Thread: V8 fuel economy
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Old 01-06-2015, 11:37 AM   #52 (permalink)
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I would have found this sooner if it was in the correct sub-forum.
Messing around with the LSx combustion chambers isn't a thing where just anyone can make measurable improvements in their garage. Milling the heads on a 4.8 will get you up to 10.0:1 SCR, but putting 4.8 pistons in a 5.3 with milled heads will get you up to 10.5:1. Still not that good.
GM did one R&D 'vette with a AFM LS2 that did 35 MPG, then didn't put that exact combo into production.
But you need to get the cranking compression around 200 PSI with the most SCR you can, this will let you spec a healthier cam to help with the 800 HP goal.
A 5.3 can't pull 65 MPH at much less than 1300 RPM in a 4th gen f-body, and a 3.25:1 axle is likely to have you considering swapping in a 2.97:1 first gear in your T56 even with turbocharging, for driveability's sake.
Those 315/35 drag radials aren't enough to hook 800 HP, and they're too wide for any MPG. Put some 225s on the 16x8s that GM put on the non-SS, there's your MPG rubber.
Toyo's 345/40R17 drag radials are cheap and sticky, but are so wide that pinching them onto your 11-inch wheels will give you no more contact patch than you have now. You can get some 9.5s widened, but then you need mini-tubs, offset trailing arms, and more. Better to just get real wrinkle-wall drag slicks for when you aren't after MPG.
Trying to have this cake and eat it too would be easier if you sell the Camaro and start over with a Miata.
Running 10.5:1 with boost is no problem if you polish the chambers, polish the piston crowns, and have a small fuel cell for race gas for when the turbos spool up. Better to try 2 small turbos than a big single for what you're after.
50 MPG is mostly about aero drag, and your 'maro has way too much of it.
I'm guessing you're probably running 275/40s on 17x9.5s on the front for some cornering ability, which is the Camaro's advantage over Mustangs, but the Miata will pull more Gs, meaning take the same curves faster, on narrower tires, further increasing its aero drag advantage. It's narrower, and that's most of its other drag advantage. A Miata being lighter means you don't need so much power to run 9s in the quarter, so the 275s that a Miata can fit without mini-tubbing it mean less need to have slicks.
There are plenty of aluminum-block 5.3s out there, and these have proven adequate to 1500 HP with no detonation ever. They're a couple hundred dollars more than the iron-block, but necessary for such a goal.
If you must, you can put the 4.8 crank and rods in to make an aluminum-block 4.8, so long as you don't start with the LS4 version, you need the truck version, not the FWD car version.
You can spec a cam that'll increase your cranking compression, and still make more HP than stock, and pull past 6500 RPM. The specs aren't what most would guess. But the turbos are there to make the HP, they can't make the cranking compression at cruise. The cam can't make the desired HP without the turbos. So don't choose a HP cam to keep the boost pressure down, just dial up the boost with the cam the MPG requires.
Once you save what you want out of the Camaro, the shell should sell for enough to buy an unloved Miata, but you may have to search far and long.
Lastly, if you could make a 502 big block pull 65 MPH at 500 RPM, it still wouldn't get the MPG of a 125 at 2000 RPM, even with peanut-purt or Feuling center-fire heads, or ThunderPower heads with half the ports blocked. It's too far below the engine's efficiency range. so the 4.8 is a good start, and it can hold big power for cheap, but there's a guy on Fiero - Pontiac's Mid-Engine Sportscar that has a turbo L67 3.8L doing low 9s and doing MPG well into the 40s. So abandoning the LSx entirely is worth considering. Copy FieroX's L67 into a Miata, and do it cheaper than your Camaro will sell for as is.
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