a/f ratios and CR's

[Guest bastaad525]

hey, this is me posting, so you should already know this, but just a forewarning, this post is gonna be long

 

 

As usual, the more I read, here, in magazines, other websites, you name it.... but the more I read the more I get confused.

 

 

My latest question/confusion is regarding fuel mixtures and what is safe and why, and how compression ratio affects this. I know the general consensus, not just here for L28ET's, but pretty widespread oppinion is that you want to run around mid 12:1 a/f ratios on a turbo charged motor to be safe. Lately I've seen some extremes... for instance I know a guy who says he recently tuned a standalone on his Evo, and had to run mid 10:1's to get rid of detonation on 91 octane. For me, the issue is compounded by my trip to the dyno, where I saw a dramatic loss in power when I richened up my a/f ratio from 13.6:1 to about 12.7:1. But then I think... for the longest time I was running 10psi w/o an I/C, and was getting around 13.2:1 air/fuel, with timing set at about stock, on 91 octane, and on several dyno pulls power was consistent, with no sign of detonation, and never the slightest knock heard on the street. I drove the car like that for a year almost daily and drove it HARD, constantly on it. Never a knock. And trust me I'm ALWAYS listening... I dont even have a working radio in my Z so my ears are always tuned in on what the Z is doing. Then when I installed the I/C and upped the boost but added NO extra fuel, running a/f ratios up in the 14:1 area, again, not one sign of ping at all, after five pulls with little cooldown time in between on a 90+ degree day, pulling up to 6000rpm. No audible ping, and no erratic power curve that would hint at ping happening that couldn't be heard.

 

 

And it's not just me... I've talked to or read posts from guys on here who knew they were running lean, not always a specific number ratio, but just looking at their setups (usually involving boost levels above 10psi and stock or near stock EFI) and comparing to my own setup and knowledge of others here you just KNEW they had to be running lean, leaner than 12.5:1 for sure, and these guys are going to the track all the time or whatever but driving these cars hard and never a problem. Jersey is one in particular who comes to mind Cronic is another one... running a bone stock zxt with bone stock EFI he had boost up at 15psi with I/C and had no problems... now hes running 20psi with extra injectors... and there have been many others. I'm not exactly presenting hard evidence, I know... but any of you who've been around here and read enough posts know I'm speaking the truth.

 

 

I figure, the compression of these motors has to have SOMETHING to do with this, but how much? I mean... a stock l28ET runs like 7.5:1 CR right? And you think... well how many guys out there say you gotta run mid 12:1 a/f ratios to be safe... and of those, how many run such a low CR motor?? Not many, I think.

 

I do know that when I built my high CR N/A motor, at about 10.3:1 CR, it pinged like crazy at around 14:1 a/f ratio when I dynoed... started pinging at about 4500rpm on stock EFI. Whereas 14:1 didn't even faze the turbo motor on stock efi at 12psi of boost... odd eh? And the guy running the Evo at 10:1 a/f ratio... aren't those up at around 9:1 CR? I know he was running a lot of boost, 20psi ish... but why so rich to be safe? Seems to me CR plays a LARGE part of determining how rich you need to run to be safe. I'm just wondering... I mean with such a low CR, do we really NEED to run so damn rich and give up all that power? I'm losing something like 40 lb ft of torque in the low end of the power band when I went from mid 13's to mid 12 afr's!!!! Also, how big of factor is the efficiency of the I/C? I know w/o the I/C, my engine DID start to ping any higher than 11psi, yet with the I/C on stock fuel 15psi was reached with no ping (admittedly it was only set there for one pull).

 

 

Maybe we dont need to run so rich on these things?

[Bernardd]

I tuned my chip to stay around 13.5 just before boost (wot) hits then it drops down to 12.5 or so to about 8psi, then beyond that I go down to 11.0-11.5. Anything leaner than that and I get knock. And with 91 octane I can't go much above 15psi. I'd say the critical thing is how much power/torque you're making and the area you make it at is most likely where the knock will occur. That's my experience so far.

Bernard

[Guest bastaad525]

Another extreme... not that you're running TOO rich, just that you get ping if you DON'T run that rich... very odd. As I said... I drove around for the longest time at leaner-than-13:1 a/f ratios and never got a ping once, including several dyno runs.

 

Jersey was up to 14 psi after installing his I/C, but before he installed bigger injectors, running stock EFI except for an upgraded pump. Now, I know every setup is different, but assuming his car runs even close to the way mine runs, he would've been at at LEAST mid 13 or low 14:1 a/f ratios at that boost level, and he took that car down the track a bazillion times w/o ping.

 

When I installed my I/C and asked what the general consensus was as to the safe level of boost I could run on the stock EFI, the common answer was 12psi... a lot of guys have run that, intercooled on stock EFI w/o any problems.

 

Cygnusx1 ran 13-14psi on stock efi and never heard a ping.

 

Cronic ran 15(!!!)psi on his ZXT on stock fuel with no ping, though oddly, he was making less power at 15psi than most people make at 12, something like 195rwhp, and still plenty rich, which makes me think maybe his boost gauge isn't working right or there is something else going on there.

