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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old February 4th, 2018, 10:43   #1
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Join Date: Apr 2016
Location: Virginia
Default Optimal intake temps for fuel economy

HI All,

I was wondering what you all felt the optimal intake temp was for the best fuel economy.

I ran across a chart from Cummins with their recommended intake temps for their stationary engines in generator applications (wished I saved the link but could not find it now) and it is possible to have intake air temps too hot or too cold.

In the colder months I installed a grill block in front of the intercooler cold air intake so I can keep intake temps more or less in the 65 to 95 degree range (of course depending on outside temps and boost levels)....Im seeing that the engine warms up faster too when blocking the air flow to the intercooler

I had though about rigging a way to manually regulate the amount of air hitting the intercooler to optimize intake temps for fuel economy....I suppose a high tech automated system would be the best but may be hard to engineer).

In the summer (or warm months) I don't block the intake to the intercooler at all since ambient is not low enough to be too cold.

Any thoughts?


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Old February 5th, 2018, 19:44   #2
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Join Date: Mar 2014
Location: St.Paul, MN
TDI(s): 02 golf ALH
Fuel Economy: 42 stock, 47-49 now

I ran mine without water in the intercooler for a while (intercooler is welded up inside the intake manifold, and the exhaust manifold will radiate quite a bit of heat into it, couldn't rest your hand on it after city driving) and it'd smoke white once fully hot, no trouble with that once I got water flowing through it.

I haven't run into issue with cold intake air yet, other than an interesting aside noticed while datalogging.
If you let it overrun from 75mph to a stop in -10 deg F, downshifting to to keep the revs up, your coolant temp will drop by 10 deg C, with the heater blower motor turned off.

ETA: I'd bet on colder intake air being better for efficiency just about all the way down to below where your coolant would freeze. It's not like a gas motor where the fuel has to be vaporized.
Interested in the math of what the heat of compression does with cooler intake air, as there's more airmass in there, but it's got a lower starting temperature...

Last edited by [486]; February 5th, 2018 at 19:49.
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Old February 6th, 2018, 00:55   #3
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Join Date: Oct 2005
Location: B.C. Canada

Good discussion of iat's here.
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Old February 6th, 2018, 07:19   #4
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Location: maine

It's difficult to differentiate the IAT effects from ambient temp effects, since there is generally a pretty strong correlation. The ideal IAT(not ambient) for fuel efficiency will probably also vary quite a bit based on load. When both ambient temps and IAT are quite low, the real benefit should come from blocking the radiator, rather than the intercooler.
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Old February 6th, 2018, 08:48   #5
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Join Date: Jan 2016
Location: Oklahoma
Fuel Economy: City mid 40s ~ Road mid 50s ~ Two (2) 61 MPG fill ups
Exclamation Excellent thread

Do the fix, pocket $8,000-ish 6, 776 miles on the clock, Pristine condition
BB = $25,625, but VW is not paying for PRIME condition nor ULTRA low miles.
Net bases in my 2015 Golf = $12,495
NADA-KBB for older CPO Golf TDi 2014 = $18,250
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Old February 6th, 2018, 09:43   #6
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Join Date: Jul 2006
Location: Waxhaw, NC
TDI(s): 2005 Variant
Fuel Economy: 44 mpg @ 70 MPH. 46 mpg @ 55 MPH

My B5 is happiest when the ambient temperature is between 75-85 degrees F. Fuel efficiency is down 10-15% with current temps just above freezing. I don't know how to tastefully put a shroud in front of the radiator to help bring engine temperatures up
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Old February 7th, 2018, 04:57   #7
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Location: uk

pipe insulation fitted tightly around each grill slat
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