Oxygen sensor

[cudabob496]

We are dealing with a gas and not a liquid and we are dealing with exhaust pulses, not a constant supply.  Therefore, outside air is easily pulled into the system and often screws up tuning of a closed loop f.i. system as the actual a/f  in the combustion chamber ends up on the rich side because the tuner was fooled by the leaner than reality a/f's being displayed.  We are dealing with an open loop system so we make the car richer than it should be instead of the car's computer doing it. As I stated above, this problem is worse at lower rpm because the exhaust pulses are not emulating a steady stream with more constant pressure.

But never mind, it does not matter whether you understand how it works.  As long as you don't have exhaust leaks before the sensor and the sensor is not close to the end of the outlet, all is good. 

I will mention, however, that on a carbureted car that 14.7-1, which is the stochiometric ratio, is usually too lean due to poor atomization of the fuel into the air stream and that somewhere around 13.0 to 14.1-1 usually provides a much more stable idle (the bigger the cam, the richer the required idle a/f) whereas about 14.2 is usually pretty close at cruise.

On a sequential port injection set up, 14.7 is the stochiometric ratio for the most complete, cleanest burn, but many of us have found that something higher than 15 may give the best gas mileage.  Carbs simply do not mix the fuel and air as well and end up needing richer numbers

I guess it is obvious that all the numbers above apply to idle or cruise and not wide open throttle which will be closer to 12-1 depending on what makes the most power without detonation

OK. I'm wondering, if there was a leak at the header/head interface, would that suck in air, always be at a positive pressure, or alternate between drawing in air, and a positive pressure?
Also, I assume my annular boosters give better atomization.

[jimynick]

Don't get side-tracked over a probably non-existent air leak. There's a good carb tuning primer in the June 2013 Mopar Action that you'd probably enjoy. 

[Strawdawg]

OK. I'm wondering, if there was a leak at the header/head interface, would that suck in air, always be at a positive pressure, or alternate between drawing in air, and a positive pressure?
Also, I assume my annular boosters give better atomization.

when the exhaust valve opens, the pulse has a positive pressure on the front end, then it goes negative.  There are quite a few articles available on line graphing the pressure change.  Scavenging pulls air from outside if the avenue exists.  This avenue can be behind the sensor, or a short distance in front.  In a good exhaust system there is virtually no back pressure so the plus side  is very low.

It is important to try to eliminate leaks in order to get a stable datum to tune from historically.   In performance tuning, there are really no magic numbers altho the ranges are usually pretty consistent.  Eliminating leaks keeps the numbers meaningful day to day.

Placing the sensor too close to the exhaust opening can sway the readings by as much as two numbers.  Minor leaks at the headers may only move the numbers a couple of tenths but one would like to try to eliminate the induced sway and look at the real number.

[cudabob496]

Don't get side-tracked over a probably non-existent air leak. There's a good carb tuning primer in the June 2013 Mopar Action that you'd probably enjoy. 

Ya, though its a possibility, I won't worry about it unless I'm seeing a high reading, indicating a possible air leak upstream of the sensor. If there was a leak at the header/head interface, there would be black soot near the leak, from when the pressure goes positive. Right now my reading is low, meaning I'm too rich. Was gonna take it out today, but raining!.

[Strawdawg]

It does not sound like you have any leaks, but, if you did have a leak, hypothetically, then your 10.5 might be actually 10.3.  Just being on the rich side does not automatically eliminate a leak.  Realistically, if you have a leak, you will probably hear it at idle as well as eventually see the evidence visually.

I deal with a lot of guys that are complaining that their car is lean because the computer is adding fuel so they keep programming it richer and the car keeps slowing down.  then they mention the crack in the header tube....which is allowing air to enter the system and fool the O2 sensor...which then screams at the computer to add fuel to get the mix right.

The car then has two problems...the spool up is slowed by the leak and the richness cools the egts down again slowing the turbo spool.  People tend to want the problem to be complicated instead of making sure that the basics are covered.

[nqkjw]

I guess it is obvious that all the numbers above apply to idle or cruise and not wide open throttle which will be closer to 12-1 depending on what makes the most power without detonation

12.7 gives best dyno numbers.
12.2 - 12.4 produces best quarter mile numbers.

[cudabob496]

It does not sound like you have any leaks, but, if you did have a leak, hypothetically, then your 10.5 might be actually 10.3.  Just being on the rich side does not automatically eliminate a leak.  Realistically, if you have a leak, you will probably hear it at idle as well as eventually see the evidence visually.

I deal with a lot of guys that are complaining that their car is lean because the computer is adding fuel so they keep programming it richer and the car keeps slowing down.  then they mention the crack in the header tube....which is allowing air to enter the system and fool the O2 sensor...which then screams at the computer to add fuel to get the mix right.

