When I sent my heads to EBS in the way-back, the cost was not prohibitive. Don't remember exactly, but not a back breaker financially.

Just talking. I had a cylinder head sensor port machined into my #6 cylinder. I think it was $60, 1997 money.

trakrat ---

1) more detonation clarification -- believe the key here (and I'm not an expert) is preventing preignition of the fuel, which is exacerbated by the longer path the spark-ignited fuel needs to travel in a wider, larger displacement cylinder. Hotter spark may help a bit, but what you're fighting (and what twinplugging will most effectively fight) is spontaneous ignition of the fuel at the far side of the cylinder before the sparkplug-ignited fuel lights it off -- spontaneous ignition at/before the piston reaches tdc is BAD.

2) compression -- octane is a measure of a fuel's ability to resist knock/preignition. Euro engines have higher compression because european gas formulations generally have a higher available maximum octane level than the US (requiring lower compression to avoid knock/preignition)

3) weight -- unclear why this is an issue. A 3.4 liter engine will provide room for more fuel/air to collect in each cylinder, resulting in a bigger explosion pushing the piston down (and therefore more power / torque). Component weight may come into play in some manner, but the justification for a bigger engine is a bigger "bang."

Unfortunately Bob... the heads are on their way back to me after getting everything done to them.
Besides that... adding all the costs to setup a twin plug (new distributor, etc...) makes things get expensive quickly.

It's not that I "won't" go to a twin plug... its just that I'm not ready. I've only driven the car for about 4 months before I had to rebuild the engine... and once that's done.. I'll ne to rebuild the suspension... and fix rust... etc...

I figure, since I'm rebuilding the engine and have to buy a new set of P&C... I thought I'd go with something slightly better than what the car came with.
But maybe that's not an option? It seems that if I don't get twin plug... then I should admit defeat and face the reality that I should just buy another set of factory 3.2 P&Cs?

Call EBS racing and see if they can source you the 98mm Mahle Max-Moritz style pistons you need. They can machine your stock cylinders to fit.

emcon5 -- good advice -- seems the key here is to go with a proven, middle of the road, setup and there are plenty of single plug 3.4l engines out there. Go with what others have successfully used in the past and don't try to "reinvent the wheel" -- Trackrat, also keep in mind that once done, you'll need to get a chip mapped to provide proper afr to your now bigger/thirstier engine

Agree.

Would be cool to have that as a hip pocket option down the line with the machine work already done.

Do call EBS and just yak with them. I like the 3.4 option. Don knows his stuff. Jon is the boss but I don't think he works the phones like in the way back.

trakrat -- if you're committed to running 100 octane gas all the time, you should have zero issue going with eurospec pistons (or much higher compression 3.4 pistons than would ordinarily be run on a car running on pump gas) -- 100 octane has much higher anti-knock properties than 93 octane pump gas.

I echo Bob Knotak's suggestion of discussing this with somebody knowledgeable about this particular issue -- somehow missed your comment about your being dedicated to running this motor on 100 octane gas exclusively

i see you referring to what Porsche did. they did not do things based on 100 octane fuel.
no one has considered the fuel you plan to use so I don't think much of the advise has been helpful.

100 octane opens the door for you.

start with fuel as I said,
figure out engine size, CR and cam based on the fuel.
with 100 octane you might be better of with a 3.2, higher CR and a cam that will work with it.

Big Bore Pistons
 

When I built the 3.3SS for my SC, I used a set of Mahle 100mm pistons. They were supposed to be 10.1:1 but wound up closer to 10l8:1, probably due to the heads being cut one or more times. These pistons have a nice, low, hemispherical dome that is less likely to block the flame travel. I am using twin plug ignition, but if you stayed under about 10:1 premium pump gas should work ok. Some pictures:

http://forums.pelicanparts.com/uploa...1536151553.jpg
http://forums.pelicanparts.com/uploa...1536151577.jpghttp://forums.pelicanparts.com/uploa...1536151652.jpghttp://forums.pelicanparts.com/uploa...1536151731.jpg

Twin Plug ignition..........
 

I used the Electromotive XDi twin plug, crank fire system. It not only is completely adjustable for timing, but eliminates the distributor completely. No expensive distributor caps or rotors to buy come tune up time! A tune up on my twin plug engine consists of an oil change, filters and 12 spark plugs! Plus, deleting the dizzy really cleans up the engine clutter! Cost for the system was about $1300.

Engine running with no distributor!
http://forums.pelicanparts.com/uploa...1536152443.jpg

I raced a single plug 10.5:1 3.4 with JE pistons, on 100 octane fuel, for several years. I also raced a 3.2 with the same fuel and comp ratio. My street SC is a 3.2ss with 9.5:1 compression and runs on 91 octane, also single plug. Both engines were built by Supertec. We've got another Carrera based single plug race engine with over 11:1 and it runs 110 octane. So in my experience those are about the limits for single plug: 91 octane - 9.5:1 or lower, 100 octane - 10.5:1 or lower, more than 10.5:1 and you'll need 110 octane and/or twin plug. A good engine builder can provide more info I'm sure, but that's my experience.

Just send your heads to "cgarr" for twin plug. You won't regret it.

You don't have to add the 2nd ignition right away, and can add later. You'll sleep better at night!

Detonation sucks!!!!

agree -- even though they're in transit back from being machined, better to incur the (relatively minimal) expense of having the heads ready for twin plug IF needed then to disassemble the top end to pull the heads post-rebuild.

Wonder what the "over/under" would be mileage-wise between the premium paid for 100 octane gas and the additional expense necessary to twinplug and run pump gas?

Well I talked with EBS Racing to see what option I have... and apparently quite a few when talking about custom JE pistons.

I need to get my heads in to do some measuring to get back with Don @ EBS racing to see what would work best.

10.0:1 Mahle 98mm single plug pistons are available - you’d need to decide on the single v twinplug though. They’d be single plug for sure in UK.

https://www.*****************/fu/prod137451/Piston-engine-set-Porsche-911-CARRERA-32L-to-34L--1984---1989--Motronic-inj---98mm-/