Lean AFR's to Increase Spool?

[mach4g63t]

I read somewhere (can't find the thread for my life) about being able to lean out your AFR in the bottom end to promote faster spool; hotter gas moves faster.

Does anyone know how valid this is, how much it will help, and how much you can lean it out to be safe? Anyone that can chyme in with experience and real world application? I just want to lean it out to help achieve full boost and then go back to a safely tuned AFR once full boost has been achieved.

[talonDSMerr]

I read about this also, and I'm pretty sure it is very valid. Another way people have went about doing this is retarding the timing during spool, which has the same basic concept behind it (gas and air will be burning closer to when the exhaust valves open creating hotter exhaust gases as well).

I have not had firsthand experience with this however, my car's in a shop getting the pedal assembly welded to eliminate play, but I'll be trying this as soon as I get it back. Good luck tuning!

[1992awdlaser]

What you want to do is retard the timing and add fuel pre-spool.

Retarding timing will make it spool faster. It does this because more of the heat from the power stroke goes into the exhaust manifold. I will try to explain each.

When timing is advanced the spark ignites the air/fuel mixture at a sooner point. This gives the mixture more time to burn in the cylinder. Most of the heat from this will be absorbed by the engine itself. The reason this makes more power is because the mixture has a longer time to burn before the piston reaches tdc. Since it has more time to burn it is going to make the pressure pushing down greater. This is where the exrta power comes from. More pressure downwards when the piston is at tdc. This is also why advancing timing can create knock or pre-ignition.

Retarding the timing makes the mixture start to burn closer to tdc. This will make less power since the mixture has less time to expand before the piston reaches tdc. Retarding will cause some of the mixture to be burning when the piston is moving down and when moving up on the exhaust stroke. Since the mixture burns later the heat has less time to be absorbed by the engine and as a result it will go into the exhaust manifold which will help spool the turbo. The mixture burning during the exhaust stroke also causes the turbo to spool sooner since the mixture is burning with the exhaust valves open. This is how antilag works, it retards timing so the mixture is burning when the exhaust valve is open.

When you add the fuel, the mixture will be burning longer since there is more fuel to burn. This will also cause the mixture to burn when the exhaust valves are open which makes the turbo spool faster. When the spark plug first goes off all the fuel doesn't burn right away. The fire spreads outward away from the spark plug. Since the flame spreads outward it takes time for all the fuel to burn. So, if you add fuel there is more fuel to burn which equals more time it takes to burn all of it. That translates into some of the fuel burning when the exhaust valves are open. This is another way of knowing how adding the fuel help.

If you don't understand i will try to make it clearer for you.

[talonDSMerr]

1992awdlaser, that was very clear. Timing-wise, it makes a lot of sense, but fuel-wise, I'm reading conflicting views. Doesn't a leaner a/f result in higher exhaust temps as well? Here's a dsmlink forum discussing: DSMLink User Group Forums

The way you explained it, it makes sense. I'm wondering whether people have tuned using both methods or not?

[1992awdlaser]

Sorry, I meant adding fuel in addition to retarding the timing.

Retarding the timing by itself will make the turbo spool faster. Adding fuel in addition to retarding the timing will make the turbo spool even faster. Adding fuel without retarding the timing isn't going to do anything for spool.

A leaner mixture might make the turbo spool a little faster. But, the way I described is the best way to get better spool. With the leaner mixture a lot of the extra heat that is generated is going to be absorbed by the block and then the cooling system will take the heat from the block. So, some of the heat will go to the exhaust manifold and help spool but the majority of it will go into the block. This is also why the leaner mixture increases the chance of knock. The block gets hot and heat creates knock. The heat from the block will be drawn away by the coolant system and the fuel in the cylinder. If too much heat is transferred to the fuel then you will either knock or get pre-ignition.

[Black_Bullet]

Why would it be worth doing.

You retard your timing and richen your afr to increase spool but then you are still sacrificing power from doing that to your timing, so question is are you gaining or loosing? How significant will the spool increase? ... Do we really want the turbo to kick in faster if it promotes compressor surge at such a low rpm? Will the gains of less timing and quicker spool even be felt, or measureable? Think of stuff like that.... Depending the turbo in the picture and the setup obviously results could vary.

Even if you did make it spool a little faster, it doesnt necessarily mean you gained much, if it takes the engine "longer" to climb to the rpm were it "starts, or finishes" spooling. Or if the lose of torque from "significantly" retarding the timing at that rpm becomes only counter productive towards whatever is gained from the slightly faster spool...

On the other hand, perhaps increasing timing, and tuning so that you can add enough torque that the engine gets to the rpm were spool takes place- quicker.
( similar to the misunderstanding that higher compression= quicker spool.)

[1992awdlaser]

When boost comes on you are adding timing back in. since you get boost at a lower rpm you are making more power sooner. The only time you are losing power is before the turbo makes boost. personally, I would rather lose a little power before boost and make the boost a couple hundred rpm sooner than have slightly more power before boost but with later spool. Since the turbo is making power sooner you will have a wider power band verse having slightly more off boost power but with a smaller power band. It is deffinately worth it if you are going to be in roll situations. For drag racing from a stop it won't really matter since you can leave the line with boost.

[Black_Bullet]

I wouldnt know; I have no reason to ponder how to decrease my spool time any further than I already have on a 16g, lol but I was just throwing thoughts out there into the melting pot for ya'l to go over.

I understand that pre boost power is irrelevant, unless you have a 42r or something that doesnt even start till 4000k or something, but im also speaking of your idea of timing retard during spool up... How significant is retarding a whole bunch of timing to decrease what, maybe 100-200 rpm of spool.... Thats what I was trying to get at.