The car then has two problems...the spool up is slowed by the leak and the richness cools the egts down again slowing the turbo spool.  People tend to want the problem to be complicated instead of making sure that the basics are covered.

ya, fairly easy to hear an exhaust leak.

[cudabob496]

12.7 gives best dyno numbers.
12.2 - 12.4 produces best quarter mile numbers.

Which supports what racers have been saying for decades, always set up to run rich at WOT for the quarter.

[cudabob496]

OK, did some testing today. Originally had 84/84 jets, and my A/F reading was close to 10 when cruising. Didn't get
a WOT reading.  Today, I'm down to 77/84, and cruising gives me about a 12.0 reading, and WOT about 13.5.
Idle around 11.5.

So, I guess I'll lower the primaries to 74, and raise the secondaries to 88 to get around 12.5 at WOT?

Maybe I'm jetting down a lot on the primaries because the annular boosters are more efficient at
fuel atomization?

Any thoughts?
thanks!

[nqkjw]

 cruising gives me about a 12.0 reading, and WOT about 13.5.

Any thoughts?
thanks!

Hmmm....be nicer if it was the other way round.That is 13.5 cruise and 12.0 WOT.
Below are just a series of thoughts on it for you to think over.
Generally I try to tune with a split of no more than around 6 sizes between primary and secondary jets.
However your AFRs support your desire for a larger split.
How abt power valves? Are you cruising with power valve(s)open?
Have you checked that PVs are the right size?
If you have no PV in the secondaries a bigger split in jet sizes would make sense.A PV is worth around 10 jet sizes.
Have you opened up the power valve channel?You are supposed use a 0.072" drill which is hard to find.A 1/16" which is 0.063" has produced good results for me.It's an old trick to allow richer power valve operation allowing you to use smaller main jets which results in better cruise economy without sacrificing off idle performance.

I'm pretty good with this sort of stuff but there are better than me out there.
I don't even know what carb you have but that is really not that important when you're tuning to AFRs.

Hope this helps.

[cudabob496]

Hmmm....be nicer if it was the other way round.That is 13.5 cruise and 12.0 WOT.
Below are just a series of thoughts on it for you to think over.
Generally I try to tune with a split of no more than around 6 sizes between primary and secondary jets.
However your AFRs support your desire for a larger split.
How abt power valves? Are you cruising with power valve(s)open?
Have you checked that PVs are the right size?
If you have no PV in the secondaries a bigger split in jet sizes would make sense.A PV is worth around 10 jet sizes.
Have you opened up the power valve channel?You are supposed use a 0.072" drill which is hard to find.A 1/16" which is 0.063" has produced good results for me.It's an old trick to allow richer power valve operation allowing you to use smaller main jets which results in better cruise economy without sacrificing off idle performance.

I'm pretty good with this sort of stuff but there are better than me out there.
I don't even know what carb you have but that is really not that important when you're tuning to AFRs.

Hope this helps.

thanks, you know a lot more than me!  Ya, I don't like such a large split on the jets, but car
is set up to make most its power above 4000 rpm
A standard 850 Holley double pumper, with choke tower ground off.
Normal power valve that came with carb, I think a 6 or 6.5 or something.
maybe I wasn't totally WOT, though i was up around 5500 rpm.
I'll take the primaries down to 74 first, to see what happens.

[jimynick]

A good point and one made in the article I mentioned, is to disconnect the secondary linkage and tune your primary AFR's until they're right, THEN hook up the secondaries and tune them based on the observed AFR's. You may have partial tip-in of the secondaries that's throwing off your #'s so much. Just my 

[cudabob496]

A good point and one made in the article I mentioned, is to disconnect the secondary linkage and tune your primary AFR's until they're right, THEN hook up the secondaries and tune them based on the observed AFR's. You may have partial tip-in of the secondaries that's throwing off your #'s so much. Just my 

Makes sense. With a 74/84 setup today, cruising at 2500 to 3000 rpm, I get into the 13's on AFR.  A lot snappier throttle response too in lower rpms.
But at WOT, I get into the mid to upper 14's, so its wanting more gas near WOT.  Gonna try a 74/88 setup tomorrow. Don't think I'll go any lower on the primaries. If my 74/88 dont get mein the 12's for AFR at WOT, may start increasing the primaries starting at 76. After all, its whats going on at WOT that I'm mainly concerned about, and I'll sacrifice gas mileage for WOT power anytime.

Maybe a Holley 850 DP in a long duration racing cam engine cannot be tuned to get 14.7 AFR when cruising and 12.5 AFR at WOT?

What is problem with having the jet number spread being more than 6? Uneven fuel distribution to cylinders?


Found this article:   http://www.amosauto.com/Articles/General/Tech/tuning-holley-four-barrel-carbs