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Old 09-09-2008, 11:06 AM
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vacuum secondaries v.s mechanical v.s. air valve

Hey guys i was looking into gettin a bigger carb last night. I am currently running a holley 600 cfm 80457. It is fairly new but i have had nothing but trouble out of it for some reason. Plus a lot of people have told me that it's probably not big enough for what im putting it on. (SBC 355 KB119 Dome pistons, Lunati bracket master 2 300* adv. dur. .515/.515 lift). It curently has stock heads from a 69 chevelle 350 on it(not permanant), however I have finally decided on the heads i am gonna get.

I am planning on getting a set of the Brodix IK 180's with 70cc chambers. This should put me around 10.45:1 compression.

Anyway, I was just wundering what are the advantages and dis-advantages to running any of these carbs. I was thinking that it may benifit me to go with a mechanical secondary 750 carb, because the Vacuum secondary 600 that i'm running now takes forever for the 4 bbl to open. Its almost like the vaccuum isn't strong enough, would my cam cause this low vacuum? I also looked into the Edelbrock 1407 performer 750 and it is an air valve secondary. I have heard that is what the old Qjet carbs were too, but i have no idea how an air valve works in comparison to a vacuum carb. Can someone please explain to me how an air valve secondary works? I don't have much expierience working on carb's but i would like to learn how to. It seems i learn something new from you guys everyday.
thanks Danny

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Old 09-09-2008, 11:26 AM
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spend a little time reading in the "Knowledge Base" link at the top of this page under the engine then carb section....

here's just one article example from the Knowledge Base that answers most of your question:

http://www.holley.com/TechService/FAQ.asp
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Old 09-09-2008, 12:30 PM
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knowledge base

Ok well that holley sight helped a little bit but when i went to the knowledge base section like you said i couldn't find a carb category. I looked in the engine section but found nothing. I've done the little paperclip zip tie test and I know my secondaries are opening but they feel as though they take to long to do so. Plus i still don't really feel like it is big enough for what i'm trying to accomplish.

Can you tell me how the Air valve secondaries work? Thats what i'm really trying to figure out. and what would be my best option. I know holley is gonna say what they gotta say to sell thier stuff, i wanna know from expierience. How would my engine perform with a mechanical or air valve sec. carb.
I also am running a TH400 and a brake booster straight off the intake manifold. I don't know if that helps you guys or not.
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Old 09-09-2008, 12:37 PM
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If the secondarys are opening, but seeming too late, you need to change the spring on top of the vacuum secondary diaphram. Holley makes an assortment of springs in various strenghts, and you can tune the secondary opening to match your application.
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Old 09-09-2008, 02:08 PM
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whoops,
carb section is under engine then "fuel delivery"

http://www.hotrodders.com/kb/carburetor

stock chebby heads flow less than 500cfm at 5700rpms, (their practical max) so your carb is plenty big until you do get better heads to be able to shift at the 6500rpms HP peak of your cam.....

operating range for that cam (246/246/108) is 2500rpms+ so if you don't have the correct dist timing for the cam and/or stall convertor and/or low enough rear gears....
it will be a PIA challenge to tune in any type of carb!
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Old 09-09-2008, 03:33 PM
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stall

yea i'm running a 33-3500 dacco stall converter. I know i could use more rear gear, but that is coming in due time. I think the rear is 3.08 now, I'd like to go to 4.11 or so. I just talked to the place that sold me my carb and it does have a lifetime warranty on it, so he said just send it in. I guess i'll do that for now till i get my better heads and stuff. money, money, money,
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Old 09-09-2008, 03:48 PM
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Try this approach, Remove the secordary diaphram housing and remove the check ball inside the housing. Don't change secondary spring, YET. If it stumble, then go for heavier spring. If car takes off with no stumble then go for a lighter spring. If the car start jerking while driving normal, the spring is too light. Your secondary are opening then closing causing the car to jerk.
Try both way with the check ball and without.
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Old 09-09-2008, 04:30 PM
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a favorite ever so true, ever so often saying: 80% of all carb problems are in fact ign timing problems....

to help that 246/246 cam get past the next to no Hg when loaded below 2500 rpms "hump"..... the motor needs 18*+ base timing to make more Hg

hard to say with your parts combo but I would have 36* all in at around 3200-3500 because of the 3.08 rear gears for no detonation

here's a good read on ign timing:
http://www.gnetworks.com/v4files/bar...withimages.pdf

for sorting out any motor it is alway find the best timing first which accounts for 70%+ of the possible performance, then tweak the carb as the last step....

if your present base timing isn't atleast 16* try more base timing before you return the carb
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Old 09-09-2008, 05:32 PM
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timing

Well I have not put a timing light on it yet. I have never even used a timing light to be honest with ya. I've just always set it by ear and by performance. I would think that if i can turn the enigine off and start it right back up (at operating temp) with out any hesitation, my timing can't be that far off. The car was running great, then I just came out to the garage one morning and the thing didn't wanna start. Then when i did get it going it won't stay at an idle unless i hold the choke butterfly 1/2 way closed even once it warmed up. I already had another thread going about carb trouble though so feel free to check it out if you want. It's surging Idle between 1800-500 rpms if the choke is wide open. The secondaries are dripping, Air and fuel was bubbling out of the seal between the primary metering block and mainbody.
All of this just happened out of the blue. Nobody touched the car. Please see the other thread.

