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Old 04-09-2017, 07:40 AM   #1 (permalink)
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Turbocharging? Vacuum? Too much thinking. Smart people unite

On 2 identical motors.. one n/a , one turbocharged.

Cruising at or closer to 0 inHg
Vs
as much vacuum as possible....

which is better for MPG?
I know cruising closer to 0 inHg is counter to what is efficient.

My thinking is while cruising at

(Example incoming)

60mph at 2000 rpm in N/A car at 16inHg (or whatever a vx cruises at)
Or
60mph at 2000 rpm in Turbo Car at 0 inHg/PSI ( give or take 1 or 2 in either direction psi or inHg)

The turbo car has more loss in Mechanical Efficiency (ME) from driving turbine wheel (albeit small) and Thermal Efficiency (TE) but it is also using free wasted energy (in the form of Hot exhaust gases) to spool turbine to Force more air into a cylinder thus increasing Volumetric Efficiency (VE). Also at closer to 0 inHg/PSI there is less pumping losses associated....

This is alll just early-morning-drive-to-work thinking....anyone want to chime in.


Also to add.. this was originally posted on my Facebook and the loss and thermal efficiency was a problem because I was referring to normal car and heat soak. However in the interest of this form a lot of people use a warm air intake so I'm wondering if the thermal efficiency will actually be a benefit as far as this conversation

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Old 04-09-2017, 07:41 AM   #2 (permalink)
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this was literal verbal diarrhea this is my morning commute and i was thinking alot apparently. If anything doesn't make sense or is missing details or something let me know I'll fill in any plot holes I can I'm just trying to figure something out
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Old 04-09-2017, 09:08 AM   #3 (permalink)
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I would think that an engine running at boost is efficient, but the problem is, it's making too much power. If the comparison was a 2 liter NA motor vs a 1 liter boosted motor, the boosted motor would be more efficient. Your comparison makes sense if we are talking about diesels.
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Old 04-09-2017, 09:10 AM   #4 (permalink)
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Ok let's specifically use the VX engine. My thinking was hotter IAT, quicker warm ups, less pumping losses, more VE while cruising would help.
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Old 04-09-2017, 09:55 AM   #5 (permalink)
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nissan bluebirds here with turbos were more economical than respective 1.8's. if you drive absolutely same pace as a NA, id think you'll get more out of a turbo, i guess?
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Old 04-09-2017, 10:14 AM   #6 (permalink)
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Acceleration and getting up to speed maybe but i mean cruising
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Old 04-09-2017, 10:28 AM   #7 (permalink)
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Once again, the problem is, running 2 other wise identical engines at different intake pressures results in one making X hp and the other making 2X hp in a gas ICE. I understand that a VX can run in lean burn, but not THAT lean. It would melt.
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Old 04-09-2017, 10:35 AM   #8 (permalink)
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I do believe that the future of ICE powered cars will be very small highly boosted gas engines as part of a hybrid system. Imagine your civic with a 500cc turbo engine and a fairly stout electric motor. It would cruise on the ICE at pretty high load into the boost a bit and use the electric assist when more power was needed. Honda already has a 500cc V-twin on the shelf. It is 300+ years old and could surely use updating but I would think this would be a fair bit more efficient than say the 900 cc 3 from the first insight.

Plus a 90 V twin sounds a damn sight cooler than an inline 3.
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Old 04-09-2017, 10:38 AM   #9 (permalink)
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Not 2X horsepower. Im talking purely cruising...not makinh power. At most 4psi. Nothing more. We will handle one subject at a time.
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Old 04-09-2017, 10:44 AM   #10 (permalink)
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OK lets jump back a few years to the early to mid nineties and we're looking at a Volvo 940. They came with either a NA 2.3 liter inline four or an intercooled turbo charged version. The NA was rated at 114hp and the Turbo motor was rated at 160hp.

The big difference was at highway speeds the fuel mileage difference between the two motors was dramatic. The Turbo version topped out at a max of maybe 27.5-28 at 55 mph. The NA 2.3L on the other hand would easily pull 32-33 mpgs at speeds of 60-65 mph.

Driving a Turbo for mileage takes some skill, one is you want to keep the turbo out of boost as much as you can. The issue is the computer tosses in extra fuel under boost, one for more power, and for another reason to cool the incoming compressed air to avoid detonation (Knocking). That is why Turbos appear to not get the best fuel economy even though they have smaller engines. You may have a 2.0 liter motor but under mid boost you're burning fuel like you have 2.5L-2.7L and under heavy boost probably more like you're driving a car with a 3.0L to 3.5L motor.

I modified a 87 2.3L Turbo Volvo wagon for fuel mileage years ago. I dropped the Cd by lowing it and removing the roof rack. Pulled the automatic and replaced it with a five speed manual. Swapped out the 3.90 automatics rear gears for 3.31's. Highway mileage jumped from 325-340 miles per 15.8 gallon tank to 425-440 miles per tank. The biggest factor was replacing the non locking torque converter automatic transmission with a manual.

If you can drop the Cd and weight of the car you can easily increase the fuel mileage of a turbo car. Less air resistance and weight to haul around the less need for you have to force the turbo into boost.

The biggest issue especially with highway mileage in a modern manual transmission turbo car is the manufactures are always lowering the OD gearing for better performance instead of fuel mileage. Most of the manuals are running 300-400 rpms higher than the same car with an automatic. Same applies most of the time to the NA version over the Turbo version.


Last edited by ALS; 04-09-2017 at 11:06 AM..
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