Lean spool for turbo freaks

General engine tech -- Drag Racing to Circle Track

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ptuomov
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Re: Lean spool for turbo freaks

Post by ptuomov » Wed May 16, 2018 9:35 am

vwchuck wrote:
Wed May 16, 2018 9:25 am
To build boost out of the powerband you add fuel and retard the timing. Then the burn happens in the exhaust system and increases your back pressure to spin the turbo faster almost instantly. This is also know as the anti-lag system in most aftermarket EMS's..
Anti-lag really only works in operating modes where the driver doesn't request maximum torque. That is, during the shift or during part throttle. That's when there's an excess of air flow that can be burned with fuel to run the turbine.

When the driver is requesting maximum torque, there's no such obvious source of excess air. For every oxygen 2 molecule that is burned in the exhaust manifold, there's one less oxygen 2 molecule burned in the combustion chamber and thus less torque applied to the crankshaft. That's the issue I'm struggling with.

Since fuel can't burn without oxygen, just adding fuel to an already rich misture isn't going to cause more reaction. It can slow down the burn, but retarding ignition timing may be a more efficient way to slow down the burn? Or may not.

One logical reason why excess fuel might work is that it may efficiently use heat of the combustion to vaporize and expand, converting heat into pressure, thereby cause the turbine run like a steam engine. But I haven't worked out the physics of that.
[b]Paradigms often shift without the clutch[/b] -- [url]https://www.youtube.com/watch?v=cxn-LxwsrnU[/url]

3pedals
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Re: Lean spool for turbo freaks

Post by 3pedals » Wed May 16, 2018 10:04 am

You are asking the tuneup to fix an engine/turbo combination problem.
What you need is an engine that has the torque desired where you are operating it, or a turbo that will spool up in that operating range
ptuomov wrote:
Wed May 16, 2018 9:35 am
vwchuck wrote:
Wed May 16, 2018 9:25 am
To build boost out of the powerband you add fuel and retard the timing. Then the burn happens in the exhaust system and increases your back pressure to spin the turbo faster almost instantly. This is also know as the anti-lag system in most aftermarket EMS's..
Anti-lag really only works in operating modes where the driver doesn't request maximum torque. That is, during the shift or during part throttle. That's when there's an excess of air flow that can be burned with fuel to run the turbine.

When the driver is requesting maximum torque, there's no such obvious source of excess air. For every oxygen 2 molecule that is burned in the exhaust manifold, there's one less oxygen 2 molecule burned in the combustion chamber and thus less torque applied to the crankshaft. That's the issue I'm struggling with.

Since fuel can't burn without oxygen, just adding fuel to an already rich misture isn't going to cause more reaction. It can slow down the burn, but retarding ignition timing may be a more efficient way to slow down the burn? Or may not.

One logical reason why excess fuel might work is that it may efficiently use heat of the combustion to vaporize and expand, converting heat into pressure, thereby cause the turbine run like a steam engine. But I haven't worked out the physics of that.

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Re: Lean spool for turbo freaks

Post by ptuomov » Wed May 16, 2018 10:24 am

3pedals wrote:
Wed May 16, 2018 10:04 am
You are asking the tuneup to fix an engine/turbo combination problem.
What you need is an engine that has the torque desired where you are operating it, or a turbo that will spool up in that operating range
If there tuning way to extend the power band to lower rpms, why wouldn't I use it? I could either get a broader power band for the current engine, or this would allow me to go up one size on the hot side and get a lower back pressure at high rpms while keeping the lower rpm end of the power band constant. In other words, why not optimize this?
[b]Paradigms often shift without the clutch[/b] -- [url]https://www.youtube.com/watch?v=cxn-LxwsrnU[/url]

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Re: Lean spool for turbo freaks

Post by 3pedals » Wed May 16, 2018 10:33 am

Please re-read my post

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Re: Lean spool for turbo freaks

Post by ptuomov » Wed May 16, 2018 10:37 am

3pedals wrote:
Wed May 16, 2018 10:33 am
Please re-read my post
Did reread it. What’s your proposed next step?

