DriftinJim

iTrader: (24)

Just wondering how many of you add some timing with your aux injection setup. I have stock twins with the normal bolt-ons (intake, full exh, bigger smic, twinpower, etc) and it seems like w/o adding at least a couple degrees of timing, I am leaving power on the table. I've read of some people adding as much as 10 degrees but I'm sure they run more intense aux setups and are pushing for every last bit. Also, are you adding it throughout the range or just before and after peak torque?

I know that adding timing typically involves dynoing and watching for torque output to fall off, but with the added safety of aux inj and starting with a conservative timing map, I feel you can add a couple degrees without approaching the edge. Just looking for others' experience.

arghx

iTrader: (3)

Yes. Leaving power on the table is how you keep your engine from blowing up, especially on engines without knock control. That's not what people want to hear and it's not a popular message. When you play with fuel, timing, etc you are in essence figuring out the marginal benefit as you get more aggressive with the tune. What that means is that I am always asking myself--how much additional power did I get for the additional timing (or leaning of the mixture)?

One method is to calculate the % change in horsepower divided by the number of additional timing degrees.
You're basically looking at first and second derivatives as you add timing, and then ultimately look for inflection points to make your decision. It's a bastardized elasticity type of calculation if you've ever taken an economics course. Call it an "improvement ratio."

A simplified scenario: I start with 378 horsepower. I add two degrees of timing and I am now at 385 horsepower. Then I add two more degrees and I'm at 391. Then I add two more degrees and now I'm at 394. Then I add 1 degree and get 396 horsepower. Where do I "stop"?

1. 2 degrees of timing: 378 --> 385 horsepower

% change (using this calculator) : +1.851 %

timing added: 2 degrees. 1.851 / 2 = .9255

2. 2 degrees of timing: 385 --> 391hp % change: +1.558

timing added: 2 degrees. 1.558 / 2 = .7790

3. 2 degrees of timing: 391 --> 394hp % change +.7672

timing added: 2 degrees. .7672 / 2 = .3836

4. 1 degree of timing: 394 --> 395hp % change +.2538

timing added: 1 degree. .2538 / 1 = .2538


Again I ask, where do we stop? Do we keep adding timing until we start losing power, assuming we don't knock first? Do you keep adding timing until you knock and then back off? You can use marginal analysis again, this time using an analysis of our improvement ratios. So I'm really asking the question: how much additional power (% wise) do I get as I add another degree of timing? And how much additional power % do I gain with this additional degree of timing compared to the last degree of timing I just added?

We don't have a given improvement ratio to start with, so we take the percent difference from run #1 and #2 and then go in order.

1. % difference in "improvement ratio" of run #1 and #2: using .9255 and .7790 , we get -15.829% .

2. % difference in "improvement ratio" of run #2 and #3: using .7790 and .3836 . we get -50.757% marginal loss in power improvement.

3. % difference in "improvement ratio" of run #3 and #4: using .3836 and .2538 . we get -33.837% marginal loss in power improvement.

And in this scenario, as you often encounter in real life, it's not a nice linear relationship and predictable relationship. The whole thing is kind of messy. So I'll ask you--how much timing would you add in the above scenario? Hint: there's no "correct" answer.

arghx

iTrader: (3)

Sure, but it's hard to draw any conclusions without knowing a lot about where they started from (the old timing map vs the new map).

You almost answered your own question. Put in practice, it's mostly rules of thumb and methodical guess-and-check. You add timing wherever the engine likes it. How much dyno time can you afford?

You have to understand the difference between tuning a boosted engine with trustworthy knock control, like an Evo or an STi, and an engine with effectively no knock control. "The edge" you speak of is a moving target. On newer engines with a programmable factory ECU and good knock control, generally speaking the knock sensor will bail you out when the weather changes and you can run a narrower safety margin.

tom94RX-7

iTrader: (5)

I know I added 3 to 4 degrees timing to a already conservative leading timing map when I was running a mix of race gas with the stock twins and high boost, and I made more power and low or no knock. I was also going to add 2 degrees to my conservative pump gas map once I get my fuel map tuned with my new single turbo and pre turbo water injection and do some testing. I figured 2 degrees would be safe. From experience I know that I lost a few HP with wi in the past so I'm hoping to gain that back with better tuning. also make sure your afr is not too rich cause you will lose a lot of HP especially with water.

DriftinJim

iTrader: (24)

Yes, I understand it becomes depreciating returns. That is why I suggested only adding a couple degrees of timing, which per your scenario (tho I understand is not exact but based in fact for example purposes), actually had the most effect. ie add a couple degrees of timing to realize safe gains to make up for the WI losses, similar to what Tom said after you. I was just wondering if others had the same idea or anyone had dynoed with any particular results.

Sgtblue

iTrader: (16)

I didn't notice any change in the "butt dyno" after WI and considered turning it off for tuning. For me it's just there for safety and I don't want to worry if the system should fail or I run out of water.