Where are the carb gains?

[mindlos]

This is probably obvious but what is it about other carbs, like Webers, that makes them give horse power gains vs. stock? Is it that they have jets capable of higher fuel flow and higher CFM or is it the fuel+air mixing?

Here's my dilemma: you are not always flooring the throttle. Actually it is wasteful, right? There is a point where you get diminishing returns on acceleration even before the pedal gets to the metal. Which means that there's something about the engine displacement limiting you that is not accomplished by giving it more fuel and air. So how do the Webers and Holleys do it?

Let me wander a bit...get a little quantitative

-1 cubic foot is about (30cm)^3 = 27000cm3
-1 liter = 1000cm3
Therefore: 1CF = 27L
-Keihin carb is say 220-240CFM (based post read) = 5940-6480 L per min
-2 L liter engine draws only 1L air (fuel very minute) per revolution because intate + compression + compustion + exhaust is 2 revolutions per cylinder.

At 6000RPM, a 2L engine draws 6000 L air which is right at the the limits of the 220-240 CFM carb. But then in the 1000-4000RPM range its not like the carb is maxed out. If the engine can chomp it and wants more the carb can give it, right? You are welcome to tell me its not a perfect world and that operational limits dont dictate sub-limit efficiency of 100%. Of what though? Supplying transient demand (during acceleration)?.

Maybe I'm looking at it wrong. I know not everything is about numbers and limits. In that case what are the designs that enable HP and acceleration gains based on all the posts I saw. After all, what use are horses if you cant get better acceleration or better performance at any load bearing operation. Unless all you want is greater max speed. I know those are not oranges to oranges either due to other factors.

[shepherd79]

OK, i think this is going to be a long post.
OK, you are wrong........ well, lets say your are right in a few spots.
Lets start with CFM. To fing out how much CFM engine pulls at certain RPM we will use this formula:
Engine Size (CID) X Maximum RPM / 3456 = CFM @100% Volumetric Efficiency
ok, so now we know the formula lets plug the numbers and see what can we get for our little 2L(119cid) engine.
119x6500(max rpm)/3456=223.81 lets round up to 224CFM
ok so we know how much can 2L engine pull at 6500rpm, lets look at carbs for a second.
Keihin, noone knows how much CFM is making. I always said it is around 150-180CFM. it is a small carb. So lets take 180cfm for example, we can see that is not even enough to fill the needs for more air and fuel. ok, we can make keihin spit more fuel but there still a problem with air.
Lets look at Weber. Again noone knows how much CFM it is. I always assumed that Weber is 1.5 times more that Keihin. which means Weber should be around 225- 270cfm. OK, compare that to the engine needs and you will see that it is clearly enough go see some major gain in HP. And i wasn't using sidedrafts. they make even more CFM which means more air and fuel.
So as you can see, with bigger carb you should be able to see major HP gain. About Acceleration, it doen't depends on just HP. it also make huge diff if you have 5speed or auto. i know i can out run auto with my stock(little modified) Keihin and 5 speed.
HP will make it run faster, it is just you need to find right combination.
The other thing is that our stock cam sucks major balls between 2K-3Krpm, but after that it is a beast in my opinion.
well this is all i have for now.
anyone has something to add or kick me in a butt for giving wrong ideas to newbie.

[A20A1]

What do you want... more response and acceleration but with a greater increase in air restriction on the top end. or More top end power with poor acceleration and or throttle response at the bottom.

The fun thing with the sidedrafts and the IR set-up is that the venturis, jets, runners, and velocity stacks can all be changed with a turn of a wrench or a screwdriver, provided you have the needed parts. You have the ultimate say is how your induction side of the head is tuned. I know EFI guys could have their tuning devices riding shotgun and fuel programs changed with the press of a button... and I admit the adaptability and control is tempting to use. But I hear efi cars on the road and I hear carbed cars on the road... and the sound and performance of a carb is just 10x more appealing to me. :D that is all.

[A20A1]

1) Weber is better at fuel/air mixing and allows the carb to be adjusted via the jets and booster venturi for added response or for a slight increase in air flow.

