Short-term power that comes from a bottle is all right for the enthusiast who doesn't mind a temporary throttle, but power on demand is what the consumer wants, and centrifugal supercharging is the answer to every high-performance-Chevy lover's fantasy.
This month we look at what's involved in setting up a centrifugally supercharged blow-through carbureted small-block. Though EFI is certainly an option (and one we'll delve into next month), we stuck to the basics to see just how much supercharged power we could extract from a budget-based engine.
Our test mule is Spot the Wonder Mutt, which has graced the pages of CHP twice before, making 403 hp and 423 lb-ft of torque naturally aspirated (Aug. '04) and 500 hp and 557 lb-ft of reliable torque (Sept. '04) when sprayed with a NitrousWorks 75hp shot. Until now many enthusiasts have avoided the blow-through carburetor system due to a misunderstood complexity of balancing increased airflow with pressurized fuel distribution, but with the expertise of Holley, ProCharger, Aeromotive, and The Carb Shop, dreams of forced induction are now a reality. This month, our trusty dyno Mutt steps up with a ProCharger D1SC centrifugal supercharger and a blower-ready 650-cfm Holley carburetor.
Spot's internals are unchanged: Hooker headers around a four-bolt short-block equipped with a forged Eagle crankshaft and H-beam connecting rods, forged Probe pistons, and a compete Milodon oiling system. Up top, a Scoggin-Dickey induction package includes valvetrain-modified iron Vortec 9.2:1 cylinders heads and an RPM Air-Gap dual-plane intake manifold with all the necessary gaskets and bolts to put everything together. Finishing the long-block off is a complete valvetrain from Crane Cams sporting self-guided 1.5:1 roller rockers and 0.509/0.528-inch lift and 222/230 degrees of duration at 0.050-inch lift on a 112-degree LSA camshaft.
The camshaft specs and cylinder head choice were part of our budget-based naturally aspirated small-block. Admittedly, the valvespring pressures on the Vortec heads are less than ideal for a supercharged motor and the camshaft profile is rather small. Though the 5,800-rpm redline definitely hurt power, we wanted to see what Spot could do when only a centrifugal supercharger was installed. The combination may not be the best, but it's a real-world financial decision that pointed Spot in the right direction.
Days after Spot finished its last nitrous dyno-test, we phoned ProCharger to find out what components we'd need to boost our engine successfully. Since finances are always an issue with the staff of CHP, we decided to direct our entire budget toward the supercharger and induction side. This meant a new camshaft and cylinder heads were out of the question, leaving us with the same engine that had been thrashed twice before. A ProCharger technical advisor recommended a centrifugal compressor that would move all the air the Vortech heads could use (and more if we decided to upgrade the induction system at a later date). The D1SC is one of ProCharger's serious street blowers that feature a self-contained (the "SC" part of the designation) oiling system, which eliminates the need for the external oiling system early style blowers have been saddled with for years.
Blower selection is extremely critical to every application, making it imperative to speak with a ProCharger technical advisor before you lay down one cent. All too often, enthusiasts want a large blower that creates tons of boost, when in reality a smaller blower will come up quickly and more efficiently, making better overall engine power. ProCharger blowers come with an instruction manual to help ease installation, including a section on modifying Holley carburetors for optimum street performance. Once we decided which blower we'd be using, our next phone call went to one of Holley's top blow-through carburetor shops where we picked up pertinent information on modifying our own carburetor.
The Carb Shop has been specializing in Holley modification for years and has the blow-though systems absolutely wired. The Carb Shop offers its own dedicated blow-through carburetor that is primarily designed for applications operating above 10 psi. While a Carb Shop blow-through carburetor is the ultimate in performance, it's also totally different internally than a conventional Holley. We wanted to take the inexpensive route and modify our own Holley carburetor to keep the cost down, so we decided to stick with ProCharger's rebuild instructions. This required us to mill the choke horn off the main body, fill in the choke-rod hole, and install nitride (nitrophyl) floats in the front and rear fuel bowls. The idea is to completely seal the carburetor's main body while increasing the durability of the internal parts that will soon be pressurized.
When a carbureted engine is put under boost, the carburetor is no longer dealing with the external atmospheric pressure it was designed to work with. Instead, it's being forced to compensate with a pressurized air/fuel environment. The internal pieces of a standard Holley carburetor were originally designed to work with the external environment, which means its internal parts were not designed to accommodate boost. However, because Holley products are well built from the outset, the internal components (except floats) can typically deal with boost pressures upward of 10 psi. Any combination nearing this level of performance or beyond may benefit from the use of a more highly modified, or custom-built blow-through carburetor.
Once the supercharger and carburetor decisions were out of the way, all we had left to do was upgrade the fuel system. This led us to Aeromotive. A technician advised us to use a boost-referenced fuel-pressure regulator along with a A1000 fuel pump. This pump is a return-style fuel pump that can handle engine combinations up to 900 hp. If a dead-head-style fuel system is already on the car, Aeromotive also offers its 11203 SS pump. According to our tech investigation, whenever an engine receives 1 psi of boost in airflow, it must also receive a 1 psi boost in fuel pressure.
