Would/do you use a spread bore carb?

I'm thinking of building another small block (meaning paying someone to) and am very interested in using a spread bore carb and intake. Mild performing 360 street engine. I REALLY miss that sound of a Qjet or TQ. I realize tuning one isn't the easiest, and parts sourcing can be difficult but if there's not a big dropoff in performance due to the intake and carb itself versus something like an edelbrock performer air airgap with Holley 3310 than I'd love to have that sound again.

Problem is I don't know if there are good intakes available, especially aluminum to accommodate a spread bore carb and if I'm sacrificing lots of power.

Also not a fan of any carb adapters but what do I know? Not much.

Spread bore carbs are a good design for a street car but good luck finding parts these days. If you can find a good carb then you'll want to track down an old Mopar Performance aluminum dual plane intake. They had a spread bore flange on them.

The new Street Demon carbs are a great carb. I have installed several and all have done a good job. They are a spread bore design that will fit both spread bore AND square bore intakes. Available in 625 and 750 CFMs.

www.holley.com/products/fuel_systems/carburetors/demon/street_demon/

www.youtube.com/watch?v=TRpmtLb_9dI

the q-jet are readily avail and there are 3 versions. 750, 800 and the elusive 840. the 840 came on 455 pontiac sw. i will be using an 800 on a magnum with aftermarket intake. i bought the q-jet book and am really excited about it.

I had a carter on my A38/E58 Aspen and a Quadrajet on my '89 Diplomat AHB and was happy with both for years, and as I mentioned oooohhhh that sound.

Enjoyed the video Dave thanks. VERY nice Demon carbs! I really like the second one with the polymer center. Both with their HUGE secondaries. Might be a better choice than the beloved carter or QJ just for simplicity. I was really impressed when he took it apart.

Thanks all. Larry

It's been awhile since I bought from them but quadrajetparts.com sells parts and kits for TQ's. Thermo Quad Parts

I read the designer of the Street Demon carburetor worked for Carter back in the 60s and 70s. He took the best attributes of the Thermoquad, the AFB and the Rochester Quadrajet when he designed the Street Demon. Cant go wrong with a new carburetor with a revamped design.

Forgot the Ford VV carbs? Nothing inherently superior in a revamped carb. It all depends on the design, new is not always better.

I have run a few of them on my SB trucks. I like the early-mid 70s q-jet to swap a mopar throttle plate or lever on and run them (327-350 chebby carbs are bestest)

I have on the shelf a

edelbrock 1905 Q-jet which list for 85 d150/auto correct throttle linkage hook up with stock 85 q-jet 318/360 engines.

NOS/NITB military surplus Holley 650cfm spredbore list for 86 d150 3604v engine. again correct throttle linkage hook up on the stock 85 d150 linkage

Mercruzer chevy 305 CI (boat q-jet non-emissions only has 2 small vac ports.1-choke pull off, 2- dist vac can & NO PCV) I ground the throttle lever off a 88 police 3184v q-jet (feedback carb) tack weld it to the throttle shaft after removing the GM style throttle lever..

IIRC, thermobog 3 piece PTK linkage will be the stock PTK linkage you will be looking for to use with a stock intake on your 79 truck you can mod the 79 2v 3 piece rods to fit and adjust out or adapt any 3 piece set up to it if need be. you can mod a 1 piece to work with a couple of the slotted flat bars bolted together (carb stud to bellcrank)


I know for sure the 85 q-jet stock 1-piece PTK linkage works with these intakes and are bolt on/adjusts with no mods.

stock 318/360 cast iron 4v intake (best on 360 heads for port size matching)
Edelbrock RPM air-gap large ports

for a small port head 273/318

stock 273 cast iron intake
273 duelquad intake
LD4B
Edelbrock performer 318/340/360
Holley Street Dominator

there are other intakes but will all be the 340/360 port size so pick your poison

my new 318 build is getting the Holley STREET Dominator with the brand new holley 650cfm spredbore. I added a QFT adjustable vac pod to it so it will be easy to control the big 2ndary opening and give that big Q-jet howl
I have a set of duel feed bowls/fuel line on another Holley 6213 800cfm spredbore I may swap on...it makes the spredbore carb LOOK HUGE sitting there on an intake.

your police cars should already have the 4v PTK and the stock 4v intake would look more correct. the hardest part is always finding the PTK linkage when swapping things around or having to mod some linkage up to work correctly

PTK working and adjusted correctly is very important or just go manual. but you know this.

