Ideal Dual-Carb FE setup for a street 390/FE?
 

FE People,
Just to double-check for a friend, what is the most ideal DUAL CARB setup going these days?... for a 2400lb Cobra with a moderate 10:1 performance FE/390 in it, with 3.25 ratio taller rear end gears and a wide ratio Toploader. The car is mainly cruised around 95% of the time. Car currently has dual 600 Holley mechanical secondary carbs. Seems to be a tad large for this engine/setup and purpose.

Current Consideration:
We are seriously considering a pair of the new Holley 450 cfm #9776 carbs instead with mechanical secondaries. Will probably be selling the Holleys to someone with a more pumped up 427 arrangement.

Any feedback with these 9776 carbs on a street 390 cruiser like this would be appreciated? Also curious if anyone has any before/after feedback with making the same change. THANKS!

450's sound like a good choice for a 390.

I had two 390 CFM carbs on mine for awhile, ran really well with synchronized throttles, crisp throttle response, great for street cruising, but seemed to run out of air above 5000 RPM. I changed to two 600's with progressive linkage and it still runs good, makes more power, and has crisp throttle response too. Did your friend have a progressive throttle setup? If not, try that first, it might fix things right up.
I would recommend vacuum secondaries for a dual carb setup. The mechanical secondaries might cause some driveability problems (you can over-carb with too much throttle, might bog at low RPM, etc.). In addition, I'm not sure whether it would be best to run 450's with a synchronized or progressive throttle setup. My 390's were like one 780, so synchro primary throttles worked just fine. The 450's would be like one 900, and might have a little too much primary throttle to be crisp on the street, so progressive might be the better setup.
Who know, lots of little variables to play with, and like you said, it is a light car so the 450's might work just right.
Let us know what the friend does and how it works.

You'll have much better driveability on the street with vacuum secondary carbs. The old Holley 1848 or 1848-1 is a 450 cfm vacuum secondary carb.

Dan

I set my engine up with a rock solid bottom end so I can run my motor up into the 7krpm range and feel reasonably confident its all going to stay together. For optimal performance at those rpms my 427 is calling for a lot of air, thus I installed twin 750 Holleys with progressive linkage and vacum secondaries, if I decide to ask my motor for big time performance it can answer the call.

http://www.clubcobra.com/photopost/d...tor_pics11.JPG

Ken,
He has progressive linkage now, and what seems to be lacking is throttle response and yes, a bit of low-end bogging, even after lots of tweakign on the carbs, PVs, jetting.

As I understand it, its partly due to a velocity issue, like flushing a toilet bowl with the dual-mechanical 600s on there now. With his taller gears, the Vacuum secondary might be good; but I gotta say, even with my own Cobra, I would never go back to vacuum secondary after experiencing a finely tuned mechanical setup. Maybe its really different with dual carbs, and that is part of what I'm reading in feedback from some of you who stress vacuum secondaries on a dual-carb setup. I wish I had about 5 different carbs to test first before buying. :)

Too bad we can't pool carbs to pass around for trials. I had two 660 center squirters (probably what decooney's friend has) on my 496 set up synchronized; as decooney stated, once a mechanical setup is dialed in, it's hard to go to vacuum secondaries; I did because of driveability issues...if I set the 660s up progressively they might work out OK on my engine, might play with that later. I'm sure they're too much for a 390, however. The vacuum secondaries are much nicer for cruising around town, which is what I do the most with the car.

Dan

When I had a 390, I was running with these two Holley, they made me 100% satisfied and they were of excellent quality. They also worked on my new block: a 482 but they were not the best combination.

I have two 600 cfm 1850 holleys .I was not happy with the rate that the spark plugs went black and started fouling. I had a local tune up shop hook it up and run it on a dyno too see if it was a ignition or fueling problem. The holleys have had the choke towers removed and were heavily romanced by Quick Fuel Technology.They came back with #73 jets . I reduced jet sizes to #66 primaries and #68 secondaries and 2.5 power valves to be sure they were closed. Testing said the engine was above 8% fuel in midrange. They said that I had enough vacume to use 6.5 powervalves. but jet size needed to be cut enough that I should start with #60 jets.They said that I was way over fueling. I'd like to get these carbs right before I switch to new 1850s with a choke for the cooler weather.

