Real-world AFRs at WOT...

midnightblue · Post by **midnightblue** » Fri Dec 09, 2005 6:47 pm

I'm interested to know what AFRs people are seeing at WOT with a well dialed in VE table.

People out there with dyno experience would be particulary good. I've read the theory on AFRs for peak torque and power, and would like to know if they agree with the practical experience.

My particualar interest is normally aspireted, push-rod V8, but I'm sure all information would be useful to the community in general.

Best regards, Matt.

natesully · Post by **natesully** » Fri Dec 09, 2005 6:55 pm

Supposedly, 12.5:1 is close enough. To really know, you need an EGT gauge and already-tuned timing (and boost, if not NA). You add/subtract fuel until your EGT is where you want it, generally in the "not melting things" range. Spark is more important though, since detonation will blow your engine up, and retarding timing is retarded and wastes power. That's what I understand anyway, the people at the local shop all say "12.5 == good", and leave it at that.

There is no real theory- you need a dyno and time to find where your particular engine will make the most power without detonating or melting. 12.5:1 is just a good guess, especially for a NA (I think- don't take my word, but that's what everyone else I know NA does.) Turbo guys like to run richer, for saftey, I've heard 11:1 and such for hardcore guys on the edge of an escaped rod.

Keithg · Post by **Keithg** » Fri Dec 09, 2005 8:21 pm

try 10.5:1... But that is only to prevent ping and only at peak
torque. Otherwise 12.5:1 near there. With a big intercooler this may
get down to 11-11.5, but to prevent ping at WOT in the summer, I need
10.5. Playing around with timing did nothing. One data point.

KeithG

On 12/9/05, natesully <magic220@gvtc.com> wrote:
>
>
> Supposedly, 12.5:1 is close enough. To really know, you need an EGT gauge
> and already-tuned timing (and boost, if not NA). You add/subtract fuel until
> your EGT is where you want it, generally in the "not melting things" range.
> Spark is more important though, since detonation will blow your engine up,
> and retarding timing is retarded and wastes power. That's what I understand
> anyway, the people at the local shop all say "12.5 == good", and leave it at
> that.
>
> There is no real theory- you need a dyno and time to find where your
> particular engine will make the most power without detonating or melting.
> 12.5:1 is just a good guess, especially for a NA (I think- don't take my
> word, but that's what everyone else I know NA does.) Turbo guys like to run
> richer, for saftey, I've heard 11:1 and such for hardcore guys on the edge
> of an escaped rod.
>
>
> ________________________________
>
> 95 Miata M-Edition, GT28 Turbo
>
>
> This post is at:
> http://www.msefi.com/viewtopic.php?p=89269#89269
>
>
Posted by email.

Enthalpy · Post by **Enthalpy** » Fri Dec 09, 2005 9:57 pm

Actually, turbos see best torque often at richer AFRs when compared with a similiar NA engine. Safety is but a side effect.

You might not believe this, but given correct ignition timing, let the AFR vary between 12.5 and 14.0 and you will see an almost indistinguishable difference in engine output! This is based on a paper published in the 1930s for NA engines that still holds true today. AFR is but a thermal management tool, as previously suggested.

LT401Vette · Post by **LT401Vette** » Sat Dec 10, 2005 9:37 am

Yes in my NA setup I have found that when I pull timing out I can pull fuel out as well and maintain the same traps. That is moving from 39* at wot down to 33-34*. Seems to make similar power at either point, but does it with less fuel when at 34. I'll be sub 12:1 at 38.

natesully · Post by **natesully** » Sat Dec 10, 2005 4:06 pm

Enthalpy wrote:Actually, turbos see best torque often at richer AFRs when compared with a similiar NA engine. Safety is but a side effect.

You might not believe this, but given correct ignition timing, let the AFR vary between 12.5 and 14.0 and you will see an almost indistinguishable difference in engine output! This is based on a paper published in the 1930s for NA engines that still holds true today. AFR is but a thermal management tool, as previously suggested.

When I added timing control to my MS, there was a HUGE difference down low compared to stock, using a pre-tuned map. It was advanced down low, which made a little knock. pulling around 3 degrees put me well out of knock land, and it the car just feels way better than it did stock. Some stock miata drivers advance their timing via turning the CAS by a few degrees and report that it makes a noticable difference.

Likewise, leaning the car up high, in boost, did not feel like much. Of course, I do not have EGT, so I am a little conservative with the AFRs at the moment, though I do have other miatas to go by.

Here's what I would like to know- my secondary fuel pump (part of a turbo kit's non-megasquirt management) failed and left me real lean in boost. Since I did not realize what was happening, I just kept flooring it for months, wondering why my turbo would not spool, before I noticed the loose ground. My engine still runs, quite well, 1/2 a year later. I'm guessing running insanely lean up high actually won't hurt your engine, as there is so little fuel that EGTs drop. However, running slightly lean (as in, "not rich enough, but still >14.7") kills engines. So, is it possible to make a car run real lean under WOT? No power, but you would get good mileage I guess

Enthalpy · Post by **Enthalpy** » Sat Dec 10, 2005 8:36 pm

natesully -
Gas engines misfire if you are too lean. Rich of the misfire limit is where engines blow up. I would REALLY like to see what your fuel pressure and/or AFRs were during this time. It's possible a bad/losse ground was causing the pump to underperform, but not die on you.

As for the ignition advance, I'm not sure what you mean by "stock map". If it's a MS map developed on another engine and changing it worked better for your engine, the answer to that is the "stock map" wasn't optimized for your individual engine in the ways you were measuring.

Pierre · Post by **Pierre** » Sun Dec 11, 2005 1:13 am

Hello,

Enthalpy wrote: You might not believe this, but given correct ignition timing, let the AFR vary between 12.5 and 14.0 and you will see an almost indistinguishable difference in engine output! This is based on a paper published in the 1930s for NA engines that still holds true today. AFR is but a thermal management tool, as previously suggested.

I have to say that with my engine, this is not correct. I have a 1600 8v (XU5 peugeot).
On the dyno, I have 5 hp more at 12/12.5 than 13/13,5 (same day, same dyno....) and it's really there while driving too.

Pierre

natesully · Post by **natesully** » Sun Dec 11, 2005 9:23 am

Enthalpy wrote:natesully -
Gas engines misfire if you are too lean. Rich of the misfire limit is where engines blow up. I would REALLY like to see what your fuel pressure and/or AFRs were during this time. It's possible a bad/losse ground was causing the pump to underperform, but not die on you.

As for the ignition advance, I'm not sure what you mean by "stock map". If it's a MS map developed on another engine and changing it worked better for your engine, the answer to that is the "stock map" wasn't optimized for your individual engine in the ways you were measuring.

I meant the map deep inside the OEM ecu. Obviously, it was not optimized, though the new miatea have knock sensors and are likely better. My fuel pressure was like, 30 psi in boost with the AFPR on

. After seeing that, using my friend to hold a gauge while I drove, I instantly realized what the problem was, lol. It did lean-misfire once, now that I think of it, but normally the car would just kind of... stop accelerating when you stomped on it.

Enthalpy · Post by **Enthalpy** » Sun Dec 11, 2005 12:06 pm

Pierre -
I suppose their might be specific circumstances where the AFR is doing some things in one specific engine it isn't doing in other engines like changing the way the air flows into the engine.

My post was meant to outline the fact that AFR is a thermal management device and not a method for gaining power. The results of this little AFR study is obviously heavily dependant on setting the timing dead on.