bigger dual quad

[woodsnake]

I may regret asking this here, but I'm going to do it any way.
In my 68 Cougar I have a 289. When I was going to school at BSU for the auto tech program, I decided that this was going to be my 'project car for school'.
So, did the brakes, rebuilt the trans, and built the motor. It's a .030 over 289 with .010 over and under on the rods and mains. I installed the RPM performer cam, .496/.520 lift, 238-248 duration. I learned a lot about push rod length, parts pushers, machine shop folks, and the usual birthing pains of the first performance motor build.
I also used the new Edelbrock dual quad set up. Two ea. 500 cfm carbs with a progressive linkage. I had the linkage set up as non progressive, I didn't like it.

So, here's my question: Even though it "looks like" 1000 cfm, do you think there is a gain in perfomance to be had by switching to a pair of 650's? The set up only runs primarily on the rear carb.
My understanding is that under WOT, the car may only draw 750 cfm between the two carbs.
A single 500 cfm is too small for my 289, and it runs primarily on exactly that.

If I went to the 650's, I Think it would be better streetability, until I stick my foot in it, and then it would be ludicrous speed out of that poor 289......


What do you think, and why?

 

[D.Hearne]

You're wrong in your assumptions. Two 500 4 bbls will not flow 1000 cfm at WOT, unless the motor is capable of drawing that much flow thru them.Those carbs are rated at 500 cfm in single form, use two at the same time and they'd be rated at half the vacuum level, cause you have twice the number of bbls. With a carb, smaller is better on the street. On the street, you're not using the carb's full potential flow, even when you floor it at a dead stop. You have to keep this in mind: for a carb to function, there has to be a pressure differential between the top of the carb and the bottom (under the venturis). Nail it at a dead stop with all 8 bbls open, and you have no pressure differential across the venturis to allow them to pull fuel from the bowls. I have a three 2 bbl setup on the 331 in my 77 Comet, and they're PLENTY of carb for it. But at the same time (these are three 250 cfm rated Holley carbs) their street manners are fantastic as it only pulls from the center carb unless you nail the pedal. You're working with carbs here, not EFI. With EFI, the fuel is pressure fed from the injectors, with a carb, the only function of the fuel pressure from the pump is to fill the fuel bowls on the carb. You have to have a vacuum from the engine across the venturis to pull fuel from the bowls. The more vacuum there is, the easier the fuel is pulled and the better it's atomized in the venturis. Thus a smaller carb is better for the street. To utilize the flow potential from bigger or multiple carbs, the engine has to be spinning in the upper rpms when all of them are opened up.

 

[woodsnake]

I think I see what you are saying,
BUT,
the linkage is NOT direct. It is NOT 8V all the time. The primary carb is in the rear.
It runs on 2V, till you move the linkage enough to engage the front carb, and then (6V) till the front secondaries open.

That's why I said "maybe 750 cfm at WOT".

Most people see two 500 carbs and say~"WOW that's WAY too much carb for that little motor!"

It's not a pair of mechanical secondary carbs,wired directly.

I read the whole 'over carbed small block' myths/rumor on S B F tech, it's a very informative thread!!

It's this one :
http://store.summitracing.com/partdetail.asp?part=SUM-CEDL318&autoview=sku

 

[D.Hearne]

It doesn't matter what linkage is on it, going to bigger carbs will make the driveabilty go down. Two 650's is too much, plain and simple. Unless you've built a strip dedicated high rpm 289. Only then (maybe) will you see a benefit of bigger carbs. And then only in the extreme upper rpms where the engine can use the added flow potential. In which case using them on the Edelbrock 2x4 intake is a mistake. Going to a Tunnel Ram is where its at. The ONLY way to find out what your motor draws in air flow at WOT is to put it on a dyno. To see if two bigger carbs is necessary, install a vacuum gauge on it now, and see if it actaully draws a vacuum at WOT. Even if it does see a vacuum at WOT, installing bigger carbs will still decrease the driveability at the lower rpms.

