choosing the right carb
Melting Slicks
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From: Great Plains Iowa
Race Director
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From: Eustis ( Area 51 Bat Cave ) Fl
The larger ( within reason ) cfm always wins...so why is that?
Race Director
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From: Eustis ( Area 51 Bat Cave ) Fl
not sure who you feel should let it go but in all due respects I have seen both DP's and VS's do very well on street cars it's all about the dialing in and I also have seen people talk a lot to hide the fact they do not actually spin wrenches, anyway, this thread still has lots that can be learned if some could just forget the egos and stop boasting and moaning....
Melting Slicks
Joined: Apr 2009
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From: Great Plains Iowa
I have seen this arithmetic before for many years and I see people call in the volumetric efficiency factor which is very real and lower the carb cfm yet I have seen this applied in real world testing both on the 1320 and on the street with seat of pants "feel" of power.
The larger ( within reason ) cfm always wins...so why is that?
The larger ( within reason ) cfm always wins...so why is that?
Ideally you would like to have ZERO manifold vacuum under full power because any vacuum would indicate a restriction (hence the vacuum). But to get additional cfm in a Holley or Edelbrock you also get larger primaries which slows the velocity down when cruising. That is why the QuadraJet is such a fantastic carburetor as it has very small primaries to give very high velocity and very large secondaries to give the needed volume. They are truly "the best of both worlds" and the reason they were used across the entire G.M. line as well as Fords.
A 454" with stock oval port heads will begin to run out of breath around 5000 rpm because of the smaller valves and ports. Increase it's cubic inches to 496" and it'll run out of breath around 4500 rpm so bigger inches require bigger valves and ports and that's why the rectangular port heads were used on the higher performance big blocks.
Le Mans Master
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The interesting thing about the 396 BBC if you have driven all three hp versions 325, 350, 375 hp the 325/350 hp had oval port heads the 350 hp a slightly hotter hydraulic cam. The oval ports were around 256cc the rectangle on the 375 hp 396, 300 cc. It had a very mild .520 lift solid lifter cam. The 375hp would kick the sh-t out of any 325/350 hp car. But big runners will kill you right.
Last edited by Little Mouse; Dec 1, 2015 at 01:16 PM.
Race Director
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
The interesting thing about the 396 BBC if you have driven all three hp versions 325, 350, 375 hp the 325/350 hp had oval port heads the 350 hp a slightly hotter hydraulic cam. The oval ports were around 256cc the rectangle on the 375 hp 396, 300 cc. It had a very mild .520 lift solid lifter cam. The 375hp would kick the sh-t out of any 325/350 hp car. But big runners will kill you right.
Last edited by MotorHead; Dec 1, 2015 at 03:20 PM.
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Le Mans Master
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Except for better exhaust manifolds on the 65 vette. the 396 375 hp in chevelle's and camaro's same thing. amazing how it gained 50 hp just put in a vette must be that price difference you have to pay for the vette lol. Then 66 they came out with a 427/ 425 hp but amazingly it made no more power same 425 hp as the 396. Then chevy built a 427 with over 12.5 compression and put a cam in that could turn another 1,000 rpm but they only got an increase in power to 430 hp lol. My 69 302 only made 290 hp at 5200 rpm so i shifted it just above that at around 5600. Ya right i'm Elvis back from the dead would you like to hear me sing.
Le Mans Master
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Race Director
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From: Eustis ( Area 51 Bat Cave ) Fl
Ideally you would like to have ZERO manifold vacuum under full power because any vacuum would indicate a restriction (hence the vacuum). But to get additional cfm in a Holley or Edelbrock you also get larger primaries which slows the velocity down when cruising. That is why the QuadraJet is such a fantastic carburetor as it has very small primaries to give very high velocity and very large secondaries to give the needed volume. They are truly "the best of both worlds" and the reason they were used across the entire G.M. line as well as Fords.
A 454" with stock oval port heads will begin to run out of breath around 5000 rpm because of the smaller valves and ports. Increase it's cubic inches to 496" and it'll run out of breath around 4500 rpm so bigger inches require bigger valves and ports and that's why the rectangular port heads were used on the higher performance big blocks.