Aeromotive's adjustable fuel-pressure regulator uses a ported orifice to read boost from the engine and adjust fuel pressure accordingly. For example, our naturally aspirated engine ran approximately 6.5 psi during its testing several months ago. In the event that this month's testing would produce 10 psi of boost, our fuel pump and pressure regulator would have to be capable of increasing the fuel pressure to the carburetor bowls until they were filled with 16.5 psi of pressure. This 1:1 increase will ensure the fuel bowls will not run dry and the carburetor has plenty of fuel to feed the engine.
ProCharger also carries an assortment of intercoolers and accessories to aid the forced induction way of life. We wanted to see what a real-world street cooler was worth in terms of temperature and power, so we plumbed an air-to-air intercooler into our intake track and installed air-temperature-sensor probes before and after the cooler. According to ProCharger, its air-to-air coolers offer 70 to 75 percent effectiveness when receiving proper airflow--that is, they remove 70 to 75 degrees of temperature for every 100 degrees above the ambient temperature generated by the supercharger. If the supercharger were to discharge air at 100 degrees above room temperature, we would ideally see incoming air temperatures of approximately 25 to 30 degrees above the room temperature with a properly flowing air-to-air intercooler. During an engine dyno-test, it is very difficult to generate airflow rates similar to the open road conditions, so we did our best to blow air through the cooler with a high-volume fan, but even that simulation falls short of an in-car installation.
Once the supercharger and cooler were mounted to blow air through our modified Holley carburetor, we trucked the business to the Vrbancic Brother's DTS dyno facility. The system was set up with a standard 7.65-inch crankshaft pulley while the largest available 5-inch pulley was fastened to the blower itself. A 66-inch, 12-rib belt was rotated over the pulleys and set in place using the kit-supplied tensioner pulley. When setting up one of these engine configurations, the most important things to maintain are the blow-off valve connection between the valve and carburetor baseplate, and the fuel-pressure-regulator connection between the hat (boosted pressure) and the regulator itself. Precautions in these areas will keep fuel to the engine under boost while allowing excess air to escape when the throttle blades are closed rapidly. Beyond these critical areas, all other connections are optional sensor outputs that are at the discretion of the engine builder.
The thing to cover before we began the flogging was the rather small 0.067-inch primary and 0.073-inch secondary jetting with a 6.5-inch primary power valve. The idea was to see just how much a proper air/fuel ratio tune-up is worth. Like any other engine combination, the amount of fuel the engine needs is relative to the amount of air the camshaft, heads, intake, and exhaust are capable of flowing. With smaller-than-needed jets installed in our carburetor, we would be able to run the engine slightly lean (for testing purposes only; not recommended) to show how much power a proper tune-up is worth. The idea was to hinder ourselves slightly from the get go and hear the dyno scream as we increased the power.
We decided to make all the pulls on 91-octane while recording data between 2,500 and 5,800 rpm. To go a step further, we wanted to make sure the engine and blower were operating at actual street temperatures, so before every pull, the air temperature, water temperature, and oil temperature were stabilized to resemble what's obtainable in a street-driven application. We also decided to test with an intercooler installed, so if a head gasket blew we would have a better chance at obtaining the high-end power data before our allotted dyno time ran out. Spot was fired to life and roared like never before. After a few impressive low-rpm pulls to find the appropriate timing, we locked the distributor out at 30 degrees total. The first full-throttle pull revealed a lean 8 psi boost but an amazing 516 lb-ft of torque at 4,300 rpm and 520 hp at 5,700 rpm with the air/fuel ratios touching the 14.5:1 range.
Maximum rpm blower temperatures hovered at 165 to 175 degrees before the intercooler and 125 to 135 degrees after the cooler. Typically, a supercharged engine like this one is happy with air/fuel ratios in the 11.8:1 to 12.0:1 range. With our air/fuel ratio so far out of whack, we headed back into the dyno cell and re-jetted the carburetor with 0.069-inch primary jets and 0.077-inch secondary jets. The increased fuel flow definitely picked up power as Spot raised the torque curve 500 rpm and made 541 lb-ft of torque and 558 hp at 5,700 rpm. The engine was still running a lean A/F ratio of 13.0:1 so we set the carburetor up with another five jet sizes (0.082) in the secondary metering system. The additional fuel brought our air/fuel ratio to a much better 12.0:1, allowing Spot to deliver 550 lb-ft of torque at 4,800 rpm and 568 hp at 5,800 rpm.
The power was now beyond par and the A/F ratios were looking pretty good. We wanted to see if there was anything left of Spot's appetite and added a bit more jet. It doesn't hurt to run a blower motor slightly rich, so we increased the secondary jet sizing to 0.084 inch. This time, torque and horsepower only increased by 1 lb-ft and 1 hp at the same peak rpm points. While we were pretty sure we had made the most amount of power possible, we cut the timing back to 26 degrees total thinking the later spark advance might gives us a little more. We lost 12 lb-ft of torque and 34 hp at the peak rpm points, so we reset total timing to 30 degrees and reconfigured with a 4.75-inch blower pulley in hopes of finding more boost and power.
