Ideal A/F ratio??

[NacsMXer]

Hey guys, I recently went to the track and my quad was cutting out on the top end with a 175 main in the FCR 39 carb and all the mods in my sig. Fresh plug was pretty black when I pulled it out.

We have a Dynojet where I work (for cars), so I recently threw my quad on the rollers just to see where the A/F was at with the exhaust sniffer.

Right before the pull, I bumped the main down 2 sizes to a 170. It pulled clean all the way to the top without cutting out but is still a tad rich. The A/F flat lines nicely all the way through but like I said is a little on the rich side at 11.4.

When we tune most forced induction cars, 11.4-11.8 A/F is what we want to see at WOT but i'm pretty sure it may be different for quads. I have heard that 14.7 A/F is the ideal range for motorcycle engines, is this true??

If it is, then I should bump my main down two sizes again to about a 165 and I should be good.

If someone who knows could confirm that for me i'd really appreciate it, thanks ;)

[bradley300]

13.5:1 is a good spot for the quads ive seen

[zrpilot]

I did the exact same thing on my 416EX with a 39MM FCR. I started with a 175 main and ended up at a 158. please link to the Dyno run to and you can see the progression of main jet changes and its effects on jetting.Dyno run link (http://www.exriders.com/vbb/showthread.php?s=&postid=2919031#post2919031)

[NacsMXer]

Cool, thanks for the info guys! Looks like I have a little ways to go to get it leaned out some. My runs were also done in 90+ degree humid weather, not exactly my ideal riding conditions so it would have been running richer than normal. I'll probably just put a 162 in it and check out the plug, i'm guessing that should get me pretty close to the 13:1 A/F range.

[zrpilot]

We need GPracer2500 to chime in!!!!

[wilkin250r]

14.7 is only the THEORETICAL ideal, by calculation of the number of molecules.

However, this is the real world, and numerous tests have shown that best power is produced right around 13.5, with little drop in power as rich as 12.5.

[NacsMXer]

Originally posted by wilkin250r
14.7 is only the THEORETICAL ideal, by calculation of the number of molecules.

However, this is the real world, and numerous tests have shown that best power is produced right around 13.5, with little drop in power as rich as 12.5.

Thanks for the input wilkin, I was thinking along the same lines. IMO, it's better to run it slightly richer than the stoichiometric "perfect" as long as it still runs hard.

[bwamos]

Yup, I like to run at ~13.5:1 also.

[GPracer2500]

One interesting tidbit about a/f ratios: We shoot for about 13.5:1 on most engines because it's been demonstrated time and again that ratio yields the best HP (again, on most engines--some high specific output air cooled engines (not so much the EX) have trouble with heat. I could see many forced induction engines having similar issues). There are several reasons for this--one of which is perfectly stoich mixtures have slower laminar burn rates than slightly rich. The reasons for that are so chemically complicated I don't fully understand it. But anyway....

It might be worth noting that when our dyno data shows us at 13.5 AFR all that's really telling us is how far away from stoich we are, NOT the actual AFR. O2 sensors don't output AFR's. They output a voltage. The processor then compares that value with a stored scale for the stoich figure of "gasoline". As wilkin250r mentioned, 1:14.7 is the ratio at which "gasoline" has the right amount of hydrocarbon and oxygen molecules to covert everything to combustion products.

The problem with that scheme is the "gasoline" used to come up with 14.7 is a reference gasoline that no one actually uses. The gasolines that we actually use could be stoich at everything from pretty close to pretty far off that figure. Any gasoline with oxygen content will have a stoich less than 14.7. Take the 10% ethanol blends (E10) that MANY of us find at the pump. If E0 is stoich at 14.7, E10 will be stoich at a little less than 14.1. If your fuel is stoich at 14.1 then 13.5 is not what you want. About 12.8 is what you should be shooting for.

BUT the signal processor for the O2 sensor doesn't know what the stoich really is for the particular gasoline you're running. It just assumes that it's 14.7 so all the figures it will give you will be referenced to that scale. In our O2 equipped EFI cars this doesn't come up because all it's paying attention to is: OK--are we lean of stoich, are we stoich, or are we rich of stoich. And then it makes the appropriate adjustments based on that. Our cars don't really care what the stoich of the fuel IS, they just care how far away from it they are.

Bottom line is the AFR that our dynos give us are mostly just telling us how far away from stoich we are--NOT the actual AFR. Which is fine, I guess. That's what we really need to know anyway. Using an AFR scale where stoich = 14.7 is just a round-about way of telling us how far from stoich we are. I'm not clear on why we even use AFR instead of lambda values. Lambda is a rich/lean scale where stoich=1, rich would be lambda=0.9, and lean would be lambda=1.1. The scale doesn't care about AFR's, it only judges how far from stoich we are. An O2 sensor can't ever tell us what the stoich value of a fuel really is--they can only tell us how far away from it we are.

[Scro]

Originally posted by GPracer2500
One interesting tidbit about a/f ratios: We shoot for about 13.5:1 on most engines because it's been demonstrated time and again that ratio yields the best HP (again, on most engines--some high specific output air cooled engines (not so much the EX) have trouble with heat. I could see many forced induction engines having similar issues). There are several reasons for this--one of which is perfectly stoich mixtures have slower laminar burn rates than slightly rich. The reasons for that are so chemically complicated I don't fully understand it. But anyway....

It might be worth noting that when our dyno data shows us at 13.5 AFR all that's really telling us is how far away from stoich we are, NOT the actual AFR. O2 sensors don't output AFR's. They output a voltage. The processor then compares that value with a stored scale for the stoich figure of "gasoline". As wilkin250r mentioned, 1:14.7 is the ratio at which "gasoline" has the right amount of hydrocarbon and oxygen molecules to covert everything to combustion products.

The problem with that scheme is the "gasoline" used to come up with 14.7 is a reference gasoline that no one actually uses. The gasolines that we actually use could be stoich at everything from pretty close to pretty far off that figure. Any gasoline with oxygen content will have a stoich less than 14.7. Take the 10% ethanol blends (E10) that MANY of us find at the pump. If E0 is stoich at 14.7, E10 will be stoich at a little less than 14.1. If your fuel is stoich at 14.1 then 13.5 is not what you want. About 12.8 is what you should be shooting for.

BUT the signal processor for the O2 sensor doesn't know what the stoich really is for the particular gasoline you're running. It just assumes that it's 14.7 so all the figures it will give you will be referenced to that scale. In our O2 equipped EFI cars this doesn't come up because all it's paying attention to is: OK--are we lean of stoich, are we stoich, or are we rich of stoich. And then it makes the appropriate adjustments based on that. Our cars don't really care what the stoich of the fuel IS, they just care how far away from it they are.

Bottom line is the AFR that our dynos give us are mostly just telling us how far away from stoich we are--NOT the actual AFR. Which is fine, I guess. That's what we really need to know anyway. Using an AFR scale where stoich = 14.7 is just a round-about way of telling us how far from stoich we are. I'm not clear on why we even use AFR instead of lambda values. Lambda is a rich/lean scale where stoich=1, rich would be lambda=0.9, and lean would be lambda=1.1. The scale doesn't care about AFR's, it only judges how far from stoich we are. An O2 sensor can't ever tell us what the stoich value of a fuel really is--they can only tell us how far away from it we are.

Sometimes I feel I need a dictionary to read your posts:p Good info!