BBC dual carbs

[john350]

hey gang,

Has any one here every tried one of the new air gap rpm edelbrock dual four intake, compared to one of there older dual four intake. I tried calling Edelbrock tech line, with out a contact person.....the tech new more about all out racing than any helpful info on there dual quad intakes.

Heres the issue, we have a custumer that likes the look of the duals on his 1969 camero, he has a cowl induction hood so there is a little room but not tons needed to install anything real tall. This is a street Rod never to be raced......well......you know how that go's ....until.....we're having some real issues getting any kind of real power out of this engine......I thought something was wrong with my dyno giving low numbers for the longest time....or maybe its was just us after screwing with it for this long and not getting good numbers. I would be ashamed to post the numbers we have gotten on the dyno. This engine has very good parts installed, the only moderate performing part or mismatched part is the intake and the carbs....also edelbrock, 2 600's.

Yesterday we were dynoing another BBc with several points less squeeze smaller roller this time a hydraulic, this engine is equiped with a single holley 850 (out of the box from Holley...not from a carb guy) and a Air gap performer rpm intake, a 1 inch spacer...the problem is this would never fit under the hood of the camero, but the torque curve was awsome..... WOW right out of the box......no tuning yet....50 plus on torque .....the smaller cam didn't make the top numbers however .....but dollar for dollar the second engine was much better.

I believe the air gap dual would just make the fit under the camero's hood, but to sell the idea to the custumer for little gains....we have enough egg on our face already. Any help...even a contact person at edelbrock.....someone had to designed that intake....they should know something about it???

[F-BIRD'88]

check this out.

www.alteredwheelbasenova.com

[john350]

thanks F-Bird for fast reply....I was looking for someone that might have had both intakes and dynoed the two to see the difference

[582r10]

I hate those edelbrock carbs. When a customer comes in with a driveability issue it's usually got one of those on it. Having said that, I've got a guy with a '67 Camaro, 402, hyd cam and the dual four set up from edelbrock and it rocks. It has electric choke so it starts immediately every time, progressive linkage, so it doesn't kill on fuel and it hauls ass pretty good for what it is, very responsive. That's about all I got, no scientific data, no dyno sheets, just the seat of the pants observation of one of those set ups in action. I still don't like those carbs though.




Roger
582r10

[john350]

Thanks 582R-for fast reply,

Well this one will go good too...no doubt ....two many stories about hp numbers....when you see low numbers you think theres something wrong.....fact is this is a 454 .040 over and hes running a mix of 93 and race gas ....aluminum heads and 11.1 to 1 squeeze a solid roller in the range of 250 in 263 ex on a 108......had numbers like 515 torque @ 4900...and 588 hp @ 6500 but theres some mid range lean spots so torque drops....then recovers .....was thinking about trying some carb spacers but didn't know were to really start ......Im still very new at dynoing ....not sure how this type of intake runner would react.....not to mention it uses the narrow bolt pattern and all the spacers I have found use the wide pattern...np just machine the spacers..... ...... the heads were CC ported and we were told would support 720 hp

Using computer engine programs we inserted all our flow numbers and cam info and were really expecting some where around 680 hp.....so you can see why he's a little disappointed......either way....that car will haul ass -as you put it with that much power.

Yesterday we ran another 454 on our Dyno it was just hard to watch a 9.2 454 .030 ...iron head mostly stock street engine....retro fitted with a hydraulic roller 232 in 244 ex and a single carb holley 850 go all upside the head of this customers machine.....552 torque @ 4500 and 533 @ 5900 ....kinda sad .....what a waste of the parts installed in the first engine. Was hoping maybe with the air gap dual quad intake we could get her into the 600's hp range at least.....but at the very least work out the horrible lean spot on the bottom while your holding the engine at WOT just before you hit run test. These Carb go very lean then start to richen up as the RPMs come up....bad.
out of the box I saw them hit 10. to 1 on the o2....and at that time the hp was only 533......lean them to clean up the top to 12.8 and the bottom fell out...now at WOT holding at 3000 is lean.....15 plus.....theres a dip for first 25% of pull then say to 3800-4100 then it gets itself into 12.8 range and holds to the end to say 6900 rpm....
......the engine with the holley.....out of the box.....didn't have to even adjust the float levels. (LOL)

[Dave Koehler]

To make yourself and the customer feel better about the base engine have you considered installing one of the other manifolds and a Holley? At the least you could verify the issue. Just sayin.

[Dodge Freak]

Those carbs need to be jetted way down just on the secondaries. It seems strange but the primaries stay close to stock.

Look on Edelbrock site for instructions for I believe dual quad intakes for Pontiac 455's? One of those intakes give the jets sizes for dual 600's

I am running dual 500 Carters 9502 on a SBM 340, an old D66 intake, non air gap. My carbs went from like .090 jets to .077 jets, power went way up. The primaries were pretty close. The motor will run just fine but be down on power with the stock carb settings.

I could guess and say its .084 secondaries jets Edelbrock recommends for 600 cfm but my memory is fuzzy

[MadBill]

I've consulted on tuning a number of engines with either Edlebrock or Quadrajet carbs and found it was always good to do a pull to medium revs on just the primaries, get the AFR correct on them and then do full pulls with the secondaries functioning, making any further fueling changes to the secondary side only. Too often one end is lean and the other rich. They don't automatically blend together in all cylinders...

