Carbureted LSX forum - Oh no, another What size carb? thread!

You all know I am an advocate for smaller rather than larger carb sizes. I know I'll get flamed, but I found this in PHR:

"...Picking the right size carburetor is an easy area to swing and miss big time! If you plan on upgrading to a different carb like one of the quadzillion Holleys out there, stick with a size that is rpm and cubic-inch appropriate for your engine. While it is true that larger carbs do tend to make more peak horsepower than smaller carbs, reality says that most engines prefer a smaller venturi carb to get better signal to the boosters, better fuel atomization, better throttle response, and actually more midrange torque. Historically, most mild 302- to 340-inch engines do well with a 600-cfm carb, 340- to 360-inch engines like a 650, 360 to 390-inch engines a 700, and only over 390 cubes would a 750 really start to perform well..."

sure if you're thinking 1970s technology......there are 800+hp 358 cube sbc motors


here's a question what came stock on a bbc in a muscle car, 750 or 850 double pumper and they ran 13s MAYBE. What carb do you pick for the typical cam only ls1 or larger cubic inch motor that can run 10s.....clearly it could take a 950 carb. I picked up .2 and some mph from a 750 to 950 swap in my pump gas street combo.

EDIT: treed!!
I am not going to flame you, but I have to add that I beleive the theory applied to old school SBC's should not be used. heres why: any LS based engine benefits over the olschool SBC by way of SUBSTANTIALLY better flowing heads. this allows theese engines to flow more total air through them than an old SBC.
My point is dont be afraid of a larger carb for an LS of any kind, compared with an old SBC.
my experience so far with my 6.0 has shown me NO poor low end performance with the 850Mighty Demon I run. I actually threw it on expecting it to fail misserably, with the engine completely stock save for external bolt ons and VVT removal, and it ran GREAT. This seems totally wrong considering that an 850 MD is equivalent to a 1000+cfm holley based on how they are flow rated. ON top of that I used a single plane intake!! even more wrong! I left my perfect holley 750DP on the shelf.
In addition, if a guy is running a dual plane intake, he should consider running a larger cfm carb compared to a single plane, as the engine only "see`s" half the carb`s flow.

I am not going to tell you that you should run an 850 Mighty Demon on a stock 6.0, if you never intend install a larger cam, but dont hadicap your results by choosing a carb size based on 1984`s carb sizing theory.
My 10.5:1, ported heads, solid cam 454 BBC couldnt handle this 850MD, I am amazed at how my 6.0L is loving it!

Carbs are rated on airflow (i.e. horsepower). Like 3pedals said LS heads are probably worth another 40-50 cubes.

For example, throw a couple of turbos into the mix. Are you still going to stick with a 650 carb?

I will side on the fence, the formula for Max RPM x cubic inches equals xxx cfm is correct...for a daily driver, so GNCLONE is right in that aspect.

As fr max power, yes, the larger carb is needed to meet expected goals.

I have a 750 vac sec on my 4.8 in my GMC, no cam, if I could get a 650 that was setup the same, I would, but the 750 was ready to go for a measly hundo.

So that being said, I run a 750 on my cammed 6.0, difference being, the vac sec 750 on the 4.8 has the heaviest secondary spring I had, works great, while the mechanical 750 on the 6.0 is balls out at WOT, the 6.0 would gain I'm sure with a 850-950, but this is what I have, I will try a SV1 at the track and see how much it can gain for sure, should be interesting results, but this argument is like anything else gentlemen, it depends on the application.

We ran a 670 street avenge on a torqer v2 cammed 6.0 in a Malibu, ran 11.58, tht was with the timing back, if I get a chance I will back door it with my 750 and see what gains can be had, but the smaller I'm sure is helping with the torque all day.

LS based engine benefits over the olschool SBC by way of SUBSTANTIALLY better flowing heads. this allows theese engines to flow more total air through them than an old SBC.

Truth ^

A 750 is the smallest I would put on a 5.3 or larger N/A LS engine.

Why? For what application? If power is conserned I agree, but a stock swap, and you want it as effective as possible for daily duty, a 600-650 would be a better choice. Like I said, cammed 6.0 with a 670 went 11.58 with only 28* timing, I will hopefully get to see if there is any gain with a 750, but I will have to back to back after I got the timing up.

I guess if you are just fetching groceries an old Rochester 2 barrel will do just fine. But for performance..................

2 barrel is taking it far back, optimal all around performance of a carb for cruising ect, just doesn't need a 750 or larger all the time. Since we found out ourselves, it is great info to back up the huge carb requirement is false, but back to back data will be awesome if I can get it.

I'll let everyone know if I break into the 11s with my little 670.

