Twin plugging is not that expensive if you use some sort of EFI that can run coil-on-plug, its the 12 plug distributor that's the expensive part.
I've run Volkswagen COP for several years now, they work fine for N/A engines, fits nicely in the valve covers and can be found at a good price.

I agree with Steve. The key words he said was "run safely." Yes, you can build an engine that will run on 10.5 compression with high octane gas and is perfectly timed and tuned and will operate fine. But, what happens when you get that random batch of "bad gas" that causes detonation? Or something happens to your timing and it accidentally gets a bit too advanced? There's very little room for error there.

In general, modern engines can run much higher compression because they actively monitor the quality of the fuel and conditions through a knock sensor. Without the ability to detect knocking (detonation, or pre-ignition of the spark), then you need to play it "safe" (as Steve said), and design down for the "bad gas days".

-Wayne

Just to clarify my intentions further, I am completely on-board with twin plugging in this situation, it's the best way to be safe. There is undoubtedly too few knowns other than engine builders stating they know what works in some situations, but with no actual data. While there is no substitute for real-world experience, the data is worth discussing.

Wayne can you comment on the VE and thermodynamic connotations? I believe it's too important an issue to assume for instance that a CIS engine swapped to 10.5:1 compression would be as safe as a ITB engine running same static compression. One is obviously filling the cylinders more effectively which results in a greater effective stroke thus higher effective compression. Simply stated, a CIS arrangement simply doesn't draw the same amount of air at its peak operating rpm, which is shown via torque in a NA motor, thus the potential fuel requirement is not same for a CIS engine running identical innards as its ITB counterpart. So mechanically speaking, given same for same components inside the engine, a CIS engine is not cranking same air at peak efficiency for same compression as an ITB engine, regardless of all internal components, thus octane requirements are different.

There is lots of discussion always about what the safe CR for single plug is. We have never built a 10.5CR single plug. We always resorted to commonly held wisdom that said it was not a good idea.

So, I would like to hear from the folks that have actually tried it or tried something similar and ran into pre ignition issues. Would love to see all of the relevant specs.

We know twin plugging a 10.5CR is safe. We are building a twin plugged, COP, moderate (relatively speaking) cammed, ITB equipped motor right now. But what are the real limits of the single plug and who can post data to support it?

single plug with J&S or twin plug

compression ratio
 

You need to actually CC the heads to determine the true compression.1st valve job with cutting the seats lowers it.It is very easy to machine the top of the piston crown flatter to dial in a safe ratio.But you need to know the C.R.to begin with and then you can apply all the various theories that abound on this forum.Twin plug over 10 to 1 for sure.A companies C.R.of their piston is just a suggestion.Putting 2.8 92mm pistons on a 2.7 motor results in a C.R.of over 12 to 1 not 10.3 due to a different head design.

Well I can confirm at least 3 of my local boys are running +10:1 CR with single plug.

1. A 2.7 with EFI, EDIS ignition and 10.1:1 JE pistons
2. A 3.0 with Bitzrace EFI and cut down Airbox, standard intake runners on a '79 with 10.5:1 Wössner pistons. Standard ignition.
3. A 3.0 with PMO carbs on 10.5:1 JE's with single plug (heads are ported). MSD ignition, but otherwise standard.

So I know it is possible. None of these are race cars, but they do see some track time.

I don't know anyone who has single plug on a 3.2 or 3.4 with high CR though.

Where are you based Nux?

Here in the UK, my 3.4 ran fine as single plug with a measured 10.45:1 CR. Always run on 98/99 Ron. I used the flat dome piston as part of the Mahle 98mm kit.

Will hopefully be back from the builder as a 3540cc on ITBs shortly, but had moved to Motec/COP in the interim.

Interesting!!!

I'm in Denmark. We have 99RON as well. Did you use a knock sensor or anything like that?

Nope, It had a live remap of the motronic ignition/fuel map after 1000miles running in, nothing more.

