A/F ratios???

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A/F ratios???

Postby k-star » Mon Feb 25, 2008 10:35 am

This is a bit long..

So I installed A/F gages on my test stand. I did it to make a video to prove that power valves ( or I should say an opened power valve) has no effect on idle richness.

I get everything hooked up and working. I have a sensor on each bank. I originally set the idle screws with the old highest vacuum/ best RPM method. So I fire the engine, run it at 1500 rpms or so to heat the o2 sensors, then return to a 900 rpm idle. I find that both side are pegged rich. So I start adjusting the idle screws until the gages are in the stitchometric range. When I get them both there the engine is idling like crap.

Engine is a basic small chevy, 302 ci, 10.0:1 static, hyd roller cam, that’s 208/221@.050 zz4 heads dual plane intake. locked timing at 30 deg.

Any ideas on why the engine would idle so much better with such a fat idle????

Keith
“If I hit you with this you’re going to be numb, that’s why they call them numb-chucks “Si Robertson

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Postby Doug Schriefer » Mon Feb 25, 2008 10:58 am

A couple of things.

1) An open Power Valve will affect the amount of fuel at idle...

2) Are you sure your WBO2 was correct? (was anything else telling you that you were rich?)

3) Most O2 systems require you to heat the sensor Before you start the engine, you may have been getting a false reading.

4) Depending on the exhaust on your stand, with the 221 duration it may have been over scavenging.

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Postby David Redszus » Mon Feb 25, 2008 11:57 am

Which O2 sensor or you using?
What reading do you get when under full load?

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Postby Ryan W. » Mon Feb 25, 2008 12:05 pm

If you are running it open header with the sensor at or near the end of the collector chances are you cannot get a good reading at idle. You will need to make some sort of extension after the sensor to make sure you do not get false readings. Are they narrow or widband sensors?

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A/F ratios

Postby k-star » Mon Feb 25, 2008 12:08 pm

I might need to give some more details.

The O2 sensors are one wire non-heated, and the gages are Autometer with the led lights. The engine has 2 chamber Flowmasters mounted to the collectors, and then it feeds a pair of big RV mufflers ( keeps the neighbors off my stink) ,then vents out side the shop. All the pipes are 3”

The O2 sensors are mounted right next to the collector flange, on the muffler side.

I can tell when the O2 sensors are cold and not reading, the gages will go max rich with no fluctuation. As soon as they warm up you will get movement out of them.

The carburetor is a modified Holley. I modified it so I can open and close the PV from a external vacuum source.

I feel everything is working correct because a ¼ turn on the idle screws will move the gage one light. So I can have it one light on the rich side, adjust the idle screw ¼ turn and have it in the stitchometric.

When I remove the vacuum to the PV and it opens it has no effect on the A/F gage. The gage reads the same at idle weather the PV is open or closed. IMO this is correct since the PV feeds the main circuit and not the idle circuit. The idle circuit gets its fuel through the restriction at the bottom of the main well. I would not think that by adding any more fuel to the main well, the restriction could allow more fuel through??? My test proved that to me…

Keith
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Re: A/F ratios

Postby Doug Schriefer » Mon Feb 25, 2008 12:24 pm

k-star wrote:
The O2 sensors are one wire non-heated, and the gages are Autometer with the led lights. The engine has 2 chamber Flowmasters mounted to the collectors, and then it feeds a pair of big RV mufflers ( keeps the neighbors off my stink) ,then vents out side the shop. All the pipes are 3”



When I remove the vacuum to the PV and it opens it has no effect on the A/F gage. The gage reads the same at idle weather the PV is open or closed. IMO this is correct since the PV feeds the main circuit and not the idle circuit. The idle circuit gets its fuel through the restriction at the bottom of the main well. I would not think that by adding any more fuel to the main well, the restriction could allow more fuel through??? My test proved that to me…

Keith


Keith,

The type of gauge you're using is not very accurate. I don't believe they are sensitive enough to measure the difference that you're making. When you have a carburetor hooked up to good flow meter, or qaulity lambda you can see the difference when the PV opens and closes.