Anyway long story short, I'm pretty sure the timing is pretty close to were it needs to be if not right on. Mabey a small turn of the Dist. is in order, but not much. I really can't get it any closer till i get the carb operating properly
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Old 09-09-2008, 07:18 PM
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di you change the rubber object that iehter engages the seconadaries early or later black is for later which is stronger which resists the vacum and a yellow one that is weaker and opens sooner..just some info sorry i cant remeber the name of the rubber piece. mark
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Old 09-10-2008, 09:10 AM
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BORROW a timing light and a vacuum guage and set up the dist correct!!!!!

your cam and mis-matched parts combo allows a very narrow timing adjustment range-window to have even a decent timing settings adjustment before you set up the carb....

consider this: at only 2,000rpms a piston is moving at 1,000ft+ per minute!!!!

unless the only micro-seconds long spark event is delivered at the precise optimum time based on where the piston is located in the cylinder while moving at XXXX ft per minute and the particular camshaft events timing....both the Hg and TQ will be way off....
just a 1* timing error can be a 2,000 psi difference in power stroke output which determines the Hg carb signal velocity and quality....
with your cam and car parts combo you need every last psi and Hg you can get below 2500rpms....

just the fact that it does start fine hot indicates the timing is not in fact nearly correct....
at 18* base, unless the starter system is in super excellent working order (or there is no CR) you will hear the motor cranking over slower because more base timing makes more compression (counter rotation direction) resistance by lighting the mix sooner while the piston is still way down in the cylinder on it's way up....

edit add:
it's great that you do want to learn how all this stuff does in fact work first....(very smart move)
here's a link that is going to make your eyeballs ache due to info overload:

http://www.simautomotive.com/

click each catagory then each magazine then each tech and build sections....
1,000's of how to and explaination articles, all the catagories are worth going thru....
(they do change/update the articles as new magazine issues come out so do save the link)

Last edited by red65mustang; 09-10-2008 at 09:46 AM.
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Old 09-10-2008, 04:22 PM
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timing

So are you saying that my motor SHOULD infact be turning over lower, and going "whamp.....whamp.....whamp....." when i'm starting it at operating temp?? I didn't quite follow what you were saying.

What i did was had it at that point, where the engine was making that "hi-compression" sound from the advance, Then I just gradually took the dist. Back the other way a hair at a time until it started and idled easily. Then i went down the road, and around prolly 5-6 miles. Took it as high as prolly 5500 rpms and then got out of it cuz my 4 bbl hadn't opened. With out my 4 bbl opening Its almost impossible to get more than that out of her cuz the TH400 is not a full manual valve body and won't stay in first.
At that point i also realized that I had another problem. I noticed that my throttle wasn't even opening all the way to WOT (by pedal) cuz the bracket that the cable goes in was bent forward somehow. Anyway, the car was running pretty good, I'm not gonna say it was perfect, cuz it wasn't. The secondaries were still dripping, among other things. (the motor was already broke in and prolly had 5-10 hrs running time.)

I know i need to put a light on it and a vac gauge, and i had/have every intention of doing it. I just didn't have a chance. After I went for that ride, I put her up for the night. I came back out 2 days later, and the thing wouldn't run unless i forced it to run. That made it really hard to do anything with the timing. Plus It should have still started and ran were it was at, because nobody had touched it since it ran 2 days prior. Before i put it up that night I let it sit for about 4 1/2 hours and cool off to see how it would start cold, and it MABEY spun over twice before starting right up. That why I said I don't think It was that far off, I'm sure it wasn't perfect, but it shouldn't have been that far off.

If i mis-understood what you were saying please clarify. If red stang insn't on here can someone else please clarify his last post for me. Was he sayin the engine should make the noise mentioned above or what???
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Old 09-10-2008, 04:39 PM
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Quote:
Originally Posted by eatonde
Hey guys i was looking into gettin a bigger carb last night. I am currently running a holley 600 cfm 80457. It is fairly new but i have had nothing but trouble out of it for some reason. Plus a lot of people have told me that it's probably not big enough for what im putting it on. (SBC 355 KB119 Dome pistons, Lunati bracket master 2 300* adv. dur. .515/.515 lift). It curently has stock heads from a 69 chevelle 350 on it(not permanant), however I have finally decided on the heads i am gonna get.

I am planning on getting a set of the Brodix IK 180's with 70cc chambers. This should put me around 10.45:1 compression.

