Lets talk spark tables

[Watson]

Okay so my L28ET is built, running, and super awesome. Its loud and goes pretty fast. I'm currently doing road tuning before I hit the dyno and I wanted to get some feedback from the interwebs to see if I'm on the right track. So the engine setup is a N42 block and head with dished pistons. I believe this should be a CR of ~ 8.5:1. The turbo is a stock L28ET turbo with a larger compressor housing (I didn't take measurements before I installed it. The PO claimed it is a Super 60) running at 7 psi. I'm running a FMIC and I believe the fuel map is pretty well dialed in. I'll post a picture of the fuel map, current timing map, and projected timing map.

 

My question is, how much advance would you, the experienced reader, run on a setup like this? I feel like my current spark map is super conservative. I did run much more advance before the current map and let me tell you, traction was a real issue. If I crank the timing way up to the projected map (in the range above 100 kpa), and given my current setup, how much of an issue do you think knock would be? You see, I don't have a knock sensor and the exhaust.... well, the exhaust exits the front fender so I may never hear knock.

 

So tell me, with your experience with turbo L28 engines, when did you experience knock? How did you know it was knock? How high would you recommend I crank up the advance?

 

Fuel map. While cruising (below 100 kpa), the AFR is ~ 14.7. The AFR for max torque (estimated at 4500 rpm, full boost) is ~ 12.

 

 

Current spark map

 

 

Projected spark map

 

[datzenmike]

First off I'm no expert on this topic but it's damn interesting to discuss.

 

 

 

7 psi is about 50 Kpa correct?

 

What's the stock timing advance on an L28? About 10-12 degrees? (just guessing based on the L 4 cylinder)

 

At full throttle at any RPM, vacuum advance isn't an issue so this leaves mechanical advance which is RPM dependent. Mechanical, plus initial (or static) advance should max out at 34-36 degrees no later than 3,000 RPMs. I see 38 to 45??? which seems a bit high?

 

Timing advance should be such that maximum cylinder pressure is reached at a sweet spot of around 17 degrees ATDC where it has maximum effect on the slow downward moving piston.  Too late and the pressure is chasing a rapidly accelerating piston. Too soon and cylinder pressure spikes and any unburnt fuel detonates.

 

Boost conditions or higher than normal cylinder filling will reduce the need for advance. In other words the fuel burns faster so start the burn later.  It would seem that some kind or timing retard should be built in to compensate for the increased compression.

[Watson]

7 psi is ~ 50 kpa but keep in mind that 100 kpa is atmospheric pressure so 150 kpa would be max boost for this application. The higher value is there in case of a boost spike scenario.

 

I get that timing should max out at 34-36 degrees but with this setup I am able to run low IATs with proper fuel and based off the butt-dyno, no noticeable surge or stuttering was felt, so it was assumed that there was no knock. In fact, the 40s range felt quite smooth for cruising so we left it at that.

 

I think there are a lot of variables to consider here. Quench area, cylinder head temps, AFR, CLT, IAT.

 

A sweet spot of 17 ATDC? That would be an interesting variable to record; the sweet spot of downward motion on a piston in relation to flame speed.

[DanielC]

"You see, I don't have a knock sensor and the exhaust.... well, the exhaust exits the front fender so I may never hear knock."

 

I am guessing you already have a lot of time, and money invested in this engine.   I would suggest you either get a knock sensor installed, so you know when you are approaching too much advance, or quiet the exhaust down, so you can hear it.

 

Or you could just have a nice story to tell about how good it was running, right before it blew up.

[Watson]

"I am guessing you already have a lot of time, and money invested in this engine."

 

Time? Yes. Money? No. Quite the exhaust down???? Sir.... But yes, a knock sensor would be nice.

 

[datzenmike]

That sweet spot is approximate. Things like longer rods delay the piston slightly longer approaching and leaving TDC but accelerate away faster. The sweet spot is where the most energy is transmitted to the piston as it starts down the cylinder but is still moving slowly.

 

Yes variables such as lower IAT and quench will reduce pre ignition compared to higher temps and the same compression with no quench but I don't think it allows more advance even in a normally aspirated engine let alone a forced induction. What if you build in some retard at high boost ? Does the butt dyno notice?

[DrRocks0]

You want that AFR much lower!  Shoot for 11-11.5!  Are you running a electric boost controller or a manual boost controller?  If you have a electric boost controller please post your target boost table, the fuel and ignition wont make any sense if I don't know where your target is.  If not can you guess your peak boost RPM?  You could have have 30 degrees at max boost and I would never know.  Also like stated above a knock sensor and an AFR would be much better.  You don't know if you are hitting your targets and the factory o2 sensor probably won't rear rich enough.  That is why you have to make fueling tables instead of letting the computer control all this ;)

 

I did a lot of tuning on subarus.... so I can give you some rule of thumb info but if there is anything specific to this motor I probably won't know it. 

 

edit: Ok after looking at your tables I am super confused lol.  totally different setup then I would do it.  If you can can you switch the x/y axis and put it in psi not kpa? 

[zed]

"I am guessing you already have a lot of time, and money invested in this engine."

 

Time? Yes. Money? No. Quite the exhaust down???? Sir.... But yes, a knock sensor would be nice.

 

call me a redneck, but that L28 sounds damn nice...