http://www.jiop.fi/ksimuseo/faq_bf109evogc.htmlhttp://www.jiop.fi/ksimuseo/faq_bf109evok.htmlhttp://www.messerschmitt-bf109.de/php-frames/merkmale-frame.phpThe K-1 through 3 remained projects only and the only K to see serial production was the K-4. The DB 605D engine was standard. This featured the enlarged supercharger of the DB 603 engine first used on the DB 605AS series that was used in the G-6/AS and later the G-14/AS series. The D featured and enlarged oil tank behind the propeller, redesigned electrical components, a relocated cold weather starting position on the right cowling, and larger cylinder heads. It also used a wider-bladed propeller than the AS series. It standardized the use of MW-50 which was feed directly into the supercharger intake. (GM-1 was never used on the K-4 as the enlarged supercharger of the 605D precluded its necessity). The fuel could be 100 or 87 octane depending on the use of MW-50 and what fuel was available. C3 and B4 are the synthetic versions of 100 and 87 octane. The engine could be altered to use either fuel.
The use of the larger supercharger caused the left main engine bearer to curve up and over the supercharger. In common with the earlier G-6/AS and G-14/AS and the G-10, the K-4 used the "refined" cowling to streamline the bulges caused by this engine bearer and the ammunition feeds for the cowling mounted MG 131 guns (used beginning with the G-6). Various styles of this "refined" cowling can be seen on the G aircraft caused mainly by the fact that all those were rebuilt aircraft from different factories and the method of dealing with the "refined" shape was accomplished differently. The K-4, since it was built as one production group, seems to have the same shaped bulges throughout its production run. Again, in common with the G-10 (and some G-14/AS aircraft), the K-4 had the small bulges under the forward cowling. This was caused by the rerouting of oil cooler lines around the larger cylinder heads of the DB 605D engine.
Three very visible differences in the K-4 compared with the G series are:
The relocated radio access hatch on the left fuselage side (higher and forward).
The relocated D/F loop from fuselage frame 3 to frame 4.
The use of a retractable tail wheel concurrent with the use of the longer shafted tailwheel.
Another visible difference was the use of a small, retractable landing gear cover for the outside of the wheel well. This had been contemplated as early as the F and G, but finally made its appearance with the K-4. Apparently these tended to give problems when used on the grass airfields the Luftwaffe used, so ground crews often removed them.