The idle circuit gets its fuel from the main well. If you change the volume of fuel in the main well it will change how much fuel flows through the idle circuit due to available volume. Now the amount of change will vary depending on your PVCR, and how you have the idle circuit tuned and setup.

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idle

Postby k-star » Mon Feb 25, 2008 12:52 pm

At idle isn't the main well stagnet??? For lack of a better term, full of fuel with no place to go??? so if the column of fuel in the main well is full,, by adding more i can't see how it would allow more to pass through the idle feed restriction... Maybe i am missing something here..

I am not sure what level of testing your at, sounds like it's a lot more sensitive then where i am at, but i think 1/4 turn on the idle screws is a good response for the gage to pick up.. If adding more fuel to the main wells affects the idle less then 1/4 turn on the adjustment screws i think it would be a mute point??? as far as an open PV affetcing idle richness???

Keith
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Postby k-star » Mon Feb 25, 2008 1:01 pm

David Redszus wrote:Which O2 sensor or you using?
What reading do you get when under full load?


I can't load the engine, it's just a test stand..

Keith
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Postby Doug Schriefer » Mon Feb 25, 2008 1:03 pm

You're still dealing with more fuel in the well. With the engine idling the fuel is not static in the mainwell. So having more fuel there will affect the idle.

I've never seen any one wire O2 sensor pick up anything even close to a decent heated unit.

Can you adjust around a PV that is open at idle? Without a doubt, but saying that it doesn't affect an engine at idle is wrong.

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carb

Postby k-star » Mon Feb 25, 2008 1:38 pm

I am not trying to be stupid, just trying to learn..

I can understand the fuel moving through the main well at idle due to it feeding the idle passages.I didn't think about that before.

So put some numbers to your testing of this issue.. what is the A/F ratio with the PV closed at idle, and what does it change to with the PV open???


I am not sure i understand your comment about my set-up not being close enough??? It will pick up a change of 1/4 turn of the idle screw. Will a heated sensor pick up a 1/8 turn????

So if i switch to heated sensors, and maybe a LM-1 what will i see different in my test???

I need to calculate the square inch area of the jets feeding the main well and the PV restriction feeding the main well and see what % difference that is, then try to get my mind around how that can force more fuel throught a fixed diameter that feeds the idle circut..

There isn't any pressure in there is it???

Keith
“If I hit you with this you’re going to be numb, that’s why they call them numb-chucks “Si Robertson

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Postby Motor Daddy » Mon Feb 25, 2008 2:15 pm

A narrow band O2 is only good at reading 14.7. A more lean mixture or more rich mixture will be darn near impossible to tell exactly what that ratio is above or below Stoich.

Look at the posted graph. It shows very clearly what I'm talking about.

The problem with setting your engine to idle at 14.7 is that it's probably too lean to have a good idle.

I use a wide band that has a digital readout. My engine idles best at 13.5:1. Anything more lean and the idle quality deteriorates rapidly. It will idle at 14.7, but the vacuum will be way down at the same idle RPM. Once a slight load is applied to the engine, such as turning the steering wheel (power steering pump load), the idle RPM will drop drastically.

I seriously doubt if your engine would prefer a 14.7 over say a 13.5 at idle. The problem with the narrow band O2 you have is that there is no way you can tell where 13.5 is and where 12.5 is, and whether opening or closing the power valve will show up on your meter. I can physically watch my XD-16 meter show when the power valve opens, as I have an in-dash vacuum gauge as well. I can go up a hill and watch the vacuum gauge drop slowly, and when the gauge reads below my power valve rating of 6.5, the A/F ratio changes about a full point from 13.8-12.8. When I open up the secondaries I see the A/F ratio stays the same. That's how I tuned the secondaries to be balanced with the primaries with the power valve open.

