Open Road

I learned something interesting today. we all know that the 928 has oiling issues, and that the oil packs in the cylinder heads. I know part of the reason. The drains in the upper block are mismatched to the drains in the lower block (girdle) with the holes in the girdle being smaller than the drain holes in the upper block. these are specifically the drains on the right side of the engine.

The drains in the upper block are approximately 26mm 12mm rectangle with rounded corners. One of the drains is an odd shape and it was too small by 2mm in all directions. The rest of the drain difference ranges between 2-5mm in width and height. One of the holes was 9.5mm by 21.5 ! Most of this is due to core shift when casting the block, but also I think there were some flaws in the castings due to the casting tech at the time. I've got about a day's worth of labor cleaning up all of the casting flaws on the lower girdle. But that is me doing everything by hand except the clearancing for the larger rod caps on my new rods.

The pressure-side is also mis-matched: the block-side gallery is huge, the head-side is about 2/3 the area.

its not just the hole sizes. its the location. the drains are partially offset, which impedes draining significantly. I opened up the lower drain parts with 2 goals improve flow, and create smoother transitions for the offset. Having a larger oil feeder get smaller might be by design to increase pressure but having a flat edge instead of a transition isn't good for flow. At some point the pressure is so high that the roughness of the transition doesn't matter I guess.

maddog2020 wrote: ↑Sat Aug 20, 2022 3:35 pm its not just the hole sizes. its the location. the drains are partially offset, which impedes draining significantly. I opened up the lower drain parts with 2 goals improve flow, and create smoother transitions for the offset.

Seems like one of those "it can't hurt things."

maddog2020 wrote: ↑Sat Aug 20, 2022 3:35 pm Having a larger oil feeder get smaller might be by design to increase pressure but having a flat edge instead of a transition isn't good for flow. At some point the pressure is so high that the roughness of the transition doesn't matter I guess.

Lord knows there's enough oil pressure to the heads.

I think it will help a lot more than "not hurting anything" One of the drains was almost 1/3 blocked. the rest were around 20% blocked, and offset.

I got a chance to look at an 88 block it it appears they resolved the issues for the S4 and later engines. I've also seen pics of the GTS blocks that had a lot less flashing, so they did improve the block over the years.