10 mpg 3.2 - can’t find where the fuel is going

[Showdown]

I don't know anything about the 964 cams you have but given that your AFR/Lambda values are pretty good looking and not showing filthy rich, plus you're still getting good mileage at highway cruise, it's totally possible that your car with those engine specs really does consume that much fuel, particularly in accelerating from a standstill.

Any unspent fuel would show up in your AFR/Lambda log and if you don't have a leak somewhere in your fuel system, then I'd say the "missing" fuel isn't missing after all, and that it's being used by your engine and you just have low MPG...

The giveaway for me is that prior to rebuild (with cam change) you were getting 18 and now after rebuild (with cam change) you're getting 10. See where I'm going with this... It's your cams.

[garment]

I went with a hotter cam (964) in my 3.2SS build. My fuel mileage plummeted too.

I don’t drive my 911 for fuel economy. The improved torque in the low end plus HP gains is worth it. The one thing I did do is go with a taller 5th gear. I get 27 MPG on the highway cruising at 80/<3K RPM. Stock CIS.

[Discseven]

Originally Posted by Showdown

I don't know anything about the 964 cams you have but given that your AFR/Lambda values are pretty good looking and not showing filthy rich, plus you're still getting good mileage at highway cruise, it's totally possible that your car with those engine specs really does consume that much fuel, particularly in accelerating from a standstill.

Any unspent fuel would show up in your AFR/Lambda log and if you don't have a leak somewhere in your fuel system, then I'd say the "missing" fuel isn't missing after all, and that it's being used by your engine and you just have low MPG...

The giveaway for me is that prior to rebuild (with cam change) you were getting 18 and now after rebuild (with cam change) you're getting 10. See where I'm going with this... It's your cams.

Julian... Yes, I know where you’re going. Of course I prefer it to be the AFM or ECU. But we’re looking for reality here and if it’s the cams, I’ll be rebuilding the rebuild. 10 mpg is intolerable. Intolerable given this engine’s capability to run double the distance for the same energy input.

Having looked into a 3.2 using 964 cams before going the 964 cam route, they seem a standard mod for 3.2s. Never have I seen any comments concerning gutter city milage from anyone doing this---until Garment! Was going to say "I can’t be the lone wolf here" but now I'm not! Garment is with me. The bite is, as you mention… “these cams not being in the engine when it got 18 city mpg.” Combined with the wideband logs that don’t fit a rich engine, "missing" fuel isn't missing after all” is an angle to this that makes sense.

Am going to take an alternate angle for the sake of looking for relevance in what you’re thinking, or counter pointing it. And Garment, with all due respect, am setting you're input aside for the time being in order to continue this vein without bias...

Car’s weight and the engine’s combustion chamber size have not changed from before engine’s rebuild to now.
Moving this weight in the city once required 1 gallon of energy to generate 18 miles of travel.
Same weight, same combustion chamber size, same motion now requires 1 gallon of energy to generate 10 miles of travel.

Moving the same weight and using 2 times the energy to travel the same distance… what’s that?

Seems to be a tremendous waste of energy occurring. Or is this a gain of power with the wideband not seeing it as waste---not seeing it as rich? If so...

What I cannot reconcile is the same combustion space efficiently burning nearly twice the energy to move the same weight the same distance.

.

[Showdown]

I hear you Karl, and I can see (and read) your optimistic thinking that maybe something else is a miss, but I suspect the reality is that the cams are just much more fuel hungry than the stock cams.

The power increase that you get with the 964 cams isn't free... and the cost is MPG.

Occam's Razor; the only thing changed was cams and now you get les sMPG ergo, it's the cams.

I wish for your sake that there was another answer and that it was as simple as tightening a bolt, replacing a plug, etc... But I'd guess that if you put the stock cams back in, your MPG would go back up...

[Mr. Merk]

I think you mentioned having the stock chip in the DME. Have you considered a chip specific or custom tailored to the new specifications?

[917_Langheck]

Originally Posted by Showdown

Occam's Razor; the only thing changed was cams and now you get less MPG ergo, it's the cams.

I wish for your sake that there was another answer and that it was as simple as tightening a bolt, replacing a plug, etc... But I'd guess that if you put the stock cams back in, your MPG would go back up...

There is a good test module; however, it's not the only thing that wasn't in the car before, or was unchanged, and presumably put back into the state it started and which is also assumed to be correctly adjusted. That is why the AFM, the ECU (DME), and injectors (are you back to the originals in this iteration?) are part of the discussion.

