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which carb??

S

shabbadoobie

Guest
First, I'd like to say hello---I'm new to the board & am happy to have found this site.
I bought a '77 L48 for a winter project (it'll have to sit inside for the season--Funky KS weather & all). Heres my first question for all you vette experts----I need more "off the line" throttle response & want to know which carb would be the best choice. I owned a 79 L48 a couple years ago that ran like a dog------That is until I replaced the Q with a new edelbrock (cant remember the CFM ) It was like night & day, I could actually have some tire spinnin' fun! Now heres my current dilema: My 77 runs OK after around 15mph or so, But idles rough & bogs down at takeoff (some) Not only is it unable to "break em loose", But it wont even powerbrake!! (I know,but it's fun). The 79 was the same way till the new carb & I'm hoping this 77 will come to life with a new carb too. I know that the HP rating isnt that high on these stock motors etc., But even so the 79 was still a lot of fun (0-60 around 6.8) I had the 77 hooked up to my G-tech last night & was getting an average of 7.4 (0-60) with the bogged down takeoff.
My L48 has headers, flowtech glasspacks & then pipes out the back--No catalytic convertors etc (true dual) other than that it is still stock. I don't care about top end speed (too chicken--what is top end on this anyway? Just curious) 1/4 times is of some concern, But I really want "off the line" grunt more than anything else. Sooooooo, Holley or edelbrock? 600-650-750 ???
I'd appreciate any & all help--Thanks
BTW, I realize my particular vette isnt gonna break any land speed records etc., But even running like it is , It tore my other car (2001 Explorer Sport Trac) a new B-ho. It was funny to watch (I later G-teched the 'Trac & came in at 9.2 (0-60) MAN it made my Vette seem like a LT1!! Thanks again
 
R

rpounds

Guest
I would be interested in hearing the answers that you get to this question.
 
C

C5Phil

Guest
Shabbadoobie ,

The dreaded “bog” is usually due to the air moving too slowly through the carburetor ventures to be effective in mixing in the proper amount of fuel. Going too big is a common problem with carburation.
The old formula “carb cfm = (cubic inches X rpm) / 3456) X VE%. Where VE is the volumetric efficiency in percent of your engine. This will probably be about 70-80% for your motor. Bottom line, your motor will only need a 600 cfm carb at 6000 rpm, if you had 100% volumetric efficiency (which you will not).
All the math aside, I would recommend a Holly 1450 series model1850, or the like for your application. With accelerator pump and power valve intact. Holly makes a half dozen or secondary springs in various tensions (they are color-coded) so you can adjust the timing of the opening of the secondarys to further prevent bog. There are also various cams available to provide different delivery curves for the accelerator pump.
For the power valve rating it should be 1 to 2 inches below the idle vacuum of your engine. For instance, if you get 7.5 inches at idle you should use a 6.5 or 5.5 power valve in your carburetor.
Using the smallest carb necessary usually results in the best atomization of the fuel, best distribution, and best throttle response. The higher speed of the air through the venturies allows the carburetor to work at peak efficiency.

Regards,
C5Phil
 
S

shabbadoobie

Guest
Oh crud, did I goof up??

You'r not going to beleive this, But I just got word that I had high bid on an item on E-bay----- I guess I'm going to be the proud owner of an Edelbrock Tunnel Ram & 2 holley 450 carbs. You guys already read my post concerning carbs------ I know the dual 4 tunnel ram will look cool & all, But will my 77 even run with it on?? The whole set-up is supposed to be in perfect condition & my high bid was 305.00 plus shipping-- I guess I should've checked BEFORE putting in a bid on the thing---------I'd really appreciate some feedback on this new development (I don't know wheather to be excited or kick myself in the butt!)
 
S

shabbadoobie

Guest
Update: Bought a new edelbrock 600 (all they had in stock & I didnt want to wait) Car runs smoother, But I want MORE ! The guy at Autozone told me if I didnt like the 600, He would take it back & sell me the 750-----What do you guys think?? Keep the 600? Get the 750? I must point out that they sold me an adapter plate for the edelbrock to fit on the Q manifold & it was the wrong one (it was an open design & the booklet said to use the four hole adapter NOT the open) BTW, the salesmen at the Autozone offered to sell me a fresh rebuilt 454 for 600.00 (complete) He was going to put it in a Camaro, But wound up selling the car. How hard would it be to put a big block in this 77? Would it be worth it? Any problem areas?
Well, thanks again & I hope you all had a good labor day
 
S

sscam69

Guest
The carb should fit the engine, run it and if it seems to have a hard time breathing put the 750.

