ok i was told a while back to go with the 500 edelbrock carb on the offenhauser duel port. i went to advance auto parts web site and they dont have a 500. but they have a 600. is that too much fuel going to the engine? im planing on putting bigger cam in , but thats down the road.

thanks for the help.

 

one more question what is the diffrence in a air valve and vacuum operated secondaries? what do they do ?
thanks again.

 

I plan to use a 390cfm on mine, but I have a 240. My guy tells me that about as big as I should go, but dunno how the extra 60 cubes might effect what carb to use.

Ron

 

Jegs or Summit will have a huge selection of carbs.

There is a EB 600 4V and a Holley 500 2V. I'm using the EB but can't really speak on long term performance just yet.

 

is the EB 600 going to be to much? how did it effect the mpg?

 

Not quite sure yet, it's running but not 100%. The carb is not the problem though.

 

A 600 is big enuf for a 400 cube engine.

A "Rule of Thumb" for any street or mild build:
MAX carb size: cubes X 1.5 = CFM
300 cubes X 1.5 = 450 CFM
240 cubes X 1.5 = 360 CFM

Holley makes some nice small 4V carbs. The smallest Edelbrock makes is a 500 CFM and it is set up for performance with a slightly richer mixture etc. The Edelbrock is set up nice if you want to split your intake manifold to improve low end throttle response.

A carb that is too big for the application will be difficult to tune for off idle throttle response. It will bog real bad. It also is harder to get the fuel metering right and will use more gas. This will also make low end throttle reponse bad.

 

I used a 500CFM carter AFB (same as the edelbrock)on my old 302 powered bronco and that thing flew. The off idle response was good, it ran good cold and the top end was great smacking the 5500 rev limiter all the time. And I could crawl hills seeing nothing but sky and never flood out. All around I loved that carb. As for your question about mechanical VS vacuum secondaries the difference is that vacuum secondaries open only when a certain throttle level and vacuum level in the primary venturies has been met, this is tuned by springs. Mechanical secondaries open based on what throttle position your at no matter what since they are actuated directly by the throttle linkage and on the EB carbs this is easy to tune/change. The EB carbs as well as the carters have a sort of air metering system in the secondaries that keeps the carb from bogging out at WOT. Anyway I hope this helps some.

 

500 cfm edel/carter is a great carb
450 holley is my second choice. these carbs are often factory set for dual aplications.
rejet a 450 like a universal 600 holley (around 60-62 jets)and they run great.

390 holley is the WORST of the small 4bbl carbs for a 300. 390's come from the factory with small light vehicles in mind. They come without checkballs in the secondary vacuum system. If they are rebuilt with checkballs added and rejetted they can work nice.

 

That's only if the primary bores are too big. That will give a weaker vacuum signal to the booster venturii and idle circuit, and will affect low-end metering response. A big carb with small primaries and big secondaries (a la Q-jet) will work fine off idle and at part throttle. The secondaries on the Q-jet can be tailored to open pretty much where you want (air valve spring tension), and the accelerator pump linkage can be bent/reholed to provide more or less shot volume and duration. The AFB carbs have a counterweighted air valve secondary (sort of like the Q-jet, only air demand works against the counterweight) that is for all practical purposes not adjustable. Nonetheless, the secondaries will open according to airflow demand, so the 500 CFM Edelbrock will not overcarb a 300 cu. in. motor. Works just great in fact, with minimal tuning required. I doubt the air valve opens fully at any point in a mildly warmed over six's RPM range.

You can even use a generic 600 CFM vacuum secondary Holley on an inline six if you jet the mains down a little (~56-60). Secondaries open later and go to full open later with a smaller engine. This means with the right secondary calibration on a too-big carb, the secondaries will not open fully at your RPM ceiling. Max airflow will be more in line (uhh huh huh...) with your engine's airflow requirements. Keep this in mind when selecting a secondary spring. Accelerator pump nozzle (I-6 with 4v manifold and header) might wanna be #28.

The point is that there is considerably more flexibility than many people suspect in making what seems like "too big" a carb work with your street beater engine. This means if all you have conveniently and affordably on hand is a 600 CFM carb, it can be made to work just fine with the 300 inline six. I speak from experience. A double-pumper and any larger carb are not even considerations, for obvious reasons. For best low-end and part-throttle response, the important things to have are small, efficient primary venturii, and good manifold vacuum to pull thru them. The Offy DP has small runners. I don't need to belabor about what this means.

A Holley 4180 boneyard special can work very well with the inline six with a little tuning, and the annular-discharge primary boosters are good for low-end and part-throttle response...

I've built "small" (302-318) motors that work better with 750 CFM carbs than with 600s. No bottom-end loss, and more on top. This is because I was using a Performer RPM or a Weiand Stealth high-flow dual plane (umm, among other things...). Dual-plane manifolds are generally more tolerant of (actually sometimes they BEG FOR...) a little more carb. The vacuum signal to the carb is stronger with a smaller plenum volume, unlike with a single-plane, which is also prone to charge-robbing in adjacent firing cylinders---5+7 on a smallblock Chevy, 6+5 on the 5.0 HO and 351, 7+8 on the older small Windsors for examples---and more uneven mixture distribution characteristics in the low-end. With a single-plane, a smaller carb is critical for any reasonable amount of low-end response.

I've only seen two performance single-plane intakes that worked halfway well in the low-end: the Edelbrock Torker II and the Weiand Team G. No substitute for a Stealth or RPM for daily driving. The Ford inline six is a whole different ball of wax, however. Any available 4-bbl manifold, whether it be a DP, C, or Clifford will improve output mostly everywhere over the stock setup. The big-six's firing arrangement, runner layout, and fewer cylinders make charge-robbing a bit less of an issue with a single-plane, and she wants to breathe, yes she do!

For example: I'm pulling 20" of idle vacuum thru a Clifford single-plane/Road Demon Jr. 525 with the stock cam (and a header). There is no loss of low-end over the teeny factory carbed log whatsoever. I can't give 'er more than maybe 1/4 throttle on wet roads at 30-40 MPH in third, or she'll get sideways, and it's feather-foot time at takeoff and around corners. Bed ballast WILL be necessary in the winter...

The Holley 390 is really more effort (than it's worth) to dial in properly for a stoked six than an Edelbrock, Carter, or Demon 500. And it'll start running out of breath when you're near the 250 hp. mark. Don't do it, you'll give yourself an unnecessary headache.

 

is the Edlebrock 500 a 4bbl?

 

yes it is.
1403 is the electric choke version
1404 is the manual choke version

 

were do i get it?

 

Summit (www.summitracing.com)

Jegs (www.jegs.com)

Ebay?

gearhead classifieds?

A buddy who just happens to have one lying around he wants to part with?

 

50_Bucks-

While you obviously know your carbs most users just don't have the parts and experience to tweak a carb to make it "work". I feel it is best to start with the properly sized carb for the application. I know you can make a big square bore carb "work" but they still are not quite right. Performance and fuel economy will suffer some. The big spread bore Q-jets etc are easier to make work because of those small primaries.

I believe that a simple modification to many 4 barrel manifolds for a six is to split the manifold front to back to reduce the manifold volume. This produces what is essentially a "dual plane" manifold that will improve low end throttle response greatly. Just like a dual plane for a V-8. To make this work the carb of course must be positioned correctly to "split" properly.