It is possible that the cam overlap is so large, and the amount of time available at low rpm so long, that twice the fuel rate is escaping out the exhaust due to scavaging, but surely the wideband would "see" that detail.

[Discseven]

Originally Posted by Showdown

I hear you Karl, and I can see (and read) your optimistic thinking that maybe something else is a miss, but I suspect the reality is that the cams are just much more fuel hungry than the stock cams.

The power increase that you get with the 964 cams isn't free... and the cost is MPG.

Occam's Razor; the only thing changed was cams and now you get les sMPG ergo, it's the cams.

I wish for your sake that there was another answer and that it was as simple as tightening a bolt, replacing a plug, etc... But I'd guess that if you put the stock cams back in, your MPG would go back up...

Julian... cams were not the "only thing." ECU and coil are not the same as when engine got 18 city. ECU has been tested but I remain dubious. Blaster coil I have not gotten a suitable Bosch black replacement to test that. If anyone knows if a coil would make this difference, sing out.

I'm prepared to dive back into the engine if it comes to nothing left but cams. Would like to rule everything "simple" out before doing a return trip. Imagine... replacing the cams only to find out... it's not the cams!

Originally Posted by Mr. Merk

I think you mentioned having the stock chip in the DME. Have you considered a chip specific or custom tailored to the new specifications?

Merk... Yes on stock chip. I've not considered a new chip. Given the relatively decent wideband logs, do you think there is a chip config that would trim fuel while also not leaning the engine out?

Originally Posted by 917_Langheck

There is a good test module; however, it's not the only thing that wasn't in the car before, or was unchanged, and presumably put back into the state it started and which is also assumed to be correctly adjusted. That is why the AFM, the ECU (DME), and injectors (are you back to the originals in this iteration?) are part of the discussion.

It is possible that the cam overlap is so large, and the amount of time available at low rpm so long, that twice the fuel rate is escaping out the exhaust due to scavaging, but surely the wideband would "see" that detail.

917... AFM is original to engine, adjusted but put back to where it was. ECU is indeed not the same as was previously in car. Lucas injectors were in engine prior to and after rebuild. These were tested against Bosch 158 originals. Both sets were tested by Mr. Injector, Bill and passed. For comparrison, Lucas were run in one side of the engine and Bosch in the other...



There is now all Bosch injectors running.

I too wonder about the overlap + cams. As you note, the wideband would "see" excess fuel delivery but it's not there. I reiterate the wideband system being gone through systematically so the data seen must be accurate.

A note for anyone heading to mod their engine. I installed these 964 cams, SSIs, and Dansk muffler on the recommendation of a "Porsche expert." There was no comment on his part about the fuel consumption cost and no questioning on my part to him or others in that regard. Was my mistake not empirically researching what affect these adjustments would have. Lesson learned and passed on.


Webcam input

Called Webcam. Spoke with Faith. Asked her what the difference is between stock 3.2 cams and 964 40/40 gind cams. She said the 964 has 20 thousandths more lift and slightly less duration. Was as I interpret this. I added I overlapped cams 2.45 rather than 2.25... and there are SSI and Dansk 2-in-1-out muffer with these cams. Config according to her would definitely not result in 10 mpg... "It's something else." She's emailing me stock and 964 40/40 technical specs. Will be interesting to see the numbers. Post them later.
.

[GH85Carrera]

Karl, when my engine was rebuild back at 150,000 miles, I went with 964 cams, ARP bolts throughout, and I did have Steve Wong burn me a new chip with all the tweaks. I have 198,000 miles now, and many cross country trips, and lots of city driving.

I don't monitor my gas mileage, but I usually get over 400 miles per tank of mileage. On pure road trips is is 500 miles.

Porsche rated the engine at 200 HP at the flywheel, and on my dyno testing I got 200 HP at the rear tires. Now on track with mostly WOT throttle I can get it down to 8 or lower.

[76FJ55]

Have you measured your idle vacuum? I'm curious what you have with those cams/timing? how steady is your idle vacuum reading?

[Showdown]

What I'm seeing is an AFR/Lambda log that's good- nothing abnormal or surprising at all. Sure, you could be a bit leaner here and there but that's not going to account for an 8mpg loss. If you were throwing fuel out the tailpipe your log would catch it and since you're not catching it, you're not wasting fuel.

If the coil or plugs or DME or ECU were acting up and not performing to spec, you'd still see that fuel in the AFR logs. And you're not.

I think the exercise of going through everything is good as you've already made some fixes and improvements to the engine, but I don't think you're going to find a culprit because there isn't one. The simple fact is that you're not wasting fuel, there's no unburnt fuel escaping the tailpipe... So a cracked vacuum hose or fouled plug should show up in your AFR/Lambda log.

