All those sharp edges on those pistons is what makes me nervous....

You'd think Wossner (and others) would put a fillet all around the crown during the machining operation.

Crazy they leave those sharp, pre-ignition prone edges on their pistons!!

Really makes me wonder why....

This thread is really fascinating. So much knowledge in here!

On a fundamental level I still don't understand why a high compression ratio needs twin plug to avoid detonation. Would someone care to make a quick explanation about the basics?

Not all engines need it. A 911 head is such that the plug is in a fundamentally bad position in an already inefficient round combustion chamber. The issue is a thermal one in that the charge of gases like to be essentially in one place as branching out takes time and causes inconsistencies.

When a big domes piston is used, such as those available in high comp 911 applications, it can create 2 almost independent thermal areas on each side of the piston dome, this is bad if effective compression is arguably above about 10:1, as in, is that static compression number ever being reached by way of airflow to the engine. Most of the time the answer is a hard no unless one is running very large ports, lots of cam, and ITB intake and great exhaust etc, and at that point a very high efficiency would only be reached at a very high rpm. If one reaches 95% or so, that is great for a 2v road engine.

Effective compression is not just reliant on the size of a piston dome, rather it relies on the volumetric efficiency (VE) of an engine. VE is basically determined by cams, intake and exhaust flow efficiencies. An individual running 10.5:1 pistons with a DC40 cam but with a single throttle body is not operating at nearly the same effective compression ie. not moving the same amount of air, as an identical engine, but with big ITBs. So the former may not have any need for twin plug as the engine is simply not ingesting enough air for that static compression to ever be reached, while the latter may very well have an issue as it is flowing more air and has a greater likelihood in reaching, with that identical internal arrangement, an efficiency greater than the approximate compression amount (9.8:1 or so) most believe 911 engines cannot handle minus twin plugs.

So when in doubt or not sure, twin plug. It's cheap-ish insurance compared to having to rebuild again. The truth is somewhere between and requires a lot more data to really know. I am of the mindset that only 911 racing engines running absurdly large ports, large cams, large intakes, and high compression absolutely required twin plugs, while with road engines, if one doesn't go too over-the-edge can probably run about any setup with single plug safely, but when in doubt or if it will make you sleep better, twin plug.

Thanks lvporschepilot for taking the time to answer. I see that you emphasize the effect of the dynamic compression ratio, which I understand, but my question was even more basic. It seems to me that in this thread it is generally implied that for a given engine there can be detonation issues beyond a certain compression ratio (static or dynamic) when single plugs are used and that those detonation issues can be alleviated to some extent by using twin plugs. Why is this? There is probably a simple explanation but I just can't figure it out from first principles.

High octane fuels can only be compressed so much before they detonate on their own, hence why race fuel is very high octane and resistant to detonation. Most engines, even race engines, are single plug due to good combustion chamber design and proper plug placement

I wonder if it has something to do with ignition advance in that twin plug needs about 10 degrees less advance than single plug all other things being equal. Obviously, the more advance, the higher the risk of detonation.

There are some threads on twin plug and detonation on the forum that may answer the question posed.

There is a very tricky component to this and that is including all the negatives about surface area and end gas temperatures in a Hemi cross flow is that the time it takes for the spark to propagate across the chamber requires more ignition timing. the twin plug will allow this to occur timely (thermodynamics place this initial light off as somewhat crucial to resist a premature explosion of end gas in the chamber) so with less lead timing we still propagate across the chamber in a more balanced pressure rendering to the piston essentially making the same power but with less tendency to detonate and peak cylinder pressure developing in the 10 to 12 degrees after TDC the best of both worlds less sensitivity to octane almost one full point and the same power.
Timing and flame propagation due to the hydrocarbon content of the fuel
reagards

Great explanation.

In my mind, you don't always have the same swirl effect of fuel entering the combustion chamber having more fuel in places than others every intake stroke. Since the flame has to travel over a large dome, the different rates of fuel lighting off may have erratic results? One event, nice burn. Next event, fuel ignites more to one side of the chamber detonating.

That has been my belief by watching the small engines run with see-through heads on YouTube. This is apples to skyscraper comparisons, but it seems to show possibly what is occurring.

Don't know about all the hypothetical, but here might be a useful datapoint: I had an auto x 2.7 motor in the late 90s. Spec: JEs, measured at 9.8, E cams. 36mm ports, weber 40 IDA and MSD. Timing to spec. Ran on 94, sometimes 96 octane -- the best street gas I could find. Following a tear down resulting from crank/bearing failure there were LOTS of signs of detonation on the pistons. I never heard a "ping," or "pink." Ever.

When 2.7s were common because they were cheap track engines, I heard of lots of people running even higher compression. But a lot of these guys worked in shops, lived for club racing and rebuilds came cheap(er). For me at the time, a rebuild came with significant financial hardship. Future engines were twin plugged and had some sort of dowel pinning.

I think your expectations matter: what you are going to use it for and how long do you expect it to last? Do you care if the lifespan is a bit shorter because things are less than optimal? And parts are damaged to the point of not being able to be reused on the next engine? There will likely be a next engine.

True wisdom right here.

Sub-audible detonation is quite real and will destroy pistons, rings, and rod bearings just as well as the kind you can hear.

Never, ever rely on your ears as the defacto 'knock-sensors' otherwise you may spend a great deal of money.

Remember the old adage, "An ounce of prevention is worth a pound of cure". :)

[QUOTE][In my mind, you don't always have the same swirl effect of fuel entering the combustion chamber having more fuel in places than others every intake stroke. Since the flame has to travel over a large dome, the different rates of fuel lighting off may have erratic results? One event, nice burn. Next event, fuel ignites more to one side of the chamber detonating.

That has been my belief by watching the small engines run with see-through heads on YouTube. This is apples to skyscraper comparisons, but it seems to show possibly what is occurring./QUOTE]

Yes very true prior to the advent of Multiple Spark Ignition research concluded adequate propagation occurred 90% of the time and due to the wearing of dwell points condensers,plugs
etc. over the range of tune up probably a lot less than that during the service span.

regards

Would you then recommend using one knock sensor per bank on a 3.0 engine to monitor/control sub-audible detonation?

Thank you for the confirmation, racing97! :)

Lets say I went with the 10.5:1 pistons. Would it be possible to shim the CR a bit down - fx to 10.1:1?

Yes but a lot of shimming lowers the deck height a bit. While it's not the greatest to not have optimal deck height it's not the worst either. Before much was known about quench and deck heights engines still ran ok. I can say for a fact that old Ferrari 365 and 512 Boxer, 246/206 Dino, and 2v 308 engines with stock pistons run .050" below deck at TDC, and they run just fine.

You could go with the smaller dome pistons yet use thinner shims to bring compression to 9.8:1 ish. There's really not much to be had from 9.8:1 - 10.1:1.

Yes, I will in any case need to measure my current CR before changing anything.

Yeah, I wouldn't loss deck height, that aids mitigating detonation too.

Nux, what did you end up doing?

I am about to order some CP 10.5 pistons for my 3.0 Carrera motor. It is using 40mm itbs, efi and race header exhaust.

Or should I only get the 9.5:1?

The cam I am looking to use is the Webcam 464/465.. second from the top
https://i.imgur.com/JGBJvqL.png