CarburetorsCarb discussion and questions. Upgrading a Third Gen carburetor, swapping TBI to carburetor, or TPI to carburetor? Need LG4 or H.O. info? Post it here.
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I have a 90 camaro it has a cam, headers, and flowmaster exhaust! the engine is a an LB9, these are the cam specs
ADV. Duration (INT/EXH): 288 / 288
Duration @ .050" (INT/EXH): 214 / 214
Valve Lift (INT/EXH): .443 / .443
Lobe Centerline: 112
Power Range: 1700-5200
I'd like to know what the best carb to go with for this application
also i know there are alot of stickies saying this but is there any way I could get some simple advice on what to do when switching tbi to carb thankk you
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That cam looks kind of crappy at first glance. Is it a flat tappet cam? If so, why are you doing that? You can put a killer roller cam in there easy.
Anyway since you've got TBI the swap is pretty easy. You'll need a new intake, a distributor/coil, and a carb. Since you're starting with nothing I'd suggest going with a Holley. Go mechanical secondaries if you've got a manual. If you've got an auto, you can do either but it's probably best to go with vacuum secondaries.
A stock q-jet would work well for you, but if you ever wanna move up to a bigger carb (which you wouldnt need unless you did some serious work to the motor...) you'll need a squarebore intake instead of a spreadbore like the q-jet.
Assuming 100% volumetric efficiency (which you probably won't have), the standard formula is: CFM = RPMs x CID/3456.
So, a 305 spinning to 5200 RPMs = 458 CFM. However, that formula also assumes a single plane intake manifold, and a dual plane divides the engine into two halves, so you can usually use a larger carb with a dual plane than the formula says. A 550 would be "big enough", but a 600 wouldn't hurt you.
FWIW, the typical q-jet is capable of flowing about 750 CFM. But, it won't open more than the engine needs, so it won't hurt you down low. You'll need a pretty healthy engine, even a pretty healthy 350, to need more than a q-jet will flow.
If your looking for a nice carb brand, i like how the edelbrock performer series run. As for your cam it looks a little small, If you have a Roller lifter motor you can use a came with more lift and still run it comfortablely on the street. If its a flat tappet i recoment a Split profile cam like a Lunati Voodoo.
__________________ "Speed has never killed anyone, suddenly becoming stationary... that’s what gets you.”-Jeremy Clarkson ,TopGear
i didnt want to use too large of a cam beacause the heads i did not touch and if i ever get a nice set of heads i will consider a larger cam but not anytime soon and i also like the edelbrock performer but could anyone tell me about the chokes on holley carbs they dont seem to have one ( but they must) and if they do then it doesnt look like the edelbrock i like holley carbs but they seem complicated i also like a carb that holds its tuning well
Most Holley Carbs use a manual Choke that you have to use a cable for. edelbrocks use an electric choke.
thank you and my edelbrock 1404 500 has manual choke do you know of any holley part carb part numbers of 600 cfm carbs. also could someone explain secondaries and mechanical compared to vacuum
do you know of any holley part carb part numbers of 600 cfm carbs. also could someone explain secondaries and mechanical compared to vacuum
Need to talk about the 2nd part of that question before the first part.
The typical 4 barrel carb operates on the front two "venturi" (the proper term for "barrel") for "normal" operation, and are therefore called the "primaries". The back two venturi are opened when you get near to wide open throttle (WOT), and are therefore called the "secondaries".
Since there is no flow through the main fuel circuit when the throttle blades are closed, there are basically 3 ways to keep the engine from "bogging" when the secondaries are opened. One is to do the same thing as on the primaries, use an "accelerator pump" to provide the needed fuel until the main circuits are full and flowing fuel - aka, the "double pumper" (which is a Holley registered trademark), or "dual squirter" if a Barry Grant Demon carb; aka "mechanical secondaries", since in this type of carb the secondary throttle blades are "mechanically" linked to the primaries on the throttle arm with no other limitations on secondary opening. The 2nd way is to limit the air flow through the secondaries via an "air valve" above the secondary throttle blades that is slowly opened by the air flow - this is what is used by q-jets and Performer carbs; although the throttle blades are mechanically linked to the throttle arm in these carbs, they don't have a secondary accelerator pump, and the air flow is controlled by the air valve as well as the throttle blades, so they are not referred to as "mechanical secondaries". The third method is to use a vacuum diaphragm to open the secondaries - there is no mechanical linkage to the secondary throttle blades to the throttle arm at all. The vacuum in the primary venturi are used as the vacuum source for the secondary vacuum diaphragm. It takes substantial flow through the primaries to produce that signal, so the secondary opening is gradual. The Holley website technical information link explains this well.
