deja Posted August 19, 2006 Share Posted August 19, 2006 After attempting to tune my 725 cfm Holley for way too long a friend gave me a 600 cfm to try. OMG its like I found 25 HP hiding somewhere. Gonna rip into the HEI tomorrow and see how clean the weights and spring mechanism looks. No clue why I'm doing all this since I plan on pulling the engine in favor of the LT1 in a month or so or when this shoulder heals up a little more. I guess I just want this one to be the best it can be. Anyone interested in a pretty clean 725 cfm Holley? Quote Link to comment Share on other sites More sharing options...
dr_hunt Posted August 19, 2006 Share Posted August 19, 2006 Save it, you'll need it for the big inch motor your going to build in the future. Too much is never enough! Quote Link to comment Share on other sites More sharing options...
deja Posted August 19, 2006 Author Share Posted August 19, 2006 Save it, you'll need it for the big inch motor your going to build in the future. Too much is never enough! LOL, you could be right. I remember only a few months ago saying " I only want a daily driver, I'll never race this car". I should have known better. Quote Link to comment Share on other sites More sharing options...
Twoeightnine Posted August 21, 2006 Share Posted August 21, 2006 May I say............told ya. I remember that you said that you didn't want to buy another one to drop your CFM. Well just keep the 600 and duck your friend! Just kidding. Yep, the small block is often overcarbed. 600 CFM MAX! Stock small blocks can even be happy with as little as 435. But like Dr Hunt says...some day you will have some big displacement and carbs dont seem to be getting any cheaper!! Congrats!! Quote Link to comment Share on other sites More sharing options...
deja Posted August 21, 2006 Author Share Posted August 21, 2006 May I say............told ya.I remember that you said that you didn't want to buy another one to drop your CFM. Well just keep the 600 and duck your friend! Just kidding. Yep' date=' the small block is often overcarbed. 600 CFM MAX! Stock small blocks can even be happy with as little as 435. But like Dr Hunt says...some day you will have some big displacement and carbs dont seem to be getting any cheaper!! Congrats!![/quote'] LOL, a well deserved "told ya so". I guess I keep thinking back to the late 60's when 302 Z28s and Boss302s were running 780 holleys from the factory. Quote Link to comment Share on other sites More sharing options...
Twoeightnine Posted August 21, 2006 Share Posted August 21, 2006 Sorry...couldn't resist! The quadrapuke 780? I had one on a 396 Impala. Would suck birds out of the sky when the massive secondaries flopped open! But bought a Z28 with an LT1 350 that sported a 650 CFM. 66 Galaxie with a....thats right.....a 289 HiPo strapping a 480 CFM. 1970 327 Nova sucking through a 600 CFM. But one of the worst cars I ever shouldn't have owned...74 guttlas Cutlass. 350 GM with a 780 Quadradump. Flow is the deal, do you still have iron exhaust? Maybe you can talk your friend out of the 600. What brand? Enjoy the ponies regardless!! Thanks for putting up with me! Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted August 21, 2006 Share Posted August 21, 2006 Funny. GM recommended a 750 cfm carb with my 6.0L.... Guess heads and intake can make as much difference as displacement. Quote Link to comment Share on other sites More sharing options...
Pyro Posted August 21, 2006 Share Posted August 21, 2006 Carb tune is also very important. Most people just throw on a carb and expect it to work. There are a bunch of things to tweek on a Holley to make it run right. Front and rear jets, power valve, accelerator pump squirters and cams, and vacuum secondary springs to name the most common. And just because the carb is a 750, doesn't mean the engine will use all 750 cfms. Vacuum secondarys seldom open all the way on smaller engines or on carbs with heavy secondary springs. GM put 750 Q-jets on 283's, but that dosen't mean the 283 would suck open the secondarys all the way. That 600 may just run better because it just happens to have a tune on it that complements your engine combo. Do you know what power valve is in your old carb, or what jets and squirter it had, or what secondary spring was in there? Do you know what vacuum your engine is pulling at idle, part throttle, full throttle? Did you read a plug and change jets? Quote Link to comment Share on other sites More sharing options...