I personally think that I have nicely matched combo for going fast (high average hp between shift point rpms), but since it’s a street car I’d like to get the more torque at low rpms if I can do so without compromising what it does when I want to go fast.
[b]Paradigms often shift without the clutch[/b] -- [url]https://www.youtube.com/watch?v=cxn-LxwsrnU[/url]

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Re: Lean spool for turbo freaks

Post by Scotthatch » Wed May 16, 2018 11:11 am

ptuomov wrote:
Wed May 16, 2018 10:37 am
3pedals wrote:
Wed May 16, 2018 10:33 am
Please re-read my post
Did reread it. What’s your proposed next step?

I personally think that I have nicely matched combo for going fast (high average hp between shift point rpms), but since it’s a street car I’d like to get the more torque at low rpms if I can do so without compromising what it does when I want to go fast.

Though I am new to turbos my understanding is you change the AR on the hot side

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Re: Lean spool for turbo freaks

Post by ptuomov » Wed May 16, 2018 11:14 am

Scotthatch wrote:
Wed May 16, 2018 11:11 am
ptuomov wrote:
Wed May 16, 2018 10:37 am
3pedals wrote:
Wed May 16, 2018 10:33 am
Please re-read my post
Did reread it. What’s your proposed next step?

I personally think that I have nicely matched combo for going fast (high average hp between shift point rpms), but since it’s a street car I’d like to get the more torque at low rpms if I can do so without compromising what it does when I want to go fast.
Though I am new to turbos my understanding is you change the AR on the hot side
If I reduce the turbine housing size or A/R I’ll of course get it to spool earlier. But now the high exhaust manifold back pressure and high wastegate mass fraction hurt the top end power.
[b]Paradigms often shift without the clutch[/b] -- [url]https://www.youtube.com/watch?v=cxn-LxwsrnU[/url]

mekilljoydammit
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Re: Lean spool for turbo freaks

Post by mekilljoydammit » Wed May 16, 2018 11:45 am

... don't have too much new insight, but why *wouldn't* you try to get your existing combination to work as well as possible? It's not "oops, I mismatched the turbo to the engine" it's "I'm happy with the midrange to top end, what's the widest I can make the powerband with those parts?"

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Re: Lean spool for turbo freaks

Post by Scotthatch » Wed May 16, 2018 11:50 am

ptuomov wrote:
Wed May 16, 2018 11:14 am
Scotthatch wrote:
Wed May 16, 2018 11:11 am
ptuomov wrote:
Wed May 16, 2018 10:37 am


Did reread it. What’s your proposed next step?

I personally think that I have nicely matched combo for going fast (high average hp between shift point rpms), but since it’s a street car I’d like to get the more torque at low rpms if I can do so without compromising what it does when I want to go fast.
Though I am new to turbos my understanding is you change the AR on the hot side
If I reduce the turbine housing size or A/R I’ll of course get it to spool earlier. But now the high exhaust manifold back pressure and high wastegate mass fraction hurt the top end power.


Everything in life is a balance


I have a car now that has this problem and nothing so far has helped .... what I am left with is A/R change or break into the engine and advance or change the camshaft ....

Which is why I am watching this thread

3pedals
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Re: Lean spool for turbo freaks

Post by 3pedals » Wed May 16, 2018 12:16 pm

ptuomov wrote:
Wed May 16, 2018 10:37 am
3pedals wrote:
Wed May 16, 2018 10:33 am
Please re-read my post
Did reread it. What’s your proposed next step?

I personally think that I have nicely matched combo for going fast (high average hp between shift point rpms), but since it’s a street car I’d like to get the more torque at low rpms if I can do so without compromising what it does when I want to go fast.
The best solution depends on what is causing the problem.
Is it the cam preventing the combo from making the low rpm torque?
Or is the turbo simply being operated below its boost threshold?
You can tune the fuel and timing to produce as much tq as possible below boost threshold, but you cannot lower boost threshold by tuning the fuel/timing.

