In reading the various threads about carburetors I am finding an ongoing problem; owners selecting a carburetor that is WAY too big for their application. A typical stock street engine is only about 75% "volumetric efficient" which is the engine's ability to fill the cylinders at high rpms. By using so-called "performance" parts the volumetric efficiency can be boosted to the 80-85% range but 90% efficiency can rarely be reached unless long-tube headers and very high-flowing mufflers are used.

Fact. A 350" engine that is 100% volumetric efficient can only swallow 608 cfm at 6000 rpm. For a healthy street engine that is say 85% efficient that drops the volume requirement down to 517 cfm at 6000 rpm. But when you factor in a realistic 5500 rpm maximum that drops the volume requirement down to only 474 cfm at that 5500 rpm. Ouch! It makes you wonder why anyone with any sense would put a 750 cfm carburetor on a 350" engine regardless of it's performance level. The 750 cfm carburetors are made for 427" and 454" engines and even then those 750 cfm carburetors are a bit too big for average street use.

And then there's the matter of using mechanical secondary "double pumper" carburetors on street engines. Due to the widely varied rpm ranges a street engine sees mechanical secondary double pumper carburetors should NEVER be used on a street engine. Those specialty carburetors are made specifically for drag and track racing where 4000+ rpm is constant and the engine can swallow that much air. To use a mechanical secondary carburetor on a street engine is a sure-fire way to create horrendous bogging problems.

 

I disagree with several of your statements, both from an engineering standpoint and from personal experience.

It all starts with you not recognizing that engine CFM and carburetor rated CFM are entirely apples and oranges.

 

I agree that sometimes a carb that is larger than necessary is selected...but that doesn't automatically make the engine less responsive...even with a DP.

4BBL carbs are rated typically at 1.5". You can put a 390 cfm carb on a serious race engine that is pulling a whole lot more than 1.5" (sometimes 7"+). 2BBL are rated at 3". That carb will flow a LOT more than 390 cfm. The air speed through the carb is insane and a lot of work is done to control it. On a "normal" engine...you can easily have 2"+ pulling on the carb and the small carb runs OK...even at WOT. But often a larger carb can slow things down a little and make the transition into the intake manifold a lot smoother and make more power as well as drive better. These ratings are just a good reference point to compare...a running engine has dramtically different operating characteristics as it accelerates.

A dual plane "likes" a larger carb since the cylinders only "see" 1 or 2BBLs at any given time vs a single plane where they "see" 2 or 4 BBLs. even in mild applications.

The volumetric efficiency numbers do tell you something...but you wouldn't want to have the carb maxxed out as far as airflow when it hits peak power. You've left yourself no room for improvement later for ANYTHING that you might do to improve power.

On the other side of the coin...a larger carb doesn't guarantee an increase in airflow through the engine. Usually the limiting factor is the heads/intake/cam/exhaust combination. I've seen a huge increase in carb size hardly move the measured airflow through an engine because the combination just couldn't move more air. Until you make a change to it the smaller carb will be just fine.

Most race type double pumpers are calibrated richer because they have no idea what you're going to bolt it to. A race engine has less idle/midrange vacuum so the carb orifices are typically larger to get the required amount of fuel through them with the lower "pull". If you put that carb on a mild engine with strong vacuum signal it's going to run richer throughout the curve.

Holley for example has done a great job with the newer "Street HP" double pumpers. They are calibrated leaner and do a great job on the average street motor out of the box.

A 5500 RPM limit is very low for a decent 350. That's dead stock territory for low-perf motors. If that's all you're doing then it won't matter. A 750 carb works pretty darn well on just about anything...evne a mild 350....heck even a decent 302. They run well on a 427 and mild 454....but both will respond to more airflow if the combo is decently spec'd. If it's a stocker....no issue.


JIM

 

Several months ago I ran a test on my 454" powered '71 with a 750 cfm #3310 Holley. I put a spring clip on the secondary lift rod then charged up a 6% grade at 135 mph (about 5000 rpm). When I got back home I checked to see how far the rod had lifted and it still had about 1/8" more lift to go; indicating the 750 cfm #3310 is sized about as good as it can be for my particular combination.

