What a difference the right size carb can make

[Twoeightnine]

What about mechanical secondaries?

 

holley's site says our light Z cars with low rear gearing are acceptable candidates for mechanical secondaries. Any one have experience with these?

Had two.

Both were adjustable as to the activation point. As for the streetability, mine seemed to be a little picky. But I must confess that my knowledge was limited at the time. Probably could have had them adjusted differently but, basicly I had it begining its swing at just past half point. That way I rev it from the line and still get good mid range torque. There were also available various eccentrics which would alter the exponentiality.

It's a performance carb made to go. And go now!

[Twoeightnine]

Neal' date='

We need a bunch more information about your engine. Maybe you should start your own post.[/quote']

Very true there. Alot could be going on.

Eliminate the coil situation by adding a hot performance set up. You know you want it, you know you need it. You seem to have doubts so, you might look there.

Seems like from there we have a few gents that are Q-jet and carb adjust ment savy. If your timing is good, and it needs to be checked through out the rmp range, and it checks out, you might have your carb as the culprit.

For me its always been to get the right size carb, jet it for its altitude, mixture with EGT, idle settings, and forget about it. All this jacking around has never been for me so that leaves me weak in the tweaking area.

Some one here will have the answer....

[Guest ZFury]

No one answered my question about 12 replys ago.... Thats okay

Because your question has nothing to do with carbs. Put your question in another thread about ignitions.

 

 

 

 

And for the rest of you old car snobs. Turn up your nose else where, and dont tell me "you need". Why do you think I dont understand physics, maby I have a math degree? Obviously fuel delivery (and other restrictions) in cars have become secrets after the late 60's or so. Vettes were suddenly loosing HP off of the factory line with all other so called sports cars. Because some group of people decided to keep these secrets for what ever reason (and there are many selfish ones), much of the information is lost, regressing the devolopment of the automotive field (mainly in the aftermarket). Plus much of the car feild today are only told what they need to know. Like this bolt goes here, this screw goes there. Then they think they know it all because they can put it together. I am sure Holly knew that their book was being written to help sheep the people. I mean, Edlebrock and many others manufactured manifolds that defies all of Hollys instructions, and so called math facts. If you look at a TB injected indy car. The 10 TB's which are about the size of the intake valve are made for each intake port. And there is enough vacuum to draw every drop of fuel in from those injectors spraying fuel into mid air. Put your finger over the intake of one of those indy cars TB's and tell me there isnt enough vacuum for a carb. Indy cars get great throttle response. The limits of TB injection are close/similar to carburetion. Just the fuel cuve is more adjustable. The main problem you will get from over carbing is that when you press the peddle own, lets say 75% of the way, the carbs are open enough to supply the motor with its maximum cfm intake. Opening the throttle more is not needed and will only change your air/fuel ratio due to the drop of vacuum in the carb. Which should equal less fuel. Which would equal a loss in HP, not a "bogging" down. Bogging is usually due to running to rich. Now for any that want argue, you will probably sound like one of these guys saying that your cars stock inatake works better than any after market set-up. That golf ball size hole that your 90mm TB pulls air through richens' the fuel mixture giving you more HP.

[jt1]

posted by Zfury

 

And for the rest of you old car snobs. Turn up your nose else where, and dont tell me "you need".

 

I've been called a lot of things, but this is a first for "snob".

 

Why do you think I dont understand physics, maby I have a math degree?

 

If you have a math degree, physics degree, or spelling degree, you hide it well.

 

Obviously fuel delivery (and other restrictions) in cars have become secrets after the late 60's or so. Vettes were suddenly loosing HP off of the factory line with all other so called sports cars. Because some group of people decided to keep these secrets for what ever reason (and there are many selfish ones), much of the information is lost, regressing the devolopment of the automotive field (mainly in the aftermarket).

 

You're saying all developments in fuel delivery since the 60's have been kept secret?

 

......a bunch of gibberish concluding with....Now for any that want argue, you will probably sound like one of these guys saying that your cars stock inatake works better than any after market set-up. That golf ball size hole that your 90mm TB pulls air through richens' the fuel mixture giving you more HP.

 

Sir, for you:

 

 

John

[COZY Z COLE]

This thread had good info at the start..... Stop the name calling NOW.... If you want to toss words at each other do it off- line and PM away... Let's get back to a civil exchange of thoughts and info...

 

 

LARRY

[Twoeightnine]

I understand the distress but, what does fuel injected Indy cars have to do with carb technology?

Of course force feeding an engine with fuel allows one to, or should I say "requires" one to have a larger air opening.

Did I miss something here?

