Positive displacement supercharger modifications

Positive displacement superchargers do not usually compress air inside the supercharger but rather better described as air pumps. Positive displacement superchargers are usually rated in liters or cubic inches (or a similar measure of displacement). The reason behind that is that a 1.5L rated positive displacement supercharger moves 1.5 liters of air per revolution and so to produce 15psi of boost (or a pressure ratio of 2.0) on a 1.5 liter engine, the supercharger would have to be geared to double the rotating speed of the engine. Thus for every round of the engine, the engine would normally ingest 1.5 liters of air, at the same time for every round of the engine, the supercharger completes two full rotations because of the gearing between the engine crank and the supercharger, and thus the supercharger puts out 3.0 liters of air for every 1 revolution of the engine.

The result is that the supercharger pumping and feeding 3.0 liters of air to a 1.5 liter engine, which results in the air being compressed inside the intake manifold between the cylinder and the supercharger, and effectively forcing the engine to breathe twice as much air, simulating a motor that is twice as large, and producing double the horsepower.

Based on this theory, the easiest way to raise the boost and thus increase the power of a supercharged car is to increase the gearing between the engine and the supercharger, making the charger spin faster and force more liters of air into the engine for each revolution.

However, superchargers are limited due to the size and weight of their rotating assembly, the tolerances between the tips of the rotating rotors, and the adiabatic efficiency of the supercharger to a certain maximum operating rpm. Exceeding the maximum rpm causes the supercharger to become inefficient, producing extremely hot output air which isn’t good for performance, possibly over-vibrating and overheating its internals and permanently damaging the rotors or having them contact the housing, and increasing the supercharger drive requirement excessively (the horsepower required to spin the supercharger to that rpm).

One of the solutions to increasing the performance of your vehicle is to upgrade to a larger supercharger, that can pump more air per revolution and so produce more boost and more power at its maximum operating rpm. However, having most supercharger kits incorporate the supercharger and intake manifold or the supercharger and throttle inlet in one mechanical package it becomes difficult to just swap to a bigger supercharger as you would with a turbocharger.

The next best solution available in such a situation is modifying the existing supercharger to make it more power friendly. Since the primary utilization of positive displacement superchargers is for factory vehicles and OEM manufacturers, there are many provisions built into the manufacturing process of the supercharger geared towards reducing the supercharger’s manufacturing cost as well as improving its NVH (noise, vibration, harmonics) performance such that the factory vehicle can adhere to local or international design standards.

Now if we’re willing to spend some money to improve our supercharger, and we’re willing to put up with (or better yet yearning for) more of that iconic supercharger whine, then supercharger performance porting becomes a viable option.

The goal of supercharger porting is to essentially turn our 1.5 liter supercharger into a 1.8 liter supercharger for example. So in simple terms, modifying the supercharger so that it is capable of pumping more air volume per revolution, and so producing more peak airflow (in CFM) at the same maximum operating rpm.

The modifications typically performed when modifying a supercharger include:

 

·         Enlarging, blending and smoothing the supercharger inlet port

·         Casting protrusion elimination

·         Raising the roof

·         Enlarging and reshaping the outlet port

·         Rotor coating

·         Plugging the silencer ports

A brief overview of the twin rotor supercharger:

The supercharger typically has two helically twisted three prong rotors encased in a dual chamber housing. Air enters the supercharger from the back of the charger through the inlet port, and is guided to the top part of the rotor housing called the roof. As the rotors turn in opposite directions, they grab the air that is available in the roof of the charge, and trap it in tight tolerance between the rotor blades edges and between the supercharger sidewalls. No compression is performed to the air, it is just trapped against the supercharger wall and pumped downwards with the rotation of the rotors.

As the rotors reach the bottom (the floor) of the supercharger they release the air into the manifold through the supercharger outlet port. The fact that the supercharger is overfeeding the engine causes the air to compress as it exists the outlet port (as explained earlier) through positive displacement of more air than the motor can breathe.

Now in light of that, let’s review our modifications:

·         Enlarging, blending and smoothing the supercharger inlet port

The supercharger inlet port is where our modifications will give us the most improvement result because it reduces restriction in the intake path of the supercharger. With reduced restriction, the supercharger can flow air more easily and that reduces horsepower drive loss and increases flow per revolution. Since this part of the supercharger has no fuel in it, and thus there is no fuel atomization requirement, the internal walls of the inlet port can be polished and smoothed. Any sharp turns or transitions between the throttle body or the supercharger inlet and the supercharger housing should be smoothed, also any rough transitions between the inlet port and the rotor housing should also be smoothed especially on the top half of the charger (opposite the outlet port) as that is where the rotors get their fresh air.

·         Casting protrusion elimination

Due to cost reductions in supercharger production, sometimes there are some materials protruding the inlet port of the supercharger. Any such protrusions should be removed, and then the housing refilled with aluminum putty filler or JB Weld, and then resurfaced smooth.