Michael,
Its interesting to note that you sent your carbs to a "specialty" carb company, they set it up and jetted it for you too, and later testing results come back indicating you are "way too high" on jets, so you lower the primaries from 73 to 66, and it's still "way over fueling". For this exact reason, I myself am not sold yet on mail order specialty carb builders who test in their lab and not on your actual car dealing with your parameters such as your exhaust restriction, your intake, your heads/porting, etc... Seems like a bit of a crap shoot to me and you are proof having to redo your carb again AFTER sending to one of these full on carb specialists. I agree, if someone is starting with a mess, there could be improvements over what they had, but may not be that much better than what you, I, and others are doing in their own cars in their own environment, elevation, etc.

Several of my own local friends have been to dyno service companies, and they get their carbs setup for max torque/hp, and there are improvements in that spectrum for sure, but in many cases it just seems like driveability actually goes down some when these carbs are reportedly optimally jetted on a dyno too. It would be cool if the car was put on a dyno, and ran through a series of runs, shifts, stops, idling, back to driving just as one would on the street which is much different than a predictable track or road coarse setting. I'll never give into programmed EFI even though its the hot setup - but its just not fun like chasing these carb blueprinting puzzles is it!

This repair station has two floor dynos for testing. A lot of their traffic is emmissions stuff but some performance. I wanted to see if the ignition was laying down on me. But at least it was on a dyno and not idle and high idle.

These carbs had the choke plates removed before I got the car. I had cut the choke towers off before having these 1850s redone so they could get refinished . Quick Fuel Technology did all sort of things to the primary metering blocks. I also had them machine and install replacable air bleeds which may be part of the cure incase I open the air bleeds up too much and need to start over . but I've got that cappability. I'll start with the jets then the air bleeds to get fuel ratio in order.

Mike,
curious to know if you try the 60 primaries with the 6.5PVs back in, and how it goes. I hadn't considered going that low on jets up front on a dual setup, but with direct 1:1 (non-progressive) linkage adjustment it would be interesting to see how it pans out for you if you try it. I will try my friend's car again once he gets his trans back in and compare. Good Luck.

According to the dyno work I have watched, you would still be overcarburetted with two 450's. The 390 CFM's have been around forever. We used to run ONE 390 on our 400 CID Chevy Sportsman road race car (those were the rules). They made over 500 HP with one 390 carb.

A 390 lower-compression cruiser needs about 650 CFM total.

My news may be a while in comming. My wife still has a friend of hers canning stuff taking up most of my bench. I plan on changing the powervalves and cleaning the air bleeds first thing. These carbs are not as they were back in 99. Then #64 jets worked for 5,000 ft elevation in Nevada.but ended up as a lean missfire down here at sea level.

Duane,

There is a way to calculate the flow demand of an engine, based on a nr. of engine specifics, such as bore, stroke, compression ratio, con-rod length, flownumbers of the intake manifold and the cyl heads, cam advance and duration (at 0.05") and the desired RPM. If you post these properties, I'll be happy ro run them through my calculator.

Off hand, I reckon that even 2 - 450 CFM carbs ma be a little big for a 390 street cruiser. Holley usually recommends to stay a tad on the small side for better respons and accelleration in the low and mid range, sacrificing a little at the top. However, properly set up, dual 450’s should work just fine.

If the 450s are vacuum secondary carbs, the secondaries will only open as much as the engine can use. Also, with dual quads, no matter what size carbs you have on, you'll get a similar rush as with a single double pumper when the primaries on the second carb open.
Food for thought,

Dan

If you feel the secondaries open then the carbs are most likely not set up correctly. Feeling the surge as the secondaries open usually means that the engine is not making enough power prior to their opening. If you plan to use the Snellings and Hellings air cleaners you may want to mill off the choke air horns so that the carb can breath. I have two 390's with mechanical secondaries and I like them a lot better than the 600 vacuums that I used to have. If you ask me on a cold morning I will tell you their perfect but on many hot summer days I get a flat spot at about 1500 RPM as I get on the gas. It is a work in progress.

Not sure about this...I've had single Holley 4bbls that you could feel a surge when the secondaries opened, and the car ran excellent. The dual 600s on my 496, however, seem to open without the surge...probably because the car is surging from the time I stomp on it. :3DSMILE: Seriously, I really don't know if it's different with dual quads or not, as everything is happening so fast with this motor...I'm a work in progress in getting used to it. :LOL:

Dan

hello
i can see by your picture that you didn't set up your linkage to work progressive. do you feel it works better with both carbs working at the same time? how is the driveability and fuel consumption?

mike
i have redone my carbs and i used #63 in the front, # 25 squiters, and the stock plates in the back were to rich (# 10) so i droped the rear plates to a #6 and the fuel ratio is dead on!!!!! as for the power valves that is set by the vac. at idle and cut it in half ( 12 inchs at idle = a 6 to 6.5 valve)
pickle