 

Fl327 ran 12psi for a long time including several track runs on stock EFI, with fuel pressure increased to 62psi at full boost/WOT... my FMU was set to boost fuel pressure to between 65-70psi and my a/f ratio was getting up to 12.7:1 at that pressure as revs approached redline. He was probably running at least a tad leaner.

 

And I'm sitting here running super rich for much of the a/f curve, losing some 40 lb ft of torque, and wondering if I really need to be?

[strotter]

Grumpy's the one you really need to communicate with regards to detonation - he has a number of posts over on ThirdGen regarding detonation. *However*, I've managed to glean just a tiny bit of info as the the sources of the problem.

 

1. Dynamic compression ratio. One of the often overlooked aspects of modifying valvetrain timing is the effective compression of the engine while it's running. We're all familiar with static C.R., and it's easy to measure, and useful as a diagnostic tool, but it's of little use thermodynamically; the dynamic C.R. is what the engine actually experiences while it's running because it takes into account the action of the valvetrain timing, so it's all that matters. You may have a calculated C.R. of 9.5:1, but if the dynamic C.R. goes much past 8:1 or so, you're going to see detonation (with iron heads. I *believe* you can add a point for aluminum, but don't quote me). Check out http://hybridz.org/nuke/index.php?name=PNphpBB2&file=viewtopic&t=30417&highlight=dynamic+compression+ratio

 

2. Detonation can be caused by a number of things, including sharp edges in the combustion chamber, carbon particles in the combustion chamber, timing errors, incorrect plugs, incorrect or damaged head gaskets, and other things I can't think of right now. Basically, as soon as the combustion changes from controlled (relatively slow) to uncontrolled (explosively, or from multiple ignition points) you get detonation.

 

3. Stoichiometric combustion is not always what you want, even under non-WOT conditions, all the time. As a matter of fact, Grumpy and other have pointed out many times (and I've noted it personally) that stoich is mostly about catalytic converters and not about power or smooth operation. Many times, motors will run smoother in "general" driving with lower AFRs, and certainly you want lower AFRs in WOT conditions. Sometimes, such as under cruising conditions, you may want your AFR to exceed stoich. There is not, however, a "perfect" number; you have to experiment and find out what you motor "wants" under different conditions. That's the beauty of fuel injection and tuning your own ECU chips, you can explicitly tune those numbers to your liking.

 

Anyway, there are a number of things with affect combustion so as to cause detonation. It has been talked to death over at thirdgen.org, I'd suggest you check it out (if you've got a free *day* or two!)

[cygnusx1]

The only time I have ever heard ping in my Z was with the N/A original motor in top gear trying to stay with an early 90's 300ZX on the highway. With this 83 Turbo Motor I run 13-14psi, 100% stock EFI and FMU, like bast said and I have never heard it ping. I also have heard that ping can be difficult to hear when it's borderline. When we had my friends modded WRX wagon on the four-wheel dyno at TurboXS, they were hearing ping with a special listening device. We couldn't hear any ping standing outside the car.

[Guest bastaad525]

Strotter - okay so how does one determine what the CR of an L28ET is while actually running? It must be extremely low. I have spent many hours going over links in Grumpy's sticky post on detonation. I do often worry that there is detonation going on that I can't hear, but this would have showed up on the dyno wouldn't it? I thought even if you couldnt' hear it, you would still see an erratic power line on the dyno chart. I'm not so much worried about detonation right now, as I have the FMU set to run very rich all the way up to redline, it never get's leaner than 12.7:1 and that was only a lean 'spike', average a/f ratio is much richer. You mention about tuning an engine for what it wants though and it just seems to me that thru my experience and that of other guys here the L28ET 'wants' a leaner mixture.... I mean... 40ft lbs of torque lost is nothing to sneeze at! With such a low static CR and running relatively low boost I'm just thinking there's a much larger margin there than most people might expect. Okay so even if there is ping going on that no one can hear... that doesn't explain why so many guys have gotten away with running their cars lean, driving them hard, for long periods of time, and never a problem from them, including myself. Okay well I've had problems but nothing I would attribute to running it too lean.

[strotter]

Basically, Dynamic CR takes into account that there's little compression going on until the valves close. There's a good description of it at http://cochise.uia.net/pkelley2/DynamicCR.html. What you do is figure out where the piston is when the intake valve closes on the intake stroke (by one means or another), then calculate the CR from *that* point. Needless to say, it's always lower than the static CR. The thing is, it *needs* to be lower. 8.25:1 is the upper limit, as I understand it.

 

With a turbo motor, I would imagine that the DCR would increase proportionally to the increase in pressure of the incoming fuel/air charge. Say, for instance, that you'd calculated DCR to be 7.5:1, given your stroke and cam. But let's also say that your pushing 7 lbs of boost, 50% above normal atmospheric. Now, your incoming charge has 1.5 times the mass, so DCR will increase some amount proportional to that - perhaps as much as the full 50%. I'm totally unclear about the mathematics, but I'm fairly sure there's going to be some significant increase.

