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Acceptable gas milege for a 1985 911 Turbo

Hello all,

I recently acquired a 1985 911 Turbo. The previous owner had the top end, clutch and transmission redone back in 2006 and have only put in 6000miles on the car since! I know the 911 Turbos are not too good on gas, but the last couple of tankfuls I am only getting about 12 miles/Imperial gallon!
That's pretty bad! I am wondering if this is the norm.
I am suspecting maybe it needs a tuneup, and also the muffler is very loud, it sounds like popping
corn as soon as I am coasting and let off the gas. My mechanic (who knows and works on a lot of Porsches) doesn't think it's a problem -- he thinks that the previous owner did some mods to the muffler to make it louder. I am thinking of getting some aftermarket muffler (Kool Vue stainless steel ones) since some of my other friends feel that there's no back pressure from the muffler, causing it to "pop". I am no mechanic, I wonder if that is one of the causes of poor fuel economy. I wanted to go back to original Porsche muffler, but it's crazy expensive ($2100 Cdn) just for the muffler itself.

Any advice would be appreciated.

Thanks

Old 10-11-2010, 12:11 AM
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I get @ that MPG and with a loud exhaust, these cars pop on decel. If your car does loud popping at WOT, then that is a problem.
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Old 10-11-2010, 12:14 AM
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i almost guarantee the PO disconnected the decel valve, that makes it pop alot. If so, its not really a problem, unless you hate the sound. Also I suspect it is running really rich.

Last edited by Groovie Ghoulie; 10-11-2010 at 01:18 AM..
Old 10-11-2010, 12:52 AM
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Bryan,

Are you telling me that 12MPG is acceptable??!!
BTW: what is WOT? Wide open throttle? No, when I am accelerating, it doesn't pop, only when
I let go of the gas and coast (at higher than 2500 RPM).
I get the most popping when I let go of gas when shifting from 2nd to 3rd gear. The popping goes away when I clutch in and let it coast, but when I engage the gear it starts popping again.
Is this normal? And why?

P
Old 10-11-2010, 12:54 AM
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Groovie Ghoulie:
Is it a simple thing to reconnect the decel valve?
Like I said, my mechanic tells me that the owner before the previous owner probably did some "mods" to the muffler. It's a stock muffler but he thinks the guy may have remove some internals to make it "louder". I have no idea why anyone likes the popping. I have grown used to it, but if fuel economy is suffering because of that, I need to get that replaced...
Old 10-11-2010, 12:57 AM
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Well, the decel valve is in my opinion not really necessary. But it can be reconnected (assuming its still on the car) by just unblocking a hose and reconnecting it into the unit.

Sometimes the decel valve makes it run WORSE:
Why didn't I do this years ago!!

And some guys want to have them, because it does cut out the popping on decel, and some claim makes the car better. personally i disconnected mine.

Could the car be running lean? see this thread:
The silent killer of 930 power and fuel economy.

If the Air Fuel Ratio is not correct, you will loose power and mileage.

I am sure some of the real know-it-alls will be in here later to help out as well.

Just use the search function to look up decel valve. lots of guys have pics and such of removing and connecting them.
Old 10-11-2010, 01:34 AM
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Quote:
Originally Posted by solarispete View Post
Bryan,

Are you telling me that 12MPG is acceptable??!!
BTW: what is WOT? Wide open throttle? No, when I am accelerating, it doesn't pop, only when
I let go of the gas and coast (at higher than 2500 RPM).
I get the most popping when I let go of gas when shifting from 2nd to 3rd gear. The popping goes away when I clutch in and let it coast, but when I engage the gear it starts popping again.
Is this normal? And why?

P
For me, that is good mileage! LOL I haven't checked my AFR as I haven't installed my LM1 yet, but I'm 99% certain it is running rich at wide open throttle as I have the euro fuel head and Andial fuel enrichment. After all, when I let off between shifts, all the extra gas goes out the exhaust and lights up the sky! I've gotten single digit MPG as I leave the car in 2nd while cruising in town so I can just romp on it.
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Old 10-11-2010, 01:43 AM
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If you don't like to popping just give it 'some' throttle while decelerating. Your mech should check your idle CO% too. IIRC 3% is about as nice as it gets but if you're after economy then 1.5% should do better but you loose some smoothness on getting on the throttle.

I have an adjustable WUR and I have dialed my part throttle for 14.5-15 AFR and it does not seem to be very hungry for gas (I have never measured consumption though). WOT is below 12 for longevity. Idle about 13.7-14 AFR.
Old 10-11-2010, 02:47 AM
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16 mpg is about the best i've seen, and that was on the open highway. Around town with stop and go and on the boost a lot, I wouldn't be surprised to see 12. At the track for a day it was probably less than 10. I don't understand why no one has pointed out that the popping is comming from the wastegate and is perfectly normal. A more civilized car routes the wastegate to the main muffler and keeps it a lot quieter but why would you want that?
Old 10-11-2010, 07:37 AM
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Why do you think the popping is from WG? WG is shut when out of boost.
Old 10-11-2010, 07:50 AM
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Guff, its not the WG. if the decel valve is disabled, the throttle plate will slam shut immediately after you lift. This dumps a crap load of unburnt fuel into the system "pig rich" as they say. this fuel will combust inside the exhaust system and cause the popping. Its not harmful, its a matter of finding it annoying or not. plus it says the AFR's could prolly use a good seeing-to.
Old 10-11-2010, 08:05 AM
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Hi guys,

