Thoughts on the Sebring Supercharger

by Bill Allan

40,000 Mile Update

It's been 40,000 miles and 44 months since a Sebring Supercharger was installed in my 1990 Miata. It was one of the original "Introductory Offer" Eaton units, that were sold at a discount, as an incentive to get these new units into the marketplace. Sebring now has over 1200 units sold and in service.

As with any new design, improvements and modifications have been made to the Sebring assembly. The heart of the system, the Eaton M-45 blower, is the same unit as is used on the General Motors line of cars, but with shorter lobes. It has not been changed - except for a modification called "S-trim". When Norm Garrett was developing a kit for the 1.8 liter Miatas, the blower generated too much heat at higher RPM's. Norm discovered that modifying the internal airflow passages not only reduced the temperature rise and increased the airflow, it also quieted the blower down at the higher speeds required for the 1.8 engine.

The distributor of the Eaton blower, Magnuson Products, 3172 Bunsen Avenue #K, Ventura, CA 93003, (805) 642-8833, will modify original Eaton M-45 blowers to include the improved "S-trim" flow features and recondition the units for less than $300. The "turn-around" for this porting and reconditioning can be as little as 48 hours! "S-trim" is presently provided with all new Sebring units.

Following are all the changes made to the Sebring kit since its original introduction:

Supercharger Bracket -

The original simple u-shaped bracket was too light and stressed the blower mounting bolts. The new bracket is quite attractive, robust, finished nicely, and simplifies achieving correct clearances and alignment. It's cost is $48, from Downing/Atlanta (770/457-6300).

Intake Elbow -

The original intake elbow was rubber. A very attractive aluminum casting was developed to replace it. To replace the rubber piece costs $85.

Idler Pulleys -

Originally, one idler-pulley, mounted on a crank-arm, was provided. Some problems developed on a few units, and the addition of the crankshaft nose pulley was troublesome to many installers, so a double-pulley arrangement, deleting the new crank pulley, was developed. The new bracket, 4-ribbed belt, pulleys, and assorted nuts and bolts cost $100 for a power steering and/or air-conditioned Miata, and $150 for a non-PS, non-A/C Miata.

Sebring also found that the idler pulleys weren't suitable for sustained 5000+ RPM driving. Replacement "blue-retainer" pulleys also proved inadequate for the higher-speed service. New design pulleys, with better bearings, are in the mill and will sell for $10 each. I haven't had this problem, so I must not drive enough above 5000 rpm!

Auxiliary Fuel Pressure Regulator -

The unit was originally furnished with a Cartech AFPR. Enough units developed a whistling noise and/or pressure surging that Norm developed his own AFPR. The new unit is a beautiful regulator, entirely finished and simply mounted. But, more importantly, it is very stable in its operation! Its cost is $219.

By-pass Actuator -

The Eaton by-pass actuator proved to be susceptible to fatigue-failure of its support bracket. This is probably due to its extreme placement, in relation to the crankshaft centerline, and the vibration inherent in a 4-cylinder engine. Some have failed due to inattention to adequate brake-line clearances. Downing/Atlanta is currently providing u-shaped bracket braces and a metal-domed actuators for failed units and lighter, plastic-housed actuators on the new units. The bracket brace is a no-charge item.

There have been a few failures of by-pass valve levers due to enlarging of the actuator/pivot-holes. Attention to lubricating this contact point seems to stop the wear. Some, including me, have found it necessary to modify the actuator rod to prevent its pushing through the lever-arm. A replacement by-pass shaft sub-assembly can be purchased for $30, if the hole enlarges excessively.

Timing Advance -

The timing specified for 92 and 93 octane gasoline is 6 and 8 degrees, respectively. This is necessary, primarily, to prevent pre-detonation with California fuels. Increasing the timing to the stock 10 degree setting or as high as 14 degrees improves power substantially at lower RPM's. To prevent pinging and the engine damage possible at these timing ranges, ignition timing controllers are offered - the MSD automatic retarder at $159, or the J&S knock-sensor/controller at $400.

Fuel Pump -

If the timing is advanced or your Miata has a marginal factory fuel pump, a high-capacity in-tank fuel pump is offered. It is a direct replacement for the existing pump. Its cost is $139.

Air Filter -

Improvements are available to the original compact, Jackson Racing air filter. A denser, higher-flow foam is now used with molded end-seals. This is still the Indy-approved design. The new units also use a two-bolt cover, which is easier for servicing the filter element than the earlier 4-bolt designs. The cost of the foam filter element is $29.95 and the complete filter assembly, with two-bolt cover (now used on new Sebring kits), is $110.

Spark Plugs -

Downing/Atlanta does not recommend the use of Splitfire plugs and feel the use of colder plugs is a good idea.

Idle Air Valve -

Philip Airey discovered that the idle air hose connection provided a constant leak of boosted air. Some experimentation showed that installing an AV-23 check valve in this line increased boost pressure 1/2 to 1 psi. Since the improvement has not been uniform among Sebring users and inconsistent in their own testing, Sebring is not yet providing this feature with their new units. The valve costs about $24 from Al's Auto Supply.

My Sebring Supercharger Experience -

The combination of being an early "pioneer" user of the Sebring unit, having an inordinate number of the potential supercharger problems, being an engineer, and basically "anal retentive" has provided me with a lot of supercharger modification experiences! In a chronological sequence, they were:

I was unable to get good initial alignment of the supercharger belt in the idler pulley. This resulted in excessive belt wear and "dusting". A belt actually broke during a rally at Gold Rush, and I mightily impressed Frank Murphy, my navigator, by replacing it without loosing significant time in the rally! (It helped that the failure occurred during a free-time period.) Replacement of the SC bracket improved things somewhat; the double idler pulley modification helped a little more; and replacing my timing belt at 60,000 miles helped even more. The belt no longer rubs against the flange of the pulleys, but is not exactly centered. (See, I am AR!) Belt wear did stop. I do run with less belt tension than specified - 50 pounds rather than 90 pounds.

