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Old 07-28-2008, 08:27 AM   #11
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Originally Posted by gungadin View Post
I was told that when it's necessary to accelerate that I should do it at the rpm of maximum torque for my engine.
Can this be true?
Maybe, but it is so nearly impossible to accomplish that it should be relegated to an interesting concept, but not a practical mode of operation.
Maintaining one best engine speed won't produce acceleration at all. You'll be below that best rpm, accelerate through that sweet spot and, then be back into a less efficient higher rpm. You can't accelerate and simultaneously hold rpm (unless you have a CVT).

If your maximum engine torque is produced at 4,000 rpm, at what accelerator pedal position is that? Max torque is produced at max fueling. Reductions in fueling make for reduced torque.
Going uphill at 4k rpm requires more torque than going downhill at 4k rpm. So the efficient engine operating condition isn't decided by rpm, it's based on rpm and load. Less fuel will be less thermally efficient than full fuel.
Then which gear for the 4k rpm? The engine's ability to convert gasoline into thermal air expansion might be more effective at some rpm than at others, but your desire is not for thermal conversion efficiency(BTU/gal/hr), but in maximizing distance per unit of fuel (mpg).

The lower the engine's peak power, and the heavier the vehicle, the easier it is to drive at full accelerator pedal and full torque at that rpm. A Porsche (except maybe a 914 or 912) will be far more likely to spin the tires under full torque application and negate any minute fuel saving of accelerating at near to full torque.
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Old 07-28-2008, 09:20 AM   #12
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Thank You!
This is exactly what I wanted to hear.
One has to sort of "be there" in my car at 4Krpm to see what that is all about.
It's like flipping a light switch. There is no way in hell that it's economial.
Like I said earlier. I think you have to be racing to enjoy the benefits of MAX torque rpm. (In my car anyway).
Cool...
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Old 07-28-2008, 03:46 PM   #13
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Quote:
Originally Posted by gungadin View Post
I'd love to have a scan gage but I just don't think there's a place to plug one in on my '76. I'm just trying to craft an artful and instinctive technique.
Nope, ScanGauge plugs into the OBDII connector which is found on 1996 and newer cars (as well as very few 1995 cars).

The best way to find out the most efficient acceleration pattern for your car is to experiment. Try a pattern for one tank, try a different pattern for the next tank. In my VW I've found that there's no substitute for low RPM; WOT + the lowest shift points I can stand does the best for me. I just use WOT and adjust my shift points for however much acceleration I want.
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Old 07-28-2008, 05:09 PM   #14
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Yeah,,,What's a WOT??
I'm sure it's something simple and I'm having a brain cramp.
Someone should start a glossary of common Acronyms and abbreviations used in this forum.
The porsche forum I belong to is nearly impossible to read for a newbie. Too bad as the most knowledgeable members use the most acronyms.
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Old 07-28-2008, 05:24 PM   #15
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WOT == Wide Open Throttle == pedal to the metal (literally; it doesn't necessarily mean fast or high RPM, just that you're shoving the pedal all the way down).

There is a glossary thread that was started a month or two ago, and under the tab bar there's this stuff:
" Mark All Forums Read - Glossary - Search The Forums - View Recent Posts"
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Old 07-28-2008, 07:21 PM   #16
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3 liter 911, hmmmmmm, hypermiling, LOL I love it.

If its fuel injection (and I think it would be), then you can play with lower engine speeds. If carbed, its probably not practical.

Use lowest practical gears where you can actually give it about 60-70% throttle with out it blasting off.

Try different rev ranges and shift points, probably about 2000-3000 RPM depending on gear ratios. Best way to understand it is to pick your rev limit, then go to a higher gear.

Your objective is to make the engine do more work at lower speeds, because your car is designed for performance not economy.

All this being said you have special considerations that don't apply to most of the rest of us.

Assuming your car is air cooled, low speed high load (sometimes called lugging) is especially dangerous and could be expensive. Air cooled engines can suffer from low speed high load driving, because you are creating more heat with less cooling.

If your 911 is air cooled and fuel injected, and you wan't max mileage, I would avoid radical efforts at higher mileage.

I would definitely glide downhill (assuming you can maintain your desired speed without issues.

regards
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Old 07-28-2008, 09:21 PM   #17
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HA! This car is a hand me down from old daddy. I was actually driving a '93 Metro 4dr 1.0L 5spd. and I gave it to my friend in AZ. He calls me often to report about how wonderful the little geo is treating him.
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3 liter 911, hmmmmmm, hypermiling, LOL I love it.

If its fuel injection (and I think it would be)Yup, then you can play with lower engine speeds. If carbed, its probably not practical.