[mikedsilva]

Originally Posted by Discseven

Called Webcam. Spoke with Faith. Asked her what the difference is between stock 3.2 cams and 964 40/40 gind cams. She said the 964 has 20 thousandths more lift and slightly less duration. Was as I interpret this. I added I overlapped cams 2.45 rather than 2.25... and there are SSI and Dansk 2-in-1-out muffer with these cams. Config according to her would definitely not result in 10 mpg... "It's something else." She's emailing me stock and 964 40/40 technical specs. Will be interesting to see the numbers. Post them later.
.

Looking at your referencing to camshaft timing through this thread.. you say 2.25mm or 2.45mm.

The spec for a lift on the overlap for a 964 cam is usually 1.26mm. If you got this wrong during the cam timing phase, you could have both cams quite advanced, or you maybe got one cam correct and the other far out of whack. I'm not sure.. but my suggestion would be to recheck your camshaft timing, and write down 1.26mm so you don't get it wrong.
Often we advance the 964 cam to 1.4 or 1.5mm to move the torque earlier. It's important to get it the same on both left and right banks.

Again, 1.26mm is the normal spec for 964 cams.

Also, above you say she told you the 964cam has MORE lift and LESS duration than a stock cam. I don't think that is quite correct.
Here are some specs I got from a competitors website...

[Discseven]

Originally Posted by GH85Carrera

Karl, when my engine was rebuild back at 150,000 miles, I went with 964 cams, ARP bolts throughout, and I did have Steve Wong burn me a new chip with all the tweaks. I have 198,000 miles now, and many cross country trips, and lots of city driving.

I don't monitor my gas mileage, but I usually get over 400 miles per tank of mileage. On pure road trips is is 500 miles.

Porsche rated the engine at 200 HP at the flywheel, and on my dyno testing I got 200 HP at the rear tires. Now on track with mostly WOT throttle I can get it down to 8 or lower.

Glen... If you're getting 400 miles a tank with 964 cams, I want what you're having!

Do you know what grind your cams are?

Originally Posted by 76FJ55

Have you measured your idle vacuum? I'm curious what you have with those cams/timing? how steady is your idle vacuum reading?

76F... Idle vacuum has not been measured. Don't have tool for that or it would have been done.

Originally Posted by Showdown

What I'm seeing is an AFR/Lambda log that's good- nothing abnormal or surprising at all. Sure, you could be a bit leaner here and there but that's not going to account for an 8mpg loss. If you were throwing fuel out the tailpipe your log would catch it and since you're not catching it, you're not wasting fuel.

If the coil or plugs or DME or ECU were acting up and not performing to spec, you'd still see that fuel in the AFR logs. And you're not.

I think the exercise of going through everything is good as you've already made some fixes and improvements to the engine, but I don't think you're going to find a culprit because there isn't one. The simple fact is that you're not wasting fuel, there's no unburnt fuel escaping the tailpipe... So a cracked vacuum hose or fouled plug should show up in your AFR/Lambda log.

Julian... I too think the wideband would show ANY air-fuel anomaly. Also agree there's nothing apparently off kilter with this engine's air-fuel. Your "No missing fuel" theory seems to hold. Then, what remains is:

Why is this engine now burning nearly twice the city fuel and doing so with apparent efficiency?

Indeed fixes have been a result of this journey. Many things have learned as well. All good.

Originally Posted by mikedsilva

Looking at your referencing to camshaft timing through this thread.. you say 2.25mm or 2.45mm.

The spec for a lift on the overlap for a 964 cam is usually 1.26mm. If you got this wrong during the cam timing phase, you could have both cams quite advanced, or you maybe got one cam correct and the other far out of whack. I'm not sure.. but my suggestion would be to recheck your camshaft timing, and write down 1.26mm so you don't get it wrong.
Often we advance the 964 cam to 1.4 or 1.5mm to move the torque earlier. It's important to get it the same on both left and right banks.

Again, 1.26mm is the normal spec for 964 cams.

Also, above you say she told you the 964cam has MORE lift and LESS duration than a stock cam. I don't think that is quite correct.
Here are some specs I got from a competitors website...

Mike... Good catch. Am going to dig into my work record to confirm. Thanks for posting timing snip. Interesting to compare snip data to Webcam's. Not an exact fit. But there are variables I did not study in detail.