I have read and read not only the manufactures information but, industry leading professionals advice. People that have no stake in carb sales.

Why would one disregard the facts? I dont get it.

 

Thanks for the disscussion however. I will keep an open mind regardless.

[grumpyvette]

i think you might find this of interest (FROM EDELBROCK, THEY MAKE INTAKES AND CARBS)

 

CFM Rules...

CFM and Carburetors: Carburetors are rated by CFM (cubic feet per minute) capacity. 4V carburetors are rated at 1.5 inches (Hg) of pressure drop (manifold vacuum) and 2V carburetors at 3 inches (Hg). Rule: For maximum performance, select a carburetor that is rated higher than the engine CFM requirement. Use 110% to 130% higher on single-plane manifolds . Example: If the engine needs 590 CFM, select a carburetor rated in the range of 650 to 770 CFM for a single-plane manifold. A 750 would be right. An 850 probably would cause driveability problems at lower RPM. A 1050 probably would cause actual loss of HP below 4500 RPM. For dual-plane manifolds use 120% to 150 % higher.

CFM and Manifolds: Manifolds must be sized to match the application. Because manifolds are made for specific engines, select manifolds based on the RPM range.

 

CFM and Camshafts: With the proper carburetor and manifold it is possible to select a cam that loses 5% to15% of the potential HP. These losses come from the wrong lift and duration which try to create air flow that does not match the air flow characteristics of the carburetor, manifold, head and exhaust so volumetric efficiency is reduced. An increase in camshaft lobe duration of 10 degrees will move the HP peak up 500 RPM but watch out, it may lose too much HP at lower RPM.

 

CFM and Cylinder Heads: Usually, cylinder heads are the limiting component in the whole air flow chain. That is why installing only a large carburetor or a long cam in a stock engine does not work. When it is not possible to replace the cylinder heads because of cost, a better matching carburetor, manifold, cam and exhaust can increase HP of most stock engines by 10 to 15 points. To break 100% Volumetric Efficiency, however, better cylinder heads or OEM “HO” level engines are usually needed.

 

CFM and Exhaust: An engine must exhaust burned gases before it can intake the next fresh charge. Cast iron, log style manifolds hamper the exhaust process. Tube style exhaust systems are preferred. But headers are often too big; especially for Performer and Performer RPM levels. Improving an engine’s Volumetric Efficiency depends on high exhaust gas velocity to scavenge the cylinder but this will not happen if the exhaust valve dumps into a big header pipe. On the newer computer controlled vehicles it is also important to ensure that all emissions control devices, and especially the O2 sensor, still work as intended.

 

CFM and Engine Control: Spark timing must be matched to Volumetric Efficiency because VE indicates the quantity of charge in each cylinder on each stroke of the engine. Different engine families require distinctly different spark advance profiles. And even engines of equal CID but different CR require their own unique spark advance profiles. Rule: Expect 0.1% to 0.5% loss in Torque for each 1 degree error in spark timing advanced or retarded from best timing. Also, detonation will occur with spark advanced only 3 degrees to 5 degrees over best timing and detonation will cause 1% to 10% torque loss, immediately, and engine damage if allowed to persist. "

 

then read this CAREFULLY

"

http://www.chevyhiperformance.com/techarticles/148_0403_seven_holley_carbs_test/

 

 

http://www.edelbrock.com/automotive/math.html

[dr_hunt]

The Q-jet is simple, read the directions that come with the rebuild kit. Tear it all the way down and I mean all the way. Soak it in carb cleaner for a few hours, rinse it off, blow with compressed air all the passages. Ok, so you install the base plate and gasket then comes the idle air jets (hopefully you have an early qjet otherwise you drill the lead coverings to access them) Install them 1.5 turns out from closed to start with. Make sure the linkage is free and the primary throttle blases cover about half the transfer slot (vertical slot in primary base plate in venturi area), Next is the primary metering rod adjustment. On some qjets it is adjustable from the front of the carb on others it is not. If it is not take a pair of wire cutters and cut about 1/8" off the little brass rod sticking out of the primary metering rod barrel thingy that holds the primary metering rods. If it's adjustable, lower it until the top of the barrel thingy is about flush with the little hard plastic retainer when you push it down. This is your cruise part throttle leaning effect, this will give you more mpg's when cruising and have no effect on WOT performance. Install the float, needle and seat and set to factory spec then install the primary metering rods and barrel, put top gasket on, should hold it all into place, don't forget the spring under the primary metering rod barrel. If you lower the float a 1/16th it'll lean it out a bit but no reason to do that, just change metering rods if you need too. The air valve for the secondaries is a little screw on the passenger side, underneath is a allen set screw to hold it in place. You release the set screw, tighten the screw until the spring just touches and starts to have tension. Tighten 3/4 turn for sbc, 1 turn for BBC. You can play with this later on the vehicle if it needs more but I doubt it will. I like to install the top of the carb with the secondary metering rods in place, but it's kind of tricky to get them in the secondary jets, make sure you get them in the jet holes right. Bolt it on and it should work fine, if you need more fuel change metering rods, GM has a chart that goes from lean to rich and they have numbers like 041 etc. the secondary metering rods are much the same as is the hangars, the performace secondary metering rod hangar is a K hangar, you'll see some j's but most are K's. Not a big deal, but the plastic cam also wears out, might change that too, they come in colors, so make sure you get the right color. The early qjets are more performance, the later ones are emissions. The best one to get is an early one '75 through '77 with the electric choke. I likey the electric choke. The heat chokes suck IMO.

