Optimal Fuel Delivery?

[Mac-Porsche]

I conducted a full engine rebuild on a '81 911SC and made some modifications. Added SSI's, 964 cams, and 10:1 - 3.1L JP Carillo P&C's.

After finishing the work, I unplugged the Lambda / O2 sensor. I don't have emissions testing where I live, so was going for a 'richer' mixture that would provide power over efficiency. Plus, read everywhere that running slightly rich is better than lean.

Adjusted the idle mixture screw and got the AFR to 13.0-13.3 at warm idle. The car then runs at 14.0-14.5 when cruising. Car runs great. Only downside I see is that fuel economy is ~13-15mpg city driving.

Looking for some thoughts, comments, opinions from others who have dabbled with AFR's... What are the pros/cons of running rich? Is there such thing as 'too' rich (I read that it can lead to carbon buildup)?

[SkiVT]

One thing to consider, do you have pure gas in AK or ethanol mix? The Internet world suggests running a richer mix with ethanol. http://www.ultra-gauge.com/customer_support/knowledgebase.php?article=29

For performance, your numbers are in the range. Including your afr at WOT would also help as that is really the performance key more than at idle, for example. WOT should be low to mid 12’s. My opinion: for most road driving conditions giving up gas mileage for a small performance gain isn’t worth it. Track driving is different. I just don’t find many daily situations where I can reach anywhere close to the cars full performance potential. It’s really a personal preference thing. I believe your CIS goes full open loop, so ignores the O2 sensor, at roughly 60% throttle. So you get all the enrichment the system can provide in that range. Before that, your trade off is gas mileage at cruising/idle which isn’t really a performance situation anyways. I believe you also need to tweak your timing to get the most out of the engine/afr mix. I am confident many will disagree with my comment on whether it’s worth it. I am simply saying the performance gain from rich afr alone isn’t worth the mileage reduction.

[Mac-Porsche]

We have pure gas. Unfortunately, the best gas we get is 90 octane. Which for a 10:1 CR, seems pretty low. I've asked some Porsche builders whether I should add octane boosters and been told it's not necessary - as gasoline quality has vastly improved since decades ago.

Thanks for the reply! The comments do help put it in perspective, especially considering this car is mostly used for road driving. However, it's not my daily driver, winters here are long, and I invested a few bucks in the engine rebuild. So I don't mind the drop in gas mileage for some occasional fun on the back roads!

I'll definitely check the afr at WOT and see what I'm getting!

[3rd_gear_Ted]

Installing an adjustable fuel pressure regulator tweaked up a couple PSI above the max pressure setting aids optimal throttle response also.

[winders]

Originally Posted by 3rd_gear_Ted

Installing an adjustable fuel pressure regulator tweaked up a couple PSI above the max pressure setting aids optimal throttle response also.

Why would this help throttle response? If you increase fuel pressure in a system that does not compensate for fuel pressure changes all you have done is make the air/fuel ratio richer than it otherwise would be. This could be good if the engine was running too lean but it's not something you do without knowing the current A/F ratio and certainly not something to do if the engine is already running on the rich side.

[AndrewCologne]

Originally Posted by Mac-Porsche

I conducted a full engine rebuild on a '81 911SC and made some modifications. Added SSI's, 964 cams, and 10:1 - 3.1L JP Carillo P&C's.

After finishing the work, I unplugged the Lambda / O2 sensor. I don't have emissions testing where I live, so was going for a 'richer' mixture that would provide power over efficiency. Plus, read everywhere that running slightly rich is better than lean.

Adjusted the idle mixture screw and got the AFR to 13.0-13.3 at warm idle. The car then runs at 14.0-14.5 when cruising. Car runs great. Only downside I see is that fuel economy is ~13-15mpg city driving.

Thats exatcly what the lambda control is good for, very efficient and proper fueling. With a good working lambda system you wont loose ANY performance as when accelerating with an open throttle beyond 35° the lambda control will be bypassed anyway and a static enrichment will be applied for performance gain.

We have pure gas. Unfortunately, the best gas we get is 90 octane. Which for a 10:1 CR, seems pretty low.

