Great reponse, Ralph! And you definitely have given me something to ponder w/r to the piston selection.

Thanks!

Thanks Ralph. I wondered if that corner made for hot spot or something.
-Chris

I just saw in "Excellence" that Mahle is making higher compression 3.4 P/C's. 10.7-1 due to the demand from engine builders. These are offered through Andial. This may make the 3.5l slightly less attractive. It's a shame they didn't offer these last year with mine. How much gain could you get going from 10.2 to 10.7 and if the 10.7 equivalent to a lower compression 3.5?

I believe Andial (and it is smart of them IMO) is trying to gain some of the market that JE Pistons has inherited almost by default. JE is a forged rather than a cast piston like past Mahle offerings. These new Mahles are also a forged piston to put them on equal footing for those who prefer a forged rather than cast piston for competition.

The other equation to consider is the higher CR these new Mahle's have. I would speculate that 10.7:1 is going to be too high for 91 octane pump gas here in California, even with twin ignition. These piston/cylinder sets I believe are being made to satisfy the race market, not the street enthusiast market that many of us are in. I doubt that you East Coast street guys could get away with 10.7:1 even with 93 octane, but who knows?. I think the general rule of thumb is 1 compression point increase over stock with twin-ignition. That would be about 10.5:1 max for a US spec Carrera.

Unless you are building a race motor, I don't think that these new offerings are the ticket. I don't think that you and I should lose any sleep over it.;)

I wonder how much those babies are going to run!:eek:

Ralph

Just to follow up a bit more on Souk's inquiry, here are the piston shape differences:

Stock 95 mm CIS/Motronic type piston shape

http://forums.pelicanparts.com/uploa...1095906823.jpg


100mm Carb/MFI/EFI piston shape with valve relief cut-outs (for more radical camshaft profiles). Usually available with a higher CR than the wedge dome shape above and may be better suited for twin-plug CIS & Motronic motors than the wedge dome.

http://forums.pelicanparts.com/uploa...1095906896.jpg


For those of you who may have never seen or don't know that there are differences between the two piston types. The real early air-cooled 962 motors used a wedge domed shaped piston like the top image when they were single ignition but than used a piston similar in shape to the bottom image when they went to twin-ignition during their development.

Ralph

My worry is a few years down the road finding cylinders as Mahle is pretty much ceasing production on alot of different piston and cylinder sets. NASCAR is where they want to be and the Porsche market is miniscule for them even though they dominate it.

Ralph
Yikes, is this true?

NASCAR info has merits, has deep pockets. BUT AFTER SEEING A REALLY GOOD PORSCHE EVENT, They'd accept and enjoy it. I've been there. Porsche would kill'em,UNLESS MEETING THEIR OPTIONS. Nascar IS GOOD

10.1 - 10.3:1 compression on 91 octane gas using a mild cam with hardly any overlap?

You're going to be building some fairly high static cylinder pressures, probably near the 180psi range... and with the high head temps on these motors you're just asking for detonation, especially on a warm day. Detonation = death, and on a 3.2/3.5, you may not even hear it until it's too late.

I have to admit, I'm not a Porsche engine guru and maybe there are some mitigating circumstances having to do with combustion chamber configuration/geometry, but consider this:

1) My stock US '88 3.2 has compression in the low 9s and it *needs* 91 octane already, at least on warm days.

2) I have a 10.5:1 compression 347 (stroked Ford 302) in my '65 Mustang that has aluminum heads and a huge cam with lots of overlap and it still needs 94 octane or better to stop the pinging. And it runs head temps that are much lower than my Carrera.

Now with engines like as in life, timing is everything. If you want to use that compression you're going to need advance and you're going to need octane to control the burn. But if you can't have the octane, you have to retard the timing. And if you can't use more advanced timing, what's the point of having the extra compression?

Maybe I've made a fool of myself already here, but all I'm saying is you might want to rethink your compression choice for a street driven car. As your engine ages you'll build up carbon, which will reduce you chamber volume and raise you compression even more. Not good.

Now I love my 347 (of course, I like my 200hp Carrera more ) and it's got an honest 380rwhp... and I ride that horse every chance I get. But having to deal with constantly worrying about it while sitting in traffic and watching the temp gauge rise or having to keep your foot out of it on a hot day can really take the fun away from driving it.

I guess it all depends on what compromises you're willing to make. If I had to do it again, I would have built the 347 with about 9.5-9.8:1, given up 15hp or so and had a much more flexible motor. In your case, if you lower it maybe all you'll be giving up is 7 or 8hp, if that, as increases in compression yield decreasing returns.

Anyway, just my $2.22. Otherwise, it sounds awesome. I'm looking forward to seeing your project move on down the road...

You certainly bring up some valid concerns. Without having some sort of knock sensor to help retard the timing, we are pretty much on our own. All I can base my decision on is my 5 year experience at Andial and speaking with other engine builders who build these types of engines. Everyone that I have spoken with seems to concur that I can get away with up to 10.3:1 on 91 octane, as long as I use twin-ignition. If I were staying with the original ignition, I don't think that I would go any further than 9.8:1.

