Nightshade

A DI 327 (I'm assuming that's the CI of the 5.3 anyway) would actually be pretty sweet. It wouldn't eat gas quite as much as the Ls3, would have the muscle car sound to it. And if this guy's calculations are right, it would kick out around the same power as the Ls2 on the concept Camaro.

AdioSS

a high compression direct injection 4.8L with AFM and VVT would do too well on fuel economy...

Capn Pete

?! ?! It would be too good on gas??? Or did you mean it would NOT be good on gas??

Before we found out about the ~300 HP 3.6L V6, a small displacement V8 seemed to make sense. Now that we (think) know we're getting a 300 HP V6, I don't think a small/low HP V8 is really necessary. So there will just be a ~100+ HP gap between the V6 and the V8 (and ~100+ HP again from the "base" V8 to the "uber" V8 ).

AdioSS

I was being sarcastic. A small displacement, very efficient, high torque, highly responsive V8, etc would just make too much sense for GM to produce... The 4.8L V8 is already SAE Certified to 295hp/305tq with only a 9.1:1 compression ratio. Direct injection and variable valve timing would allow the compression ratio to go over 11:1 which could give another 50horsepower up top and probably another 30ft-lbs down low.

And with the ZR1 going into production, there is more than a 300horsepower gap between GM's base V8 and the "uber" V8

Capn Pete

Gotcha!!! I was confused!!! (now I understand ..... why do something that would MAKE SENSE ).

teal98

What do you think the fuel economy difference would be between a 4.8l and a 6.2l V8?

To me, it's illuminating that when GM went for maximum fuel economy with the hybrid Tahoe, they actually installed a 6.0l engine (running the Atkinson cycle IIRC).

Grape Ape

I think that the reasons that the 6.0 was used probably include:

• Retaining a descent tow rating after adding 400 lbs, because otherwise why not buy a minivan or mid sized SUV?
  
• The Atkinson Cycle gives you fuel efficiency at the expense of low end torque.
  
• A smaller V8 might have lacked the guts to push the Tahoe hybrid (almost 3 tons) around in 4 cylinder mode.

AdioSS

More than the difference that put a big hold on the RWD Impala program...

AdioSS

That ALL makes a BUNCH of sense.

teal98

Which is?

AdioSS

according to Bob Lutz, the difference between FWD and RWD is 1 MPG.

I know that the 4.8L doesn't get very good mileage, especially compared to the larger engines. However, the 4.8L is only available in heavy trucks and SUVs. The 4.3L V6 and the 5.3L are the mileage leaders of the GM trucks now.

Personally, I see some flaws in the 4.8L compared to the other engines. The specific output for the 4.8L is right on par with the larger engines despite a lower compression ratio. In order to do this, it has to turn higher RPM. Generally higher RPM needs higher compression ratio to work well. If the 4.8L were to get the 5.3L aluminum block, higher compression ratio, and AFM, then I don't doubt that it could get better fuel economy. How much? It's really hard to tell. How much did the fuel economy numbers increase for the iron block, non-AFM LM7 to the current LY5? That is also hard to say because there have been several other fuel economy achievements built into the trucks.

teal98

Right. We really don't know how much it would improve.

So when you talk about how much sense it would make to produce a 4.8, you really are just guessing.

My observation is that merely reducing bore and stroke seems to make a minimal increase in mileage, while cutting power significantly. You'd have to fully optimize the engine for the new displacement, and that would be expensive, as it would essentially be a new engine.

AdioSS

Then explain to me why smaller engines are used at all? Why not use the largest engine you can physically fit into a vehicle?

teal98

Reread what I wrote. If you optimize the engine for the displacement, then you can see the gains you're looking for. One of the best ways to do that is to decrease cylinder count.

Given that, why offer two displacements on the same engine? Marketing.

Still, the reason that Mercedes, for example, offered the C250, C280, and C350 when all got about the same mileage was so that they could charge extra for the top model. It's the same story with the BMW 325i and 330i (from 2001 to 2006 in the U.S.). Note that with the current gen 3-series, the 325i and 330i both had 3 liter engines.

One of the big gains previously in efficiency with smaller cylinders (and the same count) was larger throttle openings, reducing pumping losses. But with technologies like GDI and valvetronic (BMW's name, but Audi and Infiniti have similar systems), pumping losses at small throttle openings are already greatly reduced.

besz28

the showcase vehicle will always be the corvette