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Tom wrote: Fri Jul 23, 2021 9:51 am
I always find driving my 951 helps remind me why I like it. But I'll admit I go through ups and downs with it too, especially when other cars/projects crop up. What's going on with that throttle body bracket? And do you have any issue with the long intercooler tube blowing loose under boost? When I switched over to that type of pipe, I just couldn't keep it from blowing apart eventually, so ended up adding these boost braces. I bought these, but they'd be easy enough to make...
boost-braces.gif
Hi Tom,
I see you're in CA and I was wondering how you pass smog with the MAF? I'm in Northern California and was trying to figure out a way to pass smog using one.
Hey Mark, I split this question off into its own topic because I'm a bit obsessed with CA smog -- and to bone up on my forum jockeying skills....
The bottom line is that MAF's are for off-road (track) use only in California on these cars. When I take my car in for smog checks, I put a ton of effort into making sure it will pass both the visual and sniff tests. Knock on wood, I've not failed yet. Once you've done it a hundred times, going from MAF to AFM is fairly painless. A good cat and a good tune will get you by the sniff part. There will always be smog techs willing to fake a test for money, including in the bay area, but I'm too much of a Boy Scout to ever go that route... Let me know if I can help.
Fortunately for those of us in WA State, once your car is 25 years old, there were no more smog checks... and now the whole smog check system has been disbanded.
Thanks Tom! I think that's the plan, run stock but put on a MAF when running at the track. I'll have to figure out some sort of plug adapter that I can use to make the swap easier.
himynameismark wrote: Mon Aug 16, 2021 5:31 pm
Thanks Tom! I think that's the plan, run stock but put on a MAF when running at the track. I'll have to figure out some sort of plug adapter that I can use to make the swap easier.
I pulled the connector out of a broken AFT and put it in a little plastic enclosure with the MAF wires coming out the other side -- it makes the MAF plug and play without cutting wires, etc.
As some may know, I tend to obsess on CA smog and feel like I have a pretty good handle on getting the 951 to pass. Lately, I've been trying to get my old 450SL to pass (CIS, no computer), and came across some interesting data that was new to me. With our cars, a decent O2 sensor will keep the car very close to 14.7, so that part of the equation is fairly simple to nail down. On non-O2 cars, however, it can be more of a challenge. See, e.g.,
I never really knew how close to 14.7 you needed to be in order to keep the emissions within limits. I discovered with the old mercedes that the CO% goes up very fast in response to fairly small changes in the AFR. In CA, the car needs to have a CO of roughly .6 to .8% or lower to pass. See the chart below, but that means you need to be running about 14.3 or leaner in order to pass smog (although a fresh cat may help cheat those numbers a bit). That's a much narrower window of success than I realized, and means fresh O2 sensors, good grounds, etc. are all pretty important. Just FYI for those interested.
afr to copercent table.jpg (597.72 KiB) Viewed 2345 times
Bill wrote: Tue Oct 05, 2021 7:19 am
Do you have any thoughts on high NOX?
On the 951? In Nevada? Standard advice for high NOx is that the combustion temps are getting too high, due to a lean mixture, bad EGR (n/a on the 951), overly-hot engine temps, carbon deposits in the engine, too much timing or.....wait for it....weak catalytic converter. On a practical level, if you have a wideband and can see that your car is bouncing tightly around 14.7 at idle and on very light steady cruising on a flat road, then your NOx should not be excessively high on a 951, assuming the car is running well, coolant temps are normal, the timing isn't super out of whack, no pinging, etc. In CA, the limits are pretty close to what a well-tuned 951 produces without a cat, so if everything else is good, and you're at 14.7AFR, and you still can't dip under the NOx limits, consider a new 3-way catalytic converter.
Also, there have been several studies that confirm your NOx will be up to 10-15% lower on high humidity days, with no effect on the other readings. So if you are just over the limit, getting tested on a cold, rainy, high-humidity day, might be all it takes.
I once had the chance to tune a 951 while it was on a CA smog dyno, and I simply could not get all three gasses below the limits. I'd add fuel to get the NOx under the limit, and the others would go up, and vice versa. It's like a three-sided teeter totter. We worked at it until the tech had to move on to the next customer, and got the CO and HC just under the limits, and the NOx just over the limit. That night we went home and swapped in a nearly new after market cat (DEC), and the next day, without touching anything else, the numbers were all so low across the board that the tech was worried it would look like he was cheating....
But I'll admit I go through ups and downs with it too, especially when other cars/projects crop up. What's going on with that throttle body bracket? And do you have any issue with the long intercooler tube blowing loose under boost? When I switched over to that type of pipe, I just couldn't keep it from blowing apart eventually, so ended up adding these boost braces. I bought these, but they'd be easy enough to make...
