3.2 to 3.4 - must you go twin plug?
 

I dont know if i will go for a 3.4 conversion (need to see what my cylinders look like), but i am researching.

Based on my reading, the 3.2 has non ideal ignition chamber design based on a non central plug location made necessisary due to the size of the valves. Piston design tries to minimize this issue.

Well respected engine builders (or at least 1) advised against a 3.4 without a twin plug setup. Others think it would be fine. My view is that it would be fine (assuming chip tuned), but you would be leaving something on the table without twin plugs.

Thoughts?

update: after feedback and research... the answer is yes and no.

The need for twin plug depends on the compression ratio you require, and the octane level in your fuel, and your advance curve.

compression ratio and timing advance can increase power output. The cost is the risk of detonation in the combustion chamber, which can harm your engine. This can be offset with higher octane fuel. This does not add power, but it slows the burn in the combustion chamber, lessening detonation risks with high compression ratios and advanced timing (which increases power). The higer the latter two, the higher the octane required to avoid detonation, and the more power you get.

To simplify this i will focus on 91 octane, as this is what i am limited to day to day, and what i will build my engine around. US pistons are 9.5:1 compression ratio. This balances 91 octane, and safety given the offset, non-ideal single ignition design in the 3.2. There is little room (i am not saying no room) to safely increase power any large amount (timing/compression) with 91 octane.

...Unless you twin plug. By doubling the number of spark plugs, you are solving the assymetrical design flaw of the combustion chamber. Thus with a more efficient combustion process you get a smoother more efficient combustion, which in turn allows you to push the envelope on power (timing and higher comp ratios) without increasing octane. Think of twin plugging as a "free" octane boost. Twin plug and increase octane, and the possible benefits increase further. 911 race cars, and the later 3.6 engines used this for good reason.


So where does the 3.2 to 3.4 conversion fit into the picture? Well... With a larger combustion chamber (98mm vs 95mm) you are increasing the size of the bang. More room for air and fuel... Thus a bigger bang and more power. This is a double edged sword however, as you increase the distances that the combustion spark needs to travel. Again, the assymetrical spark plugs limit full potential. Twin plugging helps to resolve this by creating two locations of spark origin.

So you need to twin plug to create a safe 3.4? No.

A man by the name of Max Moritz had Mahle design assymetrical topped 3.4l pistons in the 80's. Not unlike the assymetrical oem piston tops, they in effect curve the combustion chamber in attempt to "centre" the single plug. In essence, it allows you to eek out a bit more compression and efficiency out of the 3.4 as compared to a hemispherical or flat top piston. As such they can safely run on 91 octane fuel with a compression ratio of 9.8:1 and reasonable timing advance.

Of note, the max moritz assymetrical style piston types are not an ideal design for twin plugs. So max moritz now and twin plug later would be subopimal, but my knowledge in this area is limited.

Mahle makes a special 98mm wedge-domed piston that was specifically designed for single-ignition operation. It used to be called the Max Moritz piston named after the Porsche tuning shop they were made for.

I've used these for 30 years and they work great. :)

I know this is not an easy answer, but do they just add power proportionate to the displacement increase?

It mostly depend on the final compression you are looking for.

Of course a well twin plug system will give you more performance, but considering the much higher cost involve, I don't consider you are "leaving something on the table".

These add power through displacement and compression ratio increases.

I would be tempted to go 10.3 or 10.5:1. Will research...

Mind you, this is all pending how my cylinders look. But if they need replacing....

Just completed my 3.2 to 3.4 conversion with the Mahle MM P+C's (10.0 CR), 964 cams, SSI's, honed intake and it just dyno'ed at 234 HP at the wheels with stock chip. Single plug. More to come.

That is almost 270 hp! :eek: Stock chip? No tuned chip? What muffler? Octaine? Who did the extrude honing? I have ssi's, m+k 2:1, steve wong stage 2 chip, and plan on 964 cams. I expected 250hp on 91 octane. Intake work and the mahle 3.4's sound tempting as hell.

I might have to increase my budget. And without seeing my cylinders, i am quite sure it is best to replace them... For safety... Yes... safety.

Dyno numbers are a relative thing, vary from one unit to another, but it is a strong motor. Waiting for delivery of a custom chip with a MAF to replace my stock AFM. I decided against the SW chip as I live at 9600 ft and wanted to safely be able to drive the car anywhere without compromising on the AF ratios at high altitude. I don't know who did the extrude honing on the manifold. The muffler is a Dansk sport 2 in, single out. The dyno run was on 91 octane.

Thread on output of 3.4 with twin plug. Crazy power.
http://forums.pelicanparts.com/porsche-911-technical-forum/768929-3-4l-upgrade.html#post7633210


Thread warning of detonation (KTL had 98mm 3.2's in a 3.0). Don't advance the timing too far...

http://forums.pelicanparts.com/911-engine-rebuilding-forum/649778-3-4-mahle-98mm-9-8-cr-dual-ignition-needed.html

The 98mm Mahles are the Max Moritz variety designed for single plug ignition. I am dealing with a very experienced engine builder, but I appreciate the words of caution. I have looked at all of this before I undertook this project.

I suspect sw could adjust for altitude. What type of dyno ( mustang, dynojet)? I wonder if the altitude factored in.

The detonation was a comment more for me than you. What is the word on single plug and the 98mm max moritz and detonation? Is is an issue as it is in the 3.0 to 3.2? I would love to learn from you as we seem to be headed in similar directions.

You don't need dual plug, but why wouldn't you if you are tearing it down to the case?

Even if you install dual plug heads and then dual ignition later, at least you maximized your labor.

You were the one that got me scared. We have 91 Octane. I can mix in airplane fuel, but am not keen on such an inconveniance.

3.4 questions - Rennlist Discussion Forums

Machine now, ignition later is tempting. May research the process/cost next. Mind you, this is all pretty pie in the sky stuff for me at this point...

Good twin plug article...
http://rennsportsystems.com/letstalk-2/gasoline-detonation-twin-ignition/

My postings were/are never meant to scare people; my only objective is to help educate fellow PO's so they make sound, informed decisions about their cars.

These "MM" pistons come in relatively high compression ratios (insofar as pump gas) and need decent fuel to perform at their potential. You can make a 3.4 run nicely on 91 octane fuel, however the timing maps needed to safely run this fuel are less than ideal.

You're right, mixing avgas isn't convenient, but many people do this here in the US since 91 is prevalent in the West. :) :)

I may get some for hpde's, 3.4 or not. Isn't airplane fuel leaded? Or maybe i am misremembering.

What kind of power gains can one expect with 91 and single plug in the 3.4 vs twin plug assuming 9.8:1?

Can't help you with the power gain, but I would offer that if you do not have other sorts of anti-knock protection on your engine management, you could use a J&S Safeguard to protect the engine. I have one and love it in my CIS 3.2.

Dennis

Airplane fuel (Avgas) is definitely leaded fuel, so with a cat this might be a bad option.

Thinking about going the same route for a 3.4l. Talked to 2 well known engine builders here and nobody sees the need for twin plugs.

First was thinking about the 3.6l conversions as many have done, but for my taste it was too hack to install it nicely. The 3.4l conversion keeps the numbers matching (if that is important) and everything else around the motor can stay stock.