I am not surprised to hear this. The Chrysler 300 and Dodge Charger are the two domestic sedans that I have driven with the "Best" road feel. They have a planted feel that reminded me of my first AMG the C36. Not as good but very similar in feel. They have good steering and the best brakes in their class, all courtesy of Diamler-Benz. Now with Fiat in charge, I wonder what they will have? Best engine note and worst electrics?

As for Woody's informative disertation on engine dynamics and mean effective pressures. He is right on the mark. This sheds some light on things but also causes one to wonder what majic AMG is using? For example:

A stock 3.2 liter C32 or a SRT-6 (195 cid) dynos 287.7 rwhp @ 5850 rpm (from drag times). This works out to 258.3 ft-lbs torque @ peak hp rpm. And using the same equations it calulates the pressure to be 199.8 psi. Sounds pretty close to maxxed out. Now another C32 dynos 328.1 hp at 5900 rpm using a evosport pulley and chip set. Most of this gain is coming from increasing the boost in the intake tract. This C32 is putting out 292.1 ft-lbs at peak hp rpm. Which works out to a calculated pressure of 226 psi which exceeds the F1 engine in the earlier example. Hmmm are we driving grenades? Experience would indicate that we are not.

Still to further confuse the issue, a brilliant fellow removes the supercharger and replaces it with a turbocharger. In so doing, he runs the same boost pressure and thus the same cylinder pressures. Since his turbo is driven off the exhaust gases instead of the crankshaft like the SC'er, the turbo cars output at the rear wheels is greater to the tune of about 60 hp (according the lysholm screw supercharger map). So, his car is making something like 388 rwhp at the same pressure as the supercharged engine that made only 328 hp. So which engine is stressed the most? Neither. They are stressed the same but the turbo engine has more of its power available to do work at the wheels, thus its peak available output is greater.

Now the examples of the F1 and NASCAR engines are both describing normally aspirated engines. They run in the extreme spectrum of performance and are above the 200 psi theoretical limit and they run with little if any safety factor. The normal limits for N/A spark ignition engines is closer to 125-150 psi.

In the examples that I am using, the C32/SRT-6 is a forced induction engine. The theoretical maximum of 200 psi applies to the N/A engines, but how does it apply to the forced induction engines? It is a valid question. In fact the theoritical maximum for forced induction engines is different. A range of 180-250 psi is not out of the ordinary. But, I suspect that the calculation of pressure should be made using the crankshaft numbers before any losses are subtracted for such parasitic functions like driving the supercharger on a SC'ed engine. Because we already know that the pressures in the cylinder of a turbo or SC'ed engine will be the same at the same boost pressure (all things being equal) despite the specific output dyno numbers being quite different.

Soooo, with that in mind a turbo engine making 388 hp from a 3.2 liter (195 cid) displacement at 5900 rpm has a BMEP of: 267 psi. The same engine running the same boost with a supercharger but with 60 hp of parasitic loss to drive the SC'er off the crank will dyno 328 hp at the same rpm, more or less and will also experience the same mean effective pressure of 267 psi. This can be achieved with relative reliability because of the fact that a good bit of the pressurization is taking place outside the engine by the turbo or supercharger and that all is fairly gradual compared to the events during the compression stroke. So, is a 3.2 liter turbocharged engine that makes 428 hp at 6000 rpm on the verge of self destruction? Probably not. But there may not be that much more left on the table either.

For reference, at the end of WWII forced induction pistion engine technology was reaching its practical limits. Rolls Royce built a version of the Merlin V12 that was running 36 psi of boost on 150 octane fuel. That engine had a BMEP of 404 psi. Modern modified racing versions of those engines are stressed even further. My long winded point is that the maximum BMEP for FI engines is not the same as for N/A engines.

One last extreme example was the BMW 4 cylinder 1500cc turbo used in F1 in the 1980's. They only needed to survive for one race and were stressed to the limit. They could make 1000 hp in race trim or as much as 1400 hp for qualifying or for last lap passes by turning up the boost. They were limited to 11,200 max rpm. In race trim, they were running a BMEP of about 773psi and over 1000 psi for qualifying!

The 3.2 liter SC'ed V6 used in the C32 AMG and the Chrysler Crossfire SRT-6 was rated 349 hp at the crank and using the SC'er map this was subtracting ~45 hp to drive the supercharger, so the total output was closer to about 394 hp @ 5850 rpm. That is crank hp not wheel hp but the engine sees the same pressure eitherway. These numbers work out to a BMEP of about 273.5 psi (HP x 792,000)/(CID x RPM)=BMEP. Those numbers are above the so called normal range of 180-250 psi for forced induction engines but is well below the theoritical maximum assuming the Formula 1 turbo was at that or a bit above it. Calculating the theoretical max in this case is more complex becuse it depends on how much of the compression is taking place outside the cylinder before the compression stroke as well as intake air temperatures and density, etc, etc. It is complex. Bottom line however, it this. Forced induction is magical. It enables the engine to perform more work with less displacement. If you boost the intake pressure to one atmosphere (14.5 psi) you essentially double the torque output. If an engine makes 349 hp at 14.5 psi boost using a supercharger the same engine will make more hp with a turbocharger at the exact same boost. The difference is the amount of hp required to drive the supercharger. Spinning the supercharger faster to make more boost increases the parasitic loading and that is not a linear function. In general with our engines, the added load to increase boost from 14.5 psi to 18 psi (24% increase) is 33%. I am generalizing here cause I am too lazy to look up the supercharger maps. Point is that you cannot get unlimited boost with the SC'er because there is a point where the loads go off the chart.

Back to the cylinder pressure discussion... Regardless of what the BMEP is, it will be the same for both the Supercharged and Turbocharged engine if the boost is the same. That is why most FI engines use turbos. The supercharger is fun cause it makes boost instantly and at low RPM's and that makes it very practical for everyday use but if peak output to the wheels is the goal, then the turbo is the way to go. It is laggy and harder to modulate by the driver but once on the boil and making boost, it will accellerate faster than its supercharged counterpart. A logical question is then why to top fuel dragsters only use supercharged engines. Simple, the time it would take to spool up the turbos is too long and they would be too hard to modulate and drive at the outputs involved. Most top fuelers are not horsepower limited, they are traction limited. They can make more power if/when track conditions allow them to put it to the ground. I was once told that it would take a street hemi engine (426 hp) to drive only the supercharger of a typical topfuel drag engine.

This only scratches the surface of this subject but points out nicely that if the AMG engine is reliable at 18 psi boost using the supercharger it will also be reliable at 18 psi boost using the turbocharger. Most of the gains are coming from the reduction is parasitic losses at the crank and some possibly from improved intake charge cooling since that is a very weak link in the AMG C32/SRT-6 design. I am impressed by the elegant design and the results!

Irish