Racing Fuel Systems

I've been looking back over the engine dyno results of my 358 SBC (.040 over 350) (463HP @ 6000, 462TQ @ 4400 - uncorrected) recently and have a question concerning the meter block emulsion stack on a QF Q-750 mech sec that was used on the dyno and will be used in the car.

For most of the day - the dyno room temp was @ 50 deg F (the exhaust fan hatch was left open the night before ). The outside temp that day was high 30's (early march in NE Ohio) & the local weather station showed the Barometer at 30.15"HG, Humidity 70%, Dew Pt 16°F for the day. The Dyno/shop elevation is @ 1060Ft.

The AF Ratio (O2 Probes located in both collectors) for all the pulls was consistently in the 13.2 - 13.5 range while adjusting timing and comparing header sizes. The engine water temp was @ 140-150°F for all the pulls. The carb jetting was not adjusted because the dyno room temp was so cold and this engine would see the normal summer temps (70s to hi 80s°F) in the car. We figured that the AF would get richer as air temps increased and didn't want to spend time chasing the jetting knowing it would change later in the summer.

Ended up with the following Timing:
37 degrees mechanical (crankshaft) with 15 degrees initial @ 600rpm. Timing is all in by 2600 rpm. The idle vacuum is 12" HG @ 800rpm. I plan to use manifold vacuum for the vacuum advance. The vacuum advance starts @ 3-5” vac with full advance (crankshaft) @ 6-8” vac. Full Vacuum Adv is (crankshaft) 18 degrees. The ignition should end up with a max of 55 degrees total timing (w/ the manifold vacuum advance). I may need to limit the total advance some once the car is on the road - we'll see.

Both metering blocks are the 4 hole variety with a kill bleed/siphon break at the top of the main well. Here's the details on the carb as it was run on the dyno:
Q-750 mech sec w/Downleg Boosters (bought new in 2010)
MABs: 30’s pri & 30’s sec
Main Jets: 74’s & 65 PV - prim and 84’s & no PV – sec
IABs: 72’s pri & 72’s sec
Idle Mixture Screws: 1-1/4 turn out
Floats set to middle of bowl window

IFRs: 33’s pri & 33’s sec - Located in upper Idle well
Emulsion Stack - both primary & secondary metering blocks
Position Restriction
Kill Bleed .028”
E1 .028” - open
E2 .028” - open
E3 .028” - plugged
E4 .028” - open

Now I plan to lower the IFR's and was wondering what to do with the emulsion stack - leave them as is or change to more of an original holley emulsion setup:
Position Restriction
Kill Bleed .028”
E1 .028” - open
E2 .028” - plugged
E3 .028” - open
E4 .028” - plugged
IF I changed the emulsion stack to the holley setup above, would I also need to decrease the MAB to 27 to match the holley emulsion stack setup? I'm using a 4779-2 750DP as a base line setup.

Since the AFR stayed consistent/steady during all of the dyno pulls from 2500-6200, I'm not sure what to do with the emulsion stack. Looking for opinions and or suggestions.

Thanks,

Try both.

You didn’t state what the cars usage is, but if mostly street try a/b/a test as to how you like it as far as driving is concerned.

Have clear tuning goals in mind when tuning ie economy or more performance oriented driving. You can’t have both. What can give you a nice steady AFR curve and economy won’t give you the same throttle response and acceleration as a street performance oriented tune given both have similar average AFR curves. I’ve done it both ways and it comes down to a preference of how I want the car to feel under the right foot when driving.

Trial both and report back.

Lowered IFR on that size carb usually ends up .032” +/- .001”. If lookin for economy maybe as low as .028”

You have an AFR gauge?

An AFR gauge is number one on the list. The emulsion stack would be the last thing I would change. I disagree respectfully with Right hand drive. Performance/economy you can have both. On a well thought out engine utilizing proper timing for cruising is all it takes. The IFR will be determined by the idle vacuum ( in gear if it is an automatic). Your 12 inches idle vacuum will relate to the .033" IFR. On a 770cfm Quick fuel carb I went from a 13.5:1 cruise AFR to 15.5 AFR with no loss of performance . If anything it had a better throttle performance. On a street engine it has been well documented that rich idle/lean cruise/leaner light acceleration / to rich power wide open throttle . The only thing I see is your your timing all in by 2600 may be a little early. Too early can affect torque.

rgalajda makes some good points and well tuned carb and timing could attain performance with economy.

