Just because we could do something, does that mean we should?
….Like adding a carburetor to an M90 supercharger?
I know what you might be thinking: What’s the big deal? Gearheads combine carbs and blowers all the time, just look at the dual-quad, a 6-71 or 8-71 blowers, right? Well, yes and no, as those blowers do lend themselves to a carbureted induction system, mostly because they were designed to be top feeders.
The M90 in question for this test—however—was not!
It was originally designed by GM to supercharge their very popular 3800 V6, fuel injected motors. Because the blower was designed for EFI usage, this meant the blower housing had no top-feed provision and was instead, fed airflow (only) through a rear-mounted throttle body. Sure, we could have just attached tubing and a carb adapter to the original throttle body (we did, later), but this exercise was all about taking the wrong approach and adding a carburetor to the top of the blower!
Can you really weld up a junkyard M90 blower and add a carb? If you could, does that mean you should? (Image/Richard Holdener)
Why Mount a Carburetor to a Blower?
The decision to mount a carb on the blower came about from the theory that running fuel through the blower should simultaneously help and hurt performance.
On the plus side, the fuel should help lower charge temps, which it did, dropping IATs by as much as 60 degrees with the 2.6 inch pulley. The other potential benefit was the throttle opening itself, as the four inch tubing offered greater flow potential than the stock 70mm throttle body opening. The final plus was that the fuel might help seal up the rotor-to-housing clearances to further improve efficiency.
On the minus side, flowing fuel through the blower decreased potential airflow, meaning the blower flowed less air because it had to flow both air and fuel. The other potential downside was the 650 cfm carb—how did the total flow of the carb compare to the 2.93 inch throttle body opening?
We also wondered about the airflow coming through only a portion of the rotors with the carb, while flowing from one end of the rotor pack to the other through the throttle body.
In the end, we decided to move forward and let the dyno decide how well it worked (or didn’t).
Carburetor-to-Blower Modifications
To mount the carb on the blower, it was first necessary to have a hole on top of the blower. In truth, we (JT Fabrication) first welded the four inch section of tubing to the top of the housing, then (using a hole saw) drilled out the opening inside the tubing.
Even being careful and moving back and forth on the welding areas, the thin and porous aluminum blower casting experienced warpage. We expected as much, the question now was, how bad was it?
As it turned out, the combination of high spots and warpage eliminated the possibility of the rotor pack even being installed back into the housing—let alone spinning and providing boost. The cure was to spend some time with cartridge rolls and blue Dykem.
Little by little, we managed to first get the rotor pack back into the housing, then rotate a little, then more, then all the way around. In the end, we added a little more clearance for good measure, since the combination of rotor speed and temperature would change the running tolerances.
While there was little concern about replacing the cheap, junkyard Gen 3 M90 blower if damaged, we didn’t relish the idea of all that debris finding its way into our aluminum 5.3L test motor.
The Carbureted Blower Test
For our blower test, we decided to first run the M90-supercharged 5.3L in EFI form, then with the custom carb combo. We ran another Gen. 3 M90 equipped with the stock (size) 3.8 inch pulley breathing through a 92mm throttle body (mounted in front of the stock throttle body).
Run in this configuration, the EFI M90 combo produced less than one psi of boost on its way to 425 hp and 489 lb.-ft. of torque. This would jump up near 490 hp once we installed the 2.6 inch pulley. After running the EFI combo, we installed the carbureted M90 (also a Gen. 3) and were rewarded with 426 hp and 392 lb.-ft. of torque.
The carb’d M90 offered slightly more power than the EFI, but the real story was that it actually worked—we had our doubts!
After adding more blower speed with the 2.6 inch pulley, the peak numbers jumped to 490 hp and 499 lb.-ft. of torque. Only belt slippage with the smaller pulley stopped us from topping the 500 hp mark.
During testing, we also ran the M90 with a carb feeding the back of the blower, and even combined carb and EFI tuning to help eliminate the lean load-in condition with the carb.
