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Old 05-07-2022, 01:30 PM   #1 (permalink)
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Bu - '08 Chevrolet Malibu LS
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'98 Trans Am 40mpg

Hey all,

I bought a 1998 Trans Am with 4 speed auto for cylinder head development, and I'm doing an mpg project in the interim. The key points are decreased pumping losses, increased thermal efficiency, and lean burn.

I'm spring boarding off of Legit Street Cars' 40mpg Corvette. While his engine was basically stock, I'm taking it much further.

The Corvette has lower CdA and 6 speed manual (12% lower final drive), which is certainly an advantage over the TA. actually has the auto TA getting 15/23 vs the 6 sp Corvette at 16/25 (his stock car got 30 mpg @ 65mph). I predict the mods will be worth 30+ mpg highway running stoich, 40+ with lean burn.

To reduce pumping losses I'm looking at a 212* cam on a 118* LSA. This will reduce pumping greatly, while keeping vacuum high for good fuel vapourisation. Overlap leads to low vacuum, which causes misfire, which just dumps fuel out of the tail pipe and kills fuel economy. It should also work fine with 87 octane, which is 20% lower fuel costs vs 91.

I have 1-3/4" headers and will get a free-flowing cat-back exhaust.

I'm using 706 truck heads to bump compression to around 11.25:1, which is a 2.7% increased Brake Thermal Efficiency over 10.1:1. The heads have been ported to increase air speed. This improves mixing and atomisation for a more complete and faster burn. I should be able to remove several degrees of timing for less negative work. The preliminary engine sim is showing around 470 bhp (with no knock) and low .400 BSFC.

Alex talks about lean burn and how it raises NOx emissions. Sort of. If you run 1.1 lambda, like he does, this is true. Most charts end there, but if you keep going leaner NOx (and temps) plummet. I would like to go much leaner (24:1). A key enabler is a strong ignition system capable of 140+ mJ. Stock coils only provide around 80 mJ and ICE Ignition LS1 coils with their 10 Amp booster will generate 142. This will come later.

I should have the car up and running mid-June.

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