Originally posted at http://forums.nasioc.com/forums/showthread.php?p=13639773#post13639773
there is no “safe” psi and there is no “unsafe” psi.
with a good tune and a turbo up to the task, ej205s have produced more than double factory power output… ~400whp.
ime what kills 80% of motors is det, plain and simple. the remaining 20% is made up of lubrication issues, honest overpowering (ie rods), and old age/wear and tear.
so if you keep the det away, your engine will be happy, and it will last quite a long time.
let’s look at what happens with big boost (ie, overboosting) numbers…
1) turn up the boost.
2) compressor efficiency drops off. (notice i did not say “becomes inefficient/a hairdryer/a flamethrower.” efficiency is a point on a continuum, it’s not a binary on/off thing.)
3) as efficiency drops off, comp. discharge temp goes up.
4) undersized ic becomes overheated.
5) manifold charge air temps go up.
6) hot charge is more prone to detonate.
7) detonation causes spun bearings, snapped rods, cracked ringlands, etc.
interrupt this sequence at any one of the above steps and you stop it in its tracks. reduce the severity of any of the above characteristics and you increase the reliability of the system.
i can cause detonation with:
stock boost/fuel and too much advance.
or stock boost/advance and not enough fuel.
or stock advance/fuel and too much boost.
are there downsides to attempting to maximize one parameter at the expense of another? absolutely.
if you go for high boost, then you will increase lag. if the turbo has a higher pressure level to shoot for, then by definition it will take longer to get there. it will also probably overspin the turbo decreasing its lifespan. as stated earlier, you’re also increasing charge air temps and therefore the combustion process is more likely to run away on you and det.
if you go for lean afrs, then you will increase EGTs. used to be around here 1600 was holy crap hot. as time went on, and more and more engines seemed to be lasting at that level, it was pushed to 1650. then even 1700. i don’t watch my egt gauge when i do a highway pull, but i know it’s hot. off the 1600* scale. by that time, i’m shutting it down anyway. if you’re tuning a road racing track car, then you run it much richer so that you can keep things cool. waste of gas, throw away some power, get some longevity.
if you go for greatly advanced timing, it can be a wonderful thing… because it’s “free.” you don’t really throw anything away when you bump things up a degree.. in fact, it buys you lower egts and more torque. but what you DO throw away is your knock margin. that’s how much wiggle room you’ve got for a bad tank of gas, a heatsoaked intercooler, more load on the engine due to cargo, hills, gear selection, etc. your knock detection/control system is put more on the spot, and it’d better be up to the task. but as clark turner once said, remember one thing: timing is not a volume knob for power. on pump gas, we’re generally octane limited, and more advance does get you some torque, but it also greatly increases the peak cylinder pressures. the crank is fewer degrees ATDC by the time the charge really starts lighting off. that means those chamber pressures go way up. so does torque, which is great, but peak pressures are very hard on the pistons/rods. in an extreme example, knock is really really high peak cylinder pressure. what you really want for big power output is greater MEAN cylinder pressures (hint: cramming air mass into the chamber is a great way to get more mean pressure).
by the time i pulled out the td04 i was running 20psi in the winter, and 18–19psi in the summer. i crept up to those levels slowly, and i never got greedy with timing. i had a wideband that i constantly monitored. i had a set of detcans with which i set my knock thresholds for more accurate knock detection. i had tens of thousands of lines of datalogs and i made small changes between pulls. i took my time and tried to be as patient as i could be. could i have run more boost? sure. would i be running more boost now if i kept it on the car? yeah, i’m pretty sure i would be!
in fact, looking at the vf23 maps i have in my utec now, they aren’t that different from before i swapped turbos. i run about the same afrs, with the same advance, and about the same boost levels. the car is a LOT faster than it was with the td04. why? because it’s pumping a lot more air. i could barely eek out a 4.4 mafv on the td04 on a very cold day. now i regularly pull 4.6 mafv. 4.4mafv = 237.87g/s = 31.47lb/min, 4.6mafv = 274.75g/s = 36.35lb/min. that’s 50hp worth of air.
basically i kept turning up the boost until i had to drop too much timing to get things quiet and stable. i set myself a rule that i would not drop below 14* advance at torque peak. i didn’t read that anywhere, it’s just a metric that i determined for myself, since each car kind of has their own personality. i noticed that the car was not faster if i turned up the boost so much that i had to run 14 or less degrees at torque peak. i also made sure that when i added boost it resulted in a corresponding increase in mass air flow. without the added flow, boost is giving you NOTHING but hot charge, and high exhaust gas backpressure.
ahh, backpressure. a big result of quick spooling turbines. the bane of the td04. high backpressure ends up causing exhaust gas recirculation since the piston is not able to evacuate the chamber as effectively. now, here’s a little sticking point—egr caused by egbp will increase your knock buffer. your engine will tolerate more advance before knock. this is why the oem wrx maps have such high (like 30* or more) advance at redline… the little turbine is choking exhaust flow. are you making more power with this additional advance? no. your decreased VE is forcing you to use more advance to get any decent torque output. the charge air is diluted, the airmass in the chamber is low, and there’s all this spent gas left behind.
as VE increases, the advance necessary to develop minimum best timing decreases. the charge in the chamber is dense, the molecules are tightly packed in with one another, and the speed of flame front propigation is high. not only do you NEED less advance, but the engine will TOLERATE less advance before it knocks. look at the timing the “big turbo” guys run at redline… much less than the oem wrx maps do. again, does it mean less torque? you be the judge of that.
so i didn’t intend this to be a dissertation, but it has kind of turned into one. the bottom line is that there is more than one way to skin a cat, and you’ve got to take in as much of the system as a whole as you can. ie, if you’re running your injectors static at 19psi in the summer, and your afrs are already at 11.5:1, and you run 93 octane, you don’t have a whole lot of fuel headroom… you’ve reached the limit of the fuel system. come winter when that turbo starts compressing better and the ic starts cooling better and you pump more air, you’re gonna drift lean. so you might dial back the boost, pump a little less air, and run greater advance. OR you might go in another direction and lean it out even more to 12:1, get back 10% idc, keep boost the same and knock a degree of advance off. or, as another example, you crank up the boost to 20psi, and you find that you’re not actually getting much more MAF. you’ve reached the limit of the turbo, and less boost/more timing OR less boost/leaner afrs are better tacks.
any of those solutions are PERFECTLY ACCEPTABLE and are all part of the inherent trading off process that tuning engines involves.