wow this is a good thread thanks for the info

 

- When under boost, you don't pull timing to achieve PPP @ 13-16*ATDC, you pull timing to avoid the knock you would reach if you were at MBT. Most turbo engines are nowhere near MBT at peak boost irrespective of RPM, tuned or not.

-- In some cases you could run 0* of ignition advance and raise the boost enough to compensate. (Depending on pumping losses and how fast the mixture burns.) However your EGT would probably be well over 2,000*F and you would cook your exhaust valves. And, because the average pressure during the expansion and exhaust strokes, as well as the massflow, would be so high, you'd likely also melt a piston even without any detonation.

---- The reason you ignite the mixture so far before TDC is not just because it takes a long time for the flamefront to propogate, nor just because it takes a long time for the spark to be generated; it's because of BOTH together. The degrees of crank rotation due to the former is the Rapid Burn angle, and the latter, the Flame Development angle. Water injection is notorious for lengthening the Flame Development angle in a disproportionate fashion because the water absorbs so much of the spark's heat but does not notably slow down the laminar burn speed. Higher ignition energy tends to shorten the flame development angle. Rich mixtures tend to lengthen both angles. Conversely, lean-er (still < 14:1) mixtures tend to shorten both angles. High compression engines with compact combustion chambers tend to have a lower rapid burn angle. Low compression engines have the inverse.

If you feel like reading my very unqualified rules of thumb on tuning based on what I have read and seen ...

1: The ultimate power/torque threshold for any particular RPM is when you cannot add fuel, add timing or add boost without hitting knock, nor can you subtract fuel, subtract timing or add boost without crossing the EGT limit for your engine. Some engines reach an efficiency limit before the EGT limit, but the idea is the same.

2: Surplus fuel beyond ~13:1 is an internal coolant only, so treat it as such and use it only when you need internal cooling to avoid detonation or excessively high EGT. For example, due to thermal inertia, the internal parts of your engine and the mixture do not need as much internal cooling when you are only on the throttle for a short period of time. (<3 seconds or so) So, in many cases, you can run quite a bit leaner in the parts of the powerband where you will not spend significant amounts of time at full throttle. Also, you do not need to run 11:1 for all the boost load sites. In most cases, you'll only need to run that rich for the highest pressure load sites. Running excessively rich in the lower boost load sites is wastefull and exacerbates slow turbo spool.

3: The greater the quantity of air in the cyllinder during each combustion, the stronger the detonation will be and the hotter the engine will run as a whole. At some point you may find you are adding 3,4 or 5 psi of boost and only gaining 5-6 horsepower because of the required reduction in timing. If you reach a point where you are getting any less than 3-5 horsepower for each additional pound of boost, even if your EGT is ok, stop! Although you may gain a few horsepower, it's generally not worth the added risk and increased physical and thermal loads on the engine.

Anyway, like I said, those are my very unqualified oppinions on tuning, so just take them as they are. I am, however, always game for a good debate.

-Adrian

 

Another thing to consider is the rod stroke ratio and piston velocity when looking at timing. With the 1.5 rod ratio on a stroker engine the piston is going to stay near TDC longer and the engine will require less timing that a 2.0 would given a similar setup.

Also keep in mind combustion chamber design, actual squish from a piston with quench pads will help reduce the change for detonation since it reduces the distance for the flame front to travel.

 

Adrian,

Thank you very much for your insight. It is greatly appreciated.

strike (Dave)

 

hm. this is the first that i have heard of a quench pad. what is it? it sounds useful.

strike (Dave)

 

A quench pad is a place on the cyllinder head to which the edge of the piston comes very close. When the piston is at the top of its travel, it "squeezes" the gas between it and the quench pad towards the center of the combustion chamber where the spark plug sits. This makes the mixture near the plug turbulent and that helps the flame propogate more quickly.

Some people refer to quench as "swirl" and vice versa. Although they amount to the same effect, I tend to differentiate the two by swirl being a smooth and more controlled motion of the mixture while quench is a more abrupt, violent and turbulent motion of the mixture. However, certain piston designs, most notably the SRT-4's pistons, use the quench pads to generate a very controlled swirl in the combustion chamber.

