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GM Performance Parts LS3 Engine Build Part 3 - Late Crate Update

Want A Crazy, 900HP Street Car? Check Out The...

By Richard Holdener, Photography by Richard Holdener

With our 417 now ready for boost, we installed the turbo kit and proceeded to tune the combination at various boost levels. Using the manual wastegate controller (basically a bleed valve that tricks the wastegate into staying closed until a higher boost level is reached), we started out near 6 psi of boost. What was obvious from the first run was that the 67mm turbo was a tad on the small side of this application. Truth be told, we agreed to test this small turbo on our stroker to determine the maximum power it would support. Turns out that at 6.5 psi, the 417 produced 766 hp and 815 lb-ft of torque. The 67mm turbo was obviously a bit small for this powerful stroker, as the boost rose to a peak of nearly 9 psi then fell off rapidly. The response was impressive, but the 67mm turbo is more at home on a stock LS3 (or modified 5.3L or 5.7L). Despite the smallish turbo, the kit offered gains that exceeded 200 hp and 250 lb-ft of torque. This should give you and idea of how impressive turbo kits are on LS motors. Simply bolting on the turbo system improved the power output of the modified LS motor more than the heads, cam, and stroker kit combined. Such is the beauty of exhaust-driven forced induction, as it is entirely possible to more than double the power output of your motor, assuming all systems are capable of supporting the revised power level.

The small turbo worked well, but we had our eye on the larger 80mm Precision Turbo. After a cool-down period, we replaced the small turbo with its larger cousin. Now things were starting to get going. We were happy to have the control offered by the Turbo Smart wastegate, as the engine dyno seems to artificially load the motor and create boost control problems. The Turbo Smart wastegate kept things under control and allowed us to dial in the desired boost level. As expected, the larger 80mm turbo offered slightly less response than the smaller 67mm, but boy howdy did it make power. The 417 easily eclipsed the 900hp mark, but unfortunately, we were not able to break the magical 1,000hp barrier. The turbo kit and turbo itself were more than capable, but our 60-pound injectors called it quits at 913 hp at 12 psi. With no larger injectors at our disposal, we were forced to call it a day and officially put an end to the engine dyno portion of our LS3 upgrade series. Look for the LS3 stroker to be installed in the engine bay and run in a future issue, but know that the combination of inches and boost makes for one serious small-block.

This graph illustrates two important aspects of the turbo kit: Turbos offer huge power gains, and the 67mm was (as indicated by the guys at HP and Precision Turbo) indeed too small for this application. Sized for a stock LS application, it managed to produce 766 hp and a whopping 815 lb-ft of torque. But, it is evident from the shape of the curve that the flow rate was falling off at the top of the rev range, as both boost and power production fell past 5,500 rpm. If you are looking to make 750 hp, this is a good turbo, but if you need more, you'd better step up to something larger.
The larger 80mm Precision turbo, made more than 900 hp, but we ran into another limiting factor. Despite increasing the static fuel pressure, our 60-pound injectors were no match for the demand, and we were forced to stop at 913 hp and 925 lb-ft of torque. The turbo and accompanying kit were clearly capable of supporting more than 1,000 hp, but we'd need larger injectors (75-pounders) to do so. Keeping the tune conservative, we were able to run roughly 12 psi of boost to reach the goal of 900 hp.

Comp Cams
3406 Democrat Road
TN  38118
Probe Racing
2555 West 237th Street
CA  90505
Chevrolet Performance Parts
P.O. Box 33170
MI  48232
E3 Spark Plugs
Ponte Vedra
Demon Engines
Sante Fe Springs
Dr. J's Performance
436 S Montgomery Street
CA  92868
Lucas Oil
302 North Sheridan Street
CA  92880
HP Performance
301 E 4th St.
NM  88201
Precision Turbo & Engine
616A South Main Street
IN  46341
3400 Democrat Road
TN  38118
L&R Automotive
13731 Bora Drive
Sante Fe Springs
CA  90670
Procomp Motorsport
By Richard Holdener
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