Mahle Pistons intended for Carbs/MFI vs Motronic
 

Guys,

I'm working on the list of parts to use in the build of my twin-plugged '87 Carrera, and I need some guidance regarding pistons. I'm aiming for a reliable, street performance engine (in that order). I'd like to maintain Motronic for fuel & spark for now, but I may upgrade to Carbs or ITBs down the road. There are two Mahle 98mm piston options I'm eying up:

1. 10.4:1 static compression. Bruce Anderson writes that these are intended for "Carbs/MFI"

2. 9.8:1 static compression. These are intended for Motronic.

I'm running twin plugs and have access to 93 octane, so I'm comfortable running either. Can anyone speak to the issues/risks of running pistons intended for Carbs/MFI on a Motronic setup? If I'm thinking about going to Carbs or ITBs down the road, which route would you recommend? Are JE pistons closer to Mahle's Carbs/MFI pistons in terms of design?

If I use 9.8:1 static compression pistons and THEN move to carbs/ITBs down the road, I believe I would definitely be leaving power on the table relative to the 10.4:1 pistons.

I'm interested in hearing your thoughts. Thanks!

I see no reason to not use the higher compression pistons. You will probably find they only have the rated compression ratio at "zero" deck height. With CIS (78-83) there may have been a reason to use the CIS dome pistons. With Motronic, it shouldn't matter.

-Andy

The wedge dome Mahle pistons (aka "Max Moritz" ones) were optimized for moderate compression and proper squish with single ignition. Using twin-ignition results in uneven and improper flame fronts and that affects power as well as the propensity for detonation.

Remember, ignition advance requirements becomes assymetrical when using two plugs with those pistons and that's a problem unless one can program two simultaneous advance curves for the top & bottom plugs.

With twin-ignition, you should be using the RSR style pistons that have double valve reliefs and symmetrical domes.

Steve - As always, thank you for sharing your knowledge.

Would you say that running twin-plugs on a "Motronic" or "Max Moritz" style piston presents more risk than running Motronic EFI on a "Carb/MFI" style piston?