Thirty other guys are on the track with you, all running a 390cfm Holley carburetor mandated by the rules. One guy is out front, another is on his tail, while you feel bottled up in the middle of the pack. It is time for a change, so we decided to even up the odds in your favor and uncork all the secrets. Down in Winston Cup country, around the corner from Philip Morris in Concord, N.C., we’ll find out the inner tricks of a Keith Dorton Signature Series Holley 390 during practice and racing. Yes, they meet the rules, pass tech, and the only smoke allowed is from the tires leaving the pits. Keith, a member of Circle Track’s Technical Council, and his son Jeff preside over an amazing engine shop, Automotive Specialists. They have teamed up with Holley Performance to produce some fabulous new race-ready pieces.
A 390cfm restrictor carburetor is designed as a power limiter. It is the control valve of many racing associations, to take every one down a peg and give them an even start. It also changes many of the things you take for granted with larger carbs. Let’s install a good race-ready engine on the DTS dyno at Keith’s Automotive Specialists and connect a vacuum gauge to the intake manifold. Later, we’ll head for the track for an advanced 390 Series tuning session.
Vacuum City We are first going to run the engine with a full-size 750 or 830cfm carburetor. As we go from idle to full throttle, the vacuum is going to steadily decrease, which tells you the engine is coming closer to getting its fill and its max potential power. In effect, the throttle controls the flow through the venturi and thus, the signal.
Now leave the engine the same, but switch to the restrictor 390. You start at idle with high vacuum. As the throttle opens, the vacuum first decreases and then goes back up again. Here, the throttles and the venturii both control airflow and, as in any management fight, the results get a bit insane. Venturi size limits how much air can get in, so it now takes more suction or manifold vacuum to pull in enough air.
If you bolt a 390 on a small engine, it operates as a matching carburetor and meters normally. When you mount the same 390 on an engine that really wants an 830, all the rules of the game change. Combine this with low compression and you reach a new peak of engine inefficiency. Dorton is about to show us how to win races in spite of all this.
Take a look at a carburetor with a grain of trash under a needle or a pressure regulator that got stuck and there is a total flood, fuel every place. Here, with super vacuum under a 390, fuel is also drawn from lots of places. You are busy driving and can’t see the floods because they are hidden under the cowl and covered by the air cleaner. You look at the plugs, feel the car and pull out jet. At Holley and at Automotive Specialists, a bunch of expensive detective work included hundreds of runs on DTS dynos and close to 600 gallons of race fuel, to finally get to the bottom of the problems, one at a time.
Atomize
Atomization breaks up fuel into readily burnable mini-balls of liquid, tiny in diameter but with a huge surface area. Fuel at the surface of those little balls can now turn to vapor and burn. Horsepower and torque directly depend on good atomization. The best way to break up and condition the fuel is to pull it from the booster venturii. When fuel comes from a source other than the boosters it tends to come out in streams and cannot make as much power.
Liquid fuel rising in the main well must mix with enough air coming in from the high-speed air bleeds. Grab a metering block, study it again, don’t take anything for granted and you’ll know more than most racers. The rest of this story concentrates on metering, the fuel curve, atomization and much depends on the place and size of the emulsion holes and on booster channel size.
Not Wanted
At times a 390 can deliver lots of extra fuel, too much of a good thing. For instance, accelerator pump nozzles send a shot of fuel when you mash down on the pedal but can also spray out excess fuel at full throttle. Pump nozzles on a 390 are located in a very high-velocity area and with a big engine pulling air from small venturis, the pump shot nozzles spray streams of fuel. Jeff Dorton screened some very interesting—and expensive—high-speed movies taken with special lighting, showing what happens with fuel pullover from standard sized nozzles. You can even see sheets of fuel being drawn from one barrel to the other.
This explains the metering and fuel delivery improvement on Holley’s Keith Dorton Series 390, where the secondary pump cam, lever and nozzles are gone and the pump output is blocked with a special screw. On the primary side you’ll find a pink pump cam, but primary nozzle size is at a minimal 0.018, and it delivers enough shot to start the engine with few pedal pats. Pullover is minimized and replaced by well-atomized fuel from the boosters.
Idle Tricks
What does idle have to do with going over rich at the top end? A lot. With a conventional 750 or 830 Holley, the transition slots help with fuel until the throttle is open far enough for the main jets. Beyond the early throttle opening, fuel from the transition slots tapers off. As soon as the throttle opens wide enough, very little idle fuel is drawn out from either the curb idle ports or from the transfer slots.
It is another story with the 390. Manifold vacuum climbs back up and stays at better than seven inches at wide-open throttle, so idle can supply a substantial amount of undesired fuel at wide-open throttle. At idle, with your foot off the accelerator, the holes under the throttle plates are controlled by the idle mixture screws in the metering block. Now you can run the idle rich enough for good response.
