[hotlap]

Originally Posted by Denis

i mean i guess that answers that. im sure GM had a hp # they wanted and found that it was attainable with a 1.7. if not they would have gone with a 2.3.

The detailed write up about the LT4 engine development did say packaging was the reason for the smaller 1.7L blower.

SUPERCHARGER AND INTERCOOLER
In order to fit the supercharger within the confines of the C7 Corvette and still meet pedestrian safety regulations for vehicles sold in Europe, an intercooler design like the one found on the LS9 wasn’t going to work. There had to be area under the hood available for the bodywork to deflect and absorb impact before hitting anything solid. To keep the sleek lines of the C7 Corvette intact and still give the driver a great view of the track ahead, engineer Dan Hommes was tasked with removing 3 inches of height out of the supercharger and intercooler assembly without losing airflow or power. In engineering terms, that’s quite a challenge, as the LS9 was already very compact.

Dan Hommes was responsible for making the LT4’s supercharger/intercooler/intake assembly as compact as possible without impeding airflow. With the lid off the assembly, you can see the intercooler heat exchangers. Cool water comes in to the front port, it’s split to each side, and runs to the back and comes forward, so heat absorption is equal from front to back. Because water is better at heat absorption, the intercooler circuit runs 60 percent water and 40 percent Dexcool for better heat rejection. In comparison, the engine coolant runs a 50/50 mix.

In the LS9, the intercooler heat exchangers are in the lid, the only place to put them because the Eaton TVS 2300 supercharger rotors take up a lot of room in the lifter valley. The rocker covers weren’t going to change, so that meant the supercharger couldn’t get any wider. The solution was to use supercharger rotors 10mm smaller in diameter. The existing TVS 1650 rotor size was not quite large enough for the power levels the LT4 team was targeting. To get more air volume, Eaton built a longer set of TVS 1650 rotors that displace 1,740 cc per revolution, a size unique to the LT4. The smaller-diameter rotors mean less air per revolution, but the reduced mass and inertia allow them to turn 34 percent faster than the 2300 rotor set. In the LS9, max supercharger speed was 15,080 rpm; on the LT4, the rotors spin at 20,150 rpm at max engine speed. An added benefit of the smaller supercharger rotors is that less torque is required to drive them.


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