Absolutely loved my TQs when I was running 340s. Pulled one off my last 340 and put on a 3310. Seat-of-the-pants dyno didn't notice any difference so I put the TQ back on.
I think the reason many folks don't like them is that a Holley is just so much easier to work on although once you tinker on a TQ for a while, they are not much harder. Granted, high HP engines probably need a Holley(s). Can't recall ever seeing a big inch stroker with a single TQ or 2 TQs on a tunnel ram but that would be cool.
Had my '65 Valiant project not fell through I would have been running a 360 with a TQ.


Tim

just love those thermoquads !
it took me a looong time to figure out how to tune them, but after i figured that out, i was in hog heaven !
as said, parts for those can sometimes be hard to find, so i have a bunch of them on the shelf, and every so often, i can pick up another one for next to nothing.
if i need to use an adapter, i use the ones that are nicely cast, or i machine one out of 3/4, 1", or 1 1/2" plate chunks i have laying around here. blended in to the intake of course.
i have always wanted to try a tunnel ram with twin thermoquads, but have never had the opportunity to do so.

Factory 340/360 spread bore intakes are at least equal to eddy performer. RPM and Air Gap are a different story. Step up from stock intake if justified would be a holley street dominator spread bore intake. If you are going to run AC with a sanden style compressor, you will need a late 70's 360 intake that has the thermostat offset for it.

That said, how often will you be above 5000 rpm on the street? Not very much I suspect. A properly tuned thermoquad will give better gas mileage than any other, followed by the quadrajet. Both are excellent street carbs.

Dont be put off by others telling you how hard they are to tune or get parts for. Rebuild kits are available, and parts are available from the thousands of tq's being used for door stops or paper weights.

Won’t be running a/c. Hoping magnum heads. Shooting for something very similar to the Mopar performance crate 360/300 hp. I owned two of those in the past and was extremely impressed with the combination. Very mild-mannered for daily driving but tons of torque. Only change I would expect to make is the intake and carburetor going for a MP M1 dual plane which from what I’ve read was not all that great anyway and instead of Holley 3310 a different carb. I would love to use a TQ but the more I look at those Demons the more I like. The huge bowls are a benefit it’s a little more ethanol friendly and we have 10% ethanol here in Florida and I love the huge dump from those secondaries when you open them up. I think they call it goggle valve. Check out the video Dave posted if you don’t know much about those demon carbs.

I called Brevard Cylinder Heads yesterday (Vic) but no answer. It was afternoon so maybe that's why. Try again Monday. I'd like him to build engine.

On Demon carbs; I'm really interested in them and think I might put one on my stock '79 Power Wagon 360-4 right now. Currently has Edelbrock 1406. Granted not a performance engine but would love to go back to something closer to what it came with, with the little primaries and the big secondaries and that sound. When I look on their website they make it appear the 625cfm in the video Dave linked is not compatible as far as kickdown linkage. They show Ford/GM parts but not for my application. I'm showing my ignorance but what gives?

Larry, be sure to check ebag for refurbished ones. They pop up ever so often. That's what I grab if we have the time to wait for one to come up. And the black center ones are the way to go in areas like yours and ours where it gets hot and have alcohol in the fuel.

I have never tried the 625. All I have ever ran are the 750s because I have only put them on 360s and 440s. The only difference between the carbs are the primaries.

