AFR's prior to full boost.

[nytescion]

Just wondering what you guys are hitting for AFR prior to hitting full boost. It appears that I'm registering around 11. I've been told that if I lean it out while building boost I can spool the turbo/accelerate faster. Is this true?

 

[DSMTyson]

I have mine in the 12.0 range while I am just starting to hit boost, and while boost is building it goes 11.7-11.5-11.3-11.1(full boost). That's how I have it set on link for my openloop maxoct table.

IIRC you can help spool time by adjusting timing advance prior to spool.

 

[tkelly27]

There is a bit of a debate as to which direction to go on this.

I'll give you what I believe first, and then the other school. The facts driving this belief are that leaner air fuel mixtures create higher EGTs.

http://www.originlab.com/www/resources/images/lombard2.gif
Unfortunately that is for a 2 stroke, but it is VERY similar for a 4 stroke in that peak temp happens on the right hand side of stoich, and peak power happens on the left hand side of stoich. I don't care enough to dig into google to find it. I think it happens that peak fuel economy happens at peak temp as well.

I like to have leaner AFRs and more spark timing advance. I believe the higher EGTs are able to put more heat energy into the turbine and spool it up faster. I also believe that the engine is able to make the same or more power without detonation at atmospheric pressure levels with leaner fuel ratios and more spark advance.

I give it as much spark as it will take, and a mid 13ish fuel ratio. I've heard even leaner, up to the peak temperature fuel ratio (15's, 16's), will spool faster, but I'm not brave enough to test that without a dyno.

The other school of thought is as you suggested, richen the mixture, but you would retard the timing. The thinking here is that when the exhaust valve opens, you should be pushing out air and fuel that is still burning, and that the burning and expanding gas will drive the turbine.

I like thinking that I'm getting better spool and fuel economy, but I've never measured it either way to know. If you have a data logger and tuning software, go ahead and try them both.

I'm thinking cam timing would have an effect on which method you would use. An earlier opening exhaust cam would lend itself to the 2nd method.

 

[knochgoon24]

Here's what I'm running (93 pump):

0 psi - 14.7 - (just switching to open loop)
5 psi - 13.5
10 psi - 12.4
15 psi - 11.5
20+ psi - 11.3

Leaning out that 0-10 psi range really helped my fuel economy compared to when I had it running a full 1 AFR richer (for no good reason really). I'd like to get an EGT gauge to see what my temps are like during long pulls at lower boost and that AFR, but my plugs looked great with my tune like that.