smaller wheel = running rich?

is this true?

stock is 195/60/15 and I'm running a 205/40/17 tire, about a 3% difference.

I went to aircare(smog testing) today and did really well in everything except my carbon monoxide test in the driving portion.

the guy said that looking at the results, it's probably because I'm running a bit rich.

so I was thinking that it may be the smaller diameter wheel that I'm using. is it worth a shot just changing the rear wheels back to stock and go test? or should I go mess with something else?

these are my results

driving test

hydrocarbons: max 218.00 ppm, me 88.00 PASS
carbon monoxide: max 1.53%, me 2.42% FAIL
oxides of nitrogen: max 2226 ppm, me 276 ppm PASS

idle test

hydrocarbons: max 394ppm, me 22 ppm PASS
carbon monoxide: max 4.47%, me 0.04% PASS

so it's just the CO readings during the driving test. over by less than 1%...grrrrrr

other than the wheels, what I've done that may affect it is, 3" cone filter and 2.25" cat back exhaust. but I didn't have any real problems with running rich until after the rims went on.

TIA

rims and tires have nothing to do with fuel mixture. You are running slightly rich though under load. What RPM is the driving test conducted?

not sure

but speed is around 40km/h

so I'm guessing 2nd gear 2000-3000rpm?

damn you and your metric system, So that's about 25 MPH in 2nd gear would be right where you said. Do you have an A/F guage? if so what does it read when you're at that speed.

Maybe your O2 sensor is starting to go bad or something. Although you could dance around the subject by playing with igntion timing and inducing vacuum leaks but that's not how we fix cars is it? :slap:

Other than that i don't really have anything

edit: you could certainly try to switch your wheels back but like i said wheels and tires have nothing to do with fuel mixture or combustion efficiency

Tire heights ...

195/60/15
195*0.6 = 117 mm
2*117 + 15*25.4 = 615 mm

205/40/17
205*0.4 = 82 mm
2*82 + 17*25.4 = 595.8 mm

RPM = KPH*FD*GR*530.3030303/D
RPM = 40*4.11*1.902*530.3030303/59.58
RPM = 2783 RPM

That's providing that you have had your speedometer recalibrated ...

If not ...

RPM = 40*4.11*1.902*530.3030303/61.5
RPM = 2696 RPM

KPH = RPM*D/(FD*GR*530.3030303)
KPH = 2696*59.58/(4.11*1.902*530.3030303)
KPH = 38.75 km/h

Math is fun ...

How long has it been since you changed your air filter? And how old is the O2 sensor? This is where I'd start looking.

The tires will have no effect on the A/F ratio.

the air filter is mimms' r-active cone filter

used for only a few months and washed before installed so it should still be fine.

my dad also suggested the O2 sensor.

check the water temp sensor as well

that would cause me to run rich?

can you explain that?

if its bad then the ecu thinks the water is still cold so it adds a bit of fuel to warm it up faster

oo...okay thanks. I didn't know that :D

actually ... it might have something to do with your rims and tires.

You're not running "rich" you're just burning too much fuel ;)

I'll try and explain,

The drive clean (or air care) machines are pretty much a roller dyno.

The problem is, they may not be calibrateable. I know the guy I go to for "tests" has told me the machines are not programmable. You can't change any of the parameters. The VIN, year and engine code are entered, the computer calculates your final gear.

If your rims and tires are not stock, it can throw the calculation out of whack.

So your car is actually being run harder than it's supposed to to get to that RPM/MPH. So that could be a reason it's running a bit rich.

Now, this is just a theory ...

If it is indeed running rich all the time, there's a lot it could be.

actually ... it might have something to do with your rims and tires.

You're not running "rich" you're just burning too much fuel ;)

I'll try and explain,

The drive clean (or air care) machines are pretty much a roller dyno.

The problem is, they may not be calibrateable. I know the guy I go to for "tests" has told me the machines are not programmable. You can't change any of the parameters. The VIN, year and engine code are entered, the computer calculates your final gear.

If your rims and tires are not stock, it can throw the calculation out of whack.

So your car is actually being run harder than it's supposed to to get to that RPM/MPH. So that could be a reason it's running a bit rich.

Now, this is just a theory ...

If it is indeed running rich all the time, there's a lot it could be.

if the overall height of the tires would have increased i could see what you where saying matt to be somewhat true since you have a larger diamter and require a higher energy to rotate the larger tire, but his tire height shrunk so it is actually esier to turn his wheels then with the stock tires...

well, yes and no ... it's "easier" because it alters the final drive.

Smaller tires will increase (numerically) the rear drive ratio.

So he'd end up at a higher RPM to do 40 KPH than stock. At higher RPM, motor is using more fuel = more CO

But yeah, it's not likely ... but not impossible either ;)

now are these driving tests on a dyno or just holding the rpm at about 2500rpms

like matt said, they're on these rollers

and the guy sits on the cars and tries to hold in within the green area (between 38km/h - 45km/h from what I saw)

so it fluxuates up and down a bit as he's pressing or letting go.

hmm its got to be a bad o2 maybe

So you're saying that Indy's final drive ratio has changed because he's using smaller diameter tires ?

So how do you figure out how fast you're going after you change the tire size ?

you don't ... it's very hard to get it accurate again

Nissan offers the speedo gears to accomodate the different rear gear ratios
but any of them would be a step too far.

When your car was calibrated, it was done with a set tire size, set gear ratio etc ...

But it was also set fast. All speedos in general are set 5 km/h (or 3mph) fast. That way if there is ever a discrepancy, you can never fault the speedo if you're going too fast.

We're getting into the wiggly niggly though ... Indy's total tire size is probably not enough of a difference to be important.

Yeah in Vancouver it is on a dyno or at least it was the last time I had to aircare my car