3.4 Mahle 98mm 9.8 CR - dual ignition needed?
 

1. Is dual ignition needed for a street car with Mahle 98mm 9.8 CR pistons run with pump gas?

2. The pistons don't look like the CIS and on ones that have a rounded dome that is bulged on one side. Instead, they are flat but slanted from one side to the other. Is the high side installed on the left or right?

In california with 91 octane gas - I'd say yes. But I ran them for years in Florida - and on the track in anger with single plug. The down side with detonation is a pretty big downside.

Sounds like the Max Moritz pistons which were designed to work with single plug ignition. Posting a photo would be helpful.

wullah:

http://forums.pelicanparts.com/uploa...1326168134.jpg

Those are weird looking pistons- the offset valve reliefs. My 98mm Mahle pistons from my recent 3.2 short stroke (SC 3.0L) teardown have deeper pockets (large one is the intake side) and note the tiny pits around the perimeter of the piston

http://forums.pelicanparts.com/uploa...1326175742.jpg
http://forums.pelicanparts.com/uploa...1326175766.jpg

The tiny pits are signs of very slight detonation at the perimeter where clearance between the piston and combustion chamber/head and valve & piston is tightest. Mine has been single plug (Electromotive crankfire) for many years and the engine is a carbureted "race" engine for whatever that's worth. It was typically run on 104 or 110 race gas, occasionally 93 octane pump gas, with a lot of advance (30+ degrees). Twin plug is good insurance IMO.

BTW the "E" on the piston is for the INTAKE side. Yes I said intake because E is German for ....... I don't know. I just know what i've seen in the factory documentation like my recent read thru a 964-993 after sales training manual for engine repair

http://forums.pelicanparts.com/uploa...1326176479.jpg

Those were specifically designed & made for single-ignition and thats how they should be installed.

Steve,

Thanks for the clarification. Do you know why the valve reliefs are skewed like that? It seems like a way of replicating the original CIS & Motronic swirl-dome pistons with a taller dome that has the necessary valve relief.

Sure,......these domes are asymmetrical and the valve reliefs reflect that,...:) :)

Remember, these were made for Motronic engines which have different swirl characteristics than CIS engines and the wedge domes were intended to help improve squish at CR's above 9.5:1 with single-plug operation.

The valve reliefs can be cut a little bit for more camshaft choices.

So, if single plug, then is 91 octane ok for street purposes?

Yes, provided you're not too aggressive with timing.

Thank you for sharing your knowledge.

What are most aggressive camshafts to use with these type of pistons without cutting any material from valve reliefs and 3.2 Motronic intake? 964 or 993SS? And what about after cutting?

964 ones do not require any machine work.

Phew - 7 months for bathroom remodel. About ready to get back in to this. Sorry to beat the dual ignition thing to death, but here are a couple curve balls:

1. this engine is going into my 914 since I was concerned the 9.8 c/r wouldn't pass smog test in my non-cat '77 911. The 914 only has a fan assisted 8x10 aux oil cooler. Before, a stock 3.2 never got over 210F.

2. I will try to get a chip made by Steve or Loren who hopefully can accommodate this config. But really, I don't know what ign timing will be used.

I'd really like to get this engine right the first time. If dual ignition will really help with temps and detonation, so be it. But if it's truly not needed, fine.

You do NOT want to use twin-ignition with these pistons: single ignition only and be conservative about CR given the 91 octane fuel you have.

Steve will know what to do for Motronic software for you,...:)

You need a cooler. Check the calibration between the sender and gauge.

The engine lid needs to be double grill modified. The stock lid has a metal panel and restricts the air going to the motor.

Steve is unreal, we should show our collective appreciation. Wonder what puts a smile on Steve's face! Ideas?

Are these style of piston still available. If so in 95 x 70?

Thanks,

Steve has been very helpful and giving of his time to me. Supporting his business is certainly a good start.

Steve is definitely one of the good guys in the Porsche world. I've only owned my 911 for 14 months and I'm hooked( I'm a former Corvette and muscle car guy). Guys like him and Wayne have really helped my Porsche experience.

Dean

Gordon,

Those pistons were only made in 98mm.