As to whether the power valve effects idle mixture when open or closed, I suspect that the A/F ratio doesn't change at idle whether the power valve is opened or closed, as the power valve enriches the main circuit, and that is not active at idle (or shouldn't be, anyway). I've never checked it with the wide band, so I'm not sure, though. That's something I'll have to try this spring when it gets warmer.

If you look at the pic with the metering block gasket, you can see you would be able to control the power valve with the vacuum port on the metering block. I think I downloaded the metering block pic from the Innovate Motorsports website, from Tuner, maybe??

Image

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Postby MadBill » Mon Feb 25, 2008 4:18 pm

MD has it covered; you definitely need a wide band to get accurate data (and street cred with the results.) and work off 'lean best idle' AFR, not stoichiometric. Since you can't load the engine even as a low stall speed converter would, one thing that might help would be to run very little spark advance, say 5°. This will permit a lower stable idle speed to work from.
As far as the effect of an open vs. closed PV, I'm certain the effect will be small, if not zero. Theoretically, since the main wells are fed by both the main jets and the P.V.C.R.s, there would be a minute rise in the level and thus a rich shift, with the latter open, but I can't imagine it changing the AFR by any meaningful amount.
An effect of which I'm less certain though is the result of a PV 'fluttering' right at its opening point, i.e. an engine with a 6.5" PV idling right around 6.5" Hg. and so perhaps causing an erratic 'pump' of fuel into the wells. This could be a little more challenging to evaluate, absent some means to increase engine load.
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Postby nissan-lover » Tue Feb 26, 2008 2:28 pm

An autometer connected to a narrowband is just an expensive light show that wont give accurate readings, you can only trust a heated wideband.
At idle the mixture needs to be richer than stoic anyway, and that will peg a narrowband, if your fuel is stoichiometric at 14.5:1 then your autometer can be believed for readings between 14.1:1 and 14.8:1, outside of that it is pegged at the limits.

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Postby Lucky13 » Tue Feb 26, 2008 4:26 pm

The reason the PV being open could add to the richness of idle A/F is because of the added pressure it will add to the main well. When you change your main jets, it does infact change idle, and T-slot mixtures for the same reason, adding pressure. The carb works off of pressure differentials and I would bet Mr Tuner from the Innovate site could even provide a formula to give the results of this.

The idle feed is feed from the main (on most carbs), when the pressure in the main well changes it forces more fuel through the idle feed. I could rig up a test to do exactly what your trying to prove and see for sure but I would have to put my Holley back on to do so and make the mods. I may get a chance to do this in the next few days and can provide logs to show how much, if it does, change idle A/F. Like meantioned above it is very noticable when the PV opens when the mains are flowing, or even if they are not and the P/V only inrichens the T-Slot circuit. It is measurable to see the diff in the idle circuit when just a main jet is changed when using a Wideband system to monitor. Infact it effects it alot at ilde. I can only assume the P/V would just add that much more, but I will test to prove or dissprove for sure.


Also the 14.7 as measured wwith your setup will just simply be to lean for idle and would be rough.



Jess

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Postby MadBill » Tue Feb 26, 2008 5:12 pm

Motor Daddy wrote:...I can go up a hill and watch the vacuum gauge drop slowly, and when the gauge reads below my power valve rating of 6.5, the A/F ratio changes about a full point from 13.8-12.8..]



BTW, MD, if fuel economy is of interest (and why wouldn't it be these days?) have you considered reversing the tune sequence on your car? Dial in lean best cruise AFR (depending on cam, etc., perhaps 16.5:1) with the primary main jets (likely # 55 -60), then drill and tap the primary P.V.C.R.s to 6-32 for Holley's 142xx emulsion jets (available from 00 to 0.078") and tune the primary W.O.T. AFR with them. With this approach, PV ratings need to be fairly high to prevent weak tip-ins, but it's usually good for several miles per gallon minimum..
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