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Old 10-09-2014, 11:41 AM   #1
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Default Higher boost targets @ low rpm -> lower targets near power peak

I've read many times that ultimate power production is the limiting factor for our engines, not peak boost. This makes sense, as the manifold pressure is just a fraction of the pressure of combustion.

However, the common pressure target distribution ramps up as quickly as possible and then stays relatively constant to redline. It would seem as if this approach does not utilize the available strength of the engine at lower rpm.

Example case (imaginary numbers, could be way off):

1.8 w/ GT2560r @ 12 psi, 230 hp @ 6200, 215 wtq @ 5000

The engine should be within its safe limits at peak power and torque.

However, at 3500 rpm, the engine only produces 140 wtq. What would happen if the boost initially ramped up to 15 psi at 3500 rpm and tapered down as the torque peak approached? As long as the torque/power produced did not exceed the limits of the bottom end, would this be a valid method of producing more low end torque/power?
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Old 10-09-2014, 11:47 AM   #2
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Too tired to give you a thorough explanation.
But,
the earlier in the rev range that you make torque the more stress you're putting on the rods.
Also if you were to post a chart, most would show that while it may make 215 @ 5 it still makes close to 200 by 3500, so it's not as bad as you're describing. At least for us that are at sea level. Most of the 2560 plots I see make 200 at or just before 3500. The good setups actually make it closer to 3k

What chart are you looking at btw? This discussiong is utterly pointless if we can't all look at the same one.
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Old 10-09-2014, 12:13 PM   #3
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What chart are you looking at btw? This discussiong is utterly pointless if we can't all look at the same one.
Sorry, I wasn't referencing a specific chart. I was pulling random numbers to clarify my theoretical question. I guess that got me into trouble.

I wasn't aware of the torque being so high at ~3k. As a 1.6, mine wouldn't have that output, though. However, your comment about the lower torque limits at lower rpm satisfies some of my question.
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Old 10-09-2014, 12:22 PM   #4
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Another thing to consider is your engine's capability to generate enough exhaust flow and ingest enough intake air to even allow the turbo to hit that kind of pressure at that low of an rpm threshold. In your case, that wouldn't happen. At least never in the 1:1 gear or lower.
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Old 10-09-2014, 12:48 PM   #5
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Fair point.

This question was prompted by an experiment with open-loop boost control. I like the spool response when pulling from 2000 rpm, which I haven't yet been able to attain with the LINK closed-loop system. However, there exists the obvious problem of low-rpm overboost at WOT in 4th/5th. I was wondering how bad such overboost could actually be for the engine. I can't pretend my engine could spool those pressures at low rpm in lower gears.

Of course, my goal is to tune the LINK closed-loop better, but I was still curious.



Yes, yes, MS3x, when I can afford it.
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Old 10-09-2014, 01:03 PM   #6
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what do you mean by overboost? if you're overboosting you have a mechanical problem or a tuning problem. it's a problem that needs to be fixed, regardless of which gear and which threshold it happens at.
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Old 10-09-2014, 01:11 PM   #7
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When messing around last night, I set the WG duty cycles to hold 12 psi at WOT through the rpm range in a 2nd and 3rd gear pull. This resulted in the WG being basically sealed until 3000 with it beginning to be mostly open by 4000 rpm. It worked brilliantly in the lower gears.

Of course, when subjected to higher load at those lower rpm ranges, the fixed sealed WG allowed more than 12 psi. Much more. I didn't push it like an idiot, but I could see it wasn't stopping at 12 or 13.

I called that overboost because it exceeded my intended target. Yes, it was due to hilariously amateur but knowingly incomplete tuning.
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Old 10-09-2014, 02:08 PM   #8
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Compressive stress on the rods is a function of torque production and how close you are to MBT; if you are past MBT the peak stress goes up greatly.

Peak tensive loads are proportional to RPM squared. Other things like to break a thigh rpm - throttle shafts and oil pumps.

Thermal loads (EGT) go up with power production. However it's also a strong function of ignition timing.

So power output isn't the only stressor...
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Old 10-09-2014, 04:03 PM   #9
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Quote:
Originally Posted by JasonC SBB View Post
Compressive stress on the rods is a function of torque production and how close you are to MBT; if you are past MBT the peak stress goes up greatly.

So power output isn't the only stressor...
Jason, by, "past MBT" do you mean more idle advance than that which produces MBT?

*edit: strike through "idle"

Last edited by DNMakinson; 10-09-2014 at 09:40 PM.
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Old 10-09-2014, 08:23 PM   #10
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Not idle... more advance than MBT, produces very high rod compressive stress, even without knock.
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Old 10-09-2014, 09:38 PM   #11
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Jason,
I don't know how "idle" came out. I meant to say "spark", or just plain "advance", which you just verified was indeed what you were referring to. I presume further that the effect is to make too much expanding gas before the 90 degree ATDC which produces the maximum torque. Makes sense.

So another reason to tune to MBC, even if the engine is not knock limited.
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