3.2 top end rebuild progressing to a 3.4 - basic questions.
 

So...have a spare 86 3.2 that was in need of a top end rebuild. Was lucky enough to stumble across a Mahle conversion kit (on the cheap) and have decided on going that route.

So, as basic as basic can be, from research it seems I have a few different avenues. The work is being done by a great shop, but for my personal knowledge had some questions...

I am currently in the midwest, in the sticks, so to speak, and 91 octane is a standard, not like where I used to live in the Hill Country and 93/94, 91 non ethanol, was everywhere.

So, from a stock 3.2 that will mostly run on 91 octane here is where I am at...

What I have:
1. Mahle 98mm kit
2. 964 Cams
3. M&K bypass and muffler with original heat exchangers...would love custom headers but need good heat.

What I think I need/want(?):

4. Steve Wong Chip?
5. Twin plug?


The question has come up about twin plug vs single plug but to my understanding this isn't needed.

I am open to twin plug, not quite sure what is needed at this point, and wonder if the extra is worth it (of course it is, he says to himself).

I see a lot of threads from 10-15-20 years ago and was hoping someone could chime in with a real time or very recent take.


Engine is completely broken down and being cleaned for the top end and just wanted to get my ducks in a row for the next phase.

I see the comments on fuel quality come up quite a bit in the rebuild threads and that is why I ask.

So, with lady luck gifting me the conversion kit, just wanted to get others take on what is the simple and best answer. I drive my 911's daily and do a few cross country trips each year and just want to keep it simple with this engine upgrade.

I have another 3.2 that might become a track engine...this one the 3.4 conversion seems perfect.



Thanks for any insight...I have been scouring the 3.2 to 3.4 threads, from the main 911 site, everyday the past few weeks, and thought I might come here for a direct answer.

Again, thanks for your input-


Erik

Re twin plug: I looked at this several times, and decided to avoid the additional cost, maintenance difficulty, and complexity (i.e., more crap to go wrong). What does a twin plug gain? Maybe 10HP due to a slight increase in CR (holding other factors constant)? Unless you're going racing, not worth it, IMO.

My thoughts also, especially with this particular engine and how this car will be used. Very tempting with the engine completely broken down, but...


Thanks-
Erik

You should post pictures of your pistons. There are different designs. Pistons designed for single plug have a offset dome to concentrate the charge near the spark plug.

http://forums.pelicanparts.com/uploa...1741542760.jpg


For twin plug the piston dome is symmetrical.

http://forums.pelicanparts.com/uploa...1741542825.jpg

You can run symmetrical dome with single plug and offset with twin but they will not be optimized.

For a 3.2 it relatively easy to add a 964/993 distributor and a go twin plug. Several venders offer splitter solution to take signal from ECU to two coils.
Advantage of twin plug is you can run a little more compression safely. Plus a little more power. You also don't need to run as much timing as the spark is starting at each side of the combustion chamber. This becomes more important as the bore gets larger.

john

Thanks! Mine are at my shop/garage but are the first photo you show. There is such a back and fourth about twin plugging' these things...just trying to get more info and make the "while you are in there decision".

Erik

Unless your compression ratio is higher, general rule of thumb is one point so call it 10.5-10.8 :1 range is a goal, you will not gain any power. Make sure things actually get CC'd (measured to actually calculate compression) as often stock pistons aren't where they are published to be.

We commonly twin plug heads on race engines just-in-case we later want to change the engine config.(just fit a spark plug to fill the hole) and thus we do not have to disassemble much of the engine to accomplish this later. These are often high compression race engines, running Race fuel, so that aspect is done. I haven't done a back to back single vs twin comparison yet where something else didn't change also. It would be an interesting study though.
A couple highly experienced engine builder folks who've had dynos for decades I consider authorities shared they think on a race engine as I described, both high compression, race fuel, it will gain nothing. Hmmmm . Your case however, yes w 1 pt higher compression.

Kevin
GAS Motorsport

I'm running JE 98 mm pistons at a little less than 10:1 with 964 cam grind, single plug heads and a SW chip. Never an issue with any detonation, it's all good.

We built this twin plug (964 dizzy) 3.4 for a customer. With more aggressive cams than 964 grind, it’s a ripper. Power keeps building beyond 7k.

http://forums.pelicanparts.com/uploa...1741554207.jpg

Plenty of speculation presented and not a lot of fact, with the same "Old Wife's Tales" trotted out.

