[patrick]

I've been thinking of a carburetor upgrade in the 73 340 car. Right now it has an Edelbrock 600cfm Performer Carb with a Edelbrock Performer manifold. This was installed about 20,000 mile ago along with a total engine rebuild. Now, I'm assuming the previous owner used a performer Cam as well. But, he had it done and doesn't remember what kind it is. Only that it had a performance Cam also. Now I understand that all this stuff has to work together to get the best performance. I've been thinking Holly lately,"Street Avenger,670 to 870cfm". I'm not completely sold on Holly's but, had great luck and performance with one in the Cuda'. I don't want to drown the car with more fuel than it can take. It's real responsive but, I think it's hungry for more. What do you guy's think? Thank's Pat . B.T.W, not a drag car, just a super fun car!

[dave-r]

A carb does not add more fuel like a tap. The engine only takes what it needs.

Air is what it needs but that depends on how big the engine is and how many rpms you run it to.
Too small a carb works very well with gret throttle response but starts to fall flat at higher rpms.

Too big a carb flows enough for more rpms than you need and does not work so well at lower rpms.

We know what size it is (340) so now we need to know what rpm you take it to so we can work out what size would be best for you.

[fbernard]

A friend of mine had problems with the original Carter on his 73 340 cuda last year (the secondary counterweights had been milled, and the secondary opening was brutal and trhere was a bog ).
He switched to a Holey 3310 750cfm with vacuum secondaries. I thought it might be too big, but it works like a charm.

A thermoquad in good working order, with the small primaries, would be very nice to drive too.

[dave-r]

The Edelbrock AVS now has adjustable secondary opening rates though. I might have gone for one of those on a 340.

But not if it never goes over 5000rpm. Which is why I am wondering how Pat drives it.

[Eddie]

Pat, I would get an Edelbrock RPM AirGap manifold and a Holley 770 Street Avenger carb. Trust me on this man! The standard Edlebrock intake is a factory clone of the dual plane it had on it from the factory but in aluminum of course. The RPM series is a little better than stock. The RPM AirGap/Holley S.A. is a bomb.

[Jimiboy]

Hi Patrick!

I know what you mean, but i think that like Dave say it depends on you right foot and how you use your car. your 600 carb will do the work for you if you drive "normally" and love to just cruise the streets with your grandma' . It may give a smallblock a little xtra strenght on the lower rpm:s and save you some gasmoney...

But i totally agree with Eddie. I have Edelbrocks RPM Airgap on my 340... and i I run my car with lots of high rise rpm:s and then of course it feels like lots of fuel is the way to go! i have a Holley 650 dp with some bigger jets, that works fine for me....

If you run with smaller turbine and higher gear in your rear axle, maybe you can drop a big 750 on your engine with no worries for loosing horses on the low rpm:s... correct me if i am wrong guys? and get that Air Gap also...

[patrick]

Thank's a lot to everyone's response. Eddie, I like the holly 770 idea but, it's getting so close to the end of the season. I would like to try the 770 with the Edelbrock Performer I have. Dave, I would like to say at least 6000 RPM. I also need to get my tach to work in the off season. Car show's, drag's and cruisin's. Who's got the time to tear everything apart at peek season. If I do go with an Air Gap RPM later, at least I can get a better look at what kind of Cam I have. I do think it's a Performer Plus Jimiboy, I like how you think! Except, I'm a known Hellion around these part's. I like to go fast! And my Grandma's dead. Talk to you soon, Pat

[Jimiboy]

Damn! I knew i should not have wrote like that.. I am sorry for your grandma' Pat!

I am a bit curios about your Tach problems? Is your Tach also weird in low rpm:s like you have some sort of bad connections to ground or something like that? Mine is psycho when i use my turn signals on idle but works fine when i step on the gas(?!) That is something i must check out.... ..Later...... after this season

Good Luck with all your 'gotta do things'

[Jon]

Patrick, looks like you are getting plenty of good advice. I am using a Holly 650 DP on my 340 with an old Edlebrock LD340 manifold that needs to be replaced. Let us know your results please.

Jimiboy, what size jets are you using on that Holly? Mine are 69 primary and 74 secondary. Is that in the range of your "larger" jets? I am thinking a little bigger on the secondary as it has a slight detonation problem at WFO.

