which carbs to use ?

cobra427mnsi · #1 (**permalink**) 05-12-2003, 08:27 PM

Which carbs do you use on a 427 top oiler low rise heads dual quad engine?

Paul

SCOBRAC · #2 (**permalink**) 05-12-2003, 09:16 PM

Most economically are Holley 1850's.. 600 cfm vacuum secondary carbs. these are closest to the original, no longer produced, read really expensive OEM 550's 650's and 730 cfm factory carbs. Original rebuildable OEM Holley's aren't hard to find, but good used ones are. NOS examples have been found for $500-700 each without linkage, fuel log or air cleaners.. The 1850's are everywhere new for $239 USD. www.jegs.com

Think about what we are talking about though.. 1200 cfm.. Even a 427 cubic inch engine spinning 6500 rpm at 100% (not possible) efficiency only needs (read, can only digest) 735 cfm.. 900 cfm is 19% overcarburated.. 1200 is almost 40%.. I am not saying you can't use 1200.. but enless you are turning 9000 rpm you will never use all of it.

Other options include two holley 450's or even 390's.. Both the 450's and the 390's do not have a choke, which might be a problem in Canada, eh..

kountzecobra · #3 (**permalink**) 05-12-2003, 09:44 PM

Holley makes 390 carbs with vac secondary with electric chokes. The 450s are mech secondary carbs. Works well on my 351. Mr 0077 also has a pair of 390s on his 427 and it hauls arse!

mr0077 · #4 (**permalink**) 05-13-2003, 05:36 AM

Paul, I REALLY like the way my car runs with the 390 CFM carbs...good throttle response, and really goes well. If I EVER get the time, I plan to dial in a pair of 600 CFM Holleys I have. I'm just curious about the 600's...I would be surprised if they are an improvement for street use, but a lot of owners in the Club run them and seem pretty satisfied.

K.Wilson · #5 (**permalink**) 05-13-2003, 10:13 AM

Paul,
I have Holley 600cfm vacuum secondary on mine and after the initial set-up and some tweeking (new power valves, smaller jets etc.) it runs great!
With a vacuum secondary system you do not have an "over carb" problem as the secondary system will only open as the engine volume allows, so if the engine will only consume 800cfm that is all the carbs will provide.
Keith

mr0077 · #6 (**permalink**) 05-13-2003, 11:57 AM

Keith, for a starting place for future tuning of my 600's, what jets and power valves are you running? Mine have 7.0 power valves and #65 jets right now. TIA

K.Wilson · #7 (**permalink**) 05-13-2003, 02:06 PM

The tuning of the carb is based on a lot of factors. A good way to start is with the carb as it came from Holley. After your first start and cam break-in I usually set the float levels, idle speeds both the fast idle cam and the idle screws, then the idle mixture for max manifold vacuum. Take the vacuum in inches of mercury (in hg) and subtract approximately 2 from it and that is the number power valve you need (I run between 10.5 and 11 in hg so I use 8.5 power valves). However vacuum can vary significantly with your engine build, caused mostly by the cam, so it is very important that you take this measurement. The power valve selection has a big impact on and low rpm driveability/power.
I would install the power valve blowout preventer check valves at this time if your carbs don't have them.
The jet selection is a bit more subjective unless you have your exhaust gas measured during a Dyno run. I check plugs and exhaust pipe color and make changes accordingly.
Mine runs best with #58's in the front carb and #60's in the rear carb ( the front half of the engine runs a little cooler than the back half). If I lived in a cooler climate I would drop down another jet size or two but as I live in Alabama where 90 plus degree days are common it is safer to run on the slightly rich side.

Hope this helps, Keith

cobra427mnsi · #8 (**permalink**) 05-13-2003, 02:21 PM

Kieth

Thanks for the info. It was very informative. Are you running Holley 600's ?

Paul

cobra427mnsi · #9 (**permalink**) 05-13-2003, 02:22 PM

Kieth

Sorry, I read the last post first. That will taech me to start at the beginnig

Paul