[pboglio]

Originally Posted by Black_Bullet

Why would it be worth doing.

You retard your timing and richen your afr to increase spool but then you are still sacrificing power from doing that to your timing, so question is are you gaining or loosing? How significant will the spool increase? ... Do we really want the turbo to kick in faster if it promotes compressor surge at such a low rpm? Will the gains of less timing and quicker spool even be felt, or measureable? Think of stuff like that.... Depending the turbo in the picture and the setup obviously results could vary.

Even if you did make it spool a little faster, it doesnt necessarily mean you gained much, if it takes the engine "longer" to climb to the rpm were it "starts, or finishes" spooling. Or if the lose of torque from "significantly" retarding the timing at that rpm becomes only counter productive towards whatever is gained from the slightly faster spool...

On the other hand, perhaps increasing timing, and tuning so that you can add enough torque that the engine gets to the rpm were spool takes place- quicker.
( similar to the misunderstanding that higher compression= quicker spool.)

I agree. I have a MUCH wider power band adding fuel & timing OFF boost and leaning fuel and keeping the timing ON boost. This allows me to motor out of slow corners without bogging offboost, then wait for the big hit of torque at higher rpms on boost. This is so effective that when I dial it in right, I start ignoring the boost gauge completely.

I used to fixate on the boost gauge when tuning "offboost". Now I don't bother, I just use my ears to detect bogging/misfiring and the tachometer to see how fast the rpms are climbing.

Here's what works for me: add fuel offboost, add timing offboost, add inj deadtime, retighten those hose I.C. clamps you thought were tight 1 week ago, preload that wastegate actuator just until you get boost spikes, run a cold air intake, remove MAF and run speed density or MAFT, keep intercooler volume to an absolute minimum.

[99gst_racer]

Project_tsi and I were just discussing this yesterday. When we took my car to the dyno back in 2006, we were tuning down low for driveability and up top for peak power, and we took quite a bit of fuel out across the board. That made a huge difference on spool time and peak power. Off power throttle response increased greatly as well. I lost all of my dyno datalogs though tdue to a crash hard drive, so I had nothing but our memory to reference.

So, yesterday night, I took a drive to Taco Bell. I logged a third gear pull on my way there. While there, I took 2 points of fuel out pre-spool, and 1 point out after spool. I hit the freeway on my way back home and logged another third gear pull. I gained a couple lbs/min airflow up top, and off-boost response is a night and day difference. IIRC, I'm in the 13's for AFR for pre-spool. I ran out of fuel though, so I had to cut my tuning session short. I'm going to go back out today with one less point across the board and see how she does. So far, so good though.

1992awdlaser - Your theory sounds logical, but real world experience seems to contradict. I tried rich+timing retard before I leaned it out and it produced a slower spool, poorer off-boost throttle response.

talonDSMerr - I agree with everyone that posted in that thread on the Link boards. My results seem to mirror theirs.

[turbosax2]

Paul, does retarded or advanced timing work better pre-spool for you then? You only mentioned your fuel settings.

[1992awdlaser]

I have used the method I mentioned with a copuple of cars and it worked. I believe turboglenn also uses this same method. Once I get to my computer with logs on it I will upload them if I can find some to compare.

[talonDSMerr]

Paul, would you mind uploading your logs from yesterday night?

Eric, I think we are in agreement that retarding timing pre-spool is most effective. Running lean or rich pre-spool, I think some logs will help clarify that.

[Red97Eclipseboy]

Originally Posted by turbosax2

Paul, does retarded or advanced timing work better pre-spool for you then? You only mentioned your fuel settings.

+1. I want to try your method, too, Paul.

[99gst_racer]

Originally Posted by turbosax2

Paul, does retarded or advanced timing work better pre-spool for you then? You only mentioned your fuel settings.

I only had a chance to play with fuel last night. I just got done unloading ten gallons of 110 octane in her, so I'll be ready for some more tuning tonight. As of right now, it's on the factory timing curve, and if I remember correctly, the factory curve retards slightly around 4K (someone correct me if I'm wrong).

Tonight, I'm going to change my tranny fluid and plugs, compression test, and then go tuning. I'll lean it out a bit more and then start playing with timing a bit. I'll post up logs either tonight or tomorrow morning.

Originally Posted by talonDSMerr

Paul, would you mind uploading your logs from yesterday night?

I wish I could. I accidentally deleted last night's logs with all of the old ones. I'm still a new user to Vista - it took me like 20 minutes to figure out where they are saving to....

I'll post up tonight's though.

[1992awdlaser]

stock 1g timing map:



stock 2g timing map:

[mach4g63t]

Thanks for all the input. The DSMlink thread is really useful too. So, I guess my next questions is though is this really an applicable scenario for DSMlink? Would you want your tune to be like this in all the gears, or just in a particular low-end gear like 2nd? 1st gear, well you might as well stop and anti-lag and then go, but what about roll-ons in 2nd such? Does that make sense?

[99gst_racer]

Well, I leaned it out a bit more and took it for a spin. Pre-spool response is even better. I don't have a 'before' log to compare this to, but throttle response was better - I'd assume that would help boost come on quicker.

Anyway, here's a log. I havent even touched timing yet, so shoot me some recommendations.

[1992awdlaser]

I know it goes against what I was saying but I would try advaning the timing with your car. I say this beause of how low the stock 2g timing is. It's quite ridiculous actually when you compare it to a 1g.