Use lowest practical gears where you can actually give it about 60-70% throttle with out it blasting off. Can you explain this a bit more? That seems like alot of throttle to me. I can get loads of speed with less throttle. This thing has a really deep pedal and it's pretty linear if you know what I mean.

Try different rev ranges and shift points, probably about 2000-3000 RPMThis range works nicely depending on gear ratios. Best way to understand it is to pick your rev limit, then go to a higher gear.

Your objective is to make the engine do more work at lower speeds, because your car is designed for performance not economy.

All this being said you have special considerations that don't apply to most of the rest of us. Yeah like 12 qts of oil. Less isn't an option. Lighter weight isn't an option (20-50w) These cars actually have an allowable oil consumption spec. (1qt./2000 miles is considered average). Priveledge of adjusting the valves every 5K miles. (special gages and four valve covers.) How about a 21 US gal fuel tank. Lately I've been able to drive near 500 mi between fillups.

Assuming your car is air cooled,And Oil cooled low speed high load (sometimes called lugging) is especially dangerous and could be expensive. A DIY rebuild including all parts and machine shop work can easily go $10K Air cooled engines can suffer from low speed high load driving, because you are creating more heat with less cooling.The big air fan and oil circulation are very important.

If your 911 is air cooled and fuel injected, and you wan't max mileage, I would avoid radical efforts at higher mileage.You mean engine longevity? Well the 3.0L is considered to be the longest of all the 6cyl often going 300Kmi before the expensive teardown/rebuild...An outfit called "renegade hybrids" supplies a very nice and very cost effective conversion to chevy V8 and also I think Subaru WRX engine. and personally I can't wait... See I thought I was alllll done with VWs about 40 years ago. AND BTW I have seen 911/912 cars converted to the Suzuki 3cyl 1.0L. I love that engine sooo much.

I would definitely glide downhill (assuming you can maintain your desired speed without issues. I get it

regards
gary
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'75 Honda Goldwing GL1000 (1st yr. made) w/1100 model carbs. 49mpg.
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I miss my Metro
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Old 07-28-2008, 09:36 PM   #18
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Blasting off refers to the fact that your Porsche will accelerate at light years speed compared to my Civic VX.

Use a higher gear with a greater load for your acceleration, ONLY IF IT IS FUEL INJECTED. NOT RECOMMENDED IF CARBURETED.

Fuel injection matches fuel delivery to the engines demand. Carbureted gives you an accelerator pump that would provide too much fuel for low speed high load acceleration, which would exacerbate your heat transfer to the cylinder walls, and could greatly reduce engine life.

Basically you should drive your car like a tractor trailer truck driver drives his big rig, as long as you are not risking engine damage.

The 12 quart oil system is also a cooling system. If you have an oil temperature guage you should monitor it carefully and not drive in such a way that oil temps would rise significantly above your average temp.

Try shifting at 2200 RPM to the next higher gear and using the amount of throttle that gives you the desired rate of acceleration. If the acceleration is still too great lower the shift point or throttle position, preferrably the the shift point.

The higher the throttle position the more efficient your engine is producing power for the same amount of fuel consumed.

Also if possible set the idle speed at the lowest acceptable level (all reliability issues considered). This means your idle fuel consumption will be a low as practical.

My Civic VX idles on about .2 gallons per hour, so when I am coasting at 50 MPG, I am getting 250 MPG, which goes a long way towards improving my overall fuel economy.

Lets say you want to average 50 MPH. If practical use your highest gear (I am assuming 5th) and pulse to 55-57, then coast until you have dropped to 45-47 MPH, repeat the process. The aero of a 911 is excellent so you should glide at least 2-3 times as far as you pulse. If you have a way of reading engine vacuum you want it to be low during the pulse, so use the most throttle practical (up to 70%) on your pulses.

regards
gary
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Old 07-29-2008, 05:02 AM   #19
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Thanks for the extra schooling (everyone).
This pulsing technique is more aggressive than I have been doing.
So I'll package this all up and try it on my next tank.
Hopefully I'll have some positive results to report.
Thanks very much!
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'76 Porsche 911 w/'79 3.0L 6cyl. MSD 6AL/blaster coil. 27mpg
'75 Honda Goldwing GL1000 (1st yr. made) w/1100 model carbs. 49mpg.
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I miss my Metro
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Old 07-29-2008, 05:04 AM   #20
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Carbureted gives you an accelerator pump that would provide too much fuel for low speed high load acceleration...
I'm not sure I follow here... I don't think you are correctly explaining the action of the accelerator pump (if that's what you are trying to do)

I've run plenty of carb'd cars at low RPM & high load, so at the moment I'd have to disagree.

(The point may be moot here, I'm pretty sure the '79 3.0L is FI... wasn't the motor with that weird fuel distributor thing?)

-Bob C.
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