It's generally stated by the performance aftermarket that a smooth controlled swirl is best for emissions and mileage, while quench is best for power. I do not have enough data to confirm either theory. But I can say with some confidence that having either is better than having none at all.

In either case, the net effect is to reduce the time it takes for the mixture to burn. Why? A mixture ignited at 30*ATDC, in a hypothetical engine while cruising, will begin to burn while the piston is still going up. But, because the burn will be completed so much sooner than a burn started at 15*ATDC, the net result is more power until you reach a balancing point. The quicker the mixture burns, the less ignition advance is required, and thus the less time the mixture spends pumping against a rising piston.

Make sense?

 

Here is a picture from
magnusmotorsports

 

It makes sense and sounds incredibly useful. Does a 4G64 come with anything like that? Is it possible to add one in?

Thanks for your great responses.

strike (Dave)

 

Thanks BMan. Very cool.

strike (Dave)

 

As I clearly denoted, I provided a thumbnail sketch. Naturally, we could go on about this in great theoretical minutia for 100+ pages, but the sole purpose of my post was to explain in very general terms that what one observes when actually tuning the car is the fact that boost pressure and ignition timing dance together as part of a balancing act, and there IS a point whereby the timing retard necessary to accomodate a rise in boost pressure offsets any power gain, all else remaining constant.

 

Theory aside, approximately how many degrees BTDC is MBT on the 4G63? I'm at ~11 degrees at peak torque and just wondering how much more i can go. The 11 degrees is with 87 octane gas + water @12.5AFR, will be retuned in 5 days with 91+water. How will change MBT?
Thx in advance!
p.s. it seems that i might be able to reach mbt w/o knocking....

 

That's like asking "What time is tomorrow?". The minimum amount of spark advance required to get to MBT (or as close as you can to it given fuel limitations) varies with boost pressure.

Here are two worthwhile articles I found that may help. They pertain to normally aspirated engines, which makes things easier to understand. Throwing a turbo into the mix adds another dimension to the graph.

http://www.daytona-sensors.com/tech_tuning.html

and

http://www.stanford.edu/~bmoses/knock.html

 

Hi ND, actually it is the opposite.

Lower rod ratio, higher piston speeds around TDC, more advance required.

 

At what point does timing reduction get stupid?

Using totally unrealistic numbers (divisible by ten):

Let's say that through extensive experimentation on a load bearing dyno that I discover my engine makes max power without boost at 100 degrees advanced timing.

Let's say that my turbo will provide 100 PSI max.

When the turbo starts pushing 70 PSI, the engine starts knocking and I pull back timing to compensate. At 80 PSI, I pull back 50 degrees of timing to stop the knock. at 90 PSI, I have to pull back 100 degrees of timing (which leaves me at 0). At the full 100 PSI, I have to go negative 50 degrees of timing to prevent knock.

At what point (percentage) was the timing absurd? Please fill in more appropriate numbers if it helps. I was merely trying to work it out as percentages with easy to use numbers.

strike (Dave)

 

I still like the batter analogy.
If you swing late (retarded timing) you have to swing hard (boost). Or if you swing early you have to swing softer, (lower the boost). In either case the idea is to hit the ball at the sweet spot.
Every car is a little different, grade of gasoline, climate, drivers foot(load cell), all impact where that sweet spot is. So no, no one is going to say, "twenty lbs boost equals ten degrees" because that might put a hole in one guys car, while it works for some one else.

Now unless you have your fuel mix correct you are standing in the batters box with your hat over your eyes.


Ok, to little timing is bad, I don't know what to tell you, if 3 degrees for an instant is to low or what. I just know I wouldn't run a car that low. Let off the boost and let the timing come back up. When you get to that point in your tuning, if you want to play hardball, better do a dyno and have it tuned.



(In slowcar's case he has moved to the back of the batters box) please note he is injecting water and ...