Carzy - Spread Bore Square Bore Carb'

[jacb]

Hi all

Care to consider what may, or may not be, a crazy idea?

I have been looking at the carburetion on my Mopar 440 ………. I started by removing a very tired Thermoquad and replacing it with a Speed Demon 750 (choke tower and vac secondary). With hindsight that may not have been a wise move since this is a road car (NO TRACK USE – NIL – NADER, just the "occasional" spirited road use). Sorry to highlight the point but time and time again, carb people only talk about track use and WOT. The "stop light grand prix" is about as close as I get to the track, my car travels considerable road mileage on normal roads and fuel costs are a “real world” factor.

Since fuel in England is more expensive than liquid gold ($$$) it would seem that the old Thermoquad was actually a very good OEM choice. Small primary’s for normal daily driving with LARGE vacuum secondary’s for the fun times. But the TQ is old and hard to get bits for. Even Demon Sizzler was trying to sell his business some time ago! http://www.thermoquads.com/

To the point – Looking at the Race Demon RS - I began to think – “What about a spread bore (different size ventures) Demon for the road? Problem is I want a Removable Sleeve carb (to get the different size venturis) with a choke and Vacuum Secondaries - BG doesn’t make one, or does he? …… “The project” was on hold until I put together the following set of pictures …….

Choke tower and removable sleeves!
Note: The removable sleeves haven’t broken through the choke tower side walls.

It looks like the same casting/body is used for cast and removable sleeve Race Demon ……
The only difference would appear to be a delicate machining operation.
Removable boosters would also have to be fitted

Caution would have to be exercised during the machining operation
The milling actually breaks through the divider side walls in two places hence the flats of the RS sleeves.

So it would appear that I cold probably machine my Speed Demon 750 to accept Race Demon RS Sleeves ………………. but would it work with my vacuum secondary’s …… I don’t know enough about where the vacuum canister gets it vacuum from. I do know that BG makes a King with RS and vacuum secondary’s …… but that’s a very different body. With removable sleeves would there be the issue of sealing the vacuum passage to the secondary diaphragm with a modified Speed Demon or even and original RS Demon?

BG makes a Mighty Demon with vacuum secondary’s …… but that doesn’t have RS. However, I am beginning to think that just about any Demon main body could be modified to use vacuum secondary’s ……. Are RS Sleeved bodies the exception to that rule/observation because of the vacuum sealing issue?

OK – If you are still with me by now I guess you will be asking – Why go to the trouble/expense? Because this is not about OEM production and profit – It is a hobby. For the same reason mountain climbers climb mountains …………. Because they can. Yes, I could get a better TQ or Q-Jet and make do, but since I have a Speed Demon 750 Vac and want some specs that it doesn’t currently have at the moment, I may as well play with it a learn. You could also be saying what about the road Demon range? The problem with that range is that if you want a total wet CFM in the 700 CFM range for WOT use, then the Road Demon 725 uses the same venturi and butterfly size as a speed Demon 750, but with a lesser spec and slightly different IFR and main jets. The different sized venturi principle is however used on the smaller road Demons, JR’s etc. This "spread bore" idea would appear to validate the concept (spread bores in a square bore body) and BG are already using it but not in the larger CFM sizes.

What I dont know is are there any other “hidden” metering changes that BG uses and are not shown on the “Specification Charts” that could stop the project due to tuning issues if the RS sleeves were fitted? ….. Does the transfer slot width change with different calibrations? Are there any other internal changes that are not obvious to the naked eye which could cause problems? Yes I am aware that the DR calibration range gets holes in the butterflies and that the floats and linkages change in other calibrations, but is there anything else that would change the fuel curve if RS sleeves were fitted? Passage diameters in the main body etc? Do you know of anyone who has made a spread bore/square bore road carb before?

I believe that a carb with the following features is what I need. Hopefully it will make a good daily driver, with crisp pickup & good mileage, whilst still allowing the engine to "please" an enthusiast when the gas pedal gets pressed hard. Low down torque and mid range are much more important than WOT to me.

Specs:

·Speed Demon body fitted with removable Sleeves – Venturi sizes to be determined later
·Vacuum secondary's (with quick change canister – tuning aid)
·Choke tower remains - choke fitted …… Frequent driver used in cold England!
·Screw in boosters – due to RS sleeves
·Annular primaries – Heavy Road car with auto trans'. May leave venturi size in primaries original - anular boosters will reduce CFM
·Screw in high and low speed air bleeds – tuning aid
·Screw in IFR – tuning aid
·Four corner idle ….. may be a tuning problem if different butterfly sizes are used
·Screw in emulsion bleeds to aid tuning. 3 or 5? Does this make a huge difference in real terms? Would a modified Speed Demon metering block make a good starting point or should one recommend spending $ for the race 5 e-hole blocks?
·Idle-Ease base plate. They don’t spec this on the Race RS. Perhaps there is no room for it?
·I could even fit different sized butterflies by buying a “small” base plate and boring out the secondary's for larger butterflies – again TQ and road Demon style.


Crazy? - I don't know ............... certainly thinking out side of the box ................ any "mileage" in the idea?

[jacb]

Having had a while to look at Stan Justus work (Stallion Racing Components) as posted on the GT40 forum http://www.gt40s.com/forum/gt40-tech-engines-induction-exhaust/18991-4-bbl-carb-lola-motor.html - I have a possible solution to the issues of fitting RS sleeves with Vac Sec .....

Stan CNC machines new larger venturis on some of his carbs. Why not machine a larger venturi on the secondary side only? Would this work for a mild 440 or are these larger venturis for large motors at WOT only? RS sleeves could remove and add material, CNC machining can only remove metal.