Some of the Mahle's have built a reputation over the years for not having the same CR as they are advertised to have. I may end up at 10:1 or under yet! My buddie's 3.4L twin-plug ended up that way, supposed to be 10.5:1 and ended up just over 10. We'll find out in the next couple of weeks.

No offense, but for my sake I hope you are wrong.SmileWavy

Now how am I supposed to sleep tonight?

Ralph

Sadly, I'm afraid it is. Mahle does not have that much of an interest in further cultivating the Porsche market on their own, it will be up to the two U.S. distributors (Andial and SSF) to order large production runs of a given piston/cylinder to make them happen in the future.

The new piston/cylinder sets that Andial has released are just that: developed, stocked and sold by Andial and built to their specs by Mahle. I would venture an educated guess that they had to build 10-20 sets of each to make this happen.

When I bought my Mahle 100's with the 23mm wrist pins a few months ago, there were 11 sets left and than that would be it. The good thing for me is, the distributors are apparently only selling 2-3 sets of this particular combo per year so I figure I have a few more years before I had to worry about it (if ever).

The Mahle 98's for 3.4L are definitely more popular than 100's, but the number of units sold is still relatively small and thus these will probably become scarce in the years to come as well. Sorry to rain on the parade.

Ralph

The Mahle 98s have always been in short supply. But just how many sets will you need if the new 3.4/3.5 engines go another 150,000 miles?

Maybe I should stock up for my kid's restoration of my car :)

Hold on....aren't ALL Mahles forged ???

Wil

Exactly. Or maybe tuck away that 3.8RS motor???

Nope. A few of the real early production cars apparently used a forged piston, but most of the production cars and most of the Mahle big bore combo's pistons are cast.

Ralph

Would you mind giving a reference or two on the basis of that comment?

This from the Mahle web site today.

"Weight-optimized pistons for passenger car gasoline engines are cast or forged from high temperature resistant aluminum alloys.

As a result of advanced casting technology ECOFORM® pistons offer low weight and high structural rigidity.

Piston skirts usally have a graphite coating. For application in aluminum bores a FERROSTAN® iron coating or an iron particle-reinforced synthetic resin coating – FERROPRINT® – is applied"

Great thread - any word on the heads/valve guides/valve seals? Sounds like many 3.2's have this same issue.

I will certainly try. The first part of the statement is based on Wayne's engine book, page 55. I myself have not seen a forged piston in a 911 production car. I will be perfectly honest, I don't know squat about the earlier cars (some will argue that I don't know squat period!) and this is why I used the word "apparently" in the statement.

When I was employed at Andial in the late '80's-early 90's we didn't do very much with the early cars. It was all 2.7 to 2.9, 3.0 to 3.2, 3.2 to 3.4/3.5/3.6 and 3.3T to 3.4/3.5/3.6 conversions. These Mahle piston sets were cast, even the turbo versions. In fact, some of the turbo piston applications were simply modified NA pistons. The 98's that you put in your 3.4L conversion should have been cast units if they were from Mahle.

My memory is a bit foggy from the race side, but I believe that the 962 pistons were actually a cast rather than a forged piston as well. The reason why I say this is because the 962 piston had oil drain back holes in the ring lands, I don't know how you could make a forged piston with that feature. Maybe someone who has some more 962 experience can weigh in and correct me if I am totally off base?

Ralph

No offense taken, and I truly hope I am wrong because I'd love to see your setup work. And I have 156K on my 3.2 I am considering either a 3.6 transplant or a 3.4/3.5L twin-plug rebuild when the time comes. It's good to have options. Moreover, if the Andial guys say it's OK, don't listen to me.

I'm also fishing (subliminally of course) to see if anyone would pipe up and give the reasons why this will compression ratio work from a technical perspective. I suspect it will work with 91 octane and appropriate timing because of the double plug setup and rapid heat dissapation of all that aluminum. Generally, aluminum heads on an iron block are good for another point of compression over iron heads... so I suppose it stands to reason on an all aluminum motor as well? Even an air cooled one?

Aren't all pistons (casted or forged) machined after forging or casting? Adding the drain back holes would just be an additional step, right?

As long as we are talking pistons, I read in Excellence this morning that it is common to take some material out of the valve indentations to provide additional clearance when using a higher lift and duration cam. Are the pistions then treated?

Are the piston tops treated from the factory?

That is, are the piston surfaces treated to better endure the heat of combustion?

Don and Jon at EBS and the supplier of my 98 Mahle setup say that all the Mahle Motor Sport products are forged from the early S stuff up to and including the 98s, 100s and above.

Pete at Andial says all their Mahle products are indeed forged also.

I also have a call into Mahle NA to varify that info.

The 962 pistons were welded together and not cast BTW.

Almost all forgings are cut on a mill to final spec. There is a big difference in metalurgy between cast and forgings although both pieces typically need final machining to make a finished part. Some of the Mahle products do indeed start with the same forgings and the same SKE numbers stamped on them. But that does not make them cast.

Than I stand corrected. This is all news to me. Weird though, my buddy at PMNA says they were/are cast pistons and my friend at SSF (the other Mahle distributor) was the former parts manager at Andial and he confirms that they were cast as well. Hopefully Mahle in NC can set the record straight once and for all. Thanks for trying to get right info out there.