·         Raising the roof

Slightly raising the roof of the supercharger gives the supercharger more volume of air to grab per revolution.

·         Enlarging and reshaping the outlet port

Depending on the target pressure ratio for the application, there is an obvious relationship between the area of the inlet port and the area of the outlet port. If air enters the supercharger at 0psi (which is 1 atmosphere) and exists the supercharger at 15psi (or 2 atmosphere) then the exiting air although it has the same mass, has half the volume. So it makes sense that if our inlet port is 20 square inches large that at a target pressure ratio of 2.0 our outlet port need only be 10 square inches larger. Thus enlarging the outlet port doesn’t necessarily have much power gain potential because inherently it is less of a restriction.

When porting the supercharger outlet port, there is a limitation to how much we can enlarge the port or reshape it. The supercharger has a helical twisted rotor and for eaton this is a 60 degree twist. If you visualize a twisted rotor spinning, capturing air between the rotor and the housing, and then as the rotor’s blade reaches the floor of the rotor housing the rotors blades create a V shaped contact area.

This is why the outlet port is V shaped or triangle shaped to match the twist of the rotors and to make sure that air stays trapped properly till the rotors reach the floor of the housing to force the air into the manifold.

Enlarging and smoothing the outlet port is possible however, the maximum that we can enlarge this port to can be found by tracing a line around the exact point where the rotor touches the housing floor of the charger housing and porting to that line.

·         Rotor coating

One of the changes between the earlier and later generation Eaton superchargers is that the newer blowers have coated rotors. The rotors are coated with Teflon/Ceramic/Carbide abradeable coating. The coating does two things:

1-      Helps the rotors withstand, distribute, and manage heat better especially when using the supercharger at higher rpms.

2-      The  coating adds thickness to the rotor. When the rotor is heated by its normal operation it expands to a certain extent. If the rotor expands too much within the housing then rather than having a proper seal against the housing wall (to trap air) it may touch the wall causing the whole assembly to catastrophically seize. To prevent this seizure, there is typically a tolerance (a room for error if you will) left between the rotor tip and the housing. This room is bad for performance because it allows the air to escape rather than be trapped by the rotating rotor and thus reduces the superchargers pumping and adiabatic efficiency. To improve efficiency on the later generation superchargers the Eaton engineers have come up with an abrade-able teflon-carbon based coating that makes the rotor slightly thicker. When the rotor expands it now basically touches the housing, but this is now not a metal on metal contact, but rather a metal (housing) to carbon-teflon (rotor coating) contact. What happens here is that the harder material (which is the aluminum housing) wears down the softer material (the carbon-teflon coating) shaping it to the exact shape of the housing and giving a more exact seal between the rotor tip and the housing, better trapping air, improving supercharger flow per rotation, reducing pumping losses, and thus reducing outlet air temperatures. This idea can be used on any twin rotor charger to improve rotor seal and is really a great engineering solution.

 

·         Plugging the silencer ports

Besides the supercharger outlet port, there are typically two or more smaller ports called the silencer ports. Depending on weather whether the air in the supercharger housing or the air in the intake manifold has a higher pressure, then these ports allow air to flow in and out of the supercharger housing around the rotors to faster equalize the pressure between the two chambers. Reasons for having pressure differences between the supercharger and the manifold attached to it would be like having a throttle body in the system after the supercharger for example. If this throttle plate is rapidly opened (jumping on the throttle –full throttle) or rapidly closed (going from wide open throttle immediately to no throttle) then positive and negative pressure waves will travel back and forth between the throttle body plate up the intake system and reflect off the supercharger outlet back and forth. These pressure pulses and fluctuations caused by the dynamics of the engine are unavoidable. As we all know, sound is created from pressure waves and so these waves reflecting back and forth in the intake system create some unwanted noise in the engine bay, more so if we have a sealed system where the pressure waves are trapped to reflect back and forth multiple times. The supercharger silence ports are small bleed ports between the outlet of the supercharger that bleed air back into the supercharger housing. This is obviously bad for pumping efficiency because some of the air pumped in by the supercharger is not held as boost, but always being bled back (as a designed air leak) back into the supercharger housing. But as we said before, in situations where there are dynamic pressure changes between the supercharger housing and the rest of the intake system, having this air bleed allows the pressure to equalize faster (rather than waiting for it to reflect and die out) which improves noise performance.

Since we are interested in improving the pumping efficiency and are not required to adhere to noise, vibration, and harmonics standards like original equipment manufacturers, these ports are typically plugged with aluminum putty or JB weld and surfaced down to a smooth finish. Now that we’ve done so, 100% of the air pumped by the supercharger is trapped in the engine which increases our boost pressure and our positive displacement of air per revolution.