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Re: Lean spool for turbo freaks

Post by ptuomov » Wed May 16, 2018 2:00 pm

3pedals wrote:
Wed May 16, 2018 12:16 pm
The best solution depends on what is causing the problem.
Is it the cam preventing the combo from making the low rpm torque?
Or is the turbo simply being operated below its boost threshold?
You can tune the fuel and timing to produce as much tq as possible below boost threshold, but you cannot lower boost threshold by tuning the fuel/timing.
The cam runs very well 4000-6000 rpm and is very drivable at idle. I don't see the cam being a problem, and going smaller would hurt at 6000 rpm. The turbos don't have an independent boost threshold, but as installed of this engine, they currently produce 2.5 psi at 3000 rpm, 4 psi at 3500 rpm, and 15 psi at 4000 rpm, at which point the wastegate is already passing exhaust gas.

Tuning the engine in the 2500-4000 rpm range to maximize torque has two effects. First, the direct effect of making crankshaft torque from the air I get at given boost pressure. Second, the indirect effect of feeding more and hotter exhaust gas to the turbine to determine the boost pressure. It's actually a reasonably interesting problem to think about how to trade off the two to maximize torque below the rpm at which the wastegates open.
[b]Paradigms often shift without the clutch[/b] -- [url]https://www.youtube.com/watch?v=cxn-LxwsrnU[/url]

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Re: Lean spool for turbo freaks

Post by Big Al » Wed May 16, 2018 3:55 pm

3pedals wrote:
Wed May 16, 2018 12:16 pm
You can tune the fuel and timing to produce as much tq as possible below boost threshold, but you cannot lower boost threshold by tuning the fuel/timing.
Yes you can. If you read my old post you could understand how :)

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Re: Lean spool for turbo freaks

Post by 3pedals » Wed May 16, 2018 4:58 pm

Big Al wrote:
Wed May 16, 2018 3:55 pm
3pedals wrote:
Wed May 16, 2018 12:16 pm
You can tune the fuel and timing to produce as much tq as possible below boost threshold, but you cannot lower boost threshold by tuning the fuel/timing.
Yes you can. If you read my old post you could understand how :)
I agree 100% with your method, I use it on my drag car.
However it requires a momentary stutter, whether that is while on a t/brake, or rolling down the road.
The OP seems to think he can tune the boost threshold lower without the intentional misfire, and I disagree.

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Re: Lean spool for turbo freaks

Post by ptuomov » Wed May 16, 2018 6:17 pm

Big Al wrote:
Wed May 16, 2018 3:55 pm
3pedals wrote:
Wed May 16, 2018 12:16 pm
You can tune the fuel and timing to produce as much tq as possible below boost threshold, but you cannot lower boost threshold by tuning the fuel/timing.
Yes you can. If you read my old post you could understand how :)
This is for a manual car that's already moving when the driver changes his mind from a cruise to going fast. It's not an automatic on a standing-start drag strip.

During shifts, I have a way to keep the turbine hot. That's not a problem because I don't need to produce torque at the crankshaft and I can therefore spend the air on keeping the turbine running.

What I'm thinking about is cruising at 2000 rpm, suddenly getting some sort of bad idea, flooring it with downshifting, and accelerating as hard as possible without changing gears.
[b]Paradigms often shift without the clutch[/b] -- [url]https://www.youtube.com/watch?v=cxn-LxwsrnU[/url]

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Re: Lean spool for turbo freaks

Post by 3pedals » Wed May 16, 2018 6:40 pm

My car was a turbo stick combo before I decided to put the converter in it.
That setup had boost threshold of ~3100rpm.
No amount of timing/fuel tuning could lower that, without employing a rolling anti-lag. I simply didn't stand on it unless I was above that rpm

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