AFR's why tune so rich??

[Wildsupra]

Ok guys im a little confused about something and im gunna raise the question, hopefully someone with actual knowledge can answer it for me.

If Stoich is 14.7:1 why does everyone tune there cars to between 11-12:1? then when they see or hear about a car running upto 13:1 they all start shakin in their boots and go oooooohhh thats lean!!

Technically anything under 14.7:1 is still rich is it not?? hence 13 or 13.5:1 is still safe?
Sure anything over 14.7:1 is obviously lean and could be cause for concern but i just cant understand the "risk" in tuning to 13:1 or 13.5:1

Is it to do with the risk of misfing hence stay way way on the rich side? or do people simply not trust their tuners in the ignition department and use rich fuel ratios as a backup safety?
Also i not specifically reffering to very high powered cars either, maybe 500hp + you might wanna be extra carefull, but surely not at 200-300hp~???


Cheers
Matty

[oldcorollas]

if the air was still and fuel perfectly mixed, you would only need 14.7 times the air of fuel to burn all the fuel.

but, in high speed environment, combustion is not complete. ie, you make CO instead of CO2.. if you have 14.7 ratio, then you have O2 left over. part of reason for 12-13;1 is to use up more oxygen? (ie, increase chance of using more O2 and generate more heat.. the unburnt fuel could slightly add to the volume of gas and help a bit)

another part is cylinder and charge cooling?
another part is to cover up mistakes in timing ignition?
another is to cover for hotspots in the cylinder...

i think you may find that tuners don't trust their skills.. are aren't skilled enough.. or simply want to protect their pocket from busted engines... by tuning rich.

direct injection and more modern combustion chambers mean leaner mixtures are safer.
with stratified mixtures, very lean, like 17:1 or 20:1, can be achieved.

[Devils]

Yes, i've found alot of tuners tune to arround 12:1.

I also asked the same question you have, but the tuner i asked just avoided it and talked about lots of other things.

My belief is that it's "safe". 14.7:1 is the perfect ratio, but it's on the line. so you want to run about 14:1. This is still very close to the line but is good for N/A cars that have less variables. I.e. Only have ignition timing and ambient temps.

When it comes to turbo cars you've got alot more variables such as intake temp, intake pressure, ignition timing which all play a big roll in Combustion temps.

Putting in more fuel than is needed keeps the combustion temp down, helps avoid pre-ignition (which is the main factor) and allows you to run a bit more advance on the base timing which is good for off boost driving.

But... i am still asking the same question... why THAT rich!?

[JP]

I forget where I got this illustration from so can't remember what type of engine it's meant to be for but it might give you some idea to tuning methadology:


Also a short article here: http://innovatemotorsports.com/resources/rich.php

[Benno]

My belief is that it's "safe". 14.7:1 is the perfect ratio, but it's on the line. so you want to run about 14:1. This is still very close to the line but is good for N/A cars that have less variables. I.e. Only have ignition timing and ambient temps.

lol, go on, try running a n/a car at 14.1:1 at full noise on a summers day, u'd probably knock a hole through the tops of your pistons within the hour.

14.7:1 is stoich, which means in a perfect world situation, it's a complete burn(no fuel or oxygen left over)

but this ain't a perfect world and experience will tell you that between12.5 and 13 :1 will make much better power than a perfect burn

[Benno]

^^^ nice graph by the way.

a picture tells a thousand words

[thechuckster]

you should be able to tune AFRs according to MAP/Load and RPM

e.g... lean on cruise, stoich on light power , rich on boost.... my (rather conservative) AFR target map

[30psi 4agte]

Spot on budy........

Dont forget that certain fuels...IE some of the better race fuels have a different reccomended stoich to the run of the mill unleaded.....

Some will say they should be run in the 11's where as there are race fuels out there that want you to tune to the 16's.

It depends on the oxygen content of the fuel amoung other things.

All good race fuels will come with a chart of the amounts (%) of each chemical and its reccomended stoich level.

[Duk]

http://www.autospeed.com/cms/A_1813/...popularArticle
Chrysler were able to build a turbo engine that ran stoichiometric air/fuel ratios even at full throttle.

[Devils]

lol, go on, try running a n/a car at 14.1:1 at full noise on a summers day, u'd probably knock a hole through the tops of your pistons within the hour.

C'mon mate, we're not f*&^ing stupid here.
Depends on ignition timing as well, and i wasn't saying what the running conditions were.

As the picture points out it's an economy thing. Top find that one!!! +rep

[30psi 4agte]

http://www.autospeed.com/cms/A_1813/...popularArticle
Chrysler were able to build a turbo engine that ran stoichiometric air/fuel ratios even at full throttle.

Yep ...... ford have also done it to the XR6T . but that is opening up a hole different can of worms.

This is the link to the elf Turbo max fuel that i use. It gives a suggested stoich for best tuneable power.

Note: its in lambda format not A/F

http://www.kollevold.no/elf/turbomax.pdf

[bigjamie]

You need to have a decent safety factor when you are tuning a car. Tuners no that the closer an engine is to being lean the more horsepower it will make, they also dont want to blow your car up on the dyno so any GOOD tuner will err on the side of caution 12 - 12.5:1is usually the magic and SAFE number

[Wildsupra]

Awesome guys cheers for that, i think old corrollas spelt it out the best first up.

Well im gunna assume my current afr's of around 12.5's are not extremely dangerous as everyone keeps telling me. But i wont let em get much higher than that now.

Cheers
Matty

[JP]

that sort of AFR is dangerous on an EJ20T running the standard fuel rail setup If a tuner works mainly on Subaru's they're usually pretty cautious running anything not in the 11's.

[The Real Roadrunner]

to give you an idea on MOST fi engines they like around 12.5-7 (on pump pulp) for best power production while keeping safe EGTs. its the EGT that is the prob, yes you could tune to 14.7 but you will have to pull out a shiteload of timing for it to be safe and not burn valves etc.

with the subbie thing its because of there shit stock rail design. cyl1 may be at 10.0 and cyl4 will be at 13.0

there are so many variables it really comes down to what engine your talking about.

the reason most tuners go RICHER than 12.5 is to keep it safe.

i have a mate that tunes TT bigblocks and pulls out as much power as they will make but bare in mind that these engines only need to last 24hrs of running time and these guys want a trophy after spending 100k+ on an engine. if he blows an engine on the dyno the only thing they say is "how quick can you build another one"

on the flipside if he blows up a 2k import motor the person would want a warranty and bitch about him on every forum known to man.
pretty easy to work out why most run them richer (endurance/batch of shite fuel and other reasons)

cheers
linden