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Old 06-20-2008, 03:36 PM   #1 (permalink)
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Engine intake aerodynamics.

I know that most in here drive a gasoline powered car and this won't really help your fuel economy, but for a diesel it will help improve things. Especially if you drive a turbo diesel.

I found this article where a guy reduces his engine intake losses by 1/2 a psi. (Note: part 3 is where it gets good)


That can reduce a regular diesel engines pumping losses quite a bit and for a turbo diesel it will improve the effectiveness of the turbo.

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Old 06-20-2008, 09:14 PM   #2 (permalink)
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Some excellent pragmatic testing.
It certainly supports other data I've seen that "low restriction" air filters are inconsequential relative to other pressure drops in the circuit.

Don't tell the folks at K&N marketing........
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Old 06-20-2008, 10:16 PM   #3 (permalink)
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Most TDI Guys Claim less Smoke with more "efficient" intakes. No one that I know of reports improved MPG, although they (and I would) expect it.

Case in point 2000 NB TDI intake plugged down to 3/4" main inlet. Cleaned it out for the full 2" inlet and the smoke went down on hard accel, but MPG's were with in the tank to tank noise.
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Old 06-21-2008, 04:05 AM   #4 (permalink)
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Very interesting article. Anybody want to volunteer their Civic for some NA engine tests? I may consider some testing of filters for my 8th gen civic. Noticed that the K&N filter that I use gives lets the engine breathe a little easier at the top end, but not so sure about the sub 2500 rpm range.

Hm... I feel suckered into the K&N marketing. *scratches head*
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Old 06-21-2008, 04:55 AM   #5 (permalink)
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Thats why I haven't bought a cold air intake kit yet. I got the K&N replacement filter, and never ran it solely for it's improvement in the system. I've seen some cold air intakes that remove every accessory line from the intake. Basically it's just a smooth bore all the way into the throttle. If anyone can justify any of this for fuel efficiency, let me know. I'll do the work and let you all know where I stand. I have credit cards...

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Old 06-21-2008, 05:28 AM   #6 (permalink)
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My current intake is bone stock but the snorkel is cut so it sucks air right infront of the tire instead of looping back up into the engine bay. Cheap OEM cold air

70 mpg or die modding


Originally Posted by MetroMPG View Post
looking forward to seeing what kind of uber-sipper slinks out of the full race skunkworks.
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Old 06-21-2008, 09:15 AM   #7 (permalink)
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i've read tons of their articles before, its really great stuff, especially for the DIY guys & its mostly universal

i want to get a manohmeter gauge like they use...the water & length of tubing i made is not pratical for in car use, but i was using it to test port flow on a set of spare heads i have, then i tested a bunch of different stock mufflers i've collected over the years, very intresting results!
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Old 06-21-2008, 03:16 PM   #8 (permalink)
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I feel obligated to point out that improved air flow through your air filter does nothing for mileage on a gasoline engine. The reason being that to maintain partial throttle cruise an restriction saved in the air filter has to be made up for by the throttle while the fuel and air flow remain the same. It improves full throttle power however.

Diesel engines have no throttle so any reduction in intake restriction reduces pumping losses as well as unburnt fuel (soot).
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Old 08-02-2008, 06:29 AM   #9 (permalink)
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I know this thread is a bit old, but I have been experimenting with a Nissan diesel engine, and I spent a fair bit of time playing with the intake. I didn't use any fancy measuring equipment other than the seat of my pants.

Intake mods I have tried so far have been:

1 - Bypass the air filter.
2 - Bypass the air filter and remove intake resonator boxes.
3 - Remove all intake piping before the "throttle body" plenum intake.
4 - Dismantle the intake plenum to run open intake runners.

The results were pretty clear. Low to mid-range torque dropped noticeably as soon as the resonators were eliminated from the system. However there was noticeably more punch at the top end with #1 through #3. Intake noise is increased too. With #4 the engine was sluggish over the entire rev range. I think the open stacks were too short and turbulent. I would like to try #4 again with some longer intake runners when summer rolls around (out of interest and possible application for my race car).

These were tried out on an RD28 Nissan engine, which is a 2.8 litre straight 6, non-turbo, straight mechanical diesel. The intake is approx 3" diameter from 'throttle body' through to the air filter (except inside resonator boxes), and is approx 500mm long.

I'm not sure what is inside the resonator boxes (if anything), but they are the key to making this engine work properly. Most likely removing them caused it to run too lean.

The only mod that possibly improved economy was #1. Now that I am recording my mileage I will make an effort to try it out for a few tanks. I would imagine that a nicely ducted cold or even ram air setup would have more effect though.

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Old 08-02-2008, 04:40 PM   #10 (permalink)
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Hi All,

Nova_nz, the diameter of the runner ideally should vary with the speed of the engine. A smaller diameter for lower speeds avoids the turbulence you speak of. For ecomodding, very long smaller diameter intake pipes are better.

ConnClark, while diesels do not have throttles, many hypermilers take advantage of tall gearing and run nearly full throttle at low speeds. So reduction in intake air drag on gasoline engines can be helpful. Some of the members on this board have changed out the final drive gears to do this.

The Prius with its very wide transmission ratio range, and computer controlled throttle can run at very low RPM's at a contiuous cruise, with the throttle nearly wide open. My experience is the Prius will run 1280 rpm at 53 mph, on level ground and no wind, and temps above 70 F. 4 speed automatic transmission cars typically have to run at twice this RPM at that speed.

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