The point I was trying to make is 750 cfm carbs are WAY too big for stock L-48's and then mechanical secondaries just cause severe bogging problems when the rpm is too low. That's why I recommend using only AVS or vacuum secondaries on street engines. When adjusted correctly you never feel the secondaries "kick in" as the transition is perfectly smooth.

 

That's why Qjets have secondary air valves. But everyone knows that the BIGGER the carb, the more power it makes!

 

Back when Barry Grant was modding Holleys, before Holley sued and forced them to make their own carbs, I bought one of their modded double pumpers for my '69 427. The choke tower was removed, it was ported, and had 4 corner idle. It was rated at over 1000 cfm. It replaced the Holley 780 cfm carb that I had run for some time and been to the drag strip with. I've been tuning Holleys for a long time and had it properly jetted and the secondaries were optimized. It ran a good bit better than the Q-jet it replaced.

Putting that Barry Grant carb on the car, with no other changes, dropped my ET by .35 seconds and gained several MPH in trap speed. It also had much better throttle response on the street. Without any carb changes in the following 20 or so years that carb still kicks azz and has taken me to the ET posted in my sig, while still being flawless on the street. I'm pretty sure that the 1000+ cfm is not too much carb for the engine. The cfm guides you find online are of limited real world value.

 

The position of that clip after your test is partially controlled by the spring that you have in the vacuum secondary chamber. A softer spring and it would have opened more. I don't think you proved your point.

 

There are MANY technical issues to address regarding proper sizing of a carb to a specific engine. But, in general, I think the initial premise of this thread is correct:

MOST owners who just install a larger carb on their engine to make it "go faster" are wasting their time, money, fuel, and perhaps the life of their engine. And I'll go one step further... Unless you have a very upgraded engine or one larger than a 454 cu in, swapping-out a correctly working Q-Jet from it for any other carb, for the purpose of increasing performance, is a total waste of time. (This is regarding a [primarily] driver car.)

 

There's a post titled "expectations" about a 406. His dyno sheet shows excellent numbers and a CFM of 591 at 6500 RPM. A Qjet could handle that. My engine guy (a wise old man) told me that the only problem with Qjets is getting big engines to idle properly, and that Holleys are infinitely adjustable.
My mistake-it's the post about a "disappointed 406" not "expectations".

 

800 CFM on the car I didn't buy and forever have wished I had

http://www.holisticpage.com/camaro/camaros/302.htm

only 302 cu-in but man did it fly!

Anybody have any idea what the volumetric efficiency on that motor might have been? I had the same intake and a somewhat similar roller cam but put a 650 on it. Plenty good enough for most things but it definitely ran out of poop at the rev limit - about 6400.

 

Sorry, I'm afraid you'll have to preach the small carb sermon to another choir.


While the average enthusiast may be safer erring to the small side - smaller carbs typically being more forgiving - I'd caution against inferring that smaller is necessarily better. No, I'll take a well-tuned 750 on an L82 over a 650 any day of the week (BTDT). And, having successfully run as large as a 1050 Dom DP on street 427 BBCs, you can bet that I won't be choking my latest redux down with no 750. I may be an exception to the "rule", but make mine a 950 (whether TB or DP, pending funds), thankyouverymuch. My $.02


edit - Oh, and FWIW the L48's biggest problem is a lack of CR.

 

Back when I had the 427 in my Vette...I ran a homebuilt 1050 Dominator which mainly consisted of "streetifying" it by adding a PV, taking out some primary jet, installing progressive linkage and killing the intermediate circuit on the primary side (it was a 3 circuit carb). That thing ran flawlessley on the street, dyno, track etc. Did lots of cross country driving. Then I installed it on my 540. Ran just as well. On the 540 I tested several larger carbs up to 1400 CFM and it made almost no difference...the engine just couldn't move any more air. On the track a trick AED 1150 was a tenth and a MPH slower. Probably could have made it even if I worked on it more..but it wouldn't be any better.

I installed a 750 Dominator on it once I had rebuilt for a buddy. That's basically a 1050 with larger boosters. Definitely a huge performance decrease with the 750...even though it ran well.

Tested my 1050, the same AED 1150 and a custom Holley Shop 1250 on a 632" once....the 1150 was the best carb overall. It beat my 1050 by about 6-8 HP. The 1250 was about dead even. Again..it just wasn't the carb that was the restriction.

