Oh, Sh*t! What should my A/F ratio be with nitrous?

[bobc455]

So, I'm reading David Vizard's book on Nitrous Oxide Injection just now, and I come across the calculations that tell me that N2O to Fuel ratio should be 9.6. Ack! All of my speedpro settings are set for about 12.2:1 at WOT.

Does this mean that I should try for an effective 11.3:1 or so when running nitrous (12:1 for the air part, 9.6:1 for the nitrous part)

*Or*

Is my O2 sensor actually reading the Oxygen:fuel ratio, which should remain unchanged (or very close to 12:1 anyhow) when running nitrous?

I think I know the answer, but I need a reality check.

Thanks for help...

-Bob Cunningham
bobc@gnttype.org

 

[AnArKey]

Go with a little bit of a richer mixture than you normally would when you run nitrous. 11:5:1 would be a good ratio.

 

[bobc455]

Why?

Shouldn't the oxygen:fuel ratio remain the same (and hence the "air"/fuel ratio)?

-Bob Cunningham
bobc@gnttype.org

 

[bobc455]

Got a good answer

Okay, I feel a bit dorky for closing out my own thread, but here is the whole deal for anyone who wants to know a bit more about nitrous (it will make you more popular at social gatherings).

The A/F ratio (as indicated by an oxygen sensor) is really only indicating the oxygen/fuel ratio. Extra by-products of combustion (water, CO2, N2, etc.) will vary in ratio depending on whether you are using Air or Nitrous (or both) as an oxygen source for combustion, but theoretically the oxygen:fuel ratio shouldn't change.

The ratio of 9.649:1 of N2O/Fuel is derived the same way as the theoretically ideal Air/Fuel ratio of 14.7, because there is the same amount of oxygen in 9.649 #'s of nitrous as there is in 14.7 #'s of air.

If you want to check the calculation for yourself (or you can take my word for it), N2O is 36% Oxygen, and air is typically 23.6% oxygen (by weight). Therefore, the O2 content of 14.7 #'s of air is the same as the O2 content of 9.65 #'s of Nitrous.

It is a good idea to run a bit richer when using nitrous simply as a detonation suppressant (sp?).

-Bob Cunningham
bobc@gnttype.org
(going back to sleep now)

 

[AnArKey]

Re: Got a good answer

Originally posted by bobc455
It is a good idea to run a bit richer when using nitrous simply as a detonation suppressant (sp?).

That's why.

 

[KEVINS]

Added info on N2O: under extreme pressures and temperatures the Nitrogen molecules will break away from the Oxygen molecule. The extra Nitrogen in the cumbustion process will help "buffer" detonation. Although this has not been formally proven. I have also found that nitrous kits are designed on the rich side for safety and can be leaned out with fuel psi to gain the most power depending on application.

I have personally dropped from 6.5 psi of fuel pressure to 4.25 psi on my stang and picked up over 4 mph. This took the right spark plug to handle this though.

ks

 

[JDEstill]

Originally posted by KEVINS
Added info on N2O: under extreme pressures and temperatures the Nitrogen molecules will break away from the Oxygen molecule. The extra Nitrogen in the cumbustion process will help "buffer" detonation.

Actually Kevin, when the nitrogen and the oxygen in the nitrous dissociates you end up with a higher oxygen to nitrogen ratio than you would if you just had plain old air in there. Nitrous is 33% oxygen whereas air is just 21%. Never heard of nitrogen helping in the fight against detonation, but if true you are making things worse with nitrous; with it you are decreasing the percentage of nitrogen in the cylinder.

Oh yeah, just FYI, I'm not disagreeing with Bob's percentages, I'm used to thinking in mole percents rather than weight percents. Just one of those chemistry things ya know.

The a/f ratio targets I've heard kicked around seem to be in the 11.5-11.8 range, and that goes for high boost/no nitrous as well as nitrous assisted. No direct experience there, just what I've heard others say. FWIW. YMMV.

John