750 Fuel Demon to 750 Street Demon

[GtoGeoff]

EH,
The 'economizer' is not a channel, it is a restriction, a jet. A term not used much these days, but you will find it in older books/publications.

[GtoGeoff]

Basic,

[1] Not sure if it is that passage/hole in the pic. I have a Demon catalog & they refer to 'T.S.RES' which I assume means transfer slot restriction. Most of the models it is open, but the Race Demon Blower carb has dimensions for it, 0.063-0.076".

[2] For those that have Cliff Ruggles QJ book, he mentions IDC sizing which increases with cam duration. The IDC restriction in the QJ is at the top of the main body, right next to the IFR tube.

[3] Very poor computer skills. If you can PM me your email address, I will send some pics of IDCs & economizer info [ same thing, different name ].

[4] Might have missed it. What size engine are you working on, what cam specs & what initial timing?

[BasicQ]

To answer #4, something pertinent to the entire thread:

Engine: SBC 383
11.4:1 comp
245* & 252* @.050 int/exh ft cam .545” lift both lobes
Trickflow twisted wedge g1 heads flowing 271@ .500” 2.02” int 1.6” ex
Super Victor intake
1-3/4” 4 into 1 headers, twin 2.5” with x pipe.
110 gp/h mechanical fuel pump
MSD Digital 6 ignition 20* initial 34* total

TH350 with 4000rpm converter
9” diff with 3.89:1 gears
26” tyre
3350lbs car me in it over weigh station


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[BasicQ]

Have done he changes as outlined above. Bigger IFR and reduce MJ to 80. Decided to go .035” on the IFR thinking it would drop transition from 1 lambda and above to around .90/93 area and then look at achieving .95/97 with iab changes. That .035” IFR made it crazy rich and when I think about it a .035” low IFR might be close to .037”-.038” in the high position. There were times when AFR was sitting as low as .75 lambda but mostly mid to low .8’s. Check the video of 95-100kp/h. The .035” IFR had a huge impact as the other day with .031” IFR and 83 MJ that same speed lambda was .088/90. Compare that to now with 80 MJ and .035” IFR. You will see it go to over 1 with a quick blip of the throttle.

A quick WOT squirt showed plenty rich too at around .82/84 lambda with 91 MJ in the secondary.

Next move: go to 32 IFR and if too far back to lean drop iab.

https://youtu.be/LWViJfveXOE


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[EH179]

Yeah a 33 ifr should get you closer.

Keep in mind .83 lambda is low 12's in afr, right where BP reckon BP98 makes best power.

[BasicQ]

Yeah a 33 ifr should get you closer.

Keep in mind .83 lambda is low 12's in afr, right where BP reckon BP98 makes best power.

Don’t know why fuel companies don’t disclose that info anymore with their premium fuels. It was barely two years ago there was a product data sheet for Ultimate readily available online. I remember clearly stoichiometric being 14.4:1 and that’s what I am working with when doing calculations but I don’t recall best power ratio. Was it low 12’s?


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[GtoGeoff]

Basic,
I think you need a LOT more idle timing. Probably in the 30-40* range. What will that do? It will increase idle rpm, allowing the t/blades to be closed further at idle, resetting the blades/t slot relationship. I have cured many off idle tip/in flat spots [ leanness ] just by increasing idle timing. The extra timing produces more power [ which is why the idle rpm increases ]. The few extra ft/lbs of tq allows the engine to carry the tip/in load & not flat spot.

Below is what D. Vizard says. This is in a carb book. A carb book? Yep, because idle timing should be established before touching the carb. So important does he regard it that idle timing gets mentioned more than once.

" The optimum idle advance is typically 35-40* for a short cammed street engine and [ though not commonly realized ] as much as 50* for street/strip engine."

[BasicQ]

Tried the 32 IFR and it was an improvement but at all the steady speeds I have been testing at lambda still didn’t get much above .93/94 and still spent too much time dipping into the .8’s for cruising the streets.

Thinking something was amiss the other day I fitted the .031” ifr’s again. Nothing was amiss as afr’s replicated what I was seeing last time they were fitted. So at steady cruise at 70/80kp/h with .032” IFR lambda was around .94 and with .031” IFR almost steady at 1 on the same stretch of road at the same speeds. It’s the biggest difference I have seen with .001” difference in orifice size. Both sizes are stamped from Quickfuel and I did a thorough check of the sizes of emulsion I had when fitting the .035” so I knew what I had in between 31 and 35. So big difference in AFR with .001” but also a noticeable difference in driving. It noticeably revved better and crisper. Acceleration just felt better. I am very surprised what that one thou done, must have passed some threshold physics can only explain. So next is down on iab to reduce where it is 1 and slightly above lambda to get it down to .097/98 area.

With the primary 80 MJ AFR was better but erratic at 100k’s. It’s like the mains are spluttering a bit starting up. That might be where the large x3 .031” emulsion needs addressing.


