. Looks like we'll stick with 600-650, and probably purchase an Edelbrock w/ intake manifold.
Heidi

I have the original carb off my '77 L-48 that I'm never going to use. the car was sitting for 13 years before I bought it, but my friend said he started it a week or so before he sold the car to me and it ran fine for the short time he had it running. He found water in the oil, so he shut it down and it hasn't been run since. I bought the car 4 years ago. Carb looks to be in good shape. Make me an offer if you're interested and we'll go from there. PM me your E-mail address and I'll send you pics of it if you're interested.

 

[Chas]

Edelbrock Performer series

Intake and carburator on my '73....Works great and looks snappy too.
Chas

 

[76 Sting]

Heidi,

Most everyone is stating a 600-650 carb which is correct. You mentioned a possible Edelbrock. Well if you are interested I am installing a new engine in a few weeks and no longer need my 1405, which is an Edelbrock 600CFM and Edelbrock Performer manifold part 350-3701 (with EGR for emissions). They both have approx 500-1000 miles on them and together retail for $385 brand new. I can sell them both for $225, which is $160 less (40% savings) than buying them brand new; remember they have less than 1000 miles on them both. You will have to cover the cost of shipping and wait a little while until I install my new engine, which will happen in the next 3-4 weeks. I also have a fairly new Accell disty for sale with the same miles, basically brand new and willing to let go for $75 ($140 through Jegs).


If you are interested can send you a few snap shots of the set up.
A quick snap shot can be seen here. http://1-4u-computer-graphics.com/76_Sting.htm

 

[78SilvAnniv]

Thank you everyone.

Today we purchased an Edelbrock Performer Intake and Performer Carb, #1406. $385.00 for both.

May I ask what operational and drivers-seat-of-the-pants differences I may observe from my spreadbore Qjet small primaries and large secondaries compared to the Edelbrock square bore that appears to have the same size pri/sec holes?
Heidi

 

[Sensei]

I have tried the Edelbrock AFBs and was not happy. A properly set up q-jet is much better. I'm sure your choke is a constant source of frustration, but my brand new edelbrock auto choke had problems too. My parents bought new cars in the '70 when I was a teenager, and they had choke problems too! The best cure I have found is a manual choke conversion. $10. Pull a knob, hit the starter, push the knob half way in, drive a block or two and push it the rest of the way in. Kiss your frustrations goodbye! (all with no permanant modification!) I don't understand why auto chokes ever caught on, manuals work great! God bless, Sensei

 

[GerryLP]

Thank you everyone.

Today we purchased an Edelbrock Performer Intake and Performer Carb, #1406. $385.00 for both.

May I ask what operational and drivers-seat-of-the-pants differences I may observe from my spreadbore Qjet small primaries and large secondaries compared to the Edelbrock square bore that appears to have the same size pri/sec holes?
Heidi

Heidi,

A little late, but here is an additional carburetor selection guide. My intent is to provide additional information, and it is not to trigger a whole new discussion on which is a better carb (i.e. Q-Jet Vs Square-bore carb).

http://www.holley.com/data/TechService/Technical/Selecting%20A%20Carburetor.pdf

GerryLP:cool

 

[78SilvAnniv]

Thank you Gerry, that was interesting reading and I saved it for future reference and to show to Kenny.
Heidi

 

[wallyknoch]

Just a word of caution, make sure that you buy one with vaccum secondaries. You will not enjoy one of those double pumpers. Most of them are also available with an electric choke.

 

[78SilvAnniv]

I know what vaccum secondaries are (and we got elec choke) but what is a double pumper? I have heard the term, as in Holley Double Pumper...but I see now that I have no CLUE what is meant by the term.
Heidi

 

[JohnZ]

Heidi, a "double-pumper" has mechanical secondaries and has two accelerator pumps, one in each bowl (primary/front and secondary/rear), with two pump "squirter" nozzles - one aimed into the primary venturis and another one aimed into the secondary venturis. The standard vacuum-secondary carb has one accelerator pump in the primary bowl and one "squirter" for the primary venturis, and is a much better choice for normal street operation.

 

[78SilvAnniv]

John, thanks for the explanation.
h

 

[lightingrod]

Heidi,
I think you will be very happy with the Edelbrock setup you choose! The Edelbrock carbs are good for street going engines with a few mod's. In truth the only thing I like about a Q-Jet is the howl. You should see a little improvement with the Performer intake but the real improvement will be less headaches. Electric choke and unlike old Hollies no gaskets below fuel level and no power valves to blow out!

Enjoy,

 

[78SilvAnniv]

That's what I need...trouble-free.
h