Anyway, I was just wundering what are the advantages and dis-advantages to running any of these carbs. I was thinking that it may benifit me to go with a mechanical secondary 750 carb, because the Vacuum secondary 600 that i'm running now takes forever for the 4 bbl to open. Its almost like the vaccuum isn't strong enough, would my cam cause this low vacuum? I also looked into the Edelbrock 1407 performer 750 and it is an air valve secondary. I have heard that is what the old Qjet carbs were too, but i have no idea how an air valve works in comparison to a vacuum carb. Can someone please explain to me how an air valve secondary works? I don't have much expierience working on carb's but i would like to learn how to. It seems i learn something new from you guys everyday.
thanks Danny
An engine operating at WOT likes more CFM than the math of 1/2 displacement Xs RPM + a little % explains. This is a metering consistency issue not a CFM issue.

I guess we all develop our preferences mine are for a full out race engine or a street strip engine my carb choices a first Holley types then Carters, last would be Q-Jet. I've found that for a competition engine, that keeping the venturi size between the primary and secondary side provides better control of the mixture ratio and a better balance of cylinder fill density between the front most and rear most cylinders.

For an exclusively street driven vehicle where you want reasonable fuel economy combined with a strong but seldom used top end, I move the Q-jet to the front of the line up and put the Holley in the rear. Not to say the Holley is bad, but it takes a bit more patience to get equivalent mileage from them.

For secondary actuation on a competition engine mechanical actuation of the secondaries of a Holley is best. For some reason the vacuum secondaries consistently produce a little less power at WOT. Considering that the vacuum secondaries do have a mechanical link that pulls the throttle plates fully open at WOT one is given to wonder why these consistently give up a few top end horsepower to mechanicals. You might suppose that the vacuum actuation takes some energy from the air-stream, but this is guessing on my part. The vacuum Holley uses venturi vacuum (which is the reverse function of manifold vacuum) to pull a diaphragm against a return spring, the action of which moves a linkage attached to the secondary throttle shaft. The Carter clones and Q-Jet have mechanically linked secondaries and use use a secondary air flow plate that is opened by the force of the passing air stream against the force of a counterbalance. In this sense they function something like the Holley in that the more airflow in the venturi the greater the opening. The air plate is also connected to metering rods which control fuel flow in proportion to the plate's opening. These like the Holley vacuum secondary, show a tendency toward less WOT power than a mechanical Holley. But here you're testing across brands and engineering designs so there can be other factors involved as well. But intuitively one can imagine that another element in the air stream produces another restriction to that stream.

For a simple 10.45 compression ratio, I go a different direction in the combustion chamber with a 50ish cc chamber and flat tops. The tight chamber gives better entering mixture control and swirl. While the flat top pistons allow a faster burn with less surface area to suck out the heat before it does work.

Bogie
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Old 09-10-2008, 05:16 PM
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timing

So are you saying that my motor SHOULD infact be turning over lower, and going "whamp.....whamp.....whamp....." when i'm starting it at operating temp?? I didn't quite follow what you were saying.

What i did was had it at that point, where the engine was making that "hi-compression" sound from the advance, Then I just gradually took the dist. Back the other way a hair at a time until it started and idled easily. Then i went down the road, and around prolly 5-6 miles. Took it as high as prolly 5500 rpms and then got out of it cuz my 4 bbl hadn't opened. With out my 4 bbl opening Its almost impossible to get more than that out of her cuz the TH400 is not a full manual valve body and won't stay in first.
At that point i also realized that I had another problem. I noticed that my throttle wasn't even opening all the way to WOT (by pedal) cuz the bracket that the cable goes in was bent forward somehow. Anyway, the car was running pretty good, I'm not gonna say it was perfect, cuz it wasn't. The secondaries were still dripping, among other things. (the motor was already broke in and prolly had 5-10 hrs running time.)

I know i need to put a light on it and a vac gauge, and i had/have every intention of doing it. I just didn't have a chance. After I went for that ride, I put her up for the night. I came back out 2 days later, and the thing wouldn't run unless i forced it to run. That made it really hard to do anything with the timing. Plus It should have still started and ran were it was at, because nobody had touched it since it ran 2 days prior. Before i put it up that night I let it sit for about 4 1/2 hours and cool off to see how it would start cold, and it MABEY spun over twice before starting right up. That why I said I don't think It was that far off, I'm sure it wasn't perfect, but it shouldn't have been that far off.

If i mis-understood what you were saying please clarify. If red stang insn't on here can someone else please clarify his last post for me. Was he sayin the engine should make the noise mentioned above or what???
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Old 09-10-2008, 05:28 PM
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re: 18*
I'll try a different explaination/illustration

the starter motor is designed to have enough balls to turn over a heat soaked hot stock motor with just generally/roughly say 6*btdc of base timing no problem (the crank only needs to turn 6*'s more before the piston starts to travel down)

when you start the combustion explosion at 18*btdc, there is so much pressure built in the cylinder before the piston gets to tdc to begin to move down it works/loads the living hell out of the starter motor ....so it typically turns slower (even pulling more/max available amps)....

no, you don't want to work the starter motor and cables and batt that hard..... but your cam choice needs it to operate correct!!!

if it won't crank over at 18* when hot (weak batt/worn starter/bad cables/lack of ground source cables= not enough amps available) you can install a simple on/off dash toggle switch to turn off the 12V to the dist ign module until the motor is at max cranking speed then flip on the switch to start giving it spark
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