 

[htwheelz67]

I have never ever seen any dyno test where a dual quad setup will beat a single quad setup, even the new rpm dual quad setup makes less hp/tq than the single quad rpm setup.

It will make more noise, it will look cooler but sorry to say it will make less power, the only dual quad setup that can make more power on the street is a small runner tunnel ram like the wieand and thats at top end.

 

[D.Hearne]

I have never ever seen any dyno test where a dual quad setup will beat a single quad setup, even the new rpm dual quad setup makes less hp/tq than the single quad rpm setup.

That's because they always use a motor that's incapable of using the added flow. I've seen several dyno's of motors that do benefit from dual quads or triple duece setups and have owned and driven both in addition to that. Back in ther early 80's when I had my 427 stroked to a 454, I ran it with a single 4 (Holley 750 on a Medium riser intake) and it had plenty of power. It wasn't until I swapped on a low riser dual four intake and topped it with twin 600's that I found out what it was missing. My 331's the same way, although I've never throttled it with a four bbl carb. Now when I destroked the 427 back to a 427 and replaced the 600's with 750's (not by choice, but by necessity) It did loose power with the bigger carbs. It just couldn't fully use the twin 750's.

 

[htwheelz67]

a 750 on a 454 stroker? I bet if you ran the properly sized carb/intake you wouldnt see the difference you did.

If you ran a big dual plane or victor with a 850-1000 cfm carb, I doubt a low/med rise dual quad setup would even compare, theres a lot of technology in induction since the 80's..........

 

[D.Hearne]

I ran a "big dual plane" on it.The Medium Riser Ford intake is more than the equivalent of an RPM intake. And Holley's 3310 750 hasn't changed at all since then. And those two 600's cost me far less than a single dominator or any other tricked out 4 bbl carb. Can't say that for the triple duece, that setup cost me $1100, but the performance and sound at WOT was well worth it. Running a dual quad is a lot cheaper than three twos, simply because there's lots of cheap, "little used" carbs to use on em. As for induction tech changing in the last twenty years ? Other than the tricked out carbs available now (which can be duplicated by anyone who understands carb tuning for far less money) I don't see it. There are more aftermarket intakes out today, seems like, but they're not much better (if at all) than what Ford was making 40 years ago.

 

[rbohm]

woodsnake, do yourself a favor and stick with the dual 500cfm carbs. if you go any bigger you will have drivability problems that you cant tune out.

htwheelz67, while there have been advances in intake manifold and carb technology over the years, one thing still remains true. you need to use the right size carb for the application. when you go too big, you lose power since you cant maintain proper airflow velocity through the intake runners.

 

[jlangholzj]

just out of curiosity, would a fellow be better of running two v's set up on a progressive linkage? say maybe a couple of 250's, that way you would get some subtantial vacume from idle, but then also once the air starts flowing?

OH wait a sec! I already have one of these....It's called a holley 600 with a vacume seconadary!! Is this primarily the reason the vac. secondaries are favored?

 

[D.Hearne]

Two 2 bbls is something I'd like to try at some point. Sure, it would flow the same as a single 4 but it would have better fuel distribution mounting them on an Edelbrock 2x4 intake, plus give it the "usual" factor of multiple carbs. If I do it, it will be using twin Holley 500's on the Edelbrock Airgap 2x4 intake.

 

[rbohm]

J and DH, you guys just gave me an interesting idea for an inexpensive efi system. think about this, a dual quad intake with two cfi throttle bodies, run by a megasquirt computer. it would be much easier than a port system, and still more efficient than a stock cfi system.

 

[D.Hearne]

J and DH, you guys just gave me an interesting idea for an inexpensive efi system. think about this, a dual quad intake with two cfi throttle bodies, run by a megasquirt computer. it would be much easier than a port system, and still more efficient than a stock cfi system.

Yea, I've thought about that too, but the wiring thing has always turned me off. It would look way cooler than a single TB setup though. Sort of in the same league as the EFI "Webers" that pop up in the Cobra's.