Dynamic Daily: Dr. Jekyll Meets Mr. Hyde

By Richard Holdener/Photos By Author

This is how you make your mild, carbureted 4.8L Dr. Jekyll becomes Mr. Hyde.    

When it comes to power plants for an enthusiast vehicle, most guys have one of two distinct types, wild race car or mild daily driver. In truth, many have both, as the cantankerous race-car motor hardly makes for a reliable, or sometimes even streetable, daily driver.

Dr. Jekyll started out life as simple 4.8L LR4 from the local wrecking yard.

The same can be said of the daily driver, as a stock motor can certainly be relied on to get you safely from point A to point B, but stock just doesn’t stir the soul. Since no one wants to detune their race motor, we decided a better option was to show you how to improve the performance of an LS-based power plant designed for a daily driver, all without sacrificing drivability or reliability.

If we think of the race car as the notorious Mr. Hyde, the daily driver can be likened to the more timid Dr. Jekyll. While you never want to go full Hyde on your daily, there is nothing wrong with him making an appearance now and then, just to keep things interesting. A word of caution is in order here, as that Hyde character has a way of taking over, and pretty soon that daily driver ceases to have any resemblance to the good doctor. You were warned!

Though a daily driven, Dr. Jekyll comes in many forms, ours came from a local wrecking yard. Right off the bat, we decided the crusty, high-mileage motor was to receive carburetion, rather than the factory fuel injection. Even today, there are a great many guys out there who simply can’t tune a factory or stand-alone fuel injection system, but can change the heck out of the jets on a carburetor.

To get the motor up and running, we combined an Edelbrock Victor Jr. intake with a 4-barrel carb and plug-in ignition controller. Using these components with long-tube headers and an electric water pump, we were able to get the tired, 4.8L LR4 up and running on the dyno. Run with the stock truck cam, we tuned our way to peak numbers of 336 hp and 317 lb-ft of torque. In truth, our first choice was a dual-plane, Performer RPM for this little 4.8L, but we had none available in time to dyno test. Undaunted, we at least had a baseline for our Dr, Jekyll daily driver, but what this little motor really needed was a healthy dose of hyde.           

Unless you have been living under a rock for the last decade or so, you already know what’s coming next. The go-to performance upgrade for any LS is ALWAYS a new camshaft, but rather than go for huge power gains (which are possible), we decided to treat the good doctor to something on the mild side. What we wanted was a cam that offered sizable power gains while maintaining factory idle quality and drivability. Leafing through the Comp Cam’s catalog, we located the perfect patient, in the form of a part number 54-408-11. The mild Comp cam offered a .515/.522 lift split, a 206/212-degree duration split and 112-degree lsa. These specs put the cam on the Dr. Jekyll end of the spectrum, but fear not, Mr. Hyde will come later.

The cam was teamed with a set of 26918, beehive springs. The stock valve springs on the 4.8L had obviously seen a combination of endless mileage and minimal maintenance. We swear the oil looked like it was the original that came from GM when the motor rolled off the assembly line. It’s a wonder the little LS even ran. Equipped with the new Comp 269LRR cam, the 4.8L produced 377 hp and 337 lb-ft of torque. The cam offered peak gains near of 40 hp, all without sacrificing idle quality or any low-speed torque.

Equipped with the mild Comp cam, the power output of the 4.8L was certainly up, but not into what anyone would consider Mr. Hyde territory. What the combination needed now was some push-button power. Nothing wakes up a motor like a healthy shot of nitrous, and the best thing is that the power gains have no effect on drivability.

The mild-mannered Dr. Jekyll can cruse around and make house calls, and have Mr. Hyde appear only when summoned. To facilitate the transformation, we installed a Zex Perimeter Plate nitrous system on the carbureted 4.8L. The Zex Perimeter Plate featured 12-equally spaced dispersion holes to ensure proper distribution to each cylinder. No sense in having one cylinder doing more work than another.

The fuel and nitrous were supplied to the plate from the solenoids. Positioned in line between the solenoids and the plate were jetting to dial in the amount of nitrous (and fuel) supplied to the motor. The extra power supplied by the Zex Perimeter Plate was adjustable up to 300 hp. Though we had no intention of hitting Dr. Jekyll quite that hard, we did install jetting for an extra 125 hp! Remember here, if a little Mr. Hyde is good, a lot isn’t always better-after all, it’s a daily driver!        

To ensure adequate fuel delivery, we made sure to dial in the fuel pressure supply to the fuel solenoid. We also took the liberty of heating the supplied nitrous bottle in the old Westech warmer. With bottle pressure near 1,000 psi, we were assured to get every bit of the desired 125-hp worth of nitrous flow.

Equipped with 125-hp jetting, the power output of the nitrous injected 4.8L increased from 377 hp and 337 lb-ft of torque to 507 hp and 532 lb-ft of torque. In truth, the peak was more like 502 hp after the initial activation power spike, but the Zex system came through with almost exactly 125 extra horsepower.

Per the instructions, we also dialed the ignition timing back by 4 degrees, but ran all testing on 91-octane, premium unleaded pump gas. The cooling effect of the -129-degree nitrous oxide has a positive effect on the inlet air temp and actually helps to suppress detonation. With a hot spark coming from the FAST coil packs, we were immediately rewarded with peak power numbers of 507 hp and 532 lb-ft of torque.

If you take a look at the supplied graph, w you will see that these numbers came at the initial activation spike, but the gains settled in at an honest 125 hp over the naturally aspirated combo. Equipped with the cam and Zex nitrous, Mr. Hyde is now free to come and go as he pleases, just make sure not to let him take over and install a bigger cam and that 300-hp jetting!                          

Cammed & Carbureted 4.8L-NA vs Zex Nitrous (125-hp shot) How can you not love nitrous oxide after seeing power gains like this? After upping the power output of the carbureted 4.8L from 336 hp to 377 hp with a Comp 259 cam, we hit the mild-mannered Dr. Jekyll with 125-hp worth of Mr. Hyde. Even with the new cam, the motor idled like a stocker, but after activation of the Zex Perimeter plate system, the power output jumped from 377 hp to over 500 hp. Thanks to proper bottle and fuel pressure, the jetting designed to provide an extra 125 hp did just that! Now your daily that behaves like the good doctor can still shows its wild side once in a while.