Not What I thought

[cmegga]

Well looks like I made a big mistake on my ordering spree this spring. We did a shift kit yesterday, which is nice, but not as impressive as I expected.

Anyway we also did the retune sine I have just about 1200 on it since the build. He couldnt seem to get the carb to set right. It was aways on the lean side. So we went threw it and looked up the cat # to order some new jets and ..... To my surprise I have a 600 cfm sitting on there!! I was under the inpression it was the 750 cfm .

It looks like I ordered the wrong one and Im just noticing now. Well the question no is what to go with ?? I dont want the edelbrock again as it seemed to be a pain to tune. Im thinking a holley 750cfm.

Anyone have a better idea before I order this sucker up ?

 

[AKRAY4PLAY]

what exactly is your build?? your edelbrock should be good to 400hp. they are easy to tune if you take generous time reading the tuning manual. most parts stores stock full tuning kits for edelbrocks. the springs play a huge part in the fuel delivery curve. bottom line, they are as easy or easier to tune than a holley.

 

[vmrod]

Right now, I have a Holley 650, dual-feed, double pumper, w/manual secondaries. There is a slight vacuum leak (I can hear it when I wiggle the throttle lever.)

Anyway, I wanted to get the scoop on Edelbrock carbs. Are the good performers? What are you using AKRAY4PLAY?

 

[cmegga]

Here is the basic build on my engine. The main reason I want to switch is because we are having a hard time getting it tuned right. Im assuming it has alot to do with the fact is only a 600 cfm carb. They recomened a 750 cfm, so my first instinct is its too small.

Engine re-bored 20 over, with flat top hypertecnic pistons, new rod bearings etc.

Heads Edelbrock Perfomer RPM 60899
Cam Comp Cams Extreme Enegry CL12-242-2
Timing Chain Cloyes ‘True’ Double Roller 220-9-3100
Water Pump Edelbrock Victor 8812
Carborator Edelbrock Performer 600 CFM 350-1406
Intake Manofold Edlebrock Performer 350-2101
Header’s Headman 68300
Rocker Arms Comp CamsMagnum 249-1412-16
Lifters Comp Cams High Energy 249-812-16
Pushrods Comp Cams Magnum 1010 Chrome Moly 249-7693-16
Ignition MSD Ignition 6A 6200

 

[Evolution1980]

Are we to assume you no longer have the original Q-jet 750cfm carb?

Did you want the shift kit just to slam the shifts harder? That can be adjusted at the carb as well. There is a cable that gets adjusted to set the shift points on the tranny (at least on my TH350). My mechanic had to re-adjust mine the other day.

 

[cmegga]

The origonal carb is in veryy ruff shape. Definitly not putting that back on.

The shift kit works great from the 1 to 2nd shift, hits nice and hard But I dont really notice a diffrence at all on the higher gears.

Good idea with the shift points. I may have him pay with the shift points, I think it shifts a bit early.

 

[80convertible]

I was just reading an article (Engine Masters, summer 2001 vol. 4) on carbs, and Demon recommends 625cfm carb on a 300/350. I have the Carter 625cfm carb which is basically the same as the Edelbrock. I had it tuned while on the dyno but without changing the factory installed jets/metering rods. The carb tuned quite easily according to the mechanic. And the engine pulled some nice dyno numbers considering how old the engine is and in need of a rebuild and matched cam etc... I have to agree that this is a great carb for stock or mildly built small blocks.

Jim

 

[AKRAY4PLAY]

everything i have read and been informed of, 600 cfm should do you fine. if you get too big a carb, you are running a huge risk of never getting the thing tunned right. all carbs work off vaccume as we all know. if you go too large on the cfm your vaccume will numbers will be all over the place. it is hard to tune when you motor goes from making 20 inches of vac to nothing and then back to 8 inches. edlelbrock (carter afb, they are the same) are easy to tune for linear fuel/air curves that are needed for stock to medium engine builds. holley with the power valve set up is better suited the group of engines that fit into the medium high biuld catagory. demon carbs (barry grant) are designed for the high mod motors that do not make vaccume until 1200 rpm plus. read the tuning book and get a spring/jet kit for the edelbrock. my brother in law has a 600 edel on his 400hp 350 in his truck. it works great! i am running a 750 demon on the 406 small block making 600 hp.