By the way the stock quads could pull 750 cfm so that should give you a big hint what should be on there.

Put a peformer manifold with some headers to help with the "off the line" response you want.

Hope that helps.
 
C

C5Phil

Guest
shabbadoobie,

"The first thing to consider here is the CFM that you will need for your engine. There's a simple formula available that will put you right in the CFM ball park. The formula is:


Engine Size (CID) X Maximum RPM / 3456 =
CFM @100% Volumetric Efficiency
EXAMPLE:
350 CID X 6000 RPM = 2,100,000 / 3456 = 608 CFM


Approximately 608 CFM would be required for this engine. However, most street engines are capable of achieving only about 80% VE; a modified street engine with ported heads, headers, good intake and carburetor can achieve about 85% VE; a fully modified race engine can achieve 95% or greater VE. The CFM number arrived at with this formula must be factored by this percentage."

Factoring the 608 by 85% gives 578cfm max at 6000rpm. Your 600 CFM is probably just fine. But is the jetting correct, the power valve, etc? Did it stop your bog at launch? Are you are possibly at a point where you need to change something other than your carb to get more power? Intake manifold, cam, gears...

Regards,
C5Phil
 
S

shabbadoobie

Guest
Thanks for the info guys.
I may look into another manifold in the near future (couldn't hurt, huh?) I already have headers & flotech mufflers & the new 600 carb helped a little I guess. I hate to use this as a barometer as to my "power level", But the car wouldnt even powerbrake with the old carb, It would sit in one spot & just sound like it was going to die. Now with the new carb it STILL wont powerbrake, But it acts different----I try to powerbrake & the car starts rollin down the street at a crawl (the brakes wont hold it in one spot) I suppose that shows (among other things) that it does have more power? Again, I know a '77 isnt a really high performance year (stock) But I cant believe it cant even break the tires loose. Oh well, I'll try out the G tech thing later & see I guess. Also I mention "powerbraking"---Don't get the wrong idea, I'm not really into that per se, I just used it to test the cars "take off" (or lack thereof)
Next question-----Can a new manifold make a real "seat of the pants" difference? How hard to install?
Thanks again.
 
R

redmist

Guest
Check put the Quick Fuel Technology website. The have a tweaked 650 Holley that seems like exactly what you are looking for. http://www.quickfueltechnology.com/
Look at the specials page.
Also try putting one lighter spring (1 of 2) in your mechanical advance.
Kill the tunnel ram setup.
 
J

Joeverb

Guest
With what you described I would recommend a very thorough tune-up before anything else cause the 600cfm should be adequate for your motor as it sits.

Adapter plates are never desirable on a performance application. A performer intake or equivalent would help with low end torque.

If what your really desireing is off the line power you may consider a rear gear ratio change.
 
1

1976C3

Guest
Anything more than 650 on a stock l-48 is overkill I have ben told. *shrug* Granted the stock Q-jet is rated at 750 cfm it is set to be a econmy fuel monger stock. A good Rochester carb guy can tune it to where it is closer to its potential.

A stock 190 hp l-48 engine should do fine with a 600. It is enough cfm to quench its thirst.
 

ernbuild

Well-known member
Joined
Mar 30, 2004
Messages
82
Location
BRISBANE/AUSTRALIA
Corvette
91/ZR1
I am doing similar things to my 78 L82 engine,I found the holley internet site to very informative. with a mild cam, airgap rpm manifold ,headers 2 1/2" pipes i have gone to a 650 double pumper because holley say with a manual tranny it will work, however a 600 is what they recomend. I from aussie land ,and find that you blokes have so much more to choose from,when it comes to performance gear. information on gear is also difficult to obtain so if anybody knows wether the proform throttle bodies [no choke]
is not only worthwhile,but is it lower in height overall than the standard 650 d/pumper
i would greatly appreciate the info.
looking forward to a reply from the rev heads on the top side.
 

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