Lots and lots of reviews and information on these carbs on the 'net. Just have to Google it.

www.forabodiesonly.com/mopar/threads/750-street-demon-linkage-issue.400773/

Yep already decided I like the polymer center after watching the video you posted Have to admit I'm too anxious and want one on my truck now but don't quite understand the applications part of their website; as if I'm going to have trouble with linkage. Also, I figured the 625 was plenty for a stock smog engine

If I was sure which one to get and that it would be no issue with linkage I'd order one now for my truck.

Just saw your additional link...thanks! reading now.

Seems to me it someone that can't follow directions that's the issue in that link.

Yes, and also seems they'd have Chrysler specific applications by now.

Would you think a 625 or the 750 would be best on my stock '79 W150 360-4. Well it does have Hedman Ultra headers and dual exhaust now but stock engine.

Bolt on kickdown adapters have been the norm for decades. Hasn't been an issue for me. Holley uses them, Edelbrock uses them. Just have to remember to use one if you need it.

My experience with a 360 running a 625 or 7550 carb is limited to the Edelbrock Perfomer carb. FWIW, the 625 gave better mileage, the 750 better top end. Since I didn;'need to exceed 115 and I daily drove it I went with the smaller carb on a mildly warmed up 360.

No idea how useful that would be with a Street Demon

This is what I thought but surprised I didn't see it listed for Chrysler kickdowns on their sight but I guess they have provisions for GM/Ford and THAT is why I just need adapter. I'll try not to drag this on any more

Vacuum secondaries prefered on Power Wagon?

Looks like all you need s the throttle stud for Chrysler applications, that and not removing the shipping screw like the forabodies link did.

As for vacuum secondaries, I have never found a need for anything other than vacuum secondaries on the street. If you are considering the street demon it only comes with those.

You may have 'never found a need for anything other than vacuum secondaries on the street', but our favorite manufacturer (Mopar) used mechanical secondaries more than anything else. Only the later Holleys used vacuum. ALL of the Carters (AFBs, AVSs, Thermoquads) used mechanical secondaries. And that was on everything from 273s to Hemis.

Think Mopar might have known something you don't?

Most aftermarket carbs require some amount of mods for our Mopars. CarterBrocks will hit the choke stove on stock intakes, Holleys need the secondary springs dialed in, their Double Pumpers are pig rich on the street, all of them need adapters or fabrication on the linkage. It just comes with the territory. The Street Demons need some innovation on the linkage, too. And there are videos on Youtube to help with that. There are also clearance issues with the electric choke on some (not all) air cleaners. It is not tough, nor does it take anything like special skills or tools.

I have often used a Holley or CarterBrock Mopar throttle adapter on carbs other than what they were made for. A drill and some bits can work some real magic.

I think Mopar never found a need for mechanical secondaries on a street engine either.

What you you think the air door is on an AFB? An AVS? A TQ? A vacuum operated secondary.

Just because it doesn't use a vacuum pot to pull on linkage doesn't mean it's not a vacuum secondary.

You should be able to use the Weiand Action Plus 8007 intake.

If you feel that strongly about secondary air doors, I am pleased to inform you that the Street Demons have them, too.

Happy now?

I'm sorry to say but Vic passed away several weeks ago. I'm not sure if his wife is going to keep the business open or not.

I know Lokar makes kick down kits and there is another company that escapes me.

https://www.lokar.com/throt-cable-kickdowns.html

Oh nooooo. I had a bad feeling about this. R.I.P. Vic.

Thanks Dave and others. I'm going to order the street Demon with polymer center (as written no choice on those vac secondaries). I'll watch lots of instructional videos. I recently added the Chrysler stud to the Edelbrock 1406 to make it operate nicer/smoother so I'll see what I need when I get carb. I guess the factory spreadbore intake should work fine but maybe I'll see what's out there used and take some weight off the front end or I'll just stay with the factory four barrel intake for now. Thanks and I'll update.