You and I discussed timing this engine while I was at it. Going to 1.5mm was attempted but it proved problematic to land both sides perfectly on that. 1.45mm was perfectly matched. Am still going to check my records to be sure I did not do 2.45mm. Don't think I did but can't leave this in doubt.

• Assuming the timing is 1.45mm, and lash is correct, do you see ANY possibility a 3.2 with these 964 40/40 cams getting 10 mpg city under a conservative driving style?

What Faith told me concerning lift & duration is as I noted it. To confirm her verbal comments, I asked for the spec docs for both stock 3.2 and 964 for the 40/40 grind. She followed through and sent this...



I spliced their two time cards together so the data is side-by-side. Omitted is the "installation" reference for the 964 as their comments in this area is the same for both cams. "Stock" and "964" text is my addition for quick id. What is unknown to me is whether Webcam's "stock" cam (and their data) for 3.2 engines exactly matches Porsche's installed cams (and data). Conundrum in all this is being able to second guess so many things---or overload on cross referencing. For the sake of getting on with a comparison, second guessing is set aside here.

Webcam's measures are converted to mm.
"Stock" as I mention it below is always with caveat noted above.

Lift:

964 INTAKE valve lift compared to stock: 964 = 0.4572 mm greater than stock
964 EXHAUST valve lift compared to stock: 964 = 0.6604 mm greater than stock

Degrees of Open - Close

964 Intake = open 4 degrees longer than stock.*
964 Exhaust = open 4 degrees longer than stock.*

* Faith advised me over the phone "964 cam having higher duration and less duration than stock. According to my math, 964 cam has 4 degrees longer duration. But her "duration" is perhaps not the same as my "duration."

Something to keep in mind in all this is... what results going from 1.26mm cam timing to 1.45? (According to Faith, 18 city mpg to 10 is not possible from the 964 cam or the overlap.)

At this moment, this is as far as I can interpret Webcam's specs. Am calling Webcam Monday to be schooled.
.

[Discseven]

Rebuild note made during cam timing. 1.45mm confirmed.


.

[Discseven]

Informative read. Clue to mystery in hand not found.

[Mr. Merk]

Originally Posted by Mr. Merk

From my experience, a torn diaphragm in the fuel pressure regulator often leads to hot start issues—but definitely something worth checking. We've seen several of these regulators fail over the years.

Actually had this happen today. Pulled the vacuum line off of the driver side diaphragm and it did not show any immediate fuel or smell like it. After a few moments fuel began to emit from the nipple on top. The passenger side was full of fuel. Womp Womp.

Autozone has a regulator (GP Sorensen) that I am going to try. Still looking for an alternative for the damper.

[Flat Six]

Dunno if they're any good but I see fuel dampers on the Bay (930 110 602 30) for ~$180. With OEM $600+, might be worth a try. IIRC, its only function is to even out fuel pulses between banks.

[76FJ55]

Originally Posted by Mr. Merk

Actually had this happen today. Pulled the vacuum line off of the driver side diaphragm and it did not show any immediate fuel or smell like it. After a few moments fuel began to emit from the nipple on top. The passenger side was full of fuel. Womp Womp.

Autozone has a regulator (GP Sorensen) that I am going to try. Still looking for an alternative for the damper.

928srus offer a billet modular damper. Greg Brown at percisionmtrwerks also makes nice FPR and Dampers, but you would need to contact him directly to see what availability is. For ref. it is the same as the rear damper used on the 85-86 928.

[wazzz]

Adapt Motorsport offers a 3.2 damper for 283 euros plus shipping. Being a Swiss company, I think crazy tariffs do not apply.
They also offer various other fuel system products for our cars. Not OE products, but they seem to be good quality.

[76FJ55]

Originally Posted by Flat Six

Dunno if they're any good but I see fuel dampers on the Bay (930 110 602 30) for ~$180. With OEM $600+, might be worth a try. IIRC, its only function is to even out fuel pulses between banks.

Not specifically between banks. the intent of the damper is to damp the pulses in the fuel rails. with a batch fire system all the injectors open simultaneously, and the fuel flow from the pump can't change volume fast enough to eliminate the dip in fuel pressure. the spring loaded diaphragm of both the fuel pressure regulator and fuel pressure dampers are riven forward towards the rail and displace the stored volume of fuel to help feed the system during injector open time. then when the injectors close the regulator had closed due to the drop in rail pressure and the pump feed will repressurize the damper and regulator to build the stored fuel back up until the defined pressure is reached an then the regulator will again begin to bleed excess pressure back to the tank.

[GH85Carrera]

Karl, my cams are 100% stock 964 Cams. No regrind at all.