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[BasicQ]

Basic,
I think you need a LOT more idle timing. Probably in the 30-40* range. What will that do? It will increase idle rpm, allowing the t/blades to be closed further at idle, resetting the blades/t slot relationship. I have cured many off idle tip/in flat spots [ leanness ] just by increasing idle timing. The extra timing produces more power [ which is why the idle rpm increases ]. The few extra ft/lbs of tq allows the engine to carry the tip/in load & not flat spot.

Below is what D. Vizard says. This is in a carb book. A carb book? Yep, because idle timing should be established before touching the carb. So important does he regard it that idle timing gets mentioned more than once.

" The optimum idle advance is typically 35-40* for a short cammed street engine and [ though not commonly realized ] as much as 50* for street/strip engine."

No issue with idle. All three carbies have the IFR relocated to the lower position and all three idle at approx 9-10in in drive at 1000rpm. Remember early on when doing the preliminary setting I set the transition slot as a square and then move the idle speed screw a little either way to have the screw head slot running east-west? It provides a reference point to start with. Take a look at the pics below that show how little adjustment those idle speed screws needed for each carburetor.

[img]https://i.ibb.co/hYHTZXx/6685-AAA5-94-F9-46-BF-8362-CCE49-C6-ED2-D5.jpg[/img]

[img]https://i.ibb.co/brnH4vK/A8895-A39-5-D33-45-E7-A008-8-C1-A1-C3-BE454.jpg[/img]

[img]https://i.ibb.co/LR2DfR2/25-ADB2-DD-1175-4-C5-B-BB70-A53-D321-CBCE7.jpg[/img]

[img]https://i.ibb.co/s2nn5Ck/F89-D4553-92-AB-4404-8-B66-43-B0-D6-D5-C6-EB.jpg[/img]

Secondary screw not pictured for the carby on the engine is about +70* turned in from preliminary setting.

Probably don’t want to close the blades much more than that on any of them. Any flat spots I am having or reporting is quick stabs of the throttle to get the accelerator pumps working to gauge if they would need adjusting. Never been a transition drivability problem.

I know where you are coming from Geoff and it can be an effective tuning method on the right engine. I tried the vacuum advance to direct manifold vacuum to give the engine in the 30*s at idle a while ago. Couldn’t get the idle down and didn’t notice any difference in drivability. Vacuum advance is connected to ported and comes on quite early so all the manifold vacuum connection was doing was screwing with the idle that was good in the first place. I could have put more effort in to get the manifold vacuum working better but I just couldn’t see there would be any rewards for my efforts. Maybe with combustion chamber design, flat top pistons with only two valve reliefs, zero deck height, spark plug a little closer to cylinder center and dynamic compression ratio 9.5 - 10:1 combustion is efficient enough at idle with 20* initial.

[img]https://i.ibb.co/52zMz3G/DB278362-44-DA-4627-8652-F9-E7452-A95-ED.jpg[/img]


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[BasicQ]

Just out of pure interest thought I’d put this up from a Holley book I have. I have a 4000 converter and 383 cubes. What cfm size would Holley be recommending I have with this chart? What would you have on your combo?
See their example comes up with the typical 650-750cfm usually used on a 300-400ci engine, but that’s with stock converter or what I read a manual transmission recommendation. But up the stall speed and cfm size goes up rapidly. A meek 2000 converter with a 350 is recommending over an 850cfm carby.

[img]https://i.ibb.co/6gkrmdm/479-C2-AF6-6-E88-4-F10-8294-E09-DA204-B3-D5.jpg[/img]

[img]https://i.ibb.co/Lk0YprJ/6-EB58-C8-E-0-B74-45-AB-AECC-F729-C096-C974.jpg[/img]


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[hq308]

Surely that can't be right, according to that my little standard stroke 308 with a 3000 converter needs 850 + cfm, it's not far from 1050 on the graph.

I'm sure there's some logic behind it but I'm not seeing it lol

[EH179]

Don’t know why fuel companies don’t disclose that info anymore with their premium fuels. It was barely two years ago there was a product data sheet for Ultimate readily available online. I remember clearly stoichiometric being 14.4:1 and that’s what I am working with when doing calculations but I don’t recall best power ratio. Was it low 12’s?


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12.0:1 afr and max' compression of 10-10.5:1.

[EH179]

Mmm? that graph is a bit dodgy... 5000 converter @ 353ci would give well over 1050 cfm carb? No way.

[BasicQ]

12.0:1 afr and max' compression of 10-10.5:1.

Good info to have thanks EH!

I’ll look more toward the lower 12:1 then instead of higher 12:1s.


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[LXCHEV]

Yeah that graph is interesting - my mild little 383 with 3,500 stall = a carb well in excess of 1050cfm. Currently quite happy with it's 650 in place.