The smaller pulley delivered roughly 0.5 to 1.5 pounds more boost across the entire rpm curve and increased the 2,500-5,400-rpm torque numbers by 15 to 20 lb-ft and horsepower numbers by 10 to 15 hp. What's interesting is that the peak power numbers from 5,500 to 5,800 rpm only gained an approximate 1 lb-ft and 1 hp increase, revealing that the induction system was maxed out. Another valuable point is that additional air compression increases air temperature. In our case the inlet air temperature rose 10 degrees before and after the cooler.
After achieving such impressive numbers, we removed the intercooler that was giving us a steady 40-degree reduction in air temperature. The first clean pull increased boost from 2,500 to 3,500 rpm by 0.5 to 0.75 psi and then an additional 1 to 1.5 psi from 4,000 to 5,800 rpm. The new torque and horsepower numbers mirrored those of our 4.75-inch intercooled blower pulley numbers to approximately 4,000 rpm and then fell substantially all the way to Spot's 5,800-rpm redline. Above 4,000 we saw losses upward of 30 lb-ft of torque and 30 hp due to factors that affected the air temperature. The increased intake-charge temperature drastically altered the A/F ratio, requiring more tuning.
A glance at our sensor data showed the blower inlet air temperatures were obviously up a minimum of 40 degrees and pushing the 175-degree range while the A/F was sporting a rich 11.2:1 reading. Because the air had lost some of its (cooled) density, the A/F ratio change caused a loss of power. We re-jetted the carburetor, going two sizes smaller (0.082) on the secondary-metering side and made another pull. The A/F ratios came back to the 11.8:1 range, but the power held relatively consistent with our previous pull. The biggest change was from 5,200 to 5,800 rpm where the engine picked up a minimal amount of torque and horsepower. Before reverting to the 5-inch blower pulley, we made another pull to back up the previous information. Unfortunately, the increased air temperature mixed with our lean (but correct) A/F ratio forced a head gasket to move and dump water into the No. 1 cylinder. End of test.
We would have probably seen an approximate 1-2 pound loss in boost causing torque and horsepower to fall off anywhere between 30 lb-ft and 25 hp at the higher rpm points and about half as much power lost across the low and midrange power curve. Though our head gaskets lived through last month's nitrous massacre and numerous boosted pulls before letting go, toward the end of our test the rich A/F ratio most likely saved our head gasket. But when we leaned the engine out to obtain a proper A/F ratio, we should have pulled a few degrees of timing as well. Hot air with little fuel and a lot of timing is a receipt for disaster when running on the edge. We found the limit the hard way.
The critical thing is that testing occurred in a dyno room instead of in a car, no doubt hindering the intercooled application because we couldn't get enough air through it to produce the normal 70 to 75 percent cooling effect. We also made the engine pulls roughly 5 seconds or less, whereas a vehicle making a WOT pass typically operates for twice that or longer, thus creating a heat-soak condition that our intercooler was unable to take advantage of compared to the non-intercooled application. The weak seat pressures that we ordered from Scoggin-Dickey were designed for a naturally aspirated engine, so this inhibited rpm power and kept Spot from seeing the magic 600 hp on pump gas. Knowing that an intercooler reduces air temperature at the expense of boost is relative to the vehicle's intended use. Our engine made roughly 9 psi of boost with its intercooler and the high-rpm power numbers were outstanding because of it. However, the same setup without the intercooler made another 1 psi of boost and very similar power to roughly 4,000 rpm. Once the density of the air overcame the additional boost, the intercooled application made more power. When we compared our average 2,500 to 5,800-rpm power numbers, the non-intercooled engine produced 490 lb-ft of torque and 388 hp while the intercooled application produced 512 lb-ft and 411 hp. The peak intercooled power numbers came in at 551 lb-ft of torque and 569 hp, while the non-intercooled power delivered 536 lb-ft of torque and 502 hp. A peak torque difference of 15 lb-ft and 67 hp separated the two 4.75-inch blower pulley combinations, while the average torque difference was 22 lb-ft and 23 hp. The rules to remember are: Boost is relevant to airflow, and intake charge density equals power. More boost will increase power but lower the air density at the same time. It's a give and take situation where boost versus density must be juggled carefully.
One factor that was not revealed during our testing is the cooling effect that fuel distribution has on an intake charge. Had we installed a third air-temperature probe below the carburetor, it could have shown a temperature reduction upward of 40 degrees. This would mean our 175-degree blower temperature could be cooled approximately 40 degrees by the air-to-air intercooler before feeding the carburetor a 135-degree intake charge. Then, when fuel is added, the actual combustible intake charge could be as low as 95 degrees. Of course, all these figures are on the generous side, as they can vary according to the type of system installed on the car.