Read Steve Weiner's post here based on first hand experience and lots of dyno time:

https://forums.pelicanparts.com/911-engine-rebuilding-forum/1033435-twin-plug-not.html

My 3.4 picked up 15rwhp running twin plug over single plug with no change in compression (measured 10.4 to 1)

However the problem with trying to run twin plugs with your proposed set up Erik is that it will be costly to remap the Motronic and install either a 964/993 distributor or a special 12 lead distributor.

If you were proposing to run an aftermarket ECU like a Haltech or Motec it would be a no-brainer to go twin plug as the extra cost of 6 plugs and COP's plus a bit more harnessing is just loose change when rebuilding these engines.

Cams
 

Hi Mark-
Can you share the cams /grind that you used; or the specs? Assuming that they worked well with the Motronic (with appropriate tuning)?

A custom chip for the motronic is a couple hundred and trading a 3.2 dizzy for a 3.6 is almost a wash - maybe another hundred or two. A twelve plug dizzy is a couple thousand but unless you are doing ITB’s, the Carrera manifold is noticeable long before the 964 dizzy. Besides, the engine is for driving, not winning a beauty contest. No one modification will add gobs of power to a 3.2 but if you have a good recipe and take care of all the details, you can make a really fun engine that blows the doors off a stock build.

We're sticking with single-plug on my 3.2SS build. Running Mahle 10.2:1 PP98-014 pistons, 993SS cams (so dynamic CR around 7.8:1), etc. Running 93 but not a hyper-aggressive tune. Know a lot of folks with similar single-plug builds.

Going twin (machine shop work, dizzy, etc) was effectively the stopping point in terms of $$ + time. 250rwhp will be plenty.

Thanks for response...appreciate it.


Me and my mechanic have been digging in about more aggressive cams. I am just collecting as much info as I can now...thanks for chiming in!




I don't want my image here, on the forums, to be money no object, but this engine is completely torn down and I feel the "while you're in there" is at full force. With a full top end rebuild and new full exhaust it starts getting to the point of what's a few thousand extra...

I will be at the Hill Country Rallye next week and hoping I can find an owner with the 3.4 mod...

If I can find someone and get the "what do you regret NOT doing?" that would be great.

Thanks, really appreciate it.

Do all the oiling mods and put a bigger oil pump in (or have your pump massaged by one of the experts or at the least confirmed good). Certainly go with ARP rod bolts on a 3.2 crank and balance rotating and reciprocating components. I think that’s the min. Next level would be aftermarket rods but that could mean custom pistons and now you’re into it… but you’d have a bottom end that can support more down the line.

I dunno on twin plugging. I probably would. Seems like a worthwhile exercise even at conservative compression. Fuel may only get worse.

I thought I laid out your choice as factually as I could Erik and also took the opportunity to address some misinformation you had been given too. There are certainly lots of happy 3.4's owners with single plug Moritz pistons and remapped Motronic ECU's on this forum so I don't think you will be disappointed.

The biggest regrets I have with my 3.4 build are:
1) Not carefully planning out my build before I started buying parts. Consequently I had to backtrack and on-sell some new parts as my plans changed. This cost money and delayed work.

2) Not buying a GT3 crank when they were still affordable!


However I certainly DON'T regret the expense of dumping the Motronic and going aftermarket ECU (Motec M130) and twin plugging among other things so I don't think you can go wrong really!

Curious what the goal of using a GT3 crank would have been Peter?

🍿🍿🍿

Post pictures of your pistons. There are different designs.

I am in the process of building an engine similar to this.

My spec is:

Mahle 98mm P/C's with the higher compression dome
Turbo 74.4 mm crankshaft
Dougherty DC 35 cams (effectively 964 with a bit more overlap)
Big port heads - dual plugged
Turbo oil pump
Carillo rods
Eisenmann 42mm heat exchangers with Cargraphic exhaust with dual cats and automated cutout
PMO ITB's
Bosch CoP's
MoTec M130 engine management system including the 964 bridges for knock sensors

It is just being assembled as we speak.

Concept I was seeking was a free breathing engine with reliability as the primary goal, particularly on autobahn where you are at high load for extended periods of time.

Absolute power was not a major goal, figured the 3.4 litres on a fairly light backdated chassis was going to be plenty of entertainment for an old guy like me.