Jon

[Eddie]

Jon what is WFO? I have used the term WOT which is for wide open throttle but I like WFO better! Wide F#####G Opening! Seriously, the Edelbrock RPM Airgap is a "gem" of a manifold, according to Ulysses Gonzales, the man behind the engineering of that beauty stated that the MoPar RPM Airgap was the "best" of the dual plane designs. It even eclipsed many single planes in testing! It uses a seperation plate which isolates the runners and plenum from the valley area which of course is susceptible to hot engine oiling thereby reducing the air fuel density. Engines of course like cool dense air charge, this intake offers this in abundance. The runners have the correct taper and plenum volume for additional mods like a bigger carb and cam. Any properly functioning carby will produce, within a handful of power, close to each other in terms of sheer H.P. production so don't get caught up in that. The factory used a 900 CFM carb on it's 318's the ThermoQuad! 318 Four Barrel police package and it never bogged, at all due in part to the T.Q.'s small primaries and huge sec. systems. Get a Dave Hughes cam in there if you want more lift without a lot of intake duration and overlap. My old lady has a WFO her mouth

[Jon]

WFO in moto slang generally used during bench racing while overly emphasising your aggressive riding. (Usually ending in a crashing event).

And you better not let your wife hear you talking about her like that, or else no dinner tonight.

I like the idea of the raised plenum to keep the air/fuel mixture cool. What fills the space in the valley? Is it cast into the manifold itself? The LD 340 manifold I spoke of had the mod of the day done which was to mill the center divider out and create an open plenum. (I guess that is why, any ideas?) It is decomposing at the water passages and needs replacement soon so I am keeping an eye on threads like this one.

As for the 650 Holley it is fine for me. I am trying to tune out some detonation possibly using the secondary main jets as that is where it is occurring, at WFO.

My cam is fairly mild and I don't really want to change it until a rebuilt is needed. An older style purple shaft .484lift w/284 duration. RPM range 2000-6500 I believe.

[Eddie]

Yeah, Jon that 284/484 MoPar cam is a good one. I have to agree with Dave and Fabian as well. The Hughes cams are even better. They are ground for a .904 lobe only, while the MoPar cams are ground for .875 lifters,(their suppliers probably wanted to save on tooling so they ground cams for G.M. and FoMoCo apps as well), Comp makes then under the fast rate of lift .904 specific,(Chrysler cams), as well. Here is a pic of my Air Gap on my 5000Lb. rescue, party, going for heavyloads of groceries, ect.. I need to cause I'm in the doghouse so much To answer your question specifically, Jon, the manifold has a cast in portion that covers the valley,(China wall). Sorry for the dirty engine pics. It's my daily driver and super reliable!

[Eddie]

Engine specs are as follows: 360 Crate Magnum 380/360 Mag. I took out the 292/.509 cam and M.P. single plane intake and Holley 4150 750 D.P.. Replaced them with the following. Comp Cams Fast rate of lift 265/.456 Truck 4x4 Torque cam, Edelbrock RPM AIR GAP, Holley Truck Avenger 670 C.F.M. Guess what, it pulls harder than the old combo until 5000 or so RPM than the old setup out pulls it. But man I never hardly had it that it that high in the RPM range. This is why I recommended the intake carby combo to Pat, based on my experience with it. Hope this helps, Eddie

[Eddie]

I also use a complete MSD ignition,(dizzy, 6AL MoPar box, SC 8.5mm wires, NGK FR5 plugs). It gets 12 M.P.G. When it was new and T.B.I. and 360 it got 12 M.P.G.

[patrick]

Jimmy Boy; No sweat on grandma. She's went on to a better place, 11 year's ago now. She was 95 at the time. It's a good thing I didn't take her cruising with me. Strong heart and all, she may not have made it to 81 when I first started my mopar love affair. 66 Plymouth Fury "383". About my tach. It's been dead since I got the ole girl. I've half heartely looked at the connection's and found nothing wrong with them. I'll look closer, later.Take it easy, Pat

[patrick]

Eddie; I've heard that the Edelbrock RPM will give me more high end torque but, less low end torque. Do you think I'll be O.K. in the mean time with the Performer? Also, will I need different linkages or can I just bolt right in. Still thinking 770 Street Avenger. Take Care, Pat

[Eddie]

The AirGap will bolt right in, AR engineering sells adjusatble throttle brackets. I made my own from a Magnum throttle bracket and it works perfect. Typically, as the plenum size increases the torque production moves up the RPM range. This doesnt affect the AirGap, it makes more torque and Horsepower throughout the RPM range beating even many single plane intakes in total performance. I have an issue of M.P. magazine where they tested 7 different small block intakes. The Air Gap beat them all. The Indy Single Plane would beat it but at 5000 RPM on up. It's a race only intake. The Airgap is a wonderful intake and was engineered to beat the best of the best, I would simply wait until you have the intake and 770 carb then mount it all up together. I dont think the 770 and your current intake will make that much of a difference in performance unless your current carb has tuning issues.

