Viper et al, thanks for all the info here. I got here from a Google search and have learnt a lot. Kudos!

Because the German engines typically had higher compression ratio's....

http://www.nightrider.com/biketech/h...ompression.htm

This is another advantage to direct fuel injection and by product of a more accurate fuel metering system.

http://www.twminduction.com/faq/faq.html

An Engines rated power is a percentage range over a manufacturer's guaranteed mean. That means it has considerable variation even on the same assembly line.

Aircraft engines are built to very tight safety margins like all aircraft components. It is not like a ground vehicle where you can overbuild using large safety margins.

Stress aircraft components beyond what the engineers allow for in the Operating Instructions and they will break. It is the engineers that set those instructions based on the work an airplane is designed to perform. Fighters are built for top performance in order to destroy enemy fighters for example. There is no wiggle room in that.

If you do your research, you will find that all of the "boost increases" are accompanied by technological advances that maintain the same minimal safety margin.

To hold an aircraft specifically designed for a world record attempt as anything representative of any type of service or engine designed for normal operations is ludicrous. It is apples and oranges from which you cannot draw a single conclusion about what goes in front of the firewall on a practical aircraft. It is like drawing conclusions about capability of the car in your driveway engine by examining a top fuel dragster.

Those airplanes are specifically designed to achieve one goal and nothing else. They don't care if the engine last's 15 minutes if it only takes 5 minutes to set the record.

Many of these record breakers were completely impractical designs for anything other than that attempt. The Me-209 for example was a stability and control nightmare and was impractical for anything more than a few minutes flight of rather risky flight. That is typical of these "record breakers".

I enjoyed your post Viper2000. Fascinating.

Any one know why the German engines were so lightly boosted? Was it a design decision or the quality of the fuel they had?

Such a complicated and interesting subject it is hard for me to understand late at night... For a second I wondered if the Merlin engines superchargers had a ratio of 2.7 because that is the natural log "e"... then in a moment of inspired confusion I imagined that was also why they have optimum RPM at 2700! hehehe
I'm going to bed...

Because the German engines typically had higher compression ratio's....

This is another advantage to direct fuel injection and by product of a more accurate fuel metering system.

My idea is that it is most directly a result of their large displacement. Power developed is more or less a function of the amount of fuel burned, you "boost" (use higher pressures to inject more oxygen) to allow more fuel to be burnt. In a bigger displacement engine you simply do not need greater boost, you still get the power - the larger volume engine can take more air (=can burn more fuel) without having to resort to heavy supercharging. Even 87 octane could probably take as much as 1.6-1.7 ata, and a result practically lineally increased power output, but what's the point if the stress is too much for the engine components and the engine will have service life of 20 hours..?

Take for example the early Merlin and DB 601 at their normal 87 octane ratings.

33 liter x 1.3 ata boost pressure = possible 42.9 air volume
27 liter x ca. 1.65 ata boost pressure (roughly equivalent to +6.5 lbs/sqinch) = 44.55 air volume

So you can see they probably take in about the same amount of air, the Merlin simply makes up for smaller displacement with supercharging.

Now compression ratio was also different (6:1 on merlin, 6.7 on the DB, and later DB engines increased this to 8.5:1, the Merlin stayed the same), meaning that a DB piston is doing relatively more work, all things equal. In the end you get ca. 1000 HP on both engines, though the Merlin uses a bit more fuel for that.

The reason for that is two-fold: a, Higher CR of the DB b, the strong supercharger on the Merlin consumes more power from the engine than the DB, and that power is taken from the engine - which provides it by burning fuel (which goes to 'waste' ie. driving the s/c and not the propeller). c, More accurate fuel metering of direct fuel injection, as Crumpp noted.

Keep also in mind that power outputs are NOT primarly limited by boosts and the quality of fuel. Typically the fuel allows for higher boost pressures than used in service, but it is limited by the manufacturer because the engine components can't take the stress of the so developed power with an acceptable engine life. For example very early Merlins using 100 octane were limited to +9 lbs, but later ones could take +18 and much greater output on the same fuel.

Absolutely, there is no substitute for displacement in terms of power production.

That is what makes the article Viper posted and I quoted/linked so interesting.

It uses piston speed instead of crankshaft speed to attempt to compare the engines and fuel metering of the Rolls Royce single point injection vs the German Direct Injection systems under the same conditions. The article unfortunately does not compare the float type carburetor used during the Battle of Britain. If it did, there would be an even greater difference.

The result is at the same piston speed, the direct injection has a lower specific fuel consumption at any given brake mean effective pressure. That means it produces more power for a given amount of fuel.

It also shows an interesting graph comparing the Merlin IF it had the same compression ratio. At the same compression ratio, it is equal or better than the Jumo engine. Of course that is IF it had the same compression ratio...which is pure speculation but interesting from an academic standpoint. If frogs had pockets, would they carry pistols and shoot snakes?

A simple fix that is not so simple. Simply raising the compression ratio of the Merlin is as complicated an engineering problem as those who ask "Why didn't they just increase the wing area to lower the wingloading?"