Wire the electric choke hot wire to the alternator output wire (2 prong plug in in a GM) That way if it dies for some reason the choke doesn't come off during cold weather before the engine warms up.

 

Bolt on and adjust idle air for best vacuum not smoothest running. Timing on stock should be about 12 to 14 degrees intitial advance with vacuum disconnected. Bigger cams need more initial advance IMO.

 

Just my $.02

[Guest ZFury]

I really like these carbs. THough I have not had a chance to get one.

The Predator Carb, a variable venturi carb.

[Twoeightnine]

i think you might find this of interest (FROM EDELBROCK' date=' THEY MAKE INTAKES AND CARBS)

 

 

CFM and Cylinder Heads: Usually, cylinder heads are the limiting component in the whole air flow chain. That is why installing only a large carburetor or a long cam in a stock engine does not work. When it is not possible to replace the cylinder heads because of cost, a better matching carburetor, manifold, cam and exhaust can increase HP of most stock engines by 10 to 15 points. To break 100% Volumetric Efficiency, however, better cylinder heads or OEM “HO” level engines are usually needed.

 

CFM and Exhaust: An engine must exhaust burned gases before it can intake the next fresh charge. Cast iron, log style manifolds hamper the exhaust process. Tube style exhaust systems are preferred. But headers are often too big; especially for Performer and Performer RPM levels. Improving an engine’s Volumetric Efficiency depends on high exhaust gas velocity to scavenge the cylinder but this will not happen if the exhaust valve dumps into a big header pipe.

Awesome post, awesome read. Thanks for the links as well. Good stuff.

Headers are often too big? What is too big? I read through all of the info but could not find any recommendations. I will be pipeing next month so I was wondering. The header manufacture (sanderson) issued a 2.5 inch recommendation for the motor that I have built. Sounds about right. But does anyone have math on an alternate sizing scheem. Or should we trust the exhaust manufacturer? I would have no prob going wit a 2.0 inch. Prob even fit better.

I understand that we still have exhaust port considerations but a 2.0 inch might just do it. 2.25 for sure.

The 383 they selected was far from a mild build so the numbers they got weren't too surprising. Except for the ability to be able to use the 850 wasn't the test about right? Would one venture to guess what the number would be it they had selected a 350 chevy with iron exhaust? A 350 with a mild build? I think it would be clear.

Good stuff...thanks.

[grumpyvette]

this will get you started in the correct dirrection,(KNOWING WHAT YOUR TALKING ABOUT) BUT READ CAREFULLY,YOU NEED TO FULLY UNDERSTAND HOW THE COMPONENTS EFFECT THE RESULTS

 

http://victorylibrary.com/mopar/header-tech-c.htm

 

http://www.newcovenant.com/speedcrafter/tech/camshaft/1.htm

 

http://www.isourcecenter.com/results.php?&q=calculat%20header%20primary%20length

 

http://www.isourcecenter.com/results.php?&q=calculat%20header%20primary%20length

 

http://www.e21motorsports.com/kendanielson/header%20design%20article.htm

 

http://www.pontiacracing.net/js_header_length1.htm

 

http://www.newcovenant.com/speedcrafter/calculators/runnerarea.htm

 

http://www.newcovenant.com/speedcrafter/calculators/intake.htm

 

http://www.burnsstainless.com/TechArticles/Theory/theory.html

 

http://www.btinternet.com/~mezporting/exhaust_length.html

 

http://www.stahlheaders.com/Lit_measurement_1.htm

 

http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm

 

http://www.mustang50magazine.com/techarticles/4328/

 

http://www.grapeaperacing.com/tech/

[Z-Dreamer]

This site is also good, www.headerdesign.com

 

 

Mark

[Twoeightnine]

Thanks Gents!

I shall read away!