[ianandjess]

hey eddie do you think a thermoquad would work well on an airgap im kind of fond of the old theroquad theyve always worked well for me ive never had all these problems people say they have with them a bit of tinkering & bend a few linkages & theyre as good as gold i only have a performer at the mmoment but the air gap sounds like the manifold of choice
cheers ian

[Eddie]

The ThermoQuad is an excellent carb! None better if you can find one that hasn't been "tinkered" with to the point of it needing a complete re-built unit with a ,(and this is very important), a warped platic fuel bowl, this has been the "bane" of this carbs design. The fuel bowl warps or cracks and leaks fuel into the manifold causing hard starts and unpredictable performance. I "cut my teeth" on that carb and finding tuning parts can be troublesome although Kramer and associates has the hard parts as well as Bob Mazzolini or Pro-Max may help getting hard parts. When you find a good one keep it! The Thermoquad came with two sizes I think, 800 and 900 CFM both are excellent, however the non-electronic version is easier to tune. The electronic version has a fuel trim solenoid on the front of the carb with two wires going to it, bypassing it usually results in the fuel trim messed up, thats why I recommend the Street Avenger by Holley, it has decent sized fuel bowls to keep fuel in reserve amounts for the dragstrip or Street performance! Another important point is the AirGap has a "square bore design" the ThermoQuad a Spread bore, so using a Thermoquad will require an adapter, this is ususally no preoblem unless you already have hood clearance issues, then it may become a problem!

[Jimiboy]

thats nice to hear Pat..

Jon, at the time i am running with 69- 83

[patrick]

Hey Guy's. I've had my auto pro-mechanic nephew Mike, looking up these part's. He can get these at cost! But, what they're telling him is... I will not be happy with this combo unless I want to strap a 440 to them also. They say I should keep the performer I have, and get a Holley S.A. 670 at most. And if I insist on getting the 770 with the Airgap R.P.M. that I should get a couple of bucket's to hang off my exhaust to collect all the unburned fuel. Again, I have a 73, 340,rebuilt 20,000 mile's ago with a RV Cam, exactly witch one, I'm still unsure of. Orange box ignition, blaster coil. The whole thing was put together very nice, just think it could use a little more fuel. I think these guy's are bowtie fan's. I really like the idea of the Airgap and 770 but, these auto guy's are starting to make me second guess myself. These guy's are sale's men and I'm trying to give them money. I have a good, fresh engine. This decission is driving me nut's! Airfuel, Dave, any final thought's? Help me guy's, Pat

[fbernard]

The Air-Gap is given for 1500-6500 use. It wil not take a Thermoquad (too bad, they're fantastic for street use). Edelbrock recommends 650-800cfm AVS carbs with it, but remember Edelbrock carbs used to be called Carter ; Carter CFM and Holley CFM never matched. A 650 Carter roughly equals a 750 Holley.

I would not go over a 750 Holley (Vacuum secondaries) or a 650 DP on a small-block unless :
- your engine pulls enough vacuum to have a decent idle
- you plan to use the engine at WOT a lot (road racing, drag racing) and your cam can actually go over 6000 RPM (this means you probably have a solid cam, headers, etc.)
- you have steep gears in the rear (3.91 or more). You won't see 6000RPM much with 3.23s on the street...
- you have a manual transmission.

What this means is : if you have an hydraulic camshaft, auto trans, and 3.23 gears, get a reasonable carb or plan the other modifications that will be required to make a big carb work.

There's a simple way to calculate the CFM required :
CID * RPM / 3456 / VE = Required CFM
340*6500/3456 = 639. this means at 100% VE (volumetric efficiency) (highly tuned engine), your engine would use 639CFM at 6500 RPM. Not a CFM more. IF you factor in a volumetric efficiency of 85% (basic stock engine), you get 543CFM.

And that is at 6500RPM. Can you cam go up there? If it's hydraulic, most likely not. At 5500RPM, the CFM requirement becomes 459 at 85%VE (549 at 100%).

I helped a friend install a 390CFM road demon on a 318 (with a good cam) a few years ago. Carb was too big, we had a hard time getting enough vacuum signal to set the idle.

This same friend now has a 750 Holley 3310 (vacuum secs) on a 73 340 cuda. It works well. The very same carb (we bought 2 together, same part numbers) is installed on another friend's 426 Hemi cuda (both manual trans cars, the 340 has 3.55 gears). Despite the formula above, the Hemi works pretty well (better than it did with the dual AVS).

[dave-r]

Any carb only supplies the air and fuel that the engine takes. The only difference with a bigger carb is that it has the extra capacity to pull more air/fuel through.

The only downside to a bigger carb is that throttle response goes down.

You see. The smaller the carb the faster the air has to go through it to feed the engine. The faster the air goes the better the throttle response and the better the mixture quality.

Mixture quality is important as you get more power from the same volume of air/fuel. Bad mixture qualities tend to have fuel dropping out of suspension and running down the walls of the ports. Raw fuel does not burn. Maybe that is what they are thinking.