D.

I am in the process of building an engine similar to this.

My spec is:

Mahle 98mm P/C's with the higher compression dome
Turbo 74.4 mm crankshaft
Dougherty DC 35 cams (effectively 964 with a bit more overlap)
Big port heads - dual plugged
Turbo oil pump
Carillo rods
Eisenmann 42mm heat exchangers with Cargraphic exhaust with dual cats and automated cutout
PMO ITB's
Bosch CoP's
MoTec M130 engine management system including the 964 bridges for knock sensors

It is just being assembled as we speak.

Concept I was seeking was a free breathing engine with reliability as the primary goal, particularly on autobahn where you are at high load for extended periods of time.

Absolute power was not a major goal, figured the 3.4 litres on a fairly light backdated chassis was going to be plenty of entertainment for an old guy like me.

D.

Sounds like a nice combination and will provide you exactly what you're after.

Ah, Mike, hope you are well!

As for GT3 cranks, just a half formed daydream that a little more stroke would nice.

The reality is that I rarely drive my 3.4 nowadays and if anything, I really should redo the muffler with something more free flowing.

I generally fritter away my time with old motorcycles nowadays. Just finished the resto of my V7 Sport and have started tidying up a Katana:

http://forums.pelicanparts.com/uploa...1741637483.jpg

Now that is a build. If I was back in Germany this is what I would do also...

Hope you post the outcome, here or another thread.



Erik

Thanks to everyone posting so far. I am heading out from the Midwest to the Texas Hill Country Rallye tomorrow (March 12, 2025) and won't be back for a week. Hoping to see some unique set ups at the event and get a better feel for things, 3.4 wise.


Been some really great input and I will keep posting as my build progresses. I have two 3.2 engines and this build I may go with the basics but with the info provided I have a lot to research for the next build.

Thanks again to everyone at this point.

If not for my knowledge hopefully a thread like this will help others down the road...



Erik

Points all taken for sure. I just didn't want to be a presumed owner who just throws money at everything for a fix/build that I had no part in. I work on all my cars but this is new territory for me so getting some shop help, especially as I am in the process of moving across country.

The regrets are what I am most interested in. As we all have come to hate the phrase "while you are in there", I feel I was very much at that crossroad in real time.

I will be happy with any conversion as I am learning a lot, but I hear a lot of the "what ifs" and "should have done this" when it comes to rebuilds and mods.


Appreciate the responses!
Erik

Thanks for chiming in. Read this thread, albeit older, about the DC35's:

https://forums.pelicanparts.com/911-engine-rebuilding-forum/1039196-964-vs-dc35-cam.html

Just some engine porn to keep the faith in those waiting to get to this point.

Builder is letting me do a few things - today's task is replacing valve seals.

He also has picked up various deficiencies in some parts such as the Victor Reinz aluminum washers for the case are too soft - getting them replaced with steel and deformed nuts to replace the nylocks.

The ARP through bolt kit is very good....very nice and will last forever.

Had to clearance the Turbo pump to clear the Carillo rods. Oh yeah, bearings are all good - measured out fine and have the Calico's


http://forums.pelicanparts.com/uploa...1742408931.jpg
http://forums.pelicanparts.com/uploa...1742408945.jpg
http://forums.pelicanparts.com/uploa...1742408945.jpg


D.

The more threads I read on the 3.2 to 3.4, the more apparent it becomes that there really are two different worlds that most of the street cars are living in:

93 octane & No Cat/no smog -or- 91 octane + Cat/smog (I’m in this group)

I’m building a 3.2 to 3.4 with 964 cams, single ignition and stock exhaust & DME, with some mild head work to bring the CR down. Not expecting a lot regarding performance gains.

Is there anyone here who has consistently passed Cal smog with a 3.2 / 3.4 and a modified chip?

Good stuff!

Well, the 3.4 slightly used set didn't work out after careful checking...few items were not too far out of spec but neither I or the mechanic were comfortable with things...had hopes, but...

So ordered a new set of Mahle P&C and will be going forward with twin plugging plus a lot of other things...

Just ordered, and am lining up cam work.

Since I am completely doing a 180, so to speak, will post as progress moves forward.

Was okay with "keeping things simple" but guess that ship has sailed!


Erik

Quick question as I am trying to research and getting a better grasp on the big picture of my new direction/build.

