Target air fuel ratio?

[dsm_racing]

What is the desired a/f ratio when tuning a four stroke on 93 octane?

[SpasticR450]

I know the plug should be tan in color if the ratio is right.

[dsm_racing]

Yeah, that's what I've always heard. I plan on taking the bike to the dyno to try out some different things and I'm going to have it on a wide band, so I'm wondering what the a/f ratio should actually be.

[honda400ex2003]

I have been told about 14:1 is race engine f/a preferred. 13-13.5:1 is everyday rider tuning. 14:1 would def be right on the verge of being lean thats for sure. you would be walking the tight rope hoping not to fall off imo lol steve

[honda400ex2003]

also octane has nothing to do with f/a ratio. it has to do with the detonation of it, the higher the octane the better resistance to detonation. Higher compression requires higher octane because of this most of the time. steve

[rubbersdown]

Just read up on this in a magazine actually. The stoichiometric air-fuel ratio is 14.7:1, that is the ideal ratio for lowest emissions, but this isn't the best ratio for power. It used to be that 12.5:1 was considered the best power ratio, but with improved combustion chambers and hotter ignition systems, the ideal now is around 12.8:1 to 13.2:1.

[honda400ex2003]

you will however see a better hp with 14:1 than with 13:1 even though it may be ideal around 12.8 -13.2:1 I know my machine runs better at 14 than at 13:1 I gained almost 1 hp going from around 12.5 to just about 13.5:1. it really depends on the motor though also, some are different because of different designs of the chamber as rubber said. Others could have much less change or even more. all of the sniffers i have seen around use 14:1-14.5:1 or so as ideal- these are the ones you can buy off the shelf to monitor EGTs and A/F ratios. Im curious what the etec engines are making for f/a ratios. let me do a bit of inquiring and get back to you. just for prying eyes sake cause they are the cleanest burning 2 stroke made today. now we have a good convo started. Anyone else find any extreme cases? Rubber do you know any like what some of the fi atvs are running at? Im not sure on those either. steve

[honda400ex2003]

I posted the same question on dootalk to see what the sled guys running some of the new etecs and such are running for not only f/a but oil/fuel also. I am really curious to find out what they say about it running the sleds in alot of different conditions, most of which are much more extreme than we ride the atvs in. steve

[dsm_racing]

I run shoot for around 11.5:1 on my DSM w/ 93 octane and 25 psi of boost. It put 420 hp to the wheels out of 2.3 liters. Higher octane I run leaner. As honda400ex2003 pointed out, higher octane resists detonation. I know on cars guys will run much leaner on naturally aspirated motors. I'm just beginning to learn about bikes, though.

[honda400ex2003]

boost def leans them out, that sounds like a good ratio to be burning with a turbo. :D

Ill post my chart since i have f/a ratios with it on the bottom. the first was a 178, second a 175, third run is a 168.

http://img33.imageshack.us/img33/7978/dynorun416ex.th.jpg (http://img33.imageshack.us/i/dynorun416ex.jpg/)


I could have went to a 165 and got to right around 14:1 which was ideal according to the tuner. if i ran a 162 i would have to lower raise the needle to richen up the mid range. a 162 is only good for when it is around 80 degrees outside. I run the 168 at around 30-60, and now that i have a 165 too, ill be running that most of the summer months.

for whats its worth,
steve