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

[fallingat120mph]

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

[PeteKz]

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.

[fallingat120mph]

Originally Posted by PeteKz

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

[targa72e]

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.




For twin plug the piston dome is symmetrical.



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

[fallingat120mph]

Originally Posted by targa72e

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.




For twin plug the piston dome is symmetrical.



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

[MoreGAS]

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

[brighton911]

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.

[mepstein]

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.

[Peter M]

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.

[Tad240]

Originally Posted by mepstein

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.

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)?

[mepstein]

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.

[silverlock]

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.

[fallingat120mph]

Originally Posted by brighton911

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.

Thanks for response...appreciate it.

Originally Posted by mepstein

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.

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!

Originally Posted by Peter M

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.

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.

[Glenfield]

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.

[Peter M]

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.

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!

[mikedsilva]

Originally Posted by Peter M

I though 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?

[Jeff Alton]

🍿🍿🍿

[Kiddwayaa]

Post pictures of your pistons. There are different designs.

[Cloggie]

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.

[Cloggie]

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.