Racing Fuel Systems

After countless hours of reading up on carburetor function over the net, forums and print form (not yet delving into the engineering type books but learning from those that have) I hopped onto the theory of the old school calibrations for 4150 Holley’s that is widely adopted here. Reducing emulsion to a couple of .026” - .028” and similar with main air bleed. This has served me well over a handful of street/strip type tunes with Holley and Demon carbies but I did a little experiment with the last tune I hadn’t on others.

The carby recently tuned is a Mighty Demon 750. Lowered the IAB and cleared out the x3 .031” emulsion (.031”, blank, .031”, blank, .031” in 5 emulsion speak) in the aluminum blocks with a 2.5mm drill (excuse that wacky metric size!) and tapped for .028”, blank, .028”, blank, blank. Once I had done all the tuning and settled on calibration specs I returned to the .039” MAB to gauge differences in AFR from factory Demon specs to what I had adjusted to and settled on purely for comparison. I didn’t have x3 .031” emulsion to screw in and replicate the factory starting point so I put a third .028” in the bottom to be closer. Done a trial with the .039” MAB and then a comparison with the .028” MAB.

When out doing a test drive with the .028” and at a steady state speed and rpm that would have the mains engaged (100 kp/h @3250rpm) with the x3 .028” emulsion throttle response was better and it noticeably pulled harder if keeping the foot into it. AFR had barely changed so my first hypothesis was that adding the bottom emulsion had the reduced density and viscosity of the aerated fuel for a longer column of the main well, and therefore moved more of the main well fuel quicker and easier. So adhering to the theory that x3 .028” emulsion maybe too much for all round operation and the addition of the bottom emulsion has had a positive performance effect I went home and plugged the middle emulsion for a .028”, blank, blank, blank, .028”. Next test drive on the same road under the same conditions dulled off the response and keeping the foot into it did not pull as hard. No flat spots or hesitation, just not putting me back in my right hand drive seat as much.

So out of three emulsion set ups trialed of 1 & 3 open and the rest blocked, 1 & 5 open the rest blocked and 1, 3 & 5 open the rest blocked the latter proved the all round best performance. Those 3 test were done with .028” mab.

Again I have hypothesized that the lower density and viscosity of the main well fuel allows for faster reaction of the fuel to the changed booster signal.

Is this thought process in the ball park? What conditions would favor the three emulsion over the two emulsion setup usually favored by most of you guys?

This has got me revisiting previous carbies tunes where I stuck with 1 & 3 open the rest blocked.

Engine: SBC 383
11.4:1 comp
245* & 252* @.050 int/exh cam
Trickflow twisted wedge g1 heads flowing 271@ .500”
Super Victor intake
1-3/4” headers
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

I don't have any answers to your question as I've had similar questions myself.

We frequently hear 2 e-bleeds are usually the way to go in most cases......yet many of the more high end carbs out there (Dambest, Braswell etc.) use 3 or more? e-bleeds. I know Quickfuel and Demon also used more than 2.....a can't think they all used more emulsion as just a sales tactic. I'd like to hear their side of the that tuning approach.

I recently went to a pair of Dambest's that came with 3 e-bleeds and I have to say, they have the flattest, smoothest afr curve compared to any conventional Holley main body's/e-bleed combo I've tried so far.

Many times we hear "Depends on your combo".....well how about some scenarios where and when 3 might be better and how and why. Maybe the knowledge is limited to those few listed above.

The bottom line is setting the fuel curve for that engine that make that engine happy, and not necessarily trying to get happy AFR numbers. Don't be afraid to try smaller e-bleeds as well...

Yes, smaller emulsion. And it could be 2 or 3. Just about finding what works best for your combo. And don't forget how it affects performance.

It was a couple of weekends ago now but I had done a lot of testing on emulsion and MAB size combinations and three emulsions always performed better than two as described in my first few post. I have for now settled on .028” MAB and x3 .025” emulsion for the 750cfm. I am talking a street/strip type calibration, not everyday driving economy or all out race performance.

So back to my original question, why are three smaller sized emulsions performing better than two slightly larger sized?
I don’t know what the fuel curve looks like mapped out but I know the AFR is not adversely affected for improved performance of x3 emulsion.

I expect there will be some “it depends on your combination” answers, but why does it depend on combination? I have what I would consider an almost generic street strip combo that a lot would have similar. How many have dropped to the two emulsion combo with their over emulsioned Holley, Demon, Quickfuel etc but better is possible with three? Is it only possible to know by trial and error like I have done or are there indicators to go back up to three?

Ok, I don’t fully understand why, but with an annular booster 750 on a decent Chevy 350 I built a few years ago it had the best drivability and power with one bleed at the top. We spent almost three days on the dyno trying one, two and three bleed combos along with the air bleeds and main jest being adjusted as well. If I remember we had almost 60 dyno pulls. We also swapped bodies to down leg and it ended up wanting two bleeds at .026 if I remember correctly. That is what is currently on the engine and the engine is very happy with It.

Why that is, to be honest I do not know. But that is what very detailed testing showed us. After that experience I better understand what these guys mean by “it depends”. And most of all “test and find out”.

It would now be interesting to track test you car and see which combo produced the best time slip (if there is any noticeable change on a time slip to be had).

Paul