 

[Evolution1980]

For the sake of friendly argument, in response to the 600cfm being big enough... for whatever it's worth, I believe that all single edit: 4-barrel carb setups on any C3 were 750cfm from the factory.

I figure that if GM thought a 600cfm would suffice for their engines, they would have supplied them, as it would have been a cost-saving change.

...my less than .02 cents

 

[DDLS3]

Evolution1980

That is exactly what I thought, so I bought a 750 Edelbrock for my 81. But.........I was wrong. The 795 Q-jet spread bore (it's not a 750)is 25% primary, 75% secondary (roughly). The Edelbrock is a square bore, so it is roughly 50-50. I had to go very lean on my 750 Edelbrock to tune it in, the 600 would have been a much better choice I think.

 

[Evolution1980]

The 795 Q-jet spread bore (it's not a 750)is 25% primary, 75% secondary (roughly).

Ahhh yes, and therein lies the reason the Q-Jet is such a formidable street-racing carb. When you're puttering around town, the 25% primaries are all you need. But if you need to hop on it, watch out, 'cause those secondaries will open up and you're car will kick like a mule!

(I won't go into any more of a Q-Jet -vs- anything else because 1)my actual knowledge is limited to the above and 2) It was pretty much thoroughly covered in a thread a few months back )

 

[AKRAY4PLAY]

all good points on the different carbs to go with. the main thing you need to look at is the intended use of the carb. what all to expect from the engine at different rpm. make a pro/con list and check off the carb that best suits your needs. square bore carbs have the tendency to mellow a motor out. by that i mean the power feels more linear smooth in the delivery. a spread bore will give great throttle responce at low rpm and a hit when the secondaries open. i do like the q-jets for street cars. they seem to have the most pluses and fewest minuses for the street application. correct me if i am wrong, i have been told by old timers that the reason that the big three auto makers went with the 800cfm carbs was for certain racing applications that required drivers to run the stock carbs in the late sixties. if that is true, then it makes sense to use vaccume controlled secondaries. with vac seconds the motor will only pull what it needs and not get fussy like a mecanical secondary delivering more than an engine can consume. best of luck with the tuning. time and patience are your friends when it comes to tinkering.

 

[76L82]

I have now had V-8 cars with Rochester, Holley and Edelbrock carbs. I would have to agree with the statement that the Edelbrock is the easier to tune and probably one of the best carbs for around town and racing applications. The Holleys that I have had were real tough to get right...I always struggled getting a warm idle with them...so around town they were an absolute nightmare. I usually ended up having to set the idle around 1500 RPM so as to not lose the engine when it idled. That was tough on the gear box when you went to launch the car.

The Rochester..which currently sits on my L-82...doesn't seem to have the snap from 45mph - 100mph that the Edelbrock has...and it seems much happier when it is in daily driving scenarios... not bad for gas mileage though, but not a performer in my opinion.

Now I have an Edlebrok Q-jet on my 70 Camaro SS. I have a built 350 in her and that carb (750 cfm) has been like magic...had a 650 cfm before that...the 750 is easy to tune and has never bobbled when I punch it..no matter what speed I am going. Although I don't have an extensive background in this, for whateer it is worth, I have never seen a Rochester used in a hopped up engine package. Good luck!

 

[rpounds]

For some ideas with the Q'jet, take a look at this:

http://www.corvettefaq.com/c3/myqjet.htm


Q'jets are great carbs. It's just that a lot of people do not understand them or understand how to make them work well. For any of you running the Q'jet, the above article has some good tips.

Yes, they are 750 CFM carbs, and they were used on a variety of applications, from small blocks to big blocks, Chevy to Buick. Because of the way the Q'jet operates, it is suitable for use on an engine that would not necessarily need all of that 750 CFM. It allows the engine to pull what it needs, without being over carbureted.

Now, if your engine is 350 cubic inches and you turn it 6000 RPM, you only need 600 CFM of carburetion. Even if the volumetric efficiency of your set up was 100% (it's not), you would only need 607 CFM. The engine is in essence an air pump. It will only pump a certain amount of air, depending upon cubic inches and RPM.

Long story short, I doubt that the size of the Edelbrock is hurting you. Jetting - yes . . . size - no.

By the way . . . John Lingenfelter used the Q'jet VERY affectively in his 1/4 mile cars . . .