A 454" with stock oval port heads will begin to run out of breath around 5000 rpm because of the smaller valves and ports. Increase it's cubic inches to 496" and it'll run out of breath around 4500 rpm so bigger inches require bigger valves and ports and that's why the rectangular port heads were used on the higher performance big blocks.
or it was deleted which there was no reason that it should have been, 
the only poke I took at you in fun was you gave good info but didnt answer my question 
I have another better question that you and others will hopefully chime in on.
I wanted a q jet for the very reasons you are praising it, however,
IF the q jet design with small primaries and larger secondaries is such a wonderful great set up why dont any after market outfits make a hipo version, isnt every factory q jet stuck with emissions fittings etc?
Thread Starter
Intermediate
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I replied to you on this once toobroke but either I forgot to click "submit" or it was deleted which there was no reason that it should have been, the only poke I took at you in fun was you gave good info but didnt answer my question
I have another better question that you and others will hopefully chime in on.
I wanted a q jet for the very reasons you are praising it, however,
IF the q jet design with small primaries and larger secondaries is such a wonderful great set up why dont any after market outfits make a hipo version, isnt every factory q jet stuck with emissions fittings etc?
or it was deleted which there was no reason that it should have been, the only poke I took at you in fun was you gave good info but didnt answer my question I have another better question that you and others will hopefully chime in on.
I wanted a q jet for the very reasons you are praising it, however,
IF the q jet design with small primaries and larger secondaries is such a wonderful great set up why dont any after market outfits make a hipo version, isnt every factory q jet stuck with emissions fittings etc?
Drifting
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From: new iberia la
Melting Slicks
Joined: Jun 2003
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From: Niles, MI
I wanted a q jet for the very reasons you are praising it, however,
IF the q jet design with small primaries and larger secondaries is such a wonderful great set up why dont any after market outfits make a hipo version, isnt every factory q jet stuck with emissions fittings etc?
IF the q jet design with small primaries and larger secondaries is such a wonderful great set up why dont any after market outfits make a hipo version, isnt every factory q jet stuck with emissions fittings etc?
FS: 2 Holley 800 cfm Spreadbore Carbs
Also Edelbrock used to make a Performer RPM Quadrajet.
Last edited by Kid Vette; Dec 2, 2015 at 06:56 PM.
Melting Slicks
Joined: Mar 2008
Posts: 2,989
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From: Meriden CT
Ideally you would like to have ZERO manifold vacuum under full power because any vacuum would indicate a restriction (hence the vacuum). But to get additional cfm in a Holley or Edelbrock you also get larger primaries which slows the velocity down when cruising. That is why the QuadraJet is such a fantastic carburetor as it has very small primaries to give very high velocity and very large secondaries to give the needed volume. They are truly "the best of both worlds" and the reason they were used across the entire G.M. line as well as Fords.
A 454" with stock oval port heads will begin to run out of breath around 5000 rpm because of the smaller valves and ports. Increase it's cubic inches to 496" and it'll run out of breath around 4500 rpm so bigger inches require bigger valves and ports and that's why the rectangular port heads were used on the higher performance big blocks.
A 454" with stock oval port heads will begin to run out of breath around 5000 rpm because of the smaller valves and ports. Increase it's cubic inches to 496" and it'll run out of breath around 4500 rpm so bigger inches require bigger valves and ports and that's why the rectangular port heads were used on the higher performance big blocks.
Wow dude, the highlighted sentence is a paraphrase of my post #28 in "Engine stalls when floored", that I posted 4 hours earlier, LOL! Learning a thing or two?
You and 454luvr seem to think that the primary throttle blades just now snapped wide open, uh, all the time....when at cruise, LOL! 
Last edited by SH-60B; Dec 2, 2015 at 07:09 PM.
Melting Slicks
Joined: Apr 2009
Posts: 2,632
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From: Great Plains Iowa
Wow dude, the highlighted sentence is a paraphrase of my post #28 in "Engine stalls when floored", that I posted 4 hours earlier, LOL! Learning a thing or two?
You and 454luvr seem to think that the primary throttle blades just now snapped wide open, uh, all the time....when at cruise, LOL! What on earth are you talking about?
