air/fuel ratio

[surfcity914]

what are the correct air/fuel ratios for an air-cooled motor for idle, light throttle, and full throttle. i've been told that the stoichiometric ratio of 14.7:1 is too lean for an air cooled motor at light or full throttle, and that it should be around 12.5:1.

[John Rogers]

Most air-cooled racers run the engine slightly rich to prevent burning it up. This would be a slightly less than 14.7:1. I built one of the air/fuel ratio monitors as outlined here on the PP site and installed it on our 1987 Turbo since it has been hot-rodded and I am worried about fuel starvation and it runs slightly rich. Since I just put a new motor in my 914 I am planning to install an air/fuel monitor in it too as a safety measure.

[surfcity914]

is this ratio the same for all conditions including idle and full throttle?

[John Rogers]

Well it depends....If I am getting the car(s) smogged, then the ratio is set to be the best at the RPM/speed at the smog test settings. Otherwise I worry about full throttle on the 914 race car most since idle and midrange can be off a little and not hurt too much. On our 930 the K-Jet system keeps it pretty good through out the range and that seems to be true on my daughter's stock '74 914 too. If you have carbs, it takes some tunning to get things right and finding someone that can get all the jets right is worth the effort. After my race engine gets broken in a little more we plan to use a dyno to get the carbs set. Good luck.

[Rich Hilgersom]

Yes it should be about the same, FI will be closer than carbs. Weather permitting ;-)

Check out the CB performance gauge description.
http://216.111.121.85/cb/catalog.asp?ProductID=598

[surfcity914]

i found this article which advises to run rich. http://www.markstephenshp.com/mshp/air-fuel-ratio.html

[turbo2.0]

The ratio at idle should be around 13 to 1 for smooth idle, closer to 14 to one for a smog motor. Full throttle should be around 13 to one for maximum power/cooling, but running at 14.7 to one will result in better fuel economy/cleaner air, but will cause the engine to run hotter. The leaner an engine runs under load, the more likely it is to ping, due to the elevated combustion temperatures. This can quickly lead to engine damage if the ratio is very lean. So, to answer your question, it depends on the engine. If it is completely stock with the original fuel injection, it should run lean at idle and partial throttle, and around stoiciometric to slightly rich under full throttle, full load. If it is a higher performance motor with carbs, and smog is not a concern, it should run richer at full throttle, around 13 to one. Some turbo motors run at 12 to one at full throttle, just to get an added cushion of safety.

[udidwht]

With regards to AFR....an AFR of 16-17 is cooler running than 12:5 - 13:5

Stay out of 14:0 - 15:0 during WOT condition (under load). The further you go beyond 15:0 it will be cooler running when not under load. Cooler running than when running ROP (14.7). Driving at a part throttle condition is NOT under load.

Idling is not WOT.

Cruising at part throttle condition is not WOT.

Load = Throttle position

[Chi-Town]

There is no "short answer" for this question.

There are some basic references for non forced induction though.

WOT = 12.7-13.2 under heavy load
Cruise = 13.7-14.7 under light load
Idle = Hot-13.0-15.0 / Cold- 11.5-12.5

Now keep in mind all of these are dependent on timing.

You need to know where your base timing is set, what your advance curve looks like and your peak advance.

This is where those old SUN distributor machines come into play.

You need to measure the weights and spring tensions, the diaphragm movement per inch of vacuum in the actuator.

Even off the shelf reman distributors often have the wrong curve as they mix and match weights and springs.

Now that you've got your distributor curved correctly, timing set, and your wideband O2 installed Now you can tune your carbs or check your F.I. to make sure it's operating correctly.

Reading plugs is a good back up to the wideband and will tell you if you're too aggressive or not aggressive enough with timing.

Here's some good reading on that:
https://honda-tech.com/forums/forced-induct...%2A%2A-3063102/

Remeber always make minor adjustments and don't get crazy aggressive and your engine will last a long time.

