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Old 05-25-2002, 08:50 PM   #1
2OHOH2WRX
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Question Will less fuel in the lower RPM range bring on faster spool-up?

Will less fuel in the lower RPM range bring on faster spool-up?

If a large portion of a turbo's spool-up is due to thermal energy, will leaning out the engine in RPM ranges where there's little to no boost (boost is building) help reduce turbo lag / reduce spool-up time / lower the onset RPM of max boost?

It is a theory that I'd like some feedback on. I like my VF23, but I think I would love it if could hit max boost below 3800 RPM, so will this work? If not, would anything, in terms of Link tuning?
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Old 05-25-2002, 10:22 PM   #2
alfriedesq
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I made more spool up with rich and retard timing down low
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Old 05-25-2002, 11:26 PM   #3
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Quote:
Originally posted by alfriedesq
I made more spool up with rich and retard timing down low
I am tempted to say otherwise ie lean out and run more advance down low (relative to the limits of your state of tune, of course) but this has undesirable side effects. Theoretically, leaning out fuel and increase advance works because it also increases EGT - increases gas flow velocity = increase turbine wheel rotation, etc. However, this works fine in lower gears 1-2-3 where there isnt yet enough load to potentially overboost. But come gear 4 & 5 boost peaks very early and usually in the fuel and ignition zones that you made the above adjustments to. There is a chance detonation would occur as a result of high EGTs and the onset of peak boost (again depending on your boost levels, and also WG responsiveness).

My experience with the VF23 turbo is that it is quite lethargic compared to the VF24 or VF28. I have tuned a ver 4 WRX running a VF23 and the car exhibited no "kick" when boost peaked at around 4000 rpm @ 1.2 bar! Quite sedate compared to mine running the same boost levels and VF24.

I think Al's works safer. The higher boost you run, ignition timing needs to be appropriately retarded with fuel enrichment.
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Old 05-26-2002, 01:55 AM   #4
2OHOH2WRX
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Both interesting points.

I'm already running very rich in that range, so maybe some minor, gradual timing changes might get me closer to my goal (no AWD dyno here ).

Jan - that's the basis for my theory (less fuel --> high EGT --> faster spooling). I'm using the Link for everything but boost control. That I handle with the AVC-R; it is redundant, but I like the instant accessibility and features of the AVC-R as compared to the hassle of the Link boost control. Having said that, I think I could tame any overboosting/spiking with the AVC-R. Maybe it's my lack of experience with other turbo systems, but the 23 seems to have a pretty good kick around 4000 RPM. "Lethargic" is a very good word to describe its behavior before 4K, but after it just rips; at least that's how I perceive it, coming from the stocker. Maybe another turbo (such as the 24) would really open my eyes, but it seems that the 23 is pretty good.
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Old 05-26-2002, 11:51 AM   #5
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I gained 25 lbft at 2500 RPM on my hybrid TD04 from adding 4 degrees advance in this area and adding fuel. Most of the JECS ECUs I have logged seem to be lean during spool up and suffer from it espeically if fitted with induction kits.
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Old 05-26-2002, 12:09 PM   #6
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Like John says, low down you want plenty of fuel, and advanced ignition. This gives TORQUE.

Retarding the ignition, will give more boost, but will DRAMATICALLY reduce TORQUE, and comparatively, the car will be very slow.

The VF23 is more than capable of hitting 22psi at 3500rpm, if it's set up correctly, with fuel, ignition, and boost control, and you'll still have loads of torque.

Don't get too side tracked with boost, low down. It can be very misleading.

Happy.
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Old 05-26-2002, 08:08 PM   #7
Jan Shim
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Quote:
Originally posted by Happy
Like John says, low down you want plenty of fuel, and advanced ignition. This gives TORQUE.

Retarding the ignition, will give more boost, but will DRAMATICALLY reduce TORQUE, and comparatively, the car will be very slow.

The VF23 is more than capable of hitting 22psi at 3500rpm, if it's set up correctly, with fuel, ignition, and boost control, and you'll still have loads of torque.

Don't get too side tracked with boost, low down. It can be very misleading.

Happy.
It's true for zones below which the turbo kicks hard (peak boost) to give torque. After that, it's the turbo that gives TORQUE. The immediate fuel and ignition zones before and during peak torque are crucial to ensure you get plenty of usable power without pinging the engine. Retarding the ignition does NOT give more boost but will definitely reduce torque. Retarding ignition ALLOWS you to run more boost (relatively speaking) .. boost levels and ignition advance/retard are inversely proportional to each other. Then there's engine compression ratio which is also a factor whether you can run that much advance effectively (again without detonation the engine).

From experience, depending on the boost controller's behaviour (aggresive vs slow rising), Happy's setup with high advance might work if you're Link is controlling the boost solenoid with a rather conservative WG Sens/RPM number. Those running EBCs might find them to (boost) peak way too fast, triggering knocks!

I have even seen my stock solenoid/VF24 producing a knock in 5th gear when I slam on the throttle at 6000 rpm. Call it 'transitional knocks' or whatever, I dont believe it is acceptable. A knock is still a knock! Appropriate use of the KnockLink saves (engine) LIFE!
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Old 05-26-2002, 08:34 PM   #8
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Jan,

Yes, maybe I should have made it clearer. When I said "low down" I was referring to "low boost", or where the turbo starts to spool up.

I didn't mention compression ratio, because we were talking about stock engines, but CR certainly has an effect on an engines spool up, and ignition profile.

Transitional det' is not just a Link quirk, you'll get it with any ecu, if the ignition, and or boost controls are incorrect.

I've just found that if a turbo is spooling up as fast as it wants to, assuming the wastegate is shut, then you will get better performance by advancing the ignition, rather than retarding it, to induce the turbo to spool up even quicker.

People just assume that if they can get more boost, at a given point, it will give them more power, and this isn't always the case.

Happy.
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Old 05-26-2002, 09:58 PM   #9
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We found 5 - 8 HP on the dyno all the way up the powerband just by retarding the timing right before the turbo starts to spool hard by 4 degrees - making the EGT's hotter right before you spool up by retarding timing and running unburnt / burning fuel out the exhaust valve give you more boost slam

BUT it does make the car very letharic about town

More advance down low feels better when off boost

This is where the dyno is very helpful
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Old 05-26-2002, 11:10 PM   #10
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I'd look to the RPM point that your trubo boost starts to rise up

try retarding it a bit down there and see what happens -- wee if you can feel better mid range as the turbo spools
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Old 05-26-2002, 11:11 PM   #11
2OHOH2WRX
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Wow, thanks for all the practical responses.

Yes, I did more or less assume that more boost equals more power. Since retardation is the ECU's primary defense mechanism against prolonged knock, and it has generally been seen on this board as a bad thing, I would not have guessed that it has a certain benefit before the onset of boost (sounds like a low-grade, poor-man's anti-lag system Al ).

So... since I don't have a dyno, what should I use for AF ratios and timing to help me tune (by this I mean after looking at datalogs, what numbers I should be looking for)? This is a bit OT for this thread, but it is all in the name of getting the perfomance out of my mods...

This is what I've understood from the responses:

- advance + more fuel well before max boost --> more usable torque --> better 60' times (etc)

- some retarding right before max boost has a similar effect

- the two above combined work in synergy

Quick-n-dirty mod list: VF23, Blitz FMIC, AVC-R, Link, 550s, 255lph Walbro, Vishnu fuel rail kit, manifold-back.
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