[F-BIRD'88]

The cam is too small for your horsepower goals. So are the carbs. 268-280@.050 112 .680 -.700" twin 750's.

The air doors are not right on your 600's. I never like the 600's on anything. The 750's #1407 always works better.
Tighten the lash down on the present cam to see. You will see it wants camshaft duration, big carbs and big carb spacers. Jet as required.

[MadBill]

The 600s can't be that bad; they were on the 572" Hemi I mentioned tuning for economy (yeah, I know, but everything's relative...) in another thread. It made 765 HP with a Stage V in-line manifold.
(But I would like to see how it would do with a pair of 750s. )

[Dodge Freak]

Oh that is right, I removed the air door on the front carb. Works perfect with a 10 inch converter and 3.91 gears. No bogs, just power. The back carb has the air door weights lighten. Never tried it with both air doors removed.

I checked with a few dual carb guys before getting the 500 cfm carbs for my 340. All said two 600 would give a slight gain at top end but I would like the 500 carbs better.

I'm proud to be getting 11 mpg around town, no babying and trying for mileage. Got a feeling at a steady 35 mph, remember 3.91 gears-26 inch tires, it get pretty good mileage for what it has.

[rocks409]

A simple opinion. The Edelbrock carbs are made for a single carb. set up. Put 2 on an engine and there needs to be tuning done. The 600s are the most popular because there are so many of them at the swap meets. A good carb. for the thousands of sbc-sbf . On my stuff I generally go from .100 primary to .095,,, from .095 to .092 secondary. Sometimes the .070x.047 rods but the .070x.052 helps in most street cars. The flat spot on the dyno could be metering rods that are the edge of transistion. Too little vacuum and rods stay up in the high speed step. Causing low speed to mid range to be rich. Its a balance thing. Usually with some cam duration it is good to change to the yellow springs, might even have to go to the blues. But for my choice , the 500s provide the best all around perforance and drivability. Front secondary air valves on all of them can be removed ( 2x4) with no problem but dont try removing the rears. Bog city.

[john350]

Thanks for the great support.....I had thought of trying to enrichen the PRI's to cover the lean issue at the begining of the test pull and lean out WOT with the sec's but thought that was a little out side the box... ..like i said im still new to this dyno... ..I have spent most of the last 17 yrs working on 2 bbl oval track race engines.....

I had been renting dyno time and could only get the race engine customers to spend the extra coin... ..well after owning the dyno now.....can't believe that anyone out there building engines would choose not to have one.....

Thanks so much for the info... ..i didn't think about the air door weight......I have played....just a little with Q-jets and there sec's adjustments... ..but I really thought once this size engine was at WOT at 3000 rpm vacuum was at zero on gauge and holding it would have the sec's open... ..im still only trying to get the fuel curve right on the dyno not chasing any driviability issues yet.....knowing that i might still have to tweak it in the car....maybe......for no bog's......car is light for power output....and it appears to have plenty of bottom end.....so I believed it would be fine.......

I do have plans to try a single carb set up on the engine..once we load it back on the dyno and I will let ya know what I get ...but back to real question....why i posted.....edelbrock now offers a air gap rpm version of the dual quad intake.....the customer would perfer to stay duals....know he might be down a little from single carb.......is the air gap worth 1% OR 10%..... he might go for the change if we could get another 30- 50 hp out of this... ....but and extra 10 and were going to a single and he loses the looks he's trying for with this street rod.. ...i know ...sounds silly.......he wants the best of both worlds.....

I would just like a non tuning nightmare......but it's been a learning curve for sure.....two kinds of people out there.....either they love these carbs or they hate them....seem to me there not the real problem here....the intake is.

[Dodge Freak]

The secondaries get lean way down from being a single carb set up. The intake is just fine, the carbs are set up for single not dual carbs.

Call Edelbrock tech line if you want and let them give you the jet size, its a good 10 or more numbers smaller. There is less pull with all 8 bbl's open, there for the jet sizes go way down. That is the problem, the air doors just hurt mid range power if open to late.

[F-BIRD'88]

Clarify exactly which manifold you have on the motor now.. The old low rise C-66 or the new hi rise RPM dual quad.??

The old low rise dual quad manifold is not going to make big top end horsepower.

Carb air doors do not work on intake manifold vacuum. They work on air flow thru the carb venturri.
the 750cfm carbs always make more power than the 600's or 500's. Th air door weight (AFB) or air door spring tension (AVS) regulates the opening rate. The engine is not over carbed with AFB or AVS 750's. But the air door opening rate will need fine tuning.
Start with 2" open spacers on the high rise RPM dual quad manifold. Don't be afraid to add more carb spacer for power. The cam is way too weenie for high horsepower.
If you are running a mechanical advance timing curve, lock it out.

RE AFB air door. The air door counter weight works on weight and the offset of that weight from the centerline.
The weight adjustment (add subtract weight) sets the basic opening rate. The offset of that weight sets to opening timing and the curve of the opening of the secondaries. it is not linear. Carter used to make many many different complex (shape and mass) air door counterweights for the AFB. Experimentation is required. Edelbrock needs to come out with some more air door variations or a new "RPM" version air door with swappable/tunable weights. When you get it right and the jetting right it will be very smooth. Every motor is different. The 750 carbs has the advantage of near equal venturri diameters and high flow.
A pair of them will make the most power. Yo just need to jack 'em up and slide a real motor under 'em.