Forum » General Tuning Discussion » Timing vs boost on knock-limited fuel

Timing vs boost on knock-limited fuel

General Tuning Discussion

Discuss all things tuning in this section. News, products, problems and results. 


Page 1
Author
338 Views

Hey folks,

Forgive me if this is a super basic question, I'm deep in the learning process. I live in an area where 91AKI/95RON is the best fuel available. So in almost all circumstances I'll be knock limited on a turbo motor.

Let's say that I tune my [RB26-based] car to its spring pressure of, say, 11PSI. I find my MBT within the confines of knock at that boost pressure first and then start tuning for higher boost levels.

From general experience I know that 17-18PSI is about as far as other tuners take these motors on our terrible fuel. Obviously I'm massively generalizing here - turbo choice and engine specification will play a big role. But for the sake of this discussion let's say that tribal knowledge has taught me that that'll be my upper boost limit. And I know that as I start to raise boost I'd expect to see knock come in quickly.

Is the "best" practice to raise boost levels until knock begins? Or should I pull timing, raise boost levels to an anticipated level, and then bring timing back in until knock? Either way it begs the question of whether I'd be better off at higher boost/reduced timing or lower boost/MBT timing. Granted I anticipate this being a very circumstantial judgment call, but I'm not currently quite sure about what *process* I should employ to determine the benefit of boost vs timing on a knock-limited motor/fuel combination.

I can imagine a world where I'm running 11PSI with near MBT timing and a world where I'm running 22PSI with extremely conservative timing to reduce knock. Somewhere in between those point is probably the right combination. What should my process be to find it?

Here's the thing. There are platform-specific tuning rules of thumb on the internet that develop organically over time through various forms of media. In the old days it was mostly web sites and car forums, and now it's split among different things like video, social media, etc. What you'll find is that nobody wants to stray too far from the tried and true, because there's money on the line (from blowing the motor) and they don't want to get pilloried by their peers if something goes wrong. It's similar in the rotary community, where about 15-18psi is considered max on pump only fuel, which is certainly based on some hard-learned lessons.

If you have the dyno time/budget available, you can try different combinations of boost and timing on a dyno and see where it goes. Modern stock ECUs (think GM, anything German, definitely not Subaru) have algorithms that dynamically change timing, AFR, and valve timing to achieve the most fuel efficient way of generating torque. They have a bunch of models built into the ECU that are tuned on an engine dyno and in expensive emission labs for optimization. For hobbyists, you don't have those kinds of resources normally.

So what's the process? The normal process for an old platform with a standalone ECU like that is to lean heavily on the aforementioned tribal knowledge for a base map, put it on a chassis dyno and adjust AFR in about 0.2:1 increments (if possible) timing in 1 or 2 degree increments, and boost in about 2-3psi increments. You end up with a narrow range of combinations to try than you might really expect if you started from scratch with zero tribal knowledge.

Are you really going to run 12.5:1 when every tune out there runs 10.5:1 - 11.5:1 ? are you really going to run 23psi when nobody goes over 16? Are you going to do 20 degrees of timing when rule of thumb says 15? Sure, you could, but there's a strong disincentive to do so.

Some of this comes down to a personal preference, however on a low octane pump gas you will inevitably get to a point where you simply find a limit. What I mean by this is that as you raise the boost the engine knocks so you need to pull timing and end up making essentially the same power. This is quite apparent when it happens and on our local fuel in NZ this is often around the 22 psi mark or thereabouts.

Ultimately you're not going to see the same power at 11 psi with MBT timing that you'll see at 18 psi with more conservative timing so your power aim will drive your decision. My personal preference is to run a little more boost and a little more conservative on the timing to achieve my power targets while offering a reasonable margin of safety to the knock threshold.