21 psi at what RPM though? Because 21 psi at peak torque RPM will be a very different level of cylinder pressure than 21 psi at 7000 RPM. I think ignition advance will also likely play a role, even if only in peak pressure rather than average pressure. Do you have any more details on the cars that have had heads lift? Would love to have some data to try and tailor my tune for more longevity.
7000 RPM was just an example; the same would apply at 6000 or 5000 RPM, only to a lesser degree. The VE of an engine changes throughout its RPM range and, especially on a stock head/cams N54, will have dropped a decent amount by redline. That's why I mentioned peak torque RPM as the other extreme, ie. where VE and therefore cylinder pressure will be the highest.
Sure, but again, what RPM are we talking about? As I previously mentioned, VE changes throughout the rev range, and as a result so will cylinder pressure at a given boost level. I'm not doubting your 21 psi number, but given that we're talking about torque/cylinder pressure, a boost level on its own isn't enough information. If we're talking about a car that was hitting 26 psi at 3500 RPM, then it makes sense that it would lift a head, as that's very aggressive. But if we're talking about a car that had a slow taper up to 24 psi at redline, with no knock and reasonable ignition advance, then that's a totally different story.
Cylinder pressure for a given torque level shouldn't change with fuel type (although don't quote me on that, there may be some differences, albeit fairly small I'd imagine). Unless we're talking about knock, but that's a separate issue. My point here was that for a given boost level, if you retard ignition timing, peak cylinder pressure should drop, as you aren't starting combustion until later in the piston's stroke, meaning there's less time where the piston is traveling upwards while combustion is happening. And lower peak cylinder pressures means less chance of head lifting/rod bending/etc.
Right, and that's exactly my point. Saying "21 psi and no more" is trying to make a set formula for a situation that's much more nuanced and complex. Which is why I'm trying to have a discussion on that.
Yes, obviously, but that has no relevance in a discussion about cylinder pressure (outside of potential knock). My point specifically was that retarding ignition timing will reduce peak cylinder pressure, to which you responded:
I am simply clarifying that ignition advance plays into peak cylinder pressures an equal amount regardless of what fuel you're running. Cylinder temps are a different topic entirely.
Generally more information is good ... but I don't agree that it is conclusive if not misleading. In my experience fuel quality is a huge factor, for example. One can think that having additional data somehow more conclusively informs them. I picked 19psi for a reason ... ;-).
Certainly. I think conclusion is the wrong word, it's good that you point that out. I suppose hypothesis would work better. My intention is definitely not to come up with a conclusive "run this boost and no more" type of thing, more so to try and put as much data/info out there as we can for individuals to make what they will of it. Said data always needs to come with a "your mileage may (will) vary" clause.
Yep, 100%. Which is exactly why I was hoping for more info. It won't necessarily be conclusive, but in my opinion it's always better than not having any, as long as it's taken with the appropriate amount of consideration/grains of salt. My main point though was that boost level on its own simply isn't enough, at a bare minimum we need to be considering RPM as well when talking cylinder pressures.
Agree ... I short handed "conclusive" ... my experience ... especially with tech people ... the more "data" they acquire the more they convince themselves (a line asymptotic to truth) that they know. Yes more data is good ... but at the end of the day, the guidance will always be coarse because anything else is complexity reduced to certainty by a subset of known information assumed to be largely computed to determinism. While 21psi or 19psi are finite numbers, they intend to be a generalized target ... e.g. "if all your stuff is in reasonably working order, you should be pretty safe around this number" ... and here's the problem ... those that know enough about what it means to be "reasonably working order" DON'T need this guidance. So what does that say ... ;-)
Sure, and I have no problem with (conservative) generalized guidance, it's just that in this case a boost target on its own can be both conservative and too aggressive at the same time. Someone with big turbos may never run into issues at 19/21 psi, but someone else with stock/small turbos that will make peak boost well before 3000 RPM could take the guidance to heart and still end up lifting a head. It's not the boost level guidances themselves, more so the way they are presented, missing a key variable. I completely agree that we're likely never going to find a silver bullet of configuration/tuning that will maximize power while keeping a stock motor intact, but I think that a boost target on its own is just too oversimplified. Having more data can certainly lead to a false sense of security, but especially in this case, I think oversimplification is more dangerous.