I had the 625cfm version on my stock 360 in my 79 Power Wagon. First off, you'll need a tune kit. OOTB mine was really rich (was up to 9:1) and I'm only at 3000'. For kick down linkage, have the bouchillon kick down kit. Worked perfect with the carb, no mods required.
The thing that drove me nuts with this Carb: The steps in the metering rods were too large. I could get cruise air good, but then part throttle would be rich. Get the part throttle good, and then cruise was too lean. I probably could have massaged a set of rods to work... really? I thought these carbs were so awesome!?!?
I couldn't get enough accelerator pump shot either. Set it for a quick shot, but right at tip in, it would always have a slight flat spot. Tried bigger nozzles which helped some, maybe I needed bigger yet. I'm not a carb guru by any stretch, but I could get the thermoquad and edelbrock performer carbs to work better. Perhaps the 750 cfm would be a better way to go?
I did use the factory spread bore manifold at first, but later put on a performer manifold with a square bore to spread bore adapter. No noticeable performance gain BTW.
Just my 2¢.

i believe the other one is bouchillion. sorry i don't have a link.
i will say however, i do not like the lokar setup, and i have removed many of them and converted back to factory linkage.
in my opinion, it is a modified GM style setup, and the cable is just like bicycle stuff only with a fancy cover.
i also think the mounting bracketry is too flimsy and not designed correctly.
just my opinion.

Good pieces of info thanks. Disappointed about the tuning. Wondering too about the 750 being a better choice although seems like a lot of carb for a basic smog engine. It'd be nice to stay with factory linkages if I can with no or small modification. I'll take a pic of what's there now. As far as I know the only changes are at the carb in which I made and that's a Holley throttle extension lever and the stud for Chryslers. That just made everything work better with the 1406 carb that was on the truck when I got it.

p.s. should I not be trying to fix what's not broken?

Way back when I used the Holley TQ replacement when my TQ bowl starred leaking.
Ij just wanted to have no problems so went to Jake's Speed Shop on New Orleans he sold me the Holley made for 1972 340 with manual secondary thzt was a bolt on.
Hooked right up to what linkages were there and ran well out the box.
It was run for years and was good without ever being a problem.
It did what it was supposed to do, and was a quick swap fix.

Put a,square bore I take Street Domanater as the car was. A street used commuter everyday and 4777 when I rebuit the whole motor.

Factory installation of a 750 CFM carb on a 6 banger. 230 CI, 207 hp

FIRST YEAR 1966 PONTIAC OVERHEAD CAM 6 WITH SPRINT / SPORTS OPTION. 230 C/i, 207 Hp, 10.5:1 CR and the 4MV Rochester Quadrajet Carburetor.

^

Hughes Engines recommends the 625 CFM Street Demon on their 360 builds.

Thank you. I'm anxious to order one tomorrow but want to gather as much info as possible.

Not totally accurate, while it's true the actual throttle butterflies were mechanically actuated, the air door in the top prevented them from actually doing anything until that air valve opened as the engines ability to ingest the extra air and fuel increased creating the same effect as a Holley type of vacuum actuated carb. So TQs, QJs, AFBs and AVSs are really considered to be vacuum secondary carbs as the secondaries only work as the engine demands and not the throttle.

Not that it necessarily matters, but I don't think the Street Demon will give you the sound you desired in you initial posting.

Not totally accurate. The secondary air door has an impact on air flow only IF they are adjusted a certain way. While we all understand the function of the secondary air doors (well, maybe some do and some don't) this is the first time I have heard anyone describe the Carters as vacuum secondary carbs. There are a few AFBs that actually used a vacuum pod to operate the secondaries and there are AFBs without secondary air doors.Twist it any way you want to make an argument, but the secondary throttle plates are opened by direct linkage from the primaries (for the vast majority of Carters) and the air doors are above the boosters on most models so that they are as much an enrichment device as they are an air flow control device. The air doors have an impact on the secondaries' operation, but they do not control them. Primary control rests with the linkage and linkage alone.