Through the years I see the "oil pump" and "oil cooler" upgrade and am wondering for your build if it's a necessity or just an upgrade? I ask this respectively and am just trying to study/read at what point in a 3.2 to 3.4 build an improved oil cooler or oil pump is needed, if at all?

Just reading through B. Anderson's book (Porsche 911 Performance Handbook) over the weekend...which might be more dangerous than helpful ;)




Thanks everyone...

Erik

What brand nose bearing is that? I didn't know they made them without the notch at the top?

Hi there Mike, love your channel by the way.

I just checked over the parts list and it was a Porsche bearing....now, I bought it around 2007, so things may have changed.

D.

PS: Engine is now safely back in my garage, just trial fitting all the ancillaries and figuring out how all of this goes together. Been running into dumb stuff like forgetting to rebuild the throttle plate, so off to the bead blaster and plater go those parts

http://forums.pelicanparts.com/uploa...1744929125.jpg

Ok, slowly making progress, large PITA with the modification of the engine tin to deal with the engine moving 25 mm to the rear...that stuff is absolutely the worst sheet metal on this planet.

The wiring harness is all fabbed up along with the mounting plate for the Motec dual lambda CANBUS splitter, the MAP and the exhaust cut-out valve. Had to fiddle with the mounting of the manifold for the vacuum ports and the heater tube was a ***** with the Eisenmann/CarGraphic + cats + exhaust valve combo.

Now I am fighting putting the air horns into the air cleaner, how the hell are you supposed to tighten up those 8 mm nuts that retain the horns and the lower air cleaner housings?

There is absolutely no room for a nut driver, wrench, socket or whatever...any ideas? What's the trick?


Anyway, here are the latest pics...going out to put in the vacuum takeoff for the cutouts including check valve plus some minor parts to be bolted on.

http://forums.pelicanparts.com/uploa...1752794599.jpg


http://forums.pelicanparts.com/uploa...1752794881.jpg

http://forums.pelicanparts.com/uploa...1752794881.jpg

D.

K Nuts (AKA Jet nuts) offer unusually small heads on a flanged nut. Install them with a 1/4" universal socket on an extension.

http://forums.pelicanparts.com/uploa...1752798256.jpg
http://forums.pelicanparts.com/uploa...1752798256.jpg

I will give that a go....thank you so much!

Douwe

Henry, your suggestion was spot on. Bought the swiveled socket just like your picture - I did not know such things existed and the nuts went on quite easily.

Air cleaner now fitted, probably will cut down the snorkel a bit, although Grok tells me there is no material difference in air flow between the 57 mm diameter as it is versus the 77mm diameter I want to cut it down to.


http://forums.pelicanparts.com/uploa...1752888701.jpg


D.

Grok is an idiot.
That snorkel will work as an air restricter much like the restricters used on racing engines ti limit horse power output.
Keep in mind that the air filter housing your using was designed as a silenser on engines making less than 180hp.
Here are two ways we've handled the issue of restrictive air flow.

http://forums.pelicanparts.com/uploa...1752904520.jpghttp://forums.pelicanparts.com/uploa...1752904878.jpg

Thanks again Henry. It seemed a little counterintuitive as that hole in the snorkel looks awfully tiny compared to the 6 x 46mm throats on the PMO's.

I will probably cut the snorkel down to just before the 90 degree bend starts - it rains lots here and I sorta think a little bit of snorkel will help keep some of the water out.

Its plastic anyway, easy to cut and form....

D.

The math calculation is pretty simple
Venturi 46mm=1662mm2 x 6 = 9,972mm2
The snorkel looks to be about 60mm = 2827.4 mm2
Of course the CFM of the engine is not in direct correlation with venturi size.

Something else we've done is to cut out the opposite end of the filter housing, moved the breather hose attachment and welded in a screen. the noise level in the car goes up but the engine can breath.

I would save the intact snorkel and look for a snorkel from a 72-73 US MFI. They are already short. Intact intake parts are getting harder to find. Maybe someone is looking for your snorkel for their concours project.

http://forums.pelicanparts.com/uploa...1752943130.jpg

I tested the effect of the inlet on the dyno. It is a restriction. You can see the results in this post

https://forums.pelicanparts.com/porsche-911-technical-forum/1151593-dyno-summary-testing-early-2-4l-2-5l.html

John