Results of all of these modifications are typically in the 10 to 15% range, so depending on whether we are talking about a 200hp capable Eaton M45 or a 500hp capable Eaton M120 gains will vary between 20 and 75 hp which are sizable increases. Further performance gains can be found by increasing the supercharger rpm (now that it is able to flow more air at higher RPMs after it has been ported) and so higher gains are possible for race applications. I would be reserved though about exceeding supercharger manufacturer stated peak operating RPMs for road going vehicles that will see extensive use of the supercharger at those RPMs.

Technorati Tags: engine performance parts, Porting, Positive Displacement, Rotor, supercharger performance

Related posts:

1. A snapshot of Lysholm Autorotor Superchargers
2. What modifications do I need when adding a supercharger to a truck?
3. Supercharger Snapshot – Whipple Supercharger Performance
4. Building a supercharger car based on the Toyota A-Series engines

Posted by admin on April 24, 2009.

Tags: engine performance parts, Porting, Positive Displacement, Rotor, supercharger performance

Categories: Supercharger Performance Tips

4 Responses

1. please e-mail me on what the brand name of the coating that is used to coat the rotors & where it can be purchased.

   Thanks, Jirka

   by Jirka Kaplan on Jun 11, 2009 at 12:04 am

2. The best place to get your rotors coated is to send them to a rebuild and supercharger surfacing company like magnuson.

   As far as I know manguson has two types of coatings , one is the typical Teflon seal coating used by OEM’s on later generation roots style compressors. The other is a harder coating that is water / alcohol / methanol / fuel / nitrous injection compatible where spraying liquid before the supercharger will not hurt the rotors nor strip the coatings.

   http://www.magnusonproducts.com/reman/index.htm

   Make sure you explain to them what kind of setup you’ll be running and they’ll be able to give you the right coating service.

   As far as DIY coating I have no idea, but my best guess is that this technology trickled down from power generation turbines… here’s an example:

   http://www.calltru.com/services/coatings.html

   by admin on Jun 11, 2009 at 6:45 am

3. I assume that these modifications would apply to Lysholms as well (Autorotor/Sprintex/Kenne Bell/Whipple and eventually eaton (which has been licensed to make Lysholms, even if they’re not available yet).

   OK, something that is never stated anywhere, it seems: what sizes of superchargers are best for given engines, and what gearing (pulley sizes)? For instance I am thinking of putting Lysholms on two three of my cars: 1) 1989 FWD CE96 Corolla DX diesel commercial (no back seat, leaf rear springs) on which I’d like to replace the factory 2C-T turbo unit with a Lysholm to hopefully keep the head from cracking; enginew revs to 5000 2) 1996 AE111 Toyota Carib BZ Touring 1.6-liter 20-valve redline 8000 3) Subaru Alcyone SVX EG33 3.3-liter (year unknown) fitted to a 93 VW Microbus, assuming 7200RPM redine. For all of these I want as much power and torque as possible without affecting reliability or drivability; the vehicles will be daily drivers, not racers. Can you e-mail me suggestions, or tell me where I can get good answers?

   by Andrew Grebneff on Apr 26, 2010 at 11:07 pm

4. Andrew,

   For supercharger sizing, I’ve built the power calculator found here:

   http://www.superchargerperformance.com/the-power-calculator/power-calculator-v2-3

   Which gives you a list of the 10 ’smallest’ superchargers that can meet your power , flow, and boost requirements. The reason I list from smallest to largest is that the smaller chargers typically have less drivetrain loss associated with them so the software recommends the smallest supercharger that is capable of meeting your power / boost / flow goals with minimal drive loss.

   As far as gearing goes… the equation is very simple:

   Supercharger pulley diameter = drive (crank) pulley diamter * (engine redline / supercharger redline) * (supercharger peak horsepower / target horsepower)

   Let’s put some numbers to the equation:

   Say I have a 120mm crank pulley, a 7200 rpm redline engine, and I’m using a supercharger capable of 450hp @ 18000 rpm supercharger redline.

   If I’m shooting for 350hp from this buildup the results are as follows:

   My snout pullet = 120 * (7200 / 18000) * (450/350) = 61mm

   to check this

   when the motor is at 7200 rpms
   the supercharger rpm = engine rpm * crank pulley diameter / supercharger pulley diameter

   supercharger rpm = 7200 * 120 / 61 = 14163 rpm

   Because of intercooler pressure drop, and depending on how much boost you are running you might have to ‘over gear’ the supercharger by some 10 to 15% to get the correct boost at the motor.

   For example, building on the last example, if your target is 350hp @ 8psi …. and you have an intercooler with a 2psi pressure drop then to get your exact gearing for the supercharger you really want to run the previous equation shooting for 380hp of charger flow @ 10psi of boost

   which will leave you with 8psi in your manifold.

   The resultant pulley size if you re-run the numbers is :

   56mm as opposed to the original ‘ideal’ calculation of 61mm

   I don’t do supercharger gearing calculations in the power calculator yet, but it’s coming soon in a version update.

   by admin on Apr 27, 2010 at 8:52 am

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