Race Director
Joined: Mar 2003
Posts: 11,608
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From: Eustis ( Area 51 Bat Cave ) Fl
made to replace the q jet... I wanna hear someone say something good about them, I hear them trashed as much as q jets...
Last edited by The13Bats; Dec 3, 2015 at 08:51 AM.
Race Director
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From: California
The reason the spread bore was made was gas milage, and a hope to keep performance... with the larger secondaries.. but like a swiss army knife, pretty and very cool looking,, does a lot of stuff, but nothing best..
The reason Holly and Edelbrock made a replacement spread bore, for a while anyway, was $$$$$$ people wanted a "upgrade" or "replacement" carb, without replacing the intake, and without buying adaptors that raised the hight so on some cars it will not fit... money$$$$ that is the reason they made the carbs....
you will not find any aftermarket performance carburetor manufacture that makes a performance spread bore carb... or a performance spread bore intake... what is out there are more of "replacements" for stock...
I do not give a rats a$$ about gas milage in my Vette... some may, I do not.
The reason Holly and Edelbrock made a replacement spread bore, for a while anyway, was $$$$$$ people wanted a "upgrade" or "replacement" carb, without replacing the intake, and without buying adaptors that raised the hight so on some cars it will not fit... money$$$$ that is the reason they made the carbs....
you will not find any aftermarket performance carburetor manufacture that makes a performance spread bore carb... or a performance spread bore intake... what is out there are more of "replacements" for stock...
I do not give a rats a$$ about gas milage in my Vette... some may, I do not.
Race Director
Joined: Mar 2003
Posts: 11,608
Likes: 780
From: Eustis ( Area 51 Bat Cave ) Fl
The reason the spread bore was made was gas milage, and a hope to keep performance... with the larger secondaries.. but like a swiss army knife, pretty and very cool looking,, does a lot of stuff, but nothing best..
The reason Holly and Edelbrock made a replacement spread bore, for a while anyway, was $$$$$$ people wanted a "upgrade" or "replacement" carb, without replacing the intake, and without buying adaptors that raised the hight so on some cars it will not fit... money$$$$ that is the reason they made the carbs....
you will not find any aftermarket performance carburetor manufacture that makes a performance spread bore carb... or a performance spread bore intake... what is out there are more of "replacements" for stock...
I do not give a rats a$$ about gas milage in my Vette... some may, I do not.
The reason Holly and Edelbrock made a replacement spread bore, for a while anyway, was $$$$$$ people wanted a "upgrade" or "replacement" carb, without replacing the intake, and without buying adaptors that raised the hight so on some cars it will not fit... money$$$$ that is the reason they made the carbs....
you will not find any aftermarket performance carburetor manufacture that makes a performance spread bore carb... or a performance spread bore intake... what is out there are more of "replacements" for stock...
I do not give a rats a$$ about gas milage in my Vette... some may, I do not.
too many modern carbs will do better with far less hassles, i think I have to place the q jet in the "settling" folder.
Le Mans Master
Joined: Apr 2006
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Pauldana in 1966 when the spread bore Q- jet showed up on cars very few people shopped for a car for fuel mileage. If they did they were looking at chevy 2's with a 6 or a corvair.
I would say GM wanted the small front primary carb in 750 and later 800 form for this reason.
The small front primary carb could work well with a 327 of that time period, a two speed power glide stuffed in a heavy impala with a 3.08 rear gear.
It also had enough cfm capacity to feed almost all there engines even lower rpm big blocks. So the bean counters only had to have one carb that could do it all.
there were a few higher horse power engines they wanted holley's on so they would be legal to race with them.
In racing where engines are kept at higher rpm's all the time there is no need of a smaller front primary and the holley can be torn apart much faster.
I would say GM wanted the small front primary carb in 750 and later 800 form for this reason.
The small front primary carb could work well with a 327 of that time period, a two speed power glide stuffed in a heavy impala with a 3.08 rear gear.
It also had enough cfm capacity to feed almost all there engines even lower rpm big blocks. So the bean counters only had to have one carb that could do it all.
there were a few higher horse power engines they wanted holley's on so they would be legal to race with them.
In racing where engines are kept at higher rpm's all the time there is no need of a smaller front primary and the holley can be torn apart much faster.
Last edited by Little Mouse; Dec 3, 2015 at 12:14 PM.