Good real world info. We can only assume the engines in question are operating at maximum effective flow efficiencies which do not equate to the engine ever reaching near their static compression, or a point of detonation, whatever that point is which is going to be slightly different engine to engine given all the intake variables. The unknown with these is at what point of effective compression and diameter of cylinder bore does pump fuel become an issue? This is a thermal dynamic question which is slightly different for each engine and is highly dependent mostly on flow efficiencies and fuel.

note:
98/99 RON is equivalent to US 93 octane. 93 US is indeed closer to 98RON though

my new 10.5 CR carillo/cp pistones measured in at 10.2CR. The shape however I belive calls for twin spark so thats what I did. Obviously running MS and EDIS adding a second set of plugs was cheap and easy to do.... Shape of combustion chamber is important

Steve, the Euro Carreras have 10.3 CR on single plug, how can a 10.5 CR be far too much? There must be many more factors than just the compression ratio, like ignition timing, spark energy, cylinder bore (flame front travel), cam shafts, etc, etc.
Many US muscle cars had both larger bore and single plug ignition, with way over 10:1 CR.

Please explain!

"Many US muscle cars had both larger bore and single plug ignition, with way over 10:1 CR. "

It depends on the piston top and CC design. Flat top pistons are generally more tolerant of higher CR than a domed piston because the dome interferes with the flame front and creates pockets at the periphery that can lead to detonation.

If you measure the CR of factory engines, you'll find that there are significant disparities between advertised CR and actual CR. I've never seen any air-cooled engine from Porsche that met their published CR specs (with the exceptions being their race engines). This means that Mahle pistons are almost always several tenths below spec and this is a good thing, allowing the safe resurfacing of heads at valve job time.

For the Euro Carrera, I've found CR to be between 9.6:1 and 9.9:1, depending on head volumes and deck heights.

For American muscle cars, they were almost always VERY close to advertised CR, however they were intended for the fuel of the day (Chevron White pump). :) :)

Pent-roof or hemi chambers with central plug location do not have the same thermal inefficiencies as 911 heads. You will almost never see a twin plug with a central plug.

The issue is not just the size of the dome of the piston, it's the flow of the heads! You could have a giant piston dome in a 911 but have a very choked intake and exhaust meaning it's not ever reaching a level of effective stroke where the static compression is met. It may never detonate with that giant piston dome because it's not moving enough air to where that level of combustion has a problem. That point is that twin plugging a 911 engine isn't just a question of piston dome size, it's many other factors pertaining to how much air the thing is moving and if that air causes a problem. The safe way is to twin plug everything, but it's remains questionable and must be studied case by case whether or not it is 100% required for absolutely everything.

as steve mentioned, the stock CR ratio are optimistic. this is not uncommon, same as ferrari's, alfas etc that run cross flow hemi heads have much lower CR ratios than advertised.

yes high octane fuel with lead was abundant. but on the architecture side, many of the muscle cars ran with a wedge head, high swirl, large quench area. this makes it much less sensitive to knock or detonation then say 426 hemi. being water cooled, the thermal stability is more constant than aircooled heads.

yes a taller dome is needed for increased CR at the determent to the CC shape. 964 pistons shapes were better and flatter with a flat plateau than say a 3.0 RSR classic dome which i recall at 10-10.3. Still effective CR wasnt really that high couple with higher duration cams on the RSR.

hemi heads have inefficient large surface area and distant crevice areas. they are relatively slow burning but are high flowing. because of this they typically need more ignition advance. advancements in ignition control, twin plugs and ceramic lined ex ports helps with the elevated CR on the 3.6 motors.

my 2 cents

These are the actual Wössner 10.5:1 pistons for a 3.0 911 (95mm bore):

http://forums.pelicanparts.com/uploa...1508184386.jpg

Pic of my 98mm Single plug Mahle by comparison

Note, these have now had valve pockets machined to accommodate some more 'hirsute' cams as part of the 3.5 build - this pic was taken prior to this.

https://uploads.tapatalk-cdn.com/201...97d4a42c91.jpg