[TCracingCA]

I wish guys would get off of the 14.7!!!!!!! Hey that is a nice ratio for a good and efficient combustion in an engine! Great if you are going for economy!

We all thou are packing in more fuel and air! It depends on the squeeze, compression, intake packing, atomization, spark firing etc etc etc etc etc.

The hottest race car of the Naturally aspirated engines are up around 12.5 to 12.9s. You should get forced induction for lower air ratios with a blower or turbo to pack air, to support that high of a fuel saturation.

Therefore a good high performance build probably depending on semi-hi compression and lower, but still performance compressions, and the complimentary ignition would be 13-14! If you shoot for race car 12s, you better know what you are doing.

[David Borden]

Originally Posted by Chi-Town

There is no "short answer" for this question.

There are some basic references for non forced induction though.

WOT = 12.7-13.2 under heavy load
Cruise = 13.7-14.7 under light load
Idle = Hot-13.0-15.0 / Cold- 11.5-12.5

Now keep in mind all of these are dependent on timing.

You need to know where your base timing is set, what your advance curve looks like and your peak advance.

This is where those old SUN distributor machines come into play.

You need to measure the weights and spring tensions, the diaphragm movement per inch of vacuum in the actuator.

Even off the shelf reman distributors often have the wrong curve as they mix and match weights and springs.

Now that you've got your distributor curved correctly, timing set, and your wideband O2 installed Now you can tune your carbs or check your F.I. to make sure it's operating correctly.

Reading plugs is a good back up to the wideband and will tell you if you're too aggressive or not aggressive enough with timing.

Here's some good reading on that:
https://honda-tech.com/forums/forced-induct...%2A%2A-3063102/

Remember always make minor adjustments and don't get crazy aggressive and your engine will last a long time.

This...

If you want A/F ratio optimised, it should never be the same for idle, part throttle/cruise and WOT. You can run roughly ~13:1 for every load scenario, but you will give up fuel economy and have increase emissions and possibly a stinky idle etc.(is dependent on the specific engine)

Most cars will run pretty well at 14.7:1 for idle and part throttle, but that is 100% gas, not with ethanol blends like we have in CA. When tuning, Lambda is commonly used as it does not care what the blend is. A lambda of 1 = 14.7:1 for pure gas and roughly 14.1:1 for E10 which is around what we have here in CA.

So if you are using 14.7:1 for gas here in CA, you are most likely running lean which may explain some of the results others have posted.

[udidwht]

Originally Posted by Chi-Town

There is no "short answer" for this question.

There are some basic references for non forced induction though.

WOT = 12.7-13.2 under heavy load
Cruise = 13.7-14.7 under light load
Idle = Hot-13.0-15.0 / Cold- 11.5-12.5

Now keep in mind all of these are dependent on timing.

You need to know where your base timing is set, what your advance curve looks like and your peak advance.

This is where those old SUN distributor machines come into play.

You need to measure the weights and spring tensions, the diaphragm movement per inch of vacuum in the actuator.

Even off the shelf reman distributors often have the wrong curve as they mix and match weights and springs.

Now that you've got your distributor curved correctly, timing set, and your wideband O2 installed Now you can tune your carbs or check your F.I. to make sure it's operating correctly.

Reading plugs is a good back up to the wideband and will tell you if you're too aggressive or not aggressive enough with timing.

Here's some good reading on that:
https://honda-tech.com/forums/forced-induct...%2A%2A-3063102/

Remeber always make minor adjustments and don't get crazy aggressive and your engine will last a long time.

For optimum mileage one must shoot for 16:1 AFR but not with mechanical only distributor. Stay out of 14-15 zone, it has hottest EGTs.

This is during light to part throttle cruise only. Under load shoot for 12:4 -12:8 or so.

[Charles Freeborn]

basic rule of thumb - 13.2 @ idle and WOT / under accelerating load.
If the FI is working properly the afr should be correct at other throttle openings.