After welding and a great deal of porting and prep work, we were finally able to run the M90 supercharger in carbureted form on the LS. Thanks to the Super Richie adapter plate, we were able to bolt on the junkyard M90 blower from a 3800 V6 (adapters available for both L67 Gen 3 and L32 Gen V blowers). Run first in EFI configuration (on 91 pump gas with 21 degrees of timing), the supercharged combo produced 425 hp and 389 lb.-ft. of torque. The carb added a few extra horsepower compared to the EFI, with peaks of 426 hp and 392 lb.-ft. (at less than one psi), but things got better for the Gen. 3 blown LS when we installed the 2.6 inch blower pulley. With a falling boost curve (seven psi down to 4.25 psi), the supercharged, carbureted LS produced nearly 500 hp and 500 lb.-ft. of torque. Only belt slippage limited us from topping 500 hp. (Dyno Chart/Richard Holdener)Already a good starting point, the 5.3L L33 LS engine test mule was yanked from a local wrecking yard. Complete with 799 heads, a slightly hotter cam (compared to the pedestrian LM7), and truck intake, the all-aluminum, 5.3L produced 365 hp and 389 lb.-ft. of torque when run on the engine dyno in (near) stock trim. (Image/Richard Holdener)Prior to our testing with the carbureted M90 blower, the L33 was upgraded with a slightly hotter cam. The Elgin SS1 cam offered. 0.560″ lift, a 216/220 degree duration split, and 114 degree LSA. (Image/Richard Holdener)To supplement the camshaft upgrade, the L33 was also sporting a set of (BTR-ported) Trick Flow GenX 220 heads. In addition to the porting and milling 0.030 inch, the heads also received a BTR Extreme RPM valve spring package. (Image/Richard Holdener)Obviously the supercharged motor would require good exhaust flow, so we installed a set of 1-7/8 inch, long-tube American Racing headers feeding simple collector extensions and Magnaflow straight-through mufflers. (Image/Richard Holdener)We normally run the motors with a Meziere electric water pump, but the blower required use of the factory accessories, including the water and power steering pumps. (Image/Richard Holdener)Though we usually run ratty, old stock valve covers on these junkyard stock bottom end motors, this one received these BTR logo valve covers with AN breather fittings and custom coil brackets that allowed removal of the valve covers without having to first remove the coils—helpful for cam swaps. (Image/Richard Holdener)Because the Super Richie adapter plate was designed to work with the Holley intake, we installed the Holley Hi-Ram. (Image/Richard Holdener)Next, we bolted the Super Richie adapter plate to the Hi-Ram. This allowed installation of the junkyard M90 supercharger. (Image/Richard Holdener)We ran a variety of different pulleys with the carburetor, the smallest—and most powerful—of which was this 2.6 inch pulley from ZZ Performance. (Image/Richard Holdener)First run in EFI form, the Gen. 3 M90 was equipped with an LS 92mm throttle body feeding the (wired-open) stock 70mm (Gen. 3 3800 L67) throttle body. (Image/Richard Holdener)Tuning for each combo (timing only on the carb) came via a Holley HP management system. In EFI form the 5.3L was equipped with 80 pound Accel injectors. (Image/Richard Holdener)Run on the dyno in EFI form, the supercharged 5.3L produced 425 hp and 389 lb.-ft. of torque, though the blower only managed to provide less than one psi of boost with the 3.8 inch pulley. This would jump up to near 490 hp with the smaller 2.6 inch blower pulley. (Image/Richard Holdener)The first order of business to run the carb on the M90, was to weld on this four inch aluminum tube and then hole saw the center opening. It was not possible to weld on an aluminum carb spacer—but it would have made life easier. (Image/Richard Holdener)Unfortunately, welding the thin, porous housing resulted in warpage and high spots that prohibited even installation the rotor pack, let alone rotation once installed. (Image/Richard Holdener)After hours of marking and less-than-careful grinding, we finally got the rotor pack installed and rotating freely. With no way to measure it accurately, we wondered what was going to happen once the rotor pack was subjected to the combination of speed and temperature? (Image/Richard Holdener)We installed a Holley 650 XP carb onto a Wilson tapered combo spacer mounted on another section of tubing and flat plate adapter we happen to have on hand—used previously as a throttle body adapter. (Image/Richard Holdener)Once the fuel lines were hooked up, the carb combo fired up immediately! After some jetting (lean on the roll in, but rich overall), the combo netted 426 hp and 392 lb.-ft. of torque with the 3.8 inch pulley. This jumped to 491 hp and 499 lb.-ft. of torque with the 2.6 inch pulley, though belt slippage limited the real peak power potential. Still, the carb’d M90 worked—but not without a lot of work! (Image/Richard Holdener)We also ran a carb feeding the back of the M90 (through the throttle body). Using a simple section of tubing and our carb adapter, we wired open the stock throttle body and mounted the carb using a coupler. Interestingly enough, it made the same power as the carb on top! (Image/Richard Holdener)
Richard Holdener is a technical editor with over 25 years of hands-on experience in the automotive industry. He's authored several books on performance engine building and written numerous articles for publications like Hot Rod, Car Craft, Super Chevy, Power & Performance, GM High Tech, and many others.
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