Because a bigger carb at the same rpm has the air moving slower. It is the same volume of air but because their is no restriction it moves slower. They means not so good a mixture and not so good a throttle response.

So a carb that is small works best.

But any carb can only flow so much air before it becomes a restriction. Once airflow though the carb reaches the carbs limits it can flow no more no matter how many more rpm the engine makes.

So the carb has to be big enough to allow the engine to breath freely to the rpm you want.

It is a ballancing act. The right carb is just big enough.

Over the years various magazines, racers, and engine builders have found that a 750dp holley makes the most power on a 340/360 when used with headers and a hot camshaft. Even with stock heads.

[Eddie]

Pat, good advice going on here, can't add anything other than the fact that all the carbs mentioned will be within 10-20 H.P. of each other so dont over analyize or think too much into it. The "real H.P. "boost" will come from that R.P.M. Air Gap, then a little hotter cam if you choose that route. I would use the easiest carb to tune, to me thats a Holley Street Avenger. The 670 would be ideal for your combo. A 770 Street Avenger would be better if you had a little hotter cam and headers. The A.G. intake will give you at least 30 H.P. over the factory dual plane! A single plane and big carb,(anything over 650 CFM), and factory exhaust will waste fuel because the cam wont allow any additional fuel or air into the engine and the throttle response will become "lazy" until higher rpm is achieved. Chevy guys arent good sources for tech. They dont know their arses from from a hole in the ground If they did they would drive MoPars.

[Jon]

Pat, I will put my 2 cents in from my past experience with the 340.

I tried the "hot" solid cam with high RPM potential and ended up throwing a rod thru the pan on one engine, spinning a rod bearing on another, lost a wrist pin circlip on another,and replaced valve springs along side the road way too regularly.

Those engines were "blue printed and balance"by reputable machine shops one of which being Kieth Black in the late 70's. The rods were factory polished, shot peened, and magnafluxed with upgraded bolts.

What I finally realized once I grew up was, if I wanted to play with the big boys I would need to install upgraded components to insure the motor will survive at the RPM's need to make a small block haul ass. That would require a lot more money then I wanted to spend along with dealing with my disdain of pulling motors in and out all the time.

In my honest opinion a stock 340 is only safe to about 6000 RPM's MAX. Even that is pushing it. Stick with the milder options unless you plan on rebuilding it soon which it sounds like you are not. Put your money into the body, suspension, interior etc...

Good luck, and let me know how the new intake components work if you go that route. I am looking to replace my manifold soon too.

Jon

[dave-r]

Jon's advice is good. But I would add one thing. Rods do not often fail. It is usually the bearings or the rod bolts that fail.

Rod bearing failer would have probably happened before now if it was going to happen. If you replace the rod bolts with correctly torqued ARP bolts you will be safe at high rpms. If you actually need the high rpms.

[Jon]

Dave, mine broke in half about 3" down from the wrist pin. Subsequently the crank came around and pounded it into the the block and pan. (still have the repaired pan on the car). Not sure how the bolts may have failed though.

The engine with the spun rod bearing actually doubled up on one side. That one may have been due to the bolts stretching.

Back when I bought the rods from Chrysler they also offered a different forging which was much thicker. Including the bolts, 7/16" I believe. The extra cost and weight would have been worth it. Penny wise, pound foolish as they say

[dave-r]

Dave, mine broke in half about 3" down from the wrist pin.

Well I did say "do not often fail" not never.

I think that sort of thing is pretty rare though. And no matter how good the parts you fit. There is always something that can go wrong.

We are in the lap of the MoPar Gods.

[patrick]

Thank's Guy's! I'm pretty darned sure I'm going with the 670 street avenger, for now. Like I said before, my car is #'s matching, and I would hate to TWEEK it out TOO much. It run's great except for an annoying ping without the use of Super unleaded, plus octane boost. I've tried everything with my timing, and the most advance I can go without the pinging is 8 degree's. I normally go 10 with all my old V8 Hot Rod's. I've leaned out my carb as well. I'm pretty sure the carb is at least 8 to 10 year's old and could use a rebuild. But I've heard a lot about the street avenger's and would like to try one. Later, when the engine needs rebuilt again, or if I run in to some good cash flow , I'll get one of those Stroked out Crate engine's. Then I can tuck my #'s motor away, and keep it saif and free of blasted hole's out the side. Who know's what a #'s matching car will fetch ten year's from now $$$ ? Even though it's a 73. I've really been apreciating all the SUPERB advice I've been getting from all of you! Thank's Again, Pat

[dave-r]

Only 8 degrees?

That means you either have too much vacuum advance and/or too much advance range in your distributor.

What is your total advance set to? You should always set your advance with total (without vacuum).