As the throttle plates swing past the transition slots, the high-vacuum 390 wants to pull too much fuel, so the Keith Dorton Series Holley has a pair of convenient restrictions screwed into the main body. Equally important, at wide-open throttle, still with some intake manifold vacuum, the restrictions keep the transfer slots from adding too much fuel to what is delivered by the boosters. Other idle detailing includes a flat throttle base without idle balance slots or the need to pin them shut.
The small 390 throttle plates still need to provide enough air for idling the engine. Here, the idle bypass holes in the throttle plates are drilled larger than on a 750 or an 830. Also, the throttle plate screws are just long enough to sit flush with the shaft, for max airflow. To hold them in, use a drop of red Loctite. Crimped throttle plate screws tend to damage the shaft threads during removal.
Power Valve Channel Restrictions
In an engine with a larger 750 or 830, the power valve is closed at idle and low part throttle, with high vacuum, and then acts as a full-power enrichment device. With a 390, the power valve first acts normally, but then it turns tables and serves as a top-end lean-out. It is simply doing its job in following the engine’s vacuum changes, and, with a restrictor carburetor, it reacts to the vacuum build-up at the top end. Let’s look at what happens.
As the driver lifts at the end of the straight, manifold vacuum is high, which immediately shuts the power valve, leaning the mixture. Coming out of the turn at full throttle opens the power valve, but not for long. When the restrictor action of the 390 builds enough vacuum, part way down the straight, the power valve again shuts, leaning back down the top end. As you would expect, the low-number 2.5-inch power valve provides an almost immediate lean out, as soon as vacuum builds.
On Holley’s very tunable Keith Dorton Series 390 metering blocks, the fixed power valve channel restrictions are replaced by two screw-in jets that can be found directly under each power valve. These PVCR jets set the actual amount of extra fuel to supplement the main jets at the end of the turn and the beginning of the straight. Beyond that point, down the straight, the same power valve channel restrictions also set the lean out in the straight, effectively sending you back to main jets. With convenient screw-in jets, the Keith Dorton Series Holley is instantly dialed in for the track. First, the main jets plus the power valve channel restrictions provide fuel at wide-open throttle in midrange rpm, but the switch to main jets only cuts down the natural enrichment at top end of the 390. Typical combination is .068 main jets and .040 PVCR.
Emulsion Holes
The new five-emulsion-hole metering block designed for Holley’s Keith Dorton Series 390 is truly a wonderful piece. It will be very close to right as a bolt-on. The carburetor will allow a dialed-in car to receive a very well-tailored fuel curve, a perfect tool to work with.
Everything centers on the booster in the middle of the venturi. It supplies the signal or suction to the main well, the one with a cap on top of the metering block and the main jet feeding. The jet feeds fuel to the bottom of the main well and so does the power valve. If the suction is high enough, it lifts fuel to the booster channel and out the booster. To make things a little more elegant, outside air is introduced from a high-speed air bleed on top of the carburetor. This bleed air comes in through the dog leg channel at the main body side of the metering block.
On a standard metering block in a non-restrictor carburetor, the main well and the emulsion holes are there, but they are pre-drilled and no one pays any attention to them, except for a select few. The only way to control how much emulsion air is reaching the carburetor is to change the small screw in the high-speed air bleed. To provide the maximum amount of emulsion air accepted by NASCAR tech, the high-speed air bleeds of the new 390 Dorton Series are simply left out. The threaded screw-in hole in the main body is the metering orifice.
Air and fuel both reach the main well, and both are drawn out through the channel, thanks to booster pull. Booster suction serves as a signal and dictates how much fuel the engine receives. To run richer, larger jets feed more fuel. In addition, Keith Dorton increases fuel and emulsion air feed by reaming the booster channel, like inserting a larger straw in your drink. Where a standard channel measures 0.116, it is increased to 0.136 and on occasion to 0.144 inches, depending on the application.
Well Level
At idle or low part throttle, fuel is at the same level in the fuel bowl, and in the main well. Now you apply higher booster suction through the channel, to the top of the main well and the level in the well drops below that of the fuel bowl. The jet acts as a restrictor, and fuel level in the main well drops. A lower well level exposes lower emulsion holes that lets the air enter at a lower level. Here with five holes at different heights, the fuel in the well is like a hand on a flute, playing a tune. With a handful of assorted jets, you are in a position to dial in a much-improved fuel curve. As a trend, larger lower holes give more lift, more response and a leaner mix at the high end.
Upper holes lower the signal overall and moderate the effect of lower holes. Keep in mind that a mix of air bubbles and fuel is easier to lift up the main well than the heavier fuel alone. Emulsion-hole air helps lift fuel, atomizes it, and changes the signal the well fuel sees. To sort this all out, make a small definite change while testing on the dyno and see what happens. You also find out how much fuel the engine wants and at what rpm. That’s what fuel curves are all about. The trick is to get the correct air/fuel ratios at all points and be at the front of the pack at the end where it counts. It’s all at your fingertips with Holley’s most advanced 390.