Changing the subject, I'm thinking about going with the GMPP Hot Cam and springs set here one of these days. I haven't done much research on cams yet.

A cam swap with a 670 should get you in the 11s with a converter and a bit of gear, but how much faster with a 750 or larger is te question. I would look into a carb grind cam when you do get ready for the swap.

Man. Speedtigger accusing Zones of not going big enough. I feel like I woke up on the other side of the looking glass.

Age old battle here, I can still have talks with folks that think they need 750-850s on mild 350 old school SBCs. SBFs always liked more carb than a comparable chevy smallblock....cylinder head design? That points to whats being said about LS cylinder heads. One thing i can say,aftermarket sbc heads have came a long way,and i know alot of those motors arent sporting large carbs and still getting it done. You mix a high flowing head on a smaller cube motor you loose lots of low end,best way to make it go is to scream it and put a larger carb on it. Maybe im out of touch here, ive been hanging around 467 inch SBCs and my 565 bigblock too long,lol

Same with a vac sec, two barrel operation till the secondaries open later allow a later carb usually, that way unlike the 4 barrel mechanical, all 4 open at WOT will not operate as well on the smaller cubes, pulling through 4 Venturi vs the 2 of the vac sec.

Working the counter of a speed shop I have to stop customers all the time from trying to by a 750 SP for their mild 302 or 350. I see over carbing everyday. But it really cant be nailed down to a size, it really is more about the combo. On my Gen I 383 SBC I have a Quickfuel 850. Hasnt seen the street or track yet but the dyno shows that at least it makes KILLER midrange and topend.

Exactly, no one size fits all. But a vac sec 750 is so common I run them on everything that is not on the wild side due to availability.

Yes^ the Proform Vacuum Secondary 750 has been my favorite for LS street swaps so far.

Man. Speedtigger accusing Zones of not going big enough. I feel like I woke up on the other side of the looking glass.

LOL.

Maybe he is mellowing in his old age. :D

Carbs are rated on airflow (i.e. horsepower). Like 3pedals said LS heads are probably worth another 40-50 cubes.

For example, throw a couple of turbos into the mix. Are you still going to stick with a 650 carb?

Actually, the engine's DISPLACEMENT is a constant. The variable is RPM. I do not own an engine with 125% or 150% volumetric efficiency, perhaps you all do? An over-carbed engine still runs - just idles like crap and has no throttle response at 2000 rpm.

Chevy Highperf mag got ONE HORESPOWER on the dyno when they went from a 650 cfm to a 750 cfm (on an engine larger than mine)

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

I found this formula on an aviation based site:
"For a known engine displacement and RPM, you can calculate the engine airflow at 100% VE, in sea-level-standard-day cubic feet per minute (scfm) as follows:

100% VE AIRFLOW (scfm) = DISPLACEMENT (ci) x RPM / 3456 "

following this formula for my 6.0:
(364ci x 7500)/3456
=789.93 CFM Air Demand @ 100% VE
I have seen quite a few dyno results showing up to 105%VE+, on reasonable street builds using ported OEM, or street oriented TFS/AFR aftermarket heads (NOT Mozez or even LS7 stuff).

Its pretty simple. A smaller carb will have better low speed (air speed) throttle response, where in a larger carb will "usually" make more top end power, at the loss of low speed response.

So it goes back to the age old question. Are you building a street car, or a race car. Cause when you try to be everything to everybody, you usually wind up being nothing to nobody. I think Abe Lincoln said that.......................or maybe I just made it up, lol.

Comprimises are everywhere, choose them carefully. Eric L

All of those formulas are great, but real world results are the last word. If given combination has a history of running harder with a bigger carburetor, that is what is going to influence me the most. I will let somebody else philosophize as to why.

Its all in APPLICATION/INTENDED USE guys. You cant just say a bigger/smaller carb is better. A larger carb can run better ET's and make more dyno power and it can also give you more seat of the pants power in some areas, but it doesnt mean it is going to be as responsive on the street or the best all around driving fit. You choose your poison or try to nail the closest of best worlds for your application if you hit the strip alot.

ProRac1, you said it well!

Its all in APPLICATION/INTENDED USE guys. You cant just say a bigger/smaller carb is better. A larger carb can run better ET's and make more dyno power and it can also give you more seat of the pants power in some areas, but it doesnt mean it is going to be as responsive on the street or the best all around driving fit. You choose your poison or try to nail the closest of best worlds for your application if you hit the strip alot.

ProRac1, you said it well!

True, leaning your engine out with a carb is just as bad as leaning it out fuel injected, by computer. All good info will need in the future for my carbed lsx

Yep,that is what I was saying.