When you decide to take a trip to the land of forced induction, there are a couple of different routes you can travel. The best path you can take if you’re using a centrifugal supercharger is one that matches the size of the blower with your engine. In this article, we’re going to cover why it’s important to use a centrifugal supercharger that’s the correct size for your engine package.
Your standard centrifugal supercharger is spun by a belt that’s driven by the engine’s crankshaft to create boost. The supercharger’s impeller spins much faster than the RPM of the engine, thanks to the step-up ratio inside the supercharger’s gearbox. The air is then sent through the discharge tube into the engine’s induction system of choice.
To demonstrate how the concept of optimizing a centrifugal supercharger to an engine works, we reached out to Brian Cox at Vortech Superchargers to look at a vehicle that upgraded from a Vortech V-3 Si-Trim supercharger to a V-3 JT-Trim supercharger. The vehicle in question is a 2002 Corvette that is powered by a nasty 409 cubic-inch LS2 and happens to belong to Brian Turner, owner of Dyno Tune Motorsports. Turner took the time to make pulls with both superchargers to show the differences between the two.
“Too Big or Too Small — Neither Is Good”
Engines need to move air to make horsepower, so adding a supercharger makes that entire process much easier. Where people get into trouble with forced induction applications is they fall into the “bigger is better” trap and want to bolt the biggest supercharger they can to their engine. If you look at some of the most powerful supercharged vehicles in the drag racing world, you’ll notice they’re not really large in engine displacement, nor feature cartoonishly huge superchargers. The engines and superchargers are matched for maximum efficiency.
“If you try to put too big of a supercharger on an engine, it’s going to struggle to make horsepower. There’s only so much air an engine can move efficiently, and too big of a supercharger is going to hurt that. When the supercharger is too big for the engine, it’s going to perform poorly and just not make as much power as it could. You’ll also have issues with the supercharger running in surge, which impacts performance as well,” explains Cox.
What does it mean when a supercharger runs in surge and why is that bad? Cox adds some additional details about this condition.
“Surge generally happens when a compressor is not properly matched to the flow characteristics of the engine it is being used on, and will push the supercharger to the left side of the compressor map. This typically means the supercharger is too large for the application it’s being used on. The opposite happens when the supercharger is too small — this will cause a choking condition, and is found on the right side of the compressor map.”
A supercharger that’s too large for a specific engine is essentially going to drown the mill in air. Turner adds his thoughts on why this is a problem you want to avoid when selecting a supercharger.
“If the blower is too big, it will really drag down the overall net horsepower at peak RPM levels. It’s also going to impact how much power it makes throughout the entire power-band of the engine. The reason for this is because it keeps the supercharger’s impeller out of its speed ‘sweet spot,’ so it just can’t make good power.”
If you look at the compressor map for a supercharger you’ll be able to see where the surge line is, and that will help you determine if the supercharger is right for your application. The compressor map itself is just a visual map of the flow and pressure characteristics of the supercharger’s compressor stage. You can learn more about compressor maps and compressor efficiency right here on Vortech’s website.
An engine that’s been outfitted with an undersized supercharger is going to have its own set of problems. Enthusiasts will find themselves in the situation of having too small of a supercharger if they bolted a centrifugal supercharger kit on a vehicle that started life with a smaller displacement engine. When they move to a larger engine, that supercharger is going to struggle to keep the new engine happy.
“Much like having a supercharger that’s too large for an engine, a supercharger that’s too small will cause performance issues. The supercharger is going to see increased air discharge temperatures, it will have higher power consumption from the supercharger, and it will also run in a choke state, and that can cause the supercharger to fail,” Cox says.
Turner has run into situations on the dyno where customers have superchargers that are just too small for their application. This ultimately leads to performance levels that don’t meet the customer’s expectations and other issues as well.
“You’re never going to meet your power goals if the supercharger you’re using is too small for the engine, there’s just no way around that. The smaller supercharger is going to cause air intake temperatures to be higher than what you want to make good horsepower. You’ll also run into belt traction issues as you try to spin the blower harder to make the power you’re looking for,” Turner explains.
The Goldilocks Zone — Just The Right Size
Let’s get into why you want to avoid having a supercharger that’s either too big or too small for your engine. It all comes down to having a supercharger that’s going to function within an optimal efficiency window that matches up to what the engine likes. The major benefits of using a supercharger that’s sized correctly for an engine include: increased air charge density, lower discharge temperatures for the air flowing into the engine, and a supercharger that will last longer.
