Tuning questions

Kev · Post by **Kev** » Wed May 10, 06 4:43 pm

Been out with me O2 Sensor and Vac gauge today. Is the O2 Sensor output AC or DC. In the instructions it sez the sensor wire is positive so I'm assuming it's DC. I've gone down from 72 to 69 on the primaries and the O2 sensor still reads about 980mv, down from 1050mv. Stoich is 400mv, but obviously we err on the rich side. The plugs are a grey colour on the electrode that sez weak to me. What gives? Also, I wired my secondaries shut and there doesn't seem to be that much difference in performance so I'm assuming they are not opening. (Never having done this before is there a great difference that is felt through the seat of yer pants?) I recently cobbled a good vac sec canister together (It sucks good!) and use the purple spring. Where does the can get it's signal from? I'm making 10" vac at idle out of gear, about 8.5-9" in gear at idle. I've changed my PV from a 3.5" to a 6.5" whch seems to help. TIA Boffins!

Matt · Post by **Matt** » Wed May 10, 06 5:16 pm

o2 sensor output is DC Kev

Cheers

Matt

Mick70RR · Post by **Mick70RR** » Wed May 10, 06 5:42 pm

The output from most O2 sensors is an AC signal between 0.9V and 0.1V. The frequency is around 1 Hz. You need to take the average Voltage of several cycles. Not sure how a digital Voltmeter would respond to a signal like this. I've just bought an O2 sensor myself and I plan to build a circuit to display the mixture in the car.

Dave-R · Post by **Dave-R** » Wed May 10, 06 6:14 pm

You need an analog voltmeter to read the output from the O2 sensor. It is DC but not a smooth DC if you know what I mean.

I have a circuit diagram to make an led readout.

I also have a spare Edelbrock LED readout you can borrow if you want Kev.

Dave-R · Post by **Dave-R** » Wed May 10, 06 6:20 pm

Just looked up the voltages and Stoich is around 0.25 to 0.749 volts on a normal O2 sensor.

You want about 0.800 - 0.850 to get yourself in the best power area.

Make sure the sensor it up to full operating temp. They read lower voltages when hotter and higher voltages when colder. You also need the sensor as close to the collector as possible.

Dave-R · Post by **Dave-R** » Wed May 10, 06 6:22 pm

Oh and Kev. I have #64 jets in my little primary 2bbl center carb on the six pack. The secondary outer carbs have much much bigger jets though.

Kev · Post by **Kev** » Wed May 10, 06 6:55 pm

Yeah, I'm sorta gunning for a rich square jet set up for the Drugs. So where does the vac sec get it's signal from?

Dave-R · Post by **Dave-R** » Wed May 10, 06 7:21 pm

Sorry Kev. Missed that question. The secondary diaphram is connected to a tube leading into the primary venturies. or is it one venturi?

Anyway. When airflow through the primaries becomes great enough it creates vacuum in this tube and applies it to the top of the secondary diaphram.

Dave-R · Post by **Dave-R** » Wed May 10, 06 7:24 pm

This is the reason Stoich voltage is tricky to measure. It jumps almost instantly from around 0.2 volts to 0.8 volts.

Kev · Post by **Kev** » Wed May 10, 06 7:25 pm

Above or below the throttle plates?

Dave-R · Post by **Dave-R** » Wed May 10, 06 7:26 pm

Kev wrote:Above or below the throttle plates?

Well above mate. Otherwise manifold vacuum would pull them open wouldn't it?

Dave-R · Post by **Dave-R** » Wed May 10, 06 7:31 pm

Found a diagram in a book and scanned it for you mate. Looks like it gets a signal from the secondary side as well.

Kev · Post by **Kev** » Wed May 10, 06 8:44 pm

Found 'em! Spent the usual half an hour looking for the plunger spring clip that always has a mind of it's own! All seem clear but with or without secondaries there doesn't seem to be much difference in power. Is it really THAT noticeable?

Thanks for the help Dave, you've answered everything I was ringing you about. Ta!

Dave-R · Post by **Dave-R** » Wed May 10, 06 9:30 pm

Sorry I couldn't answer the phone mate. I was training some Post Grad students to do my job out of hours because I can't be bothered to do much overtime these days. Even if i do need the money. I am just getting too old for long hours at work.

Dave-R · Post by **Dave-R** » Wed May 10, 06 9:33 pm

Oh. Yes you will not notice much "seat of the pants" difference on just the primaries at normal driving speeds. It would soon run out of steam on the strip without the secondaries though. You can't really feel horsepower you know? Just torque. Most of that torque is made in an rpm range a small carb can cover.