Mopar's Carters were not vacuum secondary carbs.

I said they are considered vacuum secondary carbs because the secondaries function off of engine demand and not strictly off of the throttle plate butterflies. Twist however you want but look them up on Summit or Jegs and see if they are listed as mechanical or vacuum secondary carbs. You could also try pointing out where the secondary accelerator pump squirters are on those carbs.

You mean like this ?

So you use sales ads for your technical information? That helps explain your opinion on this.

And I have no idea what you are trying to say about the pump squirters. Did you read that in an advertisement, too?

Bottom line; the secondary air door is not essential to the operation of the secondaries. It can be an asset and certainly helps in some applications but is not an absolute necessity.

Straight out of the Carter manual for the TQ

Mechanical secondaries with a variable venturi feature. Good points, straight from Carter and not from a sales brochure.

Venturi: A Venturi is a system for speeding the flow of the fluid, by constricting it in a cone shaped tube. In the restriction the fluid must increase its velocity reducing its pressure and producing a partial vacuum. As the fluid leaves the constriction, its pressure increases back to the ambient or pipe level.

A venturi DOES NOT control the secondary operation. It aids it.

Thank you.

it says : "image not found" on my screen.

I have a Street Demon on my mild 440. I went to the chassis dyno guy on Friday to have the carb tuned after a recent engine build. He refused to do the job until I put a Holley on. I guess he doesn't like to take the time to disassemble the carb to change jets. Or he doesn't understand metering rod carbs. Either way, i guess I am back to dialing it in some old fashioned way.
On the prior engine a different dyno guy was ok with the Street Demon but then he changed metering rods to avoid changing the jet! It was close enough.

I wish I knew of a chassis dyno here in town. I guess it's possible there's one I don't know of but never heard of one and we're not that big. Also isn't a street demon a Holley? I guess he meant same design not "owned by".

Larry,

If you score a 9800 series TQ I will set it up for you. The attached electric choke takes care of running this beast with any intake you chose.

Anyways, they are out there, I see them occasionally pop up on eBag...not too often, but they can be found.

I have a 9801and a 9811

Read the first and second sentence in snipers attachment again. That is what I'm saying. You can't open those secondaries mechanically without some sort of restriction to delay the intake of additional air or you need a secondary accelerator pump shot to prevent a huge backfire and most likely stalling the engine. In other words, vacuum secondary carbs allow the intake of extra air and fuel as the engine demands it, not as a function of the mechanical linkage alone. Block that secondary air valve fully open and then tell us how those mechanical secondaries work.
I don't know why you want to argue this point, it's really useless and unproductive, so whatever, I'm done.

My last post on this was 6 DAYS AGO. You resurrect this, reiterate your point, and then tell me it's not worth arguing about.

And by the way, you're wrong.

I used to run a TQ and a Holley Spread Bore on my 340. Swapped them as I felt. I also used square bores as I wanted too.

For me the TQ was temperamental but when it ran it ran better then a Holley. The Holley was consistent.

I never noticed a difference in the performance for the spread bore vs square bore when I swapped them out. It always seemed to be a matter of tuning and design.

I would say for me the Carter's gave more power but since they were metered with pistons and springs they tended to "hang" and be more temperamental.

If you like that spread bore sound but holley tuning simplicity. Run a Holley Spread bore. I was just at a swap meet and there must have be 10 of them for sale for $50 each or less.

Holley owns Demon now, but the only thing in common is the secondary jets.

I was just about to ask that.

One thing I really like about the new design factors of Demon are the large bowls helping with starts after sitting for several days. Damn 10% ethanol. I get about three days parked and after that have to pour gas down carb. Would rather not have to install another pump.

One MAJOR improvement over the Thermoquads is that there is no gasket at the bottom of the fuel bowl.