10.5:1CR too much for single plug 3.2SS?
 

I have an opportunity to change my pistons in my 3.2SS with Mahle 98mm pistons allready installed, as I'm changing my cams to DC20.

I do not know the exact CR with the current pistons. Wössner makes a nice set of 98mm pistons with comp. Ratio: 10.5:1 for only 800$. Seems like the right thing to do "while your're in there" BUT: Is that too CR much for a single plug 98mm bore on 99RON octane?

What induction are you using? Carbs, Motronic, etc.

Megasquirt EFI with 42mm ITB (Triumph). Ignition is CDI+ (programmable). Exhaust is straight headers into 2 in 2 out dansk.

It's really tough to say, but most would say you need twin plug, which I completely agree with. When in doubt, twin plug.

To really delve into it though I would want more data, such as what is your port and intake valve size? How is head flow? Has it been measured? If one is really wanting to know, VE ie. cylinder filling is very important in determining what static compression can be safely run on various fuels. The volumetric efficiency of these, or any engine, will be dependent on whether or not it reaches nearly 100% or greater than 100% efficiency, which I'm just not entirely sure of with these but I have seen examples with big ported RSR motors and am certain they run above 100%, but only of course at very high rpm. Anything above 95% VE is excellent flow, but most engines, for purposes of power ranges all over the range run in the 80s.

100% volumetric efficiency means your engine dynamic compression, which with a DC20 cam is probably about 9-9.5:1 or so, meets static compression at a particular rpm which is where peak torque is made, which in your case is approx 10.5:1 with the pistons you quote. If your port size is reasonable then it is possible your engine is flowing (just as an example) at say 85-90% efficiency max placing the highest compression ratio at 9.45:1 (at 90% VE) with those pistons. That data places you entirely within acceptable ranges to run the pistons you suggest on pump fuel. Air density is also a factor. The design of 911 heads and flame front propagation is also a factor. With your ITB arrangement I expect you're flowing pretty well though.

It's based on a '78 SC, with 39mm intake ports. The heads are untouched.

I guess it's not possible for me to determine the actual and precise VE. But
even if I have say 95% VE it would still be 9.95:1CR and thus acceptable for single plug?

In theory yes, but a pointed concern is the placement of the spark plug is it lends itself to inefficient flame front propagation..... I just cannot be certain. That said, I doubt you're even at 95% VE, but high 80s or 90% may be about right. It might be worth a shot to run it and report back. I have more confidence in this as it's injected and better atomized fuel then if it were carbureted.

Yeah the Max Moritz style Mahle piston compensates for the off-center plug with an asymmetrical Dome design. The Wössner design is different.

Yeah, I'd go twin plug as 10.5 would probably be perfect for that with pump gas. Also, the DC20 cams don't have that much overlap as compared to S cam/DC40/60/80 which could have potentially helped at lower RPMs (see static v. dynamic compression discussion above).

Bottom line - if you're not running race gas, then I think 10.5 compression on single plug may be a risk for detonation especially on a hot day. If you have a knock sensor or are able to retard the timing, it may help but compression seems high for single plug. Are you in a state with 91 octane or 93?

Indeed, 10.5 CR is far too much for single-ignition and pump gas.

You'll need either twin-ignition or 100 octane unleaded race gas to run safely.

The only exception would be for long duration, narrow LC cams, however anything like that is not streetable at all. :) :)

This just isn't the case unless your heads/cams/intake/ex/flow for a very high cylinder fill ie. volumetric efficiency. Pump fuel can easily take some 10.5:1 with good thermal efficiency or flame front (hemispherical ignition or pent roof), and I don't see the above engine as one that will have such efficiencies that pump fuel would cause a particular issue on its own, rather my primary reservation is the flame front with these heads, not the effective compression.


I definitely stand by what I said before, when in doubt, twin plug. But VE must be taken into careful consideration. Let's also not forget the 3.2 Euro Carrera ran about this level of compression with single plug, but with a different piston design and a highly efficient/ported head albeit less efficient exhaust and intake which probably lead to a similar or lower VE.