Twoeightnine Posted August 21, 2006 Share Posted August 21, 2006 Is not CFM a measure of FLOW? Yes. Heads, intake, and exhaust are the all the members of flow. Yes? And its not just pure CFM, its the method of operation and the type of carb. Secondary size is part of the total CFM. Ever looked at a big vacume secondary? One would think that the primaries didn't even exist! Like dont drop your screw driver! Other factors include the vacuum requirement placed on the carb for excessories. One should account for the weakest link in the flow. It is common for moderately dressed engines to be over carbed. Quote Link to comment Share on other sites More sharing options...
Nealio240z Posted August 21, 2006 Share Posted August 21, 2006 Kinda to go along with this thread, I thought id ask my Question. At high rpm my engine is cutting out. Only when I stab it, or get on it too hard. Could this be because of my 750 Q-jet carb? Is it too much fuel at high rpm. Or is my coil saturated and not giving off enough at top end? Mild built 350...300hp Quote Link to comment Share on other sites More sharing options...
Twoeightnine Posted August 21, 2006 Share Posted August 21, 2006 Timing advance? Q-jets have delayed secondary operation. If it cuts out when you nail it, timing advance. If it cuts at the top end of the rpm sweep, she may be drowning. Quote Link to comment Share on other sites More sharing options...
deja Posted August 21, 2006 Author Share Posted August 21, 2006 Sorry...couldn't resist!The quadrapuke 780? I had one on a 396 Impala. Would suck birds out of the sky when the massive secondaries flopped open! But bought a Z28 with an LT1 350 that sported a 650 CFM. 66 Galaxie with a....thats right.....a 289 HiPo strapping a 480 CFM. 1970 327 Nova sucking through a 600 CFM. But one of the worst cars I ever shouldn't have owned...74 guttlas Cutlass. 350 GM with a 780 Quadradump. Flow is the deal' date=' do you still have iron exhaust? Maybe you can talk your friend out of the 600. What brand? Enjoy the ponies regardless!! Thanks for putting up with me![/quote'] No the 68 -69 Z28 had Holley 780 cfm 3310 carbs, not quadrajets. My 70 390 AMX had a smog 500 Autolight carb, Holley clone. I took it off and put on a Holley 780. It was like bolting on a supercharger! You're right about flow, I am still running iron exhaust. We've already made a deal on the 600, its a Holley. No problem, its like I'm learning all this stuff all over again. Too long out of the gear head world. Quote Link to comment Share on other sites More sharing options...
Pyro Posted August 21, 2006 Share Posted August 21, 2006 I'm not a Q-jet expert but I would find the adjustment that controlls the opening rate for the secondarys. On a Holley it is simple, just put in a stronger secondary spring, there are about 7 to choose from. Do a little research on the web to find out how to tune a Q-jet. Q-jets are great carbs, and your should learn them if you are going to run one. Could also be a lean condition. I'm sure there are more than a few books out there about Q-jets. The beauty of a properly tuned vacuum secondary carb is the engine will only use as much as it needs. A 750 carb will not flow 750cfm's until the secondarys are completely opened, and that doesn't happen as often as you think. Quote Link to comment Share on other sites More sharing options...
deja Posted August 21, 2006 Author Share Posted August 21, 2006 Carb tune is also very important. Most people just throw on a carb and expect it to work. There are a bunch of things to tweek on a Holley to make it run right. Front and rear jets' date=' power valve, accelerator pump squirters and cams, and vacuum secondary springs to name the most common. And just because the carb is a 750, doesn't mean the engine will use all 750 cfms. Vacuum secondarys seldom open all the way on smaller engines or on carbs with heavy secondary springs. GM put 750 Q-jets on 283's, but that dosen't mean the 283 would suck open the secondarys all the way. That 600 may just run better because it just happens to have a tune on it that complements your engine combo. Do you know what power valve is in your old carb, or what jets and squirter it had, or what secondary spring was in there? Do you know what vacuum your engine is pulling at idle, part throttle, full throttle? Did you read a plug and change jets?[/quote'] Oh yeah, I know nothing is that simple. Thats why computer controled fuel injection was invented, LOL. My plan in the near future is switching the an LT1 so I'm not going to spend a lot of tiem and money fine tuning this engine. Quote Link to comment Share on other sites More sharing options...
Mike kZ Posted August 21, 2006 Share Posted August 21, 2006 Kinda to go along with this thread' date=' I thought id ask my Question. At high rpm my engine is cutting out. Only when I stab it, or get on it too hard. Could this be because of my 750 Q-jet carb? Is it too much fuel at high rpm. Or is my coil saturated and not giving off enough at top end? Mild built 350...300hp[/quote'] You may be running out of fuel at the top end. Quote Link to comment Share on other sites More sharing options...
