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Turbo/Nitrous Tech

topic #88171

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Subject: "RE: good or bad?"

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XtremeRS	Jul-27-05 06:29 PM
Member since May 20th 2003 6329 posts,

#88283, "RE: good or bad?"
In response to In response to 30
Jul-27-05 06:55 PM by XtremeRS

Originally posted by djtrickee
That is some interesting reading. Is 10psi your max boost? or was the merely the goal of that particular "tune session". I'm just curious if you have experimented with changing the ign. timing with boost and target AFR or if you are tuning to your AFR and THEN adjusting your timing to suit that situation. I'm probably sounding confusing.

That was just a rough tuning session really. For me, 10 psi is a simple, safe boost pressure to tune at, since i have a 10 psi wastegate actuator. I then use the Profec B spec II controller to switch to high boost at 15 psi.

When tuning, the air/fuel ratio should stay more of a "constant", but as boost gets higher, slightly richening the mix will be fine. To make most power out of whatever boost you run, you need the right amount of air/fuel mix, the right amount of timing, for the given engine compression ratio, for the given amount of fuel octane. There is only so much boost pressure you can run with so much fuel octane. Sure, you can start to retard timing to keep detonation away, but now we are back to the high EGT problem again. So lets say to run 20 psi on 93 octane pump gas, we have to have like 15 degrees of timing so it doesnt get any knock. Lets say this results in a high EGT, EVEN with a rich air/fuel of 11.5:1. This situation would not make as much power as it could be. We need more timing at that pressure, but it would knock if we added timing. Basically, we have "run out" of necessary fuel octane for that boost pressure, for a "good" powerful tune.

For max torque to get transfered to the crankshaft, the combustion process needs to do the most work at around 14 degrees ATDC of crank rotation. This is how ignition timing comes into play. How much timing you need is dependant(but not exactly limited to) engine compression ratio, boost pressure, fuel octane, and to a smaller extent, air/fuel ratio. I've found that for 15 psi, 8.8:1 compression, 93 octane, S16G, we need around 18-19 degrees of timing through peak torque, then as rpm gets higher, adding 1 more degree or so up to redline. A/F on the safer rich side of around 11.5-11.8:1 This should keep EGTs in check, keep detonation at bay, and make some good power output.

It seems to me, that 15 psi on the S16G with 93 octane fuel, and 8.8:1 compression ratio, is getting really close to the limits of 93 octane fuel, for a *proper* tune. Not saying we cant run a bit more boost on 93, but A/F and timing will have to be taken away from where we can make more power if we had run a higher octane fuel. Also, a larger, more efficient turbo can possibly run a little bit more boost pressure for a given octane, mostly because they are more efficient at higher boost pressures, which results in lower intake temperatures(remember colder intake temps will also let you run more boost for a given octane) This is why the stock ECU works pretty well for 15 psi, when its doing ignition, as this is really close to where timing needs to be anyway. EGT's will be worse on this type of setup, at lower boost pressures, because the stock ECU is still putting out 17 degrees of timing, where we really need more timing. With a MAP sensor check valve(or FCD) connected, the stock ECU thinks anything over 0 psi is WOT, where it automatically puts timing at around 17 degrees advance. It has no idea of how much boost we are running. This situation is proven with the MS, because i've added like at least 5-8 degrees or so of timing from 0 psi to 5 psi, and i'll tell ya, it pulls so much better with that added timing at lower boost, and even at lower vac of 10-0 hg's. Remember that the stock ECU has timing maps made for 9.6:1 stock compression ratio, and 87 octane fuel. If you built a motor, you prob run 8.8:1 pistons, and 91-93 octane fuel. That ALONE means we need more timing THROUGHOUT the entire pressure/rpm band to make good power. Hopefully this makes sense...I tried to keep it as simple as possible, while there are still other variables to the tuning process for sure...It's really one big balancing act, and it's not as I once thought, just "tuning the air/fuel ratio to EGT's". It's just the one big problem of running boost on a engine(and ECU) that was not meant for it from the factory. This is what makes the Megasquirt(or any other fuel and ignition standalone) much, much superior in every which way, over the SFMU or 8 injector setups. There is def more power to be made over using those types of boosted fuel setups.

13.5 @108 MPH-2.2 60ft(stupid FWD!)
S16G @ 18 PSI/FMIC/Running on MegaSquirt II (Now with sequential
fuel injection)
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