Economy was poor word choice from me. I’ll try to explain rather than find the right descriptive word. The old school Holley settings will be your best bet for an efficient engine that has good AFR stability and deliver more lineally rich idle, lean cruise, leaner light acceleration through to rich WOT as rgalajda describes. Conversely, the emulsion and MAB you have in the carb now takes a little more tuning to tame for as linear an AFR curve as the old school emulsion/mab. Now, to the point of difference. I have tuned the same carb both ways (over several different sized carbs) - old school with .028” mab and emulsion of .028” X .028” X X, and the other way after a lot of trialing emulsion stacks and mab a result of .033” mab and emulsion .031” X .028” X .025”. AFR averages for steady state speeds were very close. The old school settings certainly drive quite good but when back to back test with the other ‘larger’ settings the old school style felt lazier under acceleration. There was less accelerator pump tuning required too with larger settings. The clear indicator to me was driving at 100kp/h and then hitting WOT the larger settings would see lost traction where as the old school settings in the same carb to the same AFR would not lose traction under the same conditions. I don’t think it was a power difference but speed of reaction of main well fuel delivery with the fast change of throttle position and therefore vacuum. I have settled for the emulsion/mab I came up with through trial as it delivers more in line with how I want the car to be/feel when driving through all conditions. It’s a low 11sec 3350lbs 383 sbc street/strip car that does not see long distance drives. It is accepted fuel delivery to every piston stroke may not as good as it could be but the trade off is accepted. For an everyday driver type car I wouldn’t hesitate to go the old school settings. For your objectives you may choose otherwise. There is a caveat though. One of the carbs tuned has mab of .036”, emulsion as trialed above, .029” IFR, .070” iab and 66 main jets primary all in a 650 cfm and AFR is rock solid during cruising conditions. Venturi and throttle bore size play a role there. An 850 annular is very similar.

So with the ease of how it can be accomplished with the carb you have try both settings. Bottom line is how you want the car to drive, either way you have to enjoy it.

Right hand drive

"lineally rich idle" don't know what you mean or how this applies

"for as linear an AFR curve as the old school emulsion/mab." linear is a straight line so how can a fuel curve be a straight line

Tuner wrote,

"There is no reason to run an engine at part-load any richer than as lean as it will run without misfire. Of course for each engine just what that AFR may be depends on the particular engine’s characteristics, cam, compression ratio, headers, mufflers, etc. The biggest factors are uniform AFR distribution to the individual cylinders and exhaust reversion through the valve overlap. At part-throttle, distribution is affected by throttle angle and other carb geometry as well as the manifold, manifold heat, fuel distillation curve, etc, etc.

Between about 10% to 20% and 70% to 80% load some engines will run well at part-throttle with 17/1 AFR or leaner and others start to turn bitchy at 15/1.

You need to realize that the leaner the AFR, the larger percentage of oxygen in the exhaust. When running at part-throttle the high intake vacuum is drawing hot exhaust back into the intake manifold. Leaner than stoichiometric the excess oxygen in the exhaust is returning to the cylinder in the reversion gasses and the hot oxygen improves combustion.

Obviously, the leaner it is, the larger the proportion of unburned oxygen in the exhaust will be, and up to a point (unique to each engine) the lean running usually noticeably improves the part-throttle combustion.

Depending on your particular engine’s nature, you will be surprised how much part-throttle torque improves as it is operated progressively leaner on the lean side of stoichiometric. The limit is usually reached when the leanest cylinder misfires.

Because lean part-load mixtures have a slower combustion rate, they require additional spark advance, compared to rich maximum power WOT mixtures.

Correct vacuum advance tuning is as important as carb tuning. If the timing isn’t correct, you can chase the carb tuning into a box where a rich cruising AFR is wasting gas, because, without vacuum advance, a faster-burning richer AFR compensates for retarded timing.

You have to use common sense applied to your particular engine to determine at what amount of load the engine should be switched from the lean economy AFR to the rich power AFR. The power valve opening point and the vacuum advance starting point (fully retarded vacuum amount) should usually be near the same intake vacuum.

A high compression engine that is close to stressing the available octane, such as a 10/1 mild-cam street engine on pump gas, like a muscle car engine in the late ‘60s, may need to be rich and retarded at 9 or 10 inches Hg.

A low compression engine may not need to richen up until much closer to WOT and may tolerate the vacuum advance not fully retarding until as low as 5 or 6 inches Hg.

Because lean mixtures burn slower they require the correct amount of additional timing, compared to rich WOT mixtures. The correct vacuum advance is a key to lean part-throttle tuning."