So I guess you are running a very conservative ignition advance as your engine is a 930.16 one with a very mild dizzy curve. Whats your initial advance at the pulley when setting up the ignition via moving the dizzy and strobo light? Add some octane booster and set the initial advance to 10° BTDC and do a test drive, but watch out for detonations and be aware that its noise is almost not noticable at high RPMS!

[Mac-Porsche]

Originally Posted by SkiVT

One thing to consider, do you have pure gas in AK or ethanol mix? The Internet world suggests running a richer mix with ethanol. http://www.ultra-gauge.com/customer_support/knowledgebase.php?article=29

For performance, your numbers are in the range. Including your afr at WOT would also help as that is really the performance key more than at idle

In response to your comment, I ran some tests on the road at WOT. It appears to be in the mid to upper 13's. If I hold down the throttle long enough (higher rpm's), it will slowly creep its way to lower 13's.

Everything I read, both this thread and others, suggest mid to lower 12's at WOT. So does this mean I'm running too lean? What would be the recommended fix for this? I'm hesitant to richen the idle mixture screw as I'm already getting ~13.2 afr at idle when warm (w/ O2 unplugged).

As mentioned before, I did upgrade to 10:1 P&C's, would this upgrade require a different fuel distributor from the stock US '81 911sc? (i.e. maybe from a 3.0 RoW 911sc?).

Lastly, I 'diligently' researched the whole O2 sensor "to plug or not to plug". There does appear to be a strong consensus that it really only affects idle and low throttle/cruise. I'll still probably leave it unplugged, because I like the smooth [natural] idle (and... to be really honest... because I used the only bung opening in the SSI to install my dash mounted afr meter).

[SkiVT]

Unfortunately (fortunately for me) I have an 1983 ROW 911SC so will be of limited assistance. Everything I have seen points to mid-12s as being the WOT target. Have you checked your fuel system pressure and WUR pressure (warm in particular) to ensure all is in spec? How about vacuum leaks? The "normal" process tests all those areas first and touch the mixture screw last. Your post starts with the mixture. The mixture screw changes AFR across the range of rpms so you can't materially change WOT AFR without that change hitting other points across the range.

[Mac-Porsche]

After the rebuild last summer, I had a Porsche shop check for air leaks. No leaks whatsoever. I get the drop in rpms when pulling the oil filler cap. So I believe I'm good there.

I checked the fuel pressures a few weeks ago. Had the WUR rebuilt/adjusted to be more rich when cold (to address some cold start lag and cold acceleration lag), it is now at 1.3 bars cold pressure. Warm pressure is 3.5 bars. I have a -090 WUR. Manuals suggest CCP - WCP = 1.5 - 3.5 bars for -090 (US version)... So within spec.

Given the higher compression, might I need a lower warm pressure (i.e. more fuel when warm)?

Read some other posts where people had a similar setup and went to customizable WUR's to better tackle the range.

[dhanl82]

Hello,
I have a 1982 SC that I have a second O2 sensor installed which runs an AEM air/fuel ratio gauge that I have installed in the dash so I can observe the air fuel ratio during all start up, and driving conditions.

With the Lambda O2 sensor connected I have my hot idle mixture set at about 13.8, after 1/3 throttle when the system goes to “open loop” the mixture goes rich to around 12.5 air fuel ratio, which I am fine with because I would rather have it a little rich than too lean when the engine is WOT. With the O2 sensor connected and the system working as it is designed to do the idle mixture fluctuates around 14.7 and the same is true at cruise speed below 1/3 throttle.

In my opinion after reading everything I could find on the Lambda cis injection and working with my car I can see NO reason to disconnect the O2 sensor and negate the benefits that the system was designed to provide.

If one disconnects the O2 sensor and adjusts the mixture to be too rich – not allowing the system to bring it back into the correct air fuel ratio parameters - there is a risk that this overly rich mixture will wash down the cylinder walls along with other problems.

Keep in mind that with the system connected and working as designed, the idle will always fluctuate a little bit because the O2 control system is constantly correcting the mixture as it attempts to achieve a 14.7 mixture as it reads and reacts to the signals it gets from the O2 sensor.

Hope this helps

Dave