“Optimal supercharger matching is very important to maintain compressor efficiencies. When a supercharger is operating in its efficiency range, there are benefits such as cooler air discharge temperatures, less consumed power to drive the unit, and with cooler discharge temperatures, the discharge density increases along with horsepower levels,” Cox says.
If the blower is too big, it will really drag down the overall net horsepower at peak RPM levels. – Brian Turner, Dynotune Motorsports
The best way to start the process of selecting the correct supercharger for your engine is to know your ultimate horsepower goals upfront. If you look at Turner’s Corvette, the Si-Trim supercharger is rated for a maximum of 775 horsepower, so if he was trying to make 1,000 horsepower to the tire, that supercharger wouldn’t get the job done.
“I make sure to explain to my customers that you will typically see better gains in power and airflow with a smaller unit in the lower RPM range when spinning the blower near the manufacturer’s suggested maximum impeller speed at maximum engine RPM. This is better than going with a bigger blower and spinning it slower to keep boost where you want it at peak engine speed, which I would think would have more parasitic loss and less airflow in the lower RPM range of the engine,” Turner states.
So, you’ve decided that a centrifugal supercharger is the power adder that will help you achieve your performance goals — how do you know what to look at when trying to match a blower to your engine? It can be easy to get lost with all the different ideas out there, but according to Cox, there are a few things you really want to pay attention to.
“Engine size is crucial when you’re looking at the calculation for air consumption. You also need to look at the camshaft profile which is critical. Camshaft opening and closing rates need to be considered, too, as this contributes to dynamic compression calculations. Engine speed needs to be factored into air consumption calculations. Cylinder head choice has an impact on overall engine airflow and needs to be included in the airflow calculation. The fuel you plan to use can also influence supercharger choices that are best for your application.”
Blower And Engine Optimization In The Real World
Now that we’ve covered the theory behind matching the size of a supercharger to an engine, let’s look at how this works in the real world. As we mentioned earlier, Turner’s Corvette had a Vortech V-3 Si-Trim supercharger on it for several years. Turner made pulls with the Si-Trim supercharger on the dyno and then bolted up the V-3 JT-Trim unit for a direct comparison.
Before we get into how the superchargers performed on the 408 cubic-inch LS, let’s cover the blower’s performance specs. The Si-Trim supercharger has an internal step-up ratio of 3.61:1 and has a max speed of 52,000 rpm. When at full song, the Si-Trim supports up to 26 psi of boost, and can move 1,150 cfm of air on its way to generating up to 775 horsepower. Vortech’s JT-Trim supercharger features an internal step-up ratio of 3.45:1 and a maximum impeller speed of 60,000 rpm. The JT-Trim supercharger supports up to 27 psi of boost and can move 1,450 cfm of air, and will support upwards of 1,000 horsepower.
When you look at the dyno graph for both superchargers, you’ll notice just how linear the power-band really is…this is one of the big advantages a centrifugal supercharger brings to the table. The Si-Trim supercharger’s pull netted just over 804 horsepower at 6,000 rpm and 753 lb-ft. of torque at around 5,000 rpm, while at 10 psi. Considering the Si-Trim blower is rated up to 775 horsepower, those are some impressive numbers.
If you try to put too big of a supercharger on an engine, it’s going to struggle to make horsepower. -Brian Cox, Vortech Superchargers
With the JT-Trim supercharger bolted up to the Corvette, Turner started making some pulls and adjustments to the tune to account for the larger unit. The JT-Trim allowed the 408 LS to make over 956 horsepower at 6,000 rpm, and 816 lb-ft of torque, at 5,000 rpm, while generating 16 psi of boost. The graph for the JT-Trim supercharger is almost an exact mirror of the Si-Trim.
One thing that you’ll notice about the overlayed graphs is the fact that the larger JT-Trim head unit didn’t hurt the 408’s ability to make power down low at all. This is a good indication the JT-Trim supercharger didn’t have a negative impact on the engine’s efficiency. You can also see how the JT-Trim helped the LS really make some good power up top that’s still very usable without dragging the engine down.
Both Cox and Turner stressed that if you want to optimize and maximize the power an engine can make with a supercharger, you need to do your research. You’ll want to talk with your engine builder and let them know what your power goals are when using a centrifugal supercharger so they can assist you with selecting the right parts. Cox also noted that you should consult with the supercharger company you plan on using to see which products they recommend for your goals and build.
Hopefully, you now have a better understanding of why it’s important to match up your supercharger with your engine. The last thing you want to deal with is the disappointment of a package that doesn’t make the power you need, or parts failure because you’re pushing a supercharger too hard.