As always, thanks for all your input!
My mechanic told me that the muffler on the car is currently "stock" but the previous owner
did some mods to it to make it pop and loud. I don't hear many 911's out there popping like that. Mind you, there are that many 911 TURBOS out there... My friend's non-turbo 911 of the same year is alot more "civilized", IMO.
Anyways, my question is, would a aftermarket muffler -- stainless steel Kool-Vue ones reduce the
noise and give me better fuel economy? I really can't justify paying $2100 in Canada for a original stock Porsche muffler!
I am sorry to sound like a "wimp", but 12MPG is like driving a Ferrari, Lamborghini and Viper with much bigger engines! BTW: that's imperial gallon, I am thinking US gallon would be 10...
And, the thing is that I don't floor it. I considered shifting more to third int the city around 3,000 RPM.
I do the odd "stepping" on it to kick in the turbo, but definitely not a leadfoot driver.
Where can one get a Dyno done? Is this an expensive effort?

Thanks!
Old 10-11-2010, 08:36 AM
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Just a few thoughts.

Dose your car have an O2 sensor? Is it hooked up? A lot of 930 owner's disconnect it in there search of better throttle response. With a good working O2 your AFR will be adjusted by the Lambda computer at cruse and should get you better MPG.

You might check you idle CO / AFR's. I think stock is about 1% or 14/1 and many bump it ti around 3% or 13.5/1AFR in search of better throttle response or increased fuel delivery if running a 1 bar spring.

If you have good quality fuel and are running stock boost you might, might be able to dial in some added ignition advance for a possibly better throttle response and MPG.

A good sport muffler can help to.

PS: Did your mechanic check your CIS system pressure and more importantly, control pressure?

The WUR determins your control pressure which directly effects your Air Fuel Ratios. If it is not right you might set CO/AFR at idle and be way off at cruse. Even the Lambda computor may not be able to adjust around it and you will be fighting a losing battle. Investing in a good wide band O2 & AFR gauge is a very good idea with CIS. Also it is good maintaince to have your injectors checked and cleaned.

Last edited by 911st; 10-11-2010 at 08:50 AM..
Old 10-11-2010, 08:40 AM
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TurboCharged Motors SHOULD ALWAYS be "tuned" on the rich side, the more BOOST the WG allows, the richer the mixture should be in order to prevent inadvertent detonation.

Absent a good wide band knock/ping sensor, two actually, the only protection you have against engine destruction due to detonation is to have it ALWAYS run with a RICH mixture.

Since the engine is behind you even with good hearing you would be unlikely to hear the onset of knock/ping, so your best option is to have a rich A/F ratio.
Old 10-11-2010, 09:22 AM
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Over Rich is terrible, it robs power, ruins gas consumption and makes the thing run like ass. A little rich under boost is one thing, being super rich everywhere is not good either. The car needs to be well tuned, not just say "screw it, just run it really rich and it will be good".
Old 10-11-2010, 09:26 AM
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On the rich for a turbo side on boost is good for CIS.

Rich at light load is not great.

That is why we have on boost enrichment. The factory set these up so that with 14/1 AFR at idle. They should see about 14.5/1 at cruse and about 10.5/1 AFR on boost when the mid to high 11's should be safe and make more power. However, going leaner than stock in search of power, one should make sure there fuel delivery system is well balanced and the AFR's adjusted seasonally.

Many tuned EFI turbos will run near 12/1 AFR on boost for power.

Most other make factory delivered Turbos are in the 10's on boost. We need to be richer on boost as CIS with a mechanical distributor dose not compensate for environmental conditions.

Of course, when modified the AFR's often are effected and can go richer on forst boost and then lean up top.

A good Wide Band and possibly an adjustable WUR can be nice additions if in search of power and MPG's.
Old 10-11-2010, 09:39 AM
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Hello everyone,
Not to be a arrogant about it - but I didn't buy a turbo to have a concern about my gas mileage. It is what it is, unless its like 8 under normal driving - than there may be a problem.

I have had 3 turbos and they got between 11 to 16 mpg. If I am on it and driving aggressively it probably was around 8. Monitoring the AFRs would help - which I need to do. And as others have stated - its all about the mods.

So 12 is not too bad, but a consistent 10 may show some other issue. How you drive is also part of it - at what RPM do you shift at, pushing hard when starting from a red light, going to full boost on short accelerations, etc.

There are alot of people to learn from here who have great experience. The pelicans have helped me in my recent and continuing build. Good luck and welcome to the 930 club.
Old 10-11-2010, 10:46 AM
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Please don't take this wrong:

What drew you to the 930? Why did you buy it?

I think 12 mpg is not bad for these cars. What does jump out at me is when you say this car in not civilized like the 911 or it is very loud. Those are some of the reasons most of us love these cars. What is great about these cars is the raw power that these cars produce. I personally love the rumble this car makes and have added headers and a muffler to enhance this sound. This is not a Sunday drive kind of car.

Another great thing about this car: you can change it to anything you want it to be. So if what you truly desire is a quiet, civilized you can do that. It will take research, patience and money but you can do it. These guys have enormous knowledge but be warned that we, on this site tend to go the opposite direction with our cars.
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Old 10-11-2010, 11:26 AM
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Stephanie, you're cool...

alot of us wish there were more (single) women like you!
Old 10-11-2010, 11:34 AM
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I second JFairman on Stephanie! I think the popping on decel is one of the cool aspects of these cars. Mileage is not great but a lot of that's because we can't keep from putting our foot into it. I got 19mpg on a trip once, oh well.

Old 10-11-2010, 12:28 PM
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