 

Finally, another thing about detonation; it isn't necessarily audible. Very, very often, when you're tuning with a knock sensor, it will indicate significant numbers of detonation events, and you won't even hear it. Taking some plug-cut will confirm what the sensor is indicating, though. Perhaps there's such a thing as a "stand-alone" knock sensor? Such a thing might be very useful to you, to isolate when knock is or isn't occurring.

[fl327]

L28et is a tough sucker thats for sure, on stock injectors ive run 15psi balls out at the street races, with 7gallons of 100 race gas at 50psi static fuel pressure (idle) and the almighty Jpipe!

My next engine Im thinking of going pure 80s style on, 6.9 or 7.0cr with head studs and a thick thick steel headgasket, what can I say im cheap.

[Guest bastaad525]

Strotter, you keep saying 8.x:1 is the limit for dynamic CR, but you are forgetting about the main thing I'm asking about, how air/fuel ratio or even octane affects this. Surely your 8.x number would change if you're running richer or higher octane. Or if it were leaner or too low octane, then of course you might ping at even lower DCR. This is what I'm getting at. If you run a turbo on a 10:1 CR (static CR) motor, you need to run either low boost, higher octane fuel, richer, retarded timing, or some combination of the above. With a lower CR, you can run lower octane, more boost, leaner a/f ratios, more timing, or any combination thereof. So... how to figure out what combination is safe for what CR motor. I think since a stock L28ET is much lower CR than the average motor made these days, even your average contemporary turbo motor, I think that the 'standard' safe limits maybe don't apply, at least, not as tight, to our motors. But how to figure out what the limits for THIS motor are? Everyone so dead set on running 12.5:1 a/f ratio to be safe, yet I and others seem to be able to run 13:1 or leaner all day long with no issues, and getting more power relativley to boot. How to go about changing the 'set rules' and thinking of guys running low CR L28ET's to reflect the true safe limits? How to figure out what those safe limits are?

 

Maybe I'll just turn the FMU down a bit, run my low 13:1 air/fuel and see how long I can get away with it, and enjoy my 40 more ftlbs of torque to boot Been too long since I broke something

[Guest bastaad525]

Dont want to let this topic die yet....

 

 

I know there are more guys out there who can really help put together some useful info on this... guys that have knowingly run lean or really taxed the stock EFI as far as it will go and who's motors lived to tell about it. And lived LONG afterwards. I agree with FL327 that the L28ET is a tough motor, and maybe we don't have to cheat ourselves of some of this HP 'babying' such a tough motor that really doesn't need it.

 

I've since leaned my FMU back out and am loving the reclaimed power, so far so good, nothing broken yet.

 

What I would really like is to see Jersey's car on a damn dyno which he has been putting off far too long now. I think his car, running 18psi on the stock turbo, stock EFI with 370cc injectors, and running high 12 1/4 miles, and should really be on the ragged edge of meltdown yet still puts up with his abuse run after run... could really tell us something with some good dyno pulls.

 

You listening Jersey????

[Clifton]

Max power is had around 12.2:1. Most people (500-900hp Supras) shoot for mid-high 11's to be safe. If you are running leaner than low 12's say 13:1 and do a long highway pull you would be asking for trouble. It's alot of extra heat on the pistons, they tend to melt. Running richer than mid 11's will just lower the power. You would be better of running 12:1 and lowering the boost to acheive the same power. Guys running in the 10's are just throwing away free power.

[Guest bastaad525]

Um... quite the contrary on two counts. #1, I didn't make max power at anywhere near 12.2:1 as you state, rather, I made WAY more power on my first two runs (out of six) with my air/fuel ratios averaging 13.5-14:1 during most of the run. Well, way more TORQUE, the dyno sheet I have comparing all six runs doesn't show hp numbers only the torque output and a/f ratio (dynapack dyno printouts SUCK), but the torque curve stayed consistently higher all the way up to 5000rpm. Those first two runs had a nice even torque curve, peaking early, around 3000rpm, and staying almost flat up to about 4300rpm. As I richened it up from there I consistently lost power, first I richened up to around 12.7-13.2:1 area, and lost as much as 20 ft lbs in the 3000-4000rpm range, and 10 ft lbs from 4000-4500. Not such a nice flat curve anymore. Then I richened it up even more, my aim was to get the leanest part of the fuel curve to be no leaner than 12.7:1. On my last couple tries the fuel curve started covering a bit wider of an area, a limitation of the FMU I guess... anyways, I got the curve to go from about 11.7:1-12.7:1, and lost another 20lf lbs in the 3000-4000rpm range and another 10 or so up the rest of the rev range.

 

The second 'on the contrary' is in regards to saying richen it up and lowering the boost would achieve the same power. Actually, if the pattern I'm seeing is correct, I would only loose that much more power if I did both things at once. I might gain power by lowering the boost and leaning the fuel curve out, OR, leaving it rich and upping the boost... but I dont know which would end up making more power... the small changes in the fuel curve were making HUGE differences in torque especially in the low rpm range, though not so much in the high, but still, I got more power everywhere when it was running leaner. Not that I'm going to dare leaving it running at 14:1 a/f ratios!!

[Jersey]

I'm listening