Now, I am no fan of vacuum secondary carbs on a performance engine. They are typically recommended for use with an automatic transmission, but properly sizing a mechanical secondary carb will avoid any problems, even with an automatic transmission. Similarly, I don't like Edlebrock carbs, as they are an old design with no advantages over a q-jet, and several disadvantages over a q-jet.
That being said, the 4776 Holley is a 600 CFM mechanical secondary carb, which can be converted to electric choke. A refurbished unit from Summit will run you $305.95.
so basically a carb with a single inlet might bog when the secondaries open? what are the advantages of a dual inlet over a single inlet?
Inlet probably isnt the term you'rel ooking for.
All the carbs mentioned have four barrels/venturis. For some reason you see this abbreviated as "4 bbl" or "2 bbl" for 2 barrel, which irritates me beyond belief but that's convention at this point I guess.
The difference is in how the secondaries are activated. Vacuum secondaries open as manifold vacuum drops (as it does under high load). When you open up the throttle there is an influx of air available to the motor so the negative pressure in the intake rises and approaches atmospheric pressure, and as that presure rises the secondaries open up and flow more fuel as needed. That suction(negative pressure) provided by the engine is actually what holds the secondaries shut if it helps you to think about it that way.
Mechanical secondaries are mechanically linked to your throttle pedal just like the primaries. You use mechanical secondaries with a manual because between shifts the motor is no longer loaded, and the manifold vacuum increases and the secondaries close. After you have it in the next gear you have to then mash the gas and wait for the pressure to rise enough to open the secondaries again. Makes for a terrible driving experience. But with mechanical secondaries, just putting your foot to the floor will open them up.
Mechanical secondaries tend to work better for racecars. Vacuum secondaries better for mild street cars with automatics.
As five7kid stated, you can easily run a mechanical secondary carb on an automatic car if you want to. A lot of people just prefer vacuum secondaries. Better gas mileage, a little easier to tune with regards to the secondaries as they open with engine load instead of you having to calibrate and tune it to find out exactly how much fuel it needs when you get on it.
But you'll have to tune anything you get, so dont let that stop you. But with that cam and heads... and you dont seem to act like you want a race car, I dont think you'd have any regrets with a vacuum secondary carb.
Last edited by InfernalVortex; 10-01-2009 at 03:00 PM.
thank you that helps alot, i will be using an edelbrock 1404 500 cfm carb with air valve secondaries, what are air valve secondaries and how to they work do they need tuning and if so how should i go about this I have delt with fuel injection and all this carb stuff is new to me, is a dual inlet better than a single inlet and if so why is that, i am using this edelbrock beacause it was free and id like to get this car running i will most likely be buying a different carb when funds allow me to do that it probably will be a holley double pumper but id like to know advantages of dual inlet over single inlet
Vacuum secondaries open as manifold vacuum drops (as it does under high load). When you open up the throttle there is an influx of air available to the motor so the negative pressure in the intake rises and approaches atmospheric pressure, and as that presure rises the secondaries open up and flow more fuel as needed. That suction(negative pressure) provided by the engine is actually what holds the secondaries shut if it helps you to think about it that way.
No, no, NO! That is not how vacuum secondaries work!
thank you that helps alot, i will be using an edelbrock 1404 500 cfm carb with air valve secondaries, what are air valve secondaries and how to they work do they need tuning and if so how should i go about this
Am I just talking to myself here? I already explained it.
Quote:
Originally Posted by five7kid
The 2nd way is to limit the air flow through the secondaries via an "air valve" above the secondary throttle blades that is slowly opened by the air flow - this is what is used by q-jets and Performer carbs; although the throttle blades are mechanically linked to the throttle arm in these carbs, they don't have a secondary accelerator pump, and the air flow is controlled by the air valve as well as the throttle blades, so they are not referred to as "mechanical secondaries".
as you said above my edelbrock has no accelerator pump so what keeps the engine from bogging when the secondaries are opened i am a little confused on this?, and do i need to tune the secondaries to open at the correct time on my 1404