If one started with a 750 Speed Demon (vac sec) ... As I said before, I happen to have one, the list of work would include:

• Remove boosters
• Fit annular boosters to the primary side - This would have a CFM reducing effect and offer better atomization for low speed, acceleration and mileage.
• CNC profile the Venturis in secondary side to the largest the casting will allow - What is possible in terms of wet CFM? This negates the complication of RS sleeves. Will the total flow get any where near the big Thermoquad?
• Fit new secondary boosters - probably annular. To be decided later (better atomization, auto and heavy car but CFM reduction)
• Replace Speed Demon blocks with Mighty Demon blocks for screw in bleeds etc. This would be the 3 eHole option and consideration should be given to a 5 eHole Race Demon option (not forgetting that Vac Sec's are fitted)
• Fit removable air bleed (10-32 UNF)
• Fit jets, air bleeds etc as best guess starting point for later real world tuning

Questions:

• Would larger butterflies be needed with the new maxed out Venturis?
• Can an Idlease base plate be used with the above machining?

Thoughts please ...... now be polite !

[Stan Weiss]

Originally Posted by jacb

Since fuel in England is more expensive than liquid gold ($$$) it would seem that the old Thermoquad was actually a very good OEM choice. Small primary’s for normal daily driving with LARGE vacuum secondary’s for the fun times. But the TQ is old and hard to get bits for.

Have you looked at the GM Q-Jet which has small primary’s for normal daily driving with LARGE vacuum secondary’s for the fun times. There are also a number of companies that modify them.

[jacb]

Originally Posted by Stan Weiss

Have you looked at the GM Q-Jet which has small primary’s for normal daily driving with LARGE vacuum secondary’s for the fun times. There are also a number of companies that modify them.

Hi Stan

Yes I have a Q-Jet in the garage ........... personally I like the Thermoquad more than the Q-Jet. If I was going for a true spread bore I would go back to a TQ. What I am thinking is I like the Holley square bore configuration (metering blocks verses needles) why not try different venturi sizes and enjoy some of the spread bore benefits whilst staying with the Holley/Demon metering system.

Regards

[rookie]

I want to try something like this with either the BG vac secondary Dominator or the big 4150 flange BG carb, I think you could get max power and drivability on an engine that needs 900 plus cfm, but one you could drive daily.

Maybe David will get this message to Barry Grant for some testing.

[jacb]

Originally Posted by rookie

I want to try something like this with either the BG vac secondary Dominator or the big 4150 flange BG carb, I think you could get max power and drivability on an engine that needs 900 plus cfm, but one you could drive daily.

Maybe David will get this message to Barry Grant for some testing.

Hi Rookie - Nice to know I am not alone in with this "crazy" idea. Stan Justus (Stallion Racing Components) seems to be the man for a project like this (link at the top of this thread). I am talking with him about the details cost etc of turning my standard Speed Demon into a "Spread bore, Square bore" RS carb - I will keep you posted as things develop. My thoughts are that if it doesn't work I simply change the venturis and have a working square bore RS carb.


One of the area's that I hadn't considered, as pointed out by Stan.....

"The base plate…specifically if you are planning on constructing a “true” spread-bore; i.e. w/ smaller primary throttle bores and larger secondary throttle bores. In any carburettor design which utilises a vertical transition slot to introduce fuel into the air stream during light-load “just off idle” conditions, the efficiency of these slots is quite dependent on the thickness of the throttle bore “wall” they pass through; generally the thinner, the better with something around 1mm being optimal. This is the reason why the transition circuit wells on both Demon and Holley small-throttle-bore carburettors like the 390 and 600 cfm models are elongated toward the bores themselves so as to reduce this thickness (remember, the locations of the idle and transition passages leading down to the base plate from the metering block are fixed…and especially must remain so in a RS –style carburettor). If you start with a small-throttle-bore base plate, you’ll run into this cavity when machining the secondary throttle bores to a larger size. If you start with a large-bore piece (which is probably the preferable way), you’ll be faced with having to sleeve and re-machine the primary throttle bores, cut the transition slots themselves, and also elongate the transition wells. Again, it’s all do-able, but a lot of touchy machining that – and no offence intended here -- you’re likely not to get right on the first attempt".

but ....... Stan can

With regard to talking with BG - I don't hold my breath having sent emails to tech-sales@barrygrant.com , smoore@barrygrant.com & eniconovich@barrygrant.com as yet I haven't managed to contact Doug Schriefer but I think that BG are really only interested in production models and don't want to help people thinking "out side the box" - They are in the money business! Strange thing is - quite a lot of carb shops say that getting RS boosters out of Demon is hard ..... I found this link for annular boosters Demon Carburetor Boosters - JEGS

I will keep you posted

Regards

PS Was that the Big 4500 flange ?

[rookie]

My plan was to use either the biggest 4150 with removable sleeves and install the smallest sleeves in the primary and the largest sleeves in the secondaries.

I thought about the 4500 but I would need an adapter and probably don’t need that much carb.

Another thing to try is, have sleeves machined that will fit snugly around the boosters on the primary side of the carb thereby reducing the area around the booster and forcing more air through it and increasing the booster signal.

I noticed on the base plate in the picture some one has put in fuel shear grooves for the idle and transition slots,
That is something I have done on several carbs but never knew if it really helped. ???

[jacb]

Originally Posted by rookie

I noticed on the base plate in the picture some one has put in fuel shear grooves for the idle and transition slots,
That is something I have done on several carbs but never knew if it really helped. ???

Have a look at 4 bbl Carb for Lola Motor - GT40s.com - You will find Stan's thoughts on the grooves on Post 25 GT40s.com - View Single Post - 4 bbl Carb for Lola Motor

Regards