2) Not always flooring the carb on a progressive 2-bbl carburetor is not wasteful. I think the number of fuel circuits is 4:
- IDLE (manifold vacuum controls fuel delivery here and the throttle plates keep the high vacuum signal from reaching the venturi)
- PART THROTTLE (the restriction the the plenum or runner is dropped and manifold vacuum drops the idle circuit isn't fully used and the venturi begins to recieve a vacuum signal along with the booster ventuir, and vacuum secondary.)
- WOT Idle circuit is not in use and venturi's now has full vacuum, th venturi vacuum is high enough to open the secondary all the way unless a heavier spring was used.)
- POWER VALVE (upon the drop in manifold vacuum a spring loaded plunger opens a valve releasing extra fuel.)

As you see each fuel circuit is based on engine demand and in some cases, air by way of a vacuum controlled secondary throttle.
(Vacuum secondary is controlled by engine demand based on venturi port vacuum.)

[Mike's89AccordLX]

A20A1 What is the Weber 32/36 sidedraft or downdraft? Does Weber make a better carb than the 32/36 that will just bolt on (or what I mean is, made for our cars?)

[A20A1]

the 38/38 which is usually called a 38, is a synchronous 2bbl carburetor. It's more performance then the 32/36, but as consequence gas milage will suffer. I doubt the drop in gasmilage would break the bank though.

[Mike's89AccordLX]

If I were just occasionally street racing would I go with the 32/36? How much more hp is the 38 compared to the 32/36? I just need to know some details before spending some money.

[mindlos]

Man, shepherd79, you are are confusing me with all those cubic inches. For a second I wondering what the hell is CID and where is the constant you are dividing by derived from. See... I'm just a metric brotha who live in the frozen tundra. That formula makes sense and is basically the same calc I did, barring the fact that assuming 100% volumetric efficiency may have not been smart on my part. I guess obstruction and resistance also brings the capable CFMs down.

So basically those after market carbs do their function better than the stock ones and the infinite customizations to fit your needs.

I know about the whole automatic thing and torque converter wasting some of the power and the rest of the transmission having to catch-up all the time.

I'm thinking of getting a tune-up (had the car 2 months now) and maybe shift my powerband coz I do like to get to 60MPH fast, thus I'm a heavy footed driver. I do 70-80 given there are no cops or slow traffic in the fast lane and I get nervous and flustered like I need a hit if I'm in the 50s. However, I dont race so I dont think I have enough guts to get one of those Webers.

I just get annoyed trying to pass some people sometimes when going uphill or flat coz they are driving 50. Actually I often shift down coz 2000RPMs is a true bitch on 5. Adjustable CAMs would be the shit I guess.

Thanks guys. I really just need another car to mess with to absorb all the stuff.

[shepherd79]

actually, do you have emission controls where you live?
if you don't you could remove all that vacuum crap. I did on mine, and i can nottice the diff in performance. i am not sure how much HP it gave me. probably no more than 10hp. but it has better responce than will all that crap on it. Keihin carb is not that bad for daily driving when you remove all the crap.
You could get adjustible cam gear. and advance it. it will shift the power band to lower RPM range. this way you should be able to get a little more torque and HP at lower RPM.
Oh yeah, i am sorry about confusing you with all that standart staff. i am from russia, so i am a metric guy too. but i am getting used to all this american stuff.

[mindlos]

Funny you should say, I've been waiting on the no vacuum HOWTO :D. Not to say you promised or anything but are you still gonna do it? You can make it short and sweet just for me. Just give the the number of plugs and the line #s to plug. I would hate to reinvent the wheel you already have. We have no emission control in Minnesota. I'm only dreaming about the CAM just as much as I'm dreamin about the Webers after reading all those posts. Since I cant/wont get the CAM would advancing the distributor + high octane at least bring me some noticable performance gains?

[A20A1]

I thought there was a how-to floating around in one of the carb tech threads. It may not be labled as a how-to, just inserted into some random thread to answer a question.

[shepherd79]

yeah, i need to get my shit together. i will make it as soon as possible. school is a killer this semester.
if you have AIM, just IM me one evening and i will walk you through with pictures.