I found, on installing the new idler pulleys, that Miata clearances vary enough that differing belt lengths are required from that specified by D/A. The clearances between the two idler pulleys and the PS & A/C pulleys can get quite close. The belt length has to be chosen to avoid rubbing between them. Also, considerable bending and grinding of the PS hose bracket was necessary, to avoid rubbing on the 4-ribbed belt.

I installed the aluminum elbow for appearance reasons, as well as chrome plating the cross-over pipe.

I had much trouble with whistling of the fuel pressure regulator. A replacement auxilliary fuel pressure regulator stopped the noise for a period. Then I experienced surging on wide open throttle acceleration. Much testing and experimentation with fuel pressures, diaphragms, and a gain controller didn't help. Finally Norm Garrett guessed I was getting a vacuum in my fuel return line, which produced a pulsing of the fuel pressure regulator. Putting a notch in the valve seat created a slight leak on closure and stopped the vacuum and hence the pulsation. Eventually, Norm designed a new auxiliary fuel pressure regulator, which I bought and installed, and that has been extremely successful. An added bonus is the beautiful appearance of the new AFPR.

Two by-pass actuator brackets broke due to fatigue failure. I have had no further problems since going to the light-weight plastic housing and placing a stiffener under the heads of the capscrews. The plastic design has a poor stop on the actuator shaft and I had to epoxy a washer on it, to maintain its position in the by-pass valve's lever-hole. Maintaining some grease on the bearing surface seems to have arrested the wear on the lever-arm hole.

I have had no problems with idler pulley failure, but have changed to the "blue retainer" pulleys. I plan to install the high-speed pulleys, when they become available. (More AR behavior!)

I installed the AV-23 check valve in the idle air hose and now get 7 psi boost at high RPM's. (I was seeing about 6.5psi.)

My timing is set to 10 degrees and I can hear no pinging, but I'm hard of hearing. Sometime in the future I hope to install a J&S unit and advance my timing to 14 degrees. I may install a higher pressure fuel pump at that time.

I have the two-bolt filter and highly recommend it. This is due to the close clearance in a PS Miata and its being easier to service the filter element. I'm using a K&N element and may go to the new higher rate foam in the future.

I installed a miniature SS fuel pressure gage at the AFPR, which makes checking settings a breeze! And it was only $21 plus the brass hose fittings.

You can't install a strut brace over a supercharged Miata due to there being no room for one. But Bill Cardell, of Dealer Alternative in Palisade, Colorado, had some aluminum bar units manufactured in Sweden with a rearward bend to clear the supercharger manifold. I installed one and it looks great with the chromed cross-over pipe!

Would I do it again? You betcha! The Sebring Supercharger provides a higher power engine, that is very drivable, reliable, and smooth. Sometimes I wish I had more power. You can never have enough. But, you really can't beat the torque that cubic inches provide!

Original Notes

I installed one of the introductory Sebring kits almost two years and
20,000 miles ago.  I really like mine!  It's strong and I can
accelerate like crazy at any time.  The improvement is most
substantial after about 25-30mph.  I don't spin my wheels, and that
may be part of the more impressive movement at higher speeds.  It's
fairly quiet, just a little whine 'till you really get on it.  Then
the sound really is neat!  The by-pass design is really good.
You only have the parasite load till about about 5 inches of vacuum.  Above
that point the by-pass closes and boost is instantaneous.  

Early design deficiencies have been corrected and I have most of the
updates installed - a heavier and attractive support bracket and
improved idler pulley layout. I've also installed some improvements
such as vinyl hoses to make the intake and cross-over pipes easier to
install and remove; and the classier aluminum intake elbow.
Chrome-plating the cross-over pipe really produces a sexy look!
Another appearance improvement is to run the idle-air hose under the
flow-meter rather than above it.

The only real installation problem I had was bending the
power-steering hose support-bracket out of the way.  It was a real
close fit with the supercharger drive-belt on my Miata.

A few folks, about 1-2% of the buyers including me, have had problems
with the Bell Cartech fuel pressure regulators.  Replacing the SS
seat-shim plate and putting a slight notch in the seat solved the
surging that was occuring. The thought is that a too-good seating
action was sealing the fuel-return line flow, causing a vacuum to
develop in the return line and creating an oscillation in the valve's

Another problem that has been more common is breakage of Eaton's
support-bracket for the by-pass actuator.  There are two schools of
thought here - one is the 4-cylinder engine vibrates more than the GM
V-6's and the actuator body's heavy metal mass causes a
fatigue-failure in its support bracket; the other is that there is
interference between an un-used flange on the actuator body and a
brake-line nut when the engine torques over.  Fixes are to grind the
superfluous flange off and/or go to an available redesigned actuator
made of plastic for lighter mass.  I had such a failure after 14,000
miles.  Close inspection, at Downing/Atlanta's suggestion, showed the
interference to be the cause of my case.

The installation is simple and very good-looking.  The castings are
all first-rate.  If you're not into the stealth-mode, I found the
Buick's "Supercharged" emblem to be a good-looking replacement for the
Mazda emblem on the right rear of the Miata.

In hind-sight, the only improvements I could suggest all increase the
cost of the kit: lengthen the Eaton rotors and shorten the shaft-nose
to enable slowing down the blower (to possibly reduce charge heat);
add the MSD timing control device; and provide an improved
fuel-delivery system.  I believe these would all provide more boost.
I guess I'm suffering from my cousin Tim's malady!

Bill Allen