This question has been fairly worked over in the  "CI Vs. SI" thread in this forum and by the "Air Fuel Ratio of CI" thread in the Powertrain Engineering forum.

Check them out and see if they help.

Rod

See the References Rod mentions.  But, In simple terms....

Torque comes from burning more fuel during each power stroke or engine cycle.  The limit on fuel is related to how much air can be induced into the cylinder for each engine cycle.  How do I get more air... Turbo charge or super charge.  Most diesels are turbo charged so they have more torque.  Gasoline engines are limited with turbo charging because of knock.

Power comes from torque times engine speed.  If you can rev an engine faster, you can typically get more power.  But running higher engine speeds leaves less time for complete combustion.  SI engines can rev higher than diesels because of the combustion is faster, due to two reasons.  First, ignition with a spark is quicker than ignition by compression.  Second, SI engines run premixed air/fuel charges that burn fast.  This is compared to CI engines where mixing occurs during combustion, significantly slowing the combustion process.   

In addition to what is mentioned previously, you must remember that CI engines have a much higher compression ratio.  Higher compression ratio gives more work output per cycle, which translates to higher torque.  SI engines would use higher compression ratios as well, if they were not limited by knocking.  This is why racing SI engines often use compression ratios exceeding 12:1.  However, they require high octane fuel and operate at high RPM, where knock is less of a problem.

Everybody is pretty much on target here, but I will give you something to think about in this torque/hp/CI/SI thread.

500 ci SI engine---90+ percent nitromethane fuel---5000+hp @ 8500 rpm  and they burn 3 to 5 gallons in FIVE+/- SECONDS!

Not too many dyno's around capable of testing these engines and fewer people willing to use up a $100K engine just to see what they will do.  

I am not a drag racer (since my 20's) and I don't know too much about these newer Hemis, but I am overwhelmed by their ability to turn in the seemingly impossible performances they do.


Rod

evelrod,
  I know what you mean. E.T. and top speeds thought unattainable 20 years ago are now posted at every meet. However, what is even more amazing is the use of 4 or 5 of these engines on one tractor in tractor pulling events. Talk about excess!

I was watching the tractor pulls the other day and with the big diesel engined tractors they said the wheelspin was getting up to 300mph... That kinda makes me wonder what would happen if you put that motor into a fullbodied car...


-=Whittey=-

I don't think those wheel speeds are reached until the load on the tractor is huge though. So I am not sure how you would load the tires that much on a full bodied car.

That is a good question... How much load is there on a drag engine? I'd imagine quite a bit... Considering that motor doesn't spin too fast, you'd have to use some pretty large gears, which would increase load.

Then again, how much load is the engine recieving when it's spinning that fast? The tires on a tractor puller are shaved so they don't get too much grip, but i'm sure there is still a large load. But is that large load large compared to a grippy 12" slick on a sticky track? Sure there isn't as much weight being tossed around, but that can all be made up with acceleration.


-=Whittey=-