Tune AFR to 12.5
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Evolving Member
Joined: Aug 2007
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From: wales
Tune AFR to 12.5
just wondering why can't we tune the evo to run a afr of 12.5 all the way to redline?.....(in open loop)
(I thought combustion temps are hotter either side of this number?)
so if its not about temps........???
Mark.
(I thought combustion temps are hotter either side of this number?)
so if its not about temps........???
Mark.
I don't want to get into all of the theory, but you can tune for whatever AFR you want on pump gas, as long as you adjust the other variables to compensate.
Some people tune leaner, some richer. It all depends on your own results and what you are comfortable with.
BTW, I think the temps are hottest at a certain AFR (I forgot the exact AFR or range) and they are cooler either side. I think you said it backwards. I think the same holds true for flame front speed as well. That's why tuning for a leaner AFR, especially on pump gas, may need much less timing than a richer AFR. Of course, too little timing can cause EGT issues, so the actual AFR and timing that you can safely run is determined by the knock resistance of the gas that you are using. This is the reason that you see most people running richer AFRs on pump gas.
Eric
Some people tune leaner, some richer. It all depends on your own results and what you are comfortable with.
BTW, I think the temps are hottest at a certain AFR (I forgot the exact AFR or range) and they are cooler either side. I think you said it backwards. I think the same holds true for flame front speed as well. That's why tuning for a leaner AFR, especially on pump gas, may need much less timing than a richer AFR. Of course, too little timing can cause EGT issues, so the actual AFR and timing that you can safely run is determined by the knock resistance of the gas that you are using. This is the reason that you see most people running richer AFRs on pump gas.
Eric
Last edited by l2r99gst; May 29, 2008 at 07:44 AM.
l2r ... temps are cooler on either side of stoich. 14.5 - 14.8 is the hottest I believe.
You can't run 12.5:1 because of the knock resistance of pump fuel. Even though you'd get the most power from the fuel in that area, the fuel isn't stable.
Technically, you could run 12.5:1 as l2r stated, but you couldn't run much boost and you'd have to pull a lot of timing to avoid knock. Both boost and timing advance trump fuel in terms of making power.
So, we make the decision to richen the mix to extend the burn time of the fuel and therefore make it more stable so we can advance timing and crank up the boost for more power.
You can't run 12.5:1 because of the knock resistance of pump fuel. Even though you'd get the most power from the fuel in that area, the fuel isn't stable.
Technically, you could run 12.5:1 as l2r stated, but you couldn't run much boost and you'd have to pull a lot of timing to avoid knock. Both boost and timing advance trump fuel in terms of making power.
So, we make the decision to richen the mix to extend the burn time of the fuel and therefore make it more stable so we can advance timing and crank up the boost for more power.
Right, the OP was saying the opposite, so I was correcting that. I thought I remember reading that highest thermal efficiency is around 16.x range, but I can't remember exactly or the sources, that why I was simply stating that there is an AFR or range where the burn is the hottest and it is cooler either side of that.