The car has programmable CDI, so I'm avle to retard timing. And 99RON corresponds to 93 octane.

Oh, and I'm in northern Europe - we dont't get hot days at all haha!

The reason for posting this in the first place, is that my friend is running these pistons with 10.5:1 in his 3.0 EFI engine (95mm bore). This was even recomended from Wössner!

I'm sure you will be fine if you have someone locally running a very similar arrangement with no issues. I believe this is a highly misunderstood issue in the 911 world. Twin plugging is great, but in the end we only saw it on Porsche racing motors running very high VE arrangements until the 3.6 motors. It can be argued Porsche never really made a truly hot-rod road engine I'm sure because they didn't want to have to warranty the thing.

Nux,

Listen to Steve.

He answered you question with a simple no. Based on knowledge and experience without getting into all sorts of """"""" that doesn't make any sense.

Steve is a professional engine builder with years of experience building and "hot rodding" Porsche engines, many of them air cooled. He comes on here to help and offer advice, for free.

Who would you rather take advice from??? I know who I would.

I've being doing this along time and I'd be scared to build your engine if I had went with what I'm reading here.

The overlap of the cams compensates for the hi CR to some extent. still 10/1 is high. Detonation is the problem which will cause the piston to break slowly, then the compression ring will break....rebuild time again.

High octane gas and low temps will be important

Chris

LV,

I think if you measure the CR of a Euro 3.2 motor it is in the 9's. I measured one years ago but don't have my data any longer but do recall it being consistent with what others had measured in the mid to high 9's...

All of the 10.5 motors we build have twin plug. Mind you the cams we like are wider lobe center to give us a flatter power band. We are finishing up a 3.4 with what are essentially mod S cams on 108 centers.

So, there seems to be general consensus on: I't might be possible due to ITB with EFI and better flow and fuel atomisation - combined with high octane and low temp.

However, twin plugging is recommended.

I acknowlegde and greatly appreciate the advice from Steve and others with far more knowledge and experience than me. I guess I was looking for a way to save the $$$$$ in a twin-plug conversion, but I guess a blown engine is even more $$$$$.

So I'll probably not change my current piston setup, which I guess has 9.5 or 9.8:1. I do not believe it is worth the $$$$$ to raise the CR, which requires new pistons, studs, machining of heads, twin dizzy, additional CDI etc etc.

Thanks for the help guys.

Great info, thank you.

This is an interesting subject that I believe requires further objective testing with data. Twin plug is a great way to be safe, but is it truly 100% necessary when volumetric efficiency is at best approximated? Others in the EU, per the original poster, seem to think not. Would be a great topic, from a hobbyist perspective, to explore further with CFD and thermal dynamic testing.

Throwing out a static compression figure of 10 or 11 or 12:1 is great, but without knowing flow into the engine it's impossible to know if the engine is ever actually reaching that compression. If a cylinder capacity is 100cc and it flows at 85% efficiency, it's only ever moving at best, 85cc of air, meaning if supposed '10:1' pistons are installed, it is never actually reaching 10:1.

Compression ration is only one of the factors that needs to be considered. Bore size Piston dome shape, head volume, Quench, fuel and timing control, intake and exhaust all are factors in determining what is a safe compression. My BMW turbo 6 has 10.2 compression and boost on top of that. It can run on 91 octane gas, but it takes a lot of technology and computing power to make that work.

john

The next Euro 3.2 we have apart we will measure again.

Yes, many factors play into it determining whether or not it will work. You can always retard the timing and not risk it but then you leave too much power on the table... :)

You can always twin plug the heads and not run fire the second plug. I can't remember the increase in CC of the head for a typical second plug mod, but it would lower the CR a wee bit. Steve, do you have that data in front of you?

The other key it to actually measure the CR of your build. The JE 10.5 builds we have done all were with in .1 (10.4-10.6) with a 1mm deck height. I can't say what the Mahle builds measure out at as we haven't measured one. Would love it if you could measure it and report back.

Cheers

Your BMW turbo 6 is using direct injection. Totally apples to oranges in this case. You could probably run even higher static CR and a ton of boost no problem with direct injection, and it'd be fine.