Carb. how big is 2 big?

[AirbornFox]

I am currently working on a 396 BB chevy (more like 402.23something). It has closed chamber heads off a 1969 chevelle, slightly agressive cam, and some raised pistons, am guessing around an 11.0 to 1 compression ratio. I am currently running an Edelbrock 750 carb, which is driving me NUTZ!!!! it is running rich and has been a complete pain to adjust. A friend is wanting to sell be a modified holly 850 double pumper, forget the company, but they do alot of work for the World of Outlaws. The Chevelle that the engine is in has the M20 tranny, (4spd manual) and is used for fun, on the street and on the track. I am worried that the 850 might be a little 2 big, but I also heard if tuned properly, it will run just as good, if not alot better than what I got now. SHould I stick with my edelbrock or get the 850 holly? And if this is not an ez question to answer, how do I find out how big a carb I need for my application?

 

[AirbornFox]

ok, the carb that I was looking at is by Gaerte, i think...

 

[franzh]

An 850 will work ok, as long as idle quality is unimportant, and low speed throttle response is not an issue.

The Edelbrock 750 should work great with any but the most aggressive engine/driving you can do. The carb is also supertunable, and I have been using one for ages. If it is running rich at idle, look at some plugged idle passages. What is your intake manifold vacuum? If it is not at least 12" HG at idle, you could have some problems here too.

Franz (MASE/SAE)

 

[MaxRaceSoftware]

i also had a 1968 Chevelle SS-396 375 HP 11.0:1 CR
.522/.522 solid lifter cam .022/.022 hot lash

M-22 4-speed and 5.14:1 rear gears 12-Bolt

i had a Holley 3-Barrell 950 CFM on an Edelbrock
dual-plane intake ..it ran 12.20's at 114 to 115 back in 1968-1970

i modified the 950 3-barrell back then to rear squirter
by soldering 2 copper tubes to squirt fuel in rear oval
back throttle...there were no accelerator pumps mechanism offered by Holley for the 3-Barrel back then so you had to make your carb a sort of a double-pumper

anyways it worked great with 5.14 all the way to 3.73
but less gear than 3.73 it was soggy bottom-end power curve

the dual-plane effectively "fools" the engine into thinking it has 1/2 the size carb on it..therby greatly increasing bottom end performance..i guess if i would had a "Single-Plane" manifold back then..even with the 5.14 gears ..it might have been lazy at low RPMs ???

something to think about

a 0-4779 750 carb or 0-8156 750 carb will have somewhere around 70 frt jets and 80 rear jets with a power valve in front ...the 70/80 give exactly the same A/F ratio front rear because of air bleed sizes and power valve channel

usually 70/80 jets on 750 will show 13 to 14 A/F ratio on my dyno at 600 RPM/SEC Accel rate

850 carbs like 80/80 to 83/83 jets

check float levels and seapage by needle/seat

power-valve diaphram torn ..leaking fuel drops during idle and part throttle cruising

too-low vacuum like Franzh said ..causing you to start to pull fuel from main venturis TOO EARLY into throttle

just some things to think about ???



Larry Meaux (meauxrace2@aol.com)
Meaux Racing Heads
MaxRace Software
ET_Analyst for DragRacers

 

[MaxRaceSoftware]

almost every engine we dyno test ..we find a way to hook
a vacuum port or fitting to either the carb , carb spacer,
somewhere on the intake manifold....we do this to check vacuum ...the dyno will record vac readings every 100 RPMs
and do it to .000 decimal places if you like in inches HG

with vac readings you can see REVERSION spikes ,
determine the real CFM of the carb ..on a live full-throttle run with fuel coming out of the venturis

back in early years..Holley and others offered only 1 or 2 barrell carbs ..and flow benches back then could probably only pull 3 or 4 inches H2O out of those 1 or 2 barrell carbs ...so when technology progressed far enough to make 4 barrell carbs ..the larger CFM carbs on the same flowbenches could only get around 1.5 inches Hg ..so Holley used 1.5 to rate 4 barrells and 3.0 for 1 and 2 barrell carbs

but now in modern times with a bunch of dynos all over the country .....just about every engine you find makes peak HP
at or below 1.0 to 1.1 inches HG ....once you climb above
1.3 inches Hg...the carb becomes like a "RESTRICTOR-PLATE"
to the engine

if i were Holley ..i would rate all my carbs at 1.0 inch of Hg ....i know the customers would complain because for example an 800 cfm carb at 1.5 would now be 653.2 cfm at 1.0 inch hg. ...racers would frown on this ..but it would be closer to real live engine Peak-HP conditions !!!

if you dyno an engine with an 800 CFM carb on gasoline
and then modify that same carb for methanol or purchase another 800 cfm methanol carb to test...on the flowbench both carbs might flow exactly 800 CFM ..but on the dyno with gasoline coming out of the venturis -VS- methanol coming out of the venturis..the methanol carb will
actually flow LESS by 10 to 15 percent measured VE
..so you would see only 700 to 720 actual dyno CFM with methanol




Larry Meaux (meauxrace2@aol.com)
Meaux Racing Heads
MaxRace Software
ET_Analyst for DragRacers

 

[MaxRaceSoftware]

if you have a 750 CFM carb happily working on your engine ...and you decide to switch over to methanol ..
then don't purchase another 750 CFM methanol carb ..you would actually be going backwards !!!

you would need a 850 to 900 CFM methanol carb to be EVEN with your 750 CFM gasoline carb !!!! to make a fair comparison bewtween the two !!

then you would see the torque and power increase on the order of 10 to 11 percent gains going from gasoline to methanol

the biggest single mistake i see racers do is to purchase the same EXACT size CFM methanol carb as their old gasoline carb CFM ...what happens is the racer pickups up a bunch of torque switching to methanol over gasoline...but they get dissappointed that the Peak HP numbers are not a bunch better than their old gasoline carb

you see this at the dragstrip every weekend across the USA
the methanol carb racers get great 60 Ft times and pretty good 1/4 mile ET times but their MPH doesn't correlate to the ET like a gasoline carb...the gasoline carb will run more MPH most of the times because the methanol guy
isn't feeding enough AIR to the engine TopEnd Larry Meaux (meauxrace2@aol.com)
Meaux Racing Heads
MaxRace Software
ET_Analyst for DragRacers

 

[MaxRaceSoftware]

CFM = CID * RPM_at_Peak_HP * .000289352

756.08 CFM = 402 * 6500 *.000289352

thats at 100.00 % percent Ve volumetric efficiency

but are you satified with only 100.0 Ve ??
with a dual-plane intake..that 100.0 percent isn't too bad a reading

but with a single-plane intake and a little bit of bowl-work porting in the heads you should need
105.0 + VE ...so you would need or be able to use
a 794 CFM carb at most ...so the 850 CFM carb on a single-plane intake manifold is going to be too big !!!

the 850 CFM carb on a Dual_Plane intake is going to be pretty good for power ...maybe not so good for gas mileage
because of venturi size even with a dual-plane
Larry Meaux (meauxrace2@aol.com)
Meaux Racing Heads
MaxRace Software
ET_Analyst for DragRacers