Is stoichiometric air/fuel ok for these air cooled engines?

[justinmcgiver]

So when I tune a FI bike i shoot for stoichiometric air fuel(14.7 : 1) or as close to it as I can get. I'm in the process of tuning an '80 gs1000 with V&H pipes and a dynojet stage 3 kit. I found the A/F mix to be way rich with their set up. About 7.4:1. Just wondering if I need to keep it a little rich to keep engine temps down with the air cooling... Anyone know what proper a/f ratio is for these old beasts? Thanks
Justin

[Nessism]

So when I tune a FI bike i shoot for stoichiometric air fuel(14.7 : 1) or as close to it as I can get. I'm in the process of tuning an '80 gs1000 with V&H pipes and a dynojet stage 3 kit. I found the A/F mix to be way rich with their set up. About 7.4:1. Just wondering if I need to keep it a little rich to keep engine temps down with the air cooling... Anyone know what proper a/f ratio is for these old beasts? Thanks
Justin

Max power for most engines is not at 14.7, it's lower - not sure exactly sure what but 13'ish comes to mind. I don't think it matters whether or not if the engine is air cooled, although running a smig rich might be a good idea to keep the temps in check.

[Billy Ricks]

Max power for most engines is not at 14.7, it's lower - not sure exactly sure what but 13'ish comes to mind. I don't think it matters whether or not the engine is air cooled, although running a smig rich might be a good idea to keep the temps in check.

That's about what I remember the ratio being.

[themess]

In the early 80s (?), the EPA mandated "lean" tuning for motor vehicles. GSes started being delivered with caps over mixture adjustment screws that had been set at the factory for lean burning. I'm guessing that the mixture was pretty close to 14.7:1. Some people recommend a slightly richer mixture for easier starting and improved rideability. My 1982 GS650G never had the carbs touched, never had any gas treatment or carb cleaner, and when I crashed it in 1997, it still ran perfectly.

[focus frenzy]

just did some digging around and found some interesting data.

Stoichiometric ratio or chemically correct ratio for a gasoline engine is 14:1
14:1 is where both air and fuel are consumed equally at balance rate. 14 lbs air to 1 lb fuel.

a study using air cooled airplane engine found that a lean ratio of 15.5:1 produced the best fuel economy and ratios leaner than that did not produce any improvement out to 18.5:1 the leanest a engine can run evenly.

a article on air cooled Volkswagens list that 10% rich of ideal (12.6:1) produces the most power but a ratio 10% lean from ideal (15.4:1) produced the best economy

[49er]

I can't give any figure for best economy, but the best HP and torque figures on my 850 were produced at 13.4-1. When richened to 12.4-1, I lost 2 HP and 1.5 ft lbs of torque. Dynojet charts have a red median line at 13-1.

[Zook]

The best way to find out is to read the spark plugs. They will tell the tale.

In GA aircraft you adjust to a little less than peak EGT at cruise altitude.
A little richer for climb.

[49er]

The best way to find out is to read the spark plugs. They will tell the tale.

In GA aircraft you adjust to a little less than peak EGT at cruise altitude.
A little richer for climb.

Geez Zooks, you know how to kill a thread! I thought that was strickly my domain!!

What are the GA recommended EGT's at cruise altitude? Don't they vary from model to model, differing altitudes, different barametric pressures? How much richer is recommended for climbing?

EGT senors need to be positioned accurately to be a reliable tuning tool. The adjacent castings on each cylinder need to be identical to allow for simalar heat disapation too. I guess that is true for aircraft engines anyway.

Justin, I'm not having any heating problems at 10.5-1 CR and 13.4-1 AFR until ambient air gets around 35+ deg C. When that occurs, I just up the octane rating. I don't run an oil cooler either.

[dirtyfingers]

Stoichiometric 14.7:1 is under ideal conditions, ie. air pressure, temp, and humidity. In the real world 14.7 is too lean, even at wide open throttle. Generally where I live engines make the most power in the 12-13:1 range, and idle at way richer than that, some exceed our O2 sensors capability.

[Ola]

If you want to dive deep into the mysteries of mixture, I really recommend this column by John Deakin.

http://www.avweb.com/news/pelican/182084-1.html

Don't skim through it! If busy, bookmark it and wait until you have a nice pot of coffee and some spare time. It's written for general aviation, but the fuel and air molecules don't know if the wheels are touching the ground or not. As for accurate EGTs, he shows how it's not the absolute temp reading that matters, but the relative setting above and below peak which helps you set correct mixture for a given power setting. It made me wish for an easily adjustable mixture setting and EGT gauges on my GSX.

The VW FSI engines have different modes, injecting fuel at different points in the stroke depending on load condition. At low speed cruise, the ratios can be in the 40-50:1 range.

[Zook]

Justin, I'm not having any heating problems at 10.5-1 CR and 13.4-1 AFR until ambient air gets around 35+ deg C. When that occurs, I just up the octane rating.

snip

If you are getting heat related pinging at 35 degrees then you are running way too lean. That's why it heats up so easily. Lean makes heat.

[justinmcgiver]

hanks for the replies. I'm going to shoot for high 12s to low 13s across the board and call it good.

[49er]

If you are getting heat related pinging at 35 degrees then you are running way too lean. That's why it heats up so easily. Lean makes heat.

At 13.4-1, I am above the Dynojet guide of 13-1 and on the lean side. The plug reads confirm that.

The only time I have got slight pinging, was on a steep climb in top gear, under 3/4 throttle. The ambient air at that time was 38 deg and I was running your 87 PON. That's equivalant to our 91 RON. Total advance is also 39 deg, two above stock. Probably should be running 95 RON, but midrange torque is stronger running 91. I usually conpromise by alternating the octane rating at fillup time, during the hotter months. I try to refill by around 1/2 tank, which probably gives an average rating of around 93 RON.

[themess]

If you want to dive deep into the mysteries of mixture, I really recommend this column by John Deakin.

http://www.avweb.com/news/pelican/182084-1.html

Thanks for the link! Looks as if reading it through the first time will take an hour, and it should probably be read several times. I'm forwarding it to two private pilots I know, too.

[82Shafty]

You have to determine at what throttle postion you are at when you want said AFR's.


WOT on NA is usually in the low to mid 12 range. Once you get into the 13's you better be prepared to let off quick if she spikes.

Boost, low to mid 11's AFR under WOT

Cruising, optimal burn was considered to be your stoich mixture, but it depends on the engine as some will burn a tad better closer to mid 15's, this is a light load, part throttle condition at a steady rpm.

During Transitions you can get down to 8 momentarily.

Here, check this link out, maybe you'll find something of interest.

http://www.airfuelratio.com/manual.htm