Combining Xu9 Cylinder Heads And Blocks

[BXtzi 0]

Dear experts on 205gtidrivers

 

I am restoring a BX TZi configured as it was sold in North Scandinavia and perhaps Switzerland too (DDZ engine with the Fenix 1b monopoint system). I have to change the block of the engine due to a crack that leaks coolant. For that purpose, I bought a D6A engine with the same block as other 8V XU9 engines, but upon separation of the parts I started thinking (dangerous): I have seen posts on combining heads and blocks from XU engines on this site. I have, however, not come across any that combined the cylinder head from a DDZ (most likely similar to D6B?) with a D6A block that comes with flat piston crowns. The head from the D6A engine clearly has a larger combustion chamber volume than that of my initial DDZ engine, which in turn then has the asymmetric, chute-like cut-outs in the piston crowns that I first thought were to the benefit of direct injection engines (to mix petrol and air upon injection). It seems that these differences causes the D6A engine with flat piston crowns but larger combustion chamber and the DDZ engine with non-flat piston crowns and smaller combustion chamber to have about the same compression ratio.

 

So here is my question: will the increased compression ratio from combining the flat pistons of the D6A and the small combustion chamber of the DDZ improve the performance of the engine or will it rather impose problems like 'pinking' and lack of control from the engine management system? I suspect that there will be no problems with flat pistons when it comes to the valves of the DDZ head actually protruding into the cylinder volume as these open, since the valves in this position also reach into area covered by the piston crowns of the non-flat original pistons. Are any of you familar with the effect of this or a similar combination (I speculate that both the head and the pistons that I am talking about here occur in other versions of the XU9 engine and maybe the XU5?)

[petert 586]

If you can control the detonation, use the flat piston with the smallest combustion chamber. For example, it's common here to use a DFZ piston with a D6B/DKZ/XU5M head, which gives high 10 CR. However, without a recurved dizzy or programmable ECU and 98 octane fuel it's a disaster.

 

Valve to piston contact won't happen, unless the belt breaks.

Edited May 19, 2017 by petert

[BXtzi 0]

OK, thank you. I feared that I would end up there somewhere. I shall return to the original piston crowns then.

[Chuchy 10]

If you want to use the standard cam, the flat top pistons gives you too much compression, it will detonate.

 

With the standard cam, the best option are the catalyc mi16 DFZ pistons, you will be around 10/10.5:1.