Interestingly on the dyno...back to back....the 1050 made near identical power throughout the curve as a Holley EFI system with a 2000 CFM throttle body. Just shows the 1050 wasn't a restriction..which we knew from previous carb swapping...and that even though the throttle body was larger and restriction dropped a little...there still wasn't any more demand for airflow than the engine could use.

I've got a 750 DP that I've used on a stock Ford 302 Mustang GT, several 327's and 350's, a 396, a 427 and a 454. It ran well on all of them, but I would say it was the nearest "properly sized" to the 350's....but it wasn't a restriction on the 427 either and it was running 11's on motor and deep 10's/high 9's on N2O.

A Holley is very easy to adjust and play with...but most issues people see are because they stray too far from how Holley built it. The DO know how to build a carb and if you just tweak what needs to be tweaked you'll be happy.

Q-Jet's are great....but they have to be good ones...it's easy to get weird stuff happening when mixing matching parts. Heck...I even ran a tunnel ram with 2 of them one time and they were awesome!

JIM

 

Being new to the world of V8 petrol engines I'm learning fast about a lot of things. I'm particularly interested in the carb discussions as my mild? Stock? 350 is equipped with a Holley 1850 that is older than the car. As I would like to upgrade the engine eventually to a 383 I have held off doing anything about the carb other than to get it running as well as I can (which is considerably better than when I got the car) but I still feel something more modern should be an improvement but what to use that would fit what I have now and what it might become. I was leaning towards a Quickfuel BD-1957 but now am unsure if it will suit a 383.
My intention with the engine is to build it for torque with as much of that below 3500 rpm as I can get but when you get into discussions about down leg boosters etc per Dave Vizard and bigger carbs being able to be used with suitable boosters etc. well then, what to do? The manifold is an Edelbrock Performer EPS that I would like to stick with as it just fits under the stock hood with the Holley and a drop base air cleaner.

 

And the small head and cam choir. Any hotrooder / mechanic worth his salt can tell you what carb to use on any street / strip engine size and configuration and then tune it to the engine it is sitting on.

Ask in any serious performance shop if they use "The formula" for selling carburetors and they will laugh at you. I wish it would permanently die as it does no one any good.

 

Did you ever come back down?

 

 

L-48 owners with TH350's need vacuum-secondary or AVS carburetors; NOT mechanical-secondary double pumpers. They buy a "double pumper" because they want to fit in with the "in" crowd; not realizing they'd be much better off without that style of racing carburetor.

 

So we're only talking about an L-48 with a T350? What's that thing got....like 23 HP??

No reason to waste money on anything but a good Q-jet rebuild (BTW- 750 cfm) until you really want to start making it run.

But that said....I'd throw a 750 DP on it in a heartbeat without even worrying about it if I had one lying around. I can guarantee you I (or anyone else who wants to learn) can make it come off the line even with a stock converter and 2.73 gears without hesitation when going WOT with basic tuning.

I suppose I've said all I've got to say about it. For folks who want to stick with vacuum secondary carbs because they "read it on the interweb"......all can say is.....

"Wanna Race?"


To the above fellow with the 1850 carb....it's a great little basic replacement carb for stock stuff...or maybe two of them on a tunnel ram. But you can go a LONG way with something better. When you go 383 you're going to want at least a 750 cfm carb. At least I would...especially on a typical dual plane intake.

 

A Holley 750, with vacuum secondaries is very forgiving of being mismatched to the engine. Much more so than a double pumper.

The vast majority put way too much carb on the car. It is generally better to select smaller carb than bigger. As a general rule you will get off the line quicker with smaller.

The trick is to tune and properly match the carb to your condition. In this regard the Holley is far superior in ease, understanding, options, availability of parts, choices and information.

It is interesting to keep in mind that GM put 600 cfm performance Holleys on their small block performance motors.

 

I have a Q-jet with L-46 i thought mine would be 650 cfm ??
didn't the BB cars get 750 cfm Holleys ???

BTW.... i did a stock rebuild on my Q-jet, what an AWESOME learning experience that was !
Had trouble with it but got is sorted out and it runs even better than when i started, which is always a good thing !

Sorry for the hi-jack.
-ALF out....