Twoeightnine Posted August 21, 2006 Share Posted August 21, 2006 Carbs can be a bitch unless one gets lucky.Or does so serious matching. A good breathing engine can consume a lot more fuel than it needs. this can lead one to believe that the engine needs that much. Ok, Ok every one loves numbers and formulas, so here you go. One of the first things to determine is what size carburetor do you need? A number of factors come into play. What cubic inch size is your motor? What do you do with it? Race, street use, towing, street use with occasional trips to the track? What type of intake manifold do you have, split plenum, open plenum, tunnel ram, individual runner? How fast do you spin the motor? What is the volumetric efficiency of the motor? Do you have a manual or auto transmission? What is the rear gear ratio? Do you want to get the best gas mileage possible or do you want to develop the most power possible? Keep in mind that a carburetor is just one part of the engine combination. All of the parts need to work together. Putting a larger carburetor on is NOT going to immediately put 100 more horsepower at your disposal. The carb needs to work with the other parts you have chosen and your intentions concerning how the vehicle is going to be used. One general rule of thumb uses a formula to determine the CFM requirements of your engine. It goes like this: You need to know the CUBIC INCHES of the motor. You also need the maximum RPMs the motor will be spun to. Finally you also need the VOLUMETRIC EFFICIENCY PERCENTAGE (VE%) of the engine. The first two items (CUBIC INCHES and RPMs), are relatively easy to determine. The engine VE% is another matter. If an engine could use all of the air it ingested, it would have a VE% of 100%. Many performance engines reach this level. Certain race engines can actually exceed this and reach a VE% of over 100% at certain points in their RPM range. Most production engines and most street performance engines have VE levels below 100%. In fact, stock, production, low performance motors will fall around 75%-85% volumetric efficiency. The math formula is: CARB CFM = cubic inches X rpms ---------------------------- X VE% 3456 So if you had a stock, low performance production motor of 350 cubic inches and you wanted to spin it to 5 000 rpms max and it had a VE% of 80%, the formula would determine a required carb CFM of 405 CFM. If you had a warmed over street performance motor of the same size, but it was capable of 7000 max rpms and it had better heads, camshaft, headers and a performance intake that raised the VE% to 95%, the formula would give you a minimum required carb CFM size of 673 CFM. Quote Link to comment Share on other sites More sharing options...
Twoeightnine Posted August 21, 2006 Share Posted August 21, 2006 Laymans terms. Take your cubic inch displacement, times that by your TOP RPM requirements, divide that figure by 3456. Times the sum by the volumetric efficiency. Dont for get your decimal. Example 75% VE =.75. Or you can just read the first three numbers in your final sum. Remember to be realistic about your VE. A well tuned built engine with all of the goodies will fall about 95%. Some what stock...75% or less. A tired stock engine...65%. Quote Link to comment Share on other sites More sharing options...
dr_hunt Posted August 21, 2006 Share Posted August 21, 2006 Timing advance?Q-jets have delayed secondary operation. If it cuts out when you nail it' date=' timing advance. If it cuts at the top end of the rpm sweep, she may be drowning.[/quote'] Q jets are completely adjustable, unlike a holley that you have to change springs on for the secondaries to open. The qjet has a screw and a allen set screw on the passenger side top to adjust the spring tension on the secondaries. 3/4 turn from touching for a big block and 1 turn for a small block. Of course that's just factory settings. My monte went 12's with a qjet on a 406. Quote Link to comment Share on other sites More sharing options...
Twoeightnine Posted August 21, 2006 Share Posted August 21, 2006 Q jets are completely adjustable' date=' unlike a holley that you have to change springs on for the secondaries to open. The qjet has a screw and a allen set screw on the passenger side top to adjust the spring tension on the secondaries. 3/4 turn from touching for a big block and 1 turn for a small block. Of course that's just factory settings. My monte went 12's with a qjet on a 406.[/quote'] A q-jet is a sweet thing when they are working properly. I'll venture a guess that you may have had the rpms up just a tad before launch. hmmm? I'll say this, the sweetest sounding engine I have ever had the fortune to punch was the afore mentioned 396 with a 780 QuadraJet! It just seemed made for that block!! Good stuff. Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted August 22, 2006 Share Posted August 22, 2006 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? Quote Link to comment Share on other sites More sharing options...
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