Grill block affected milleage
 

Trying to figure out what happened, my mileage was worse with my grill blocked anyone have any idea's, this car rolls almost forever so its not mech. All highway(80) 400 miles rolling hills hardly any wind... my only idea is more air is pushed around the side of the car producing a biger wake---Also I found if I remove my passanger mirror there is a lot more wind noise

1986 Saab 900 with SPG aero package lowers CD down to .38 from .42

Running a grill block, a large flat skid plate (for rocks and smooth air) and a large whale tail( it changes rear angle from 25 degrees to 16 degrees)

Ideas:

- possibly faulty "test" methodology (what are you comparing it to? Identical speed & route? Identical weather & traffic conditions?)

- blocked too much, so your electric fan was running a lot more (significant electrical load)

- blocked too much, IAT's were elevated enough to cause timing to back down and lost efficiency

What did you use to cover the hole with? Possibly it had less acoustic insulation than when the mirror was in place. The A pillar vortex can make a lot of noise.

Some people will refer to a mythical Chevy Caprice which supposedly had a better Cd with the mirrors ON, but I'm doubtful that any car wouldn't benefit from a passenger mirror delete.

Somebody partially syphoned your tank?

Heck could be more cumulative headwinds throughout the tank.

maybe it has to do with where the engine air pickup is. on some cars the engine picks up it's air in front of the radiator, in some older car from inside the engine bay, but in both cases blocking the grill will cause warmer air to be ingested by the engine, and also a decrease in pressure might cause the engine to have to work harder to breath enough air.

in some cases warm air is a good thing for economy but it also leads to a decrease in performance wich could work out negative for FE

since the grillblock seems to work for aero you might considder rerouting your air pickup point to an area where there's higher pressure (the front of the car,) or maybe alter the grillblock so a little opening is situated in fort on the air intake so it car breath unrestricted.

I had something similar happen my cee'd recently. I blocked the upper grille and the mpg suffered. Its a turbo intercooled diesel so the extra intake heat may have something to do with it. The intake is just behind the grille but at the top of the radiator, drawing air from in front of it. I left the grille block in place and inserted a cardboard radiator block to get the engine hotter. The mpg improved as i reckon the air pressure went up in that area, forcing air into the intake.

By the way, i'm still not getting the mpg i expected. Bleedin diesels! Bleedin warranty too; i cant do bugger all ecomodding to it!

ollie

Hello,

Can you post pictures, please? It's hard to speculate based on a description -- and what were the MPG figures you got before and after?

Your instincts are right that bringing in hot air may have hurt your mileage. Diesels are very sensitive to intake air temperature. Hot intake air in a diesel is pure power loss since the limiting factor in diesel engine power is how much O2 there is in the cylinder for combustion. The more O2 the more power at a given fuel rate, and hot air simply has fewer O2 molecules than cold air.

Here is a copy of a post I made on a diesel forum I belong to:

I got to thinking that this was an interesting little experiment so I did it today. I have a van so my under-hood temps will not be directly comparable to yours, but here goes...

Engine inlet temperatures with and without cool air snorkle:

45 mph: snorkle = 84F, engine compartment air = [103F]
65 mph: 84F [101F]
Stop and go less than 30 mph: 88F [136F]

The first number is using the snorkle to pull in cool outside air and the number in brackets is the air temperature of the air pulled from the hot engine compartment with the snorkle disconnected.

The formula used to correct volume given temperature change is called Boyle's Law: (P1*V1)/T1 = (P2*V2)/T2

Using that formula the results from my temperature measurements are:
When the intake air is 19F degrees cooler the engine gets 3.2% more oxygen (nice)
When the intake air is 17F degrees cooler the engine gets 3.0% more oxygen (nice)
When the intake air is 48F degrees cooler the engine gets 8.1% more oxygen (WOW)

For those that do not want to do the arithmetic (and you know who you are ;) ) here are two handy charts I made. To find out how much air (mass, not volume) you are missing out on by not ducting to a cool air source, find the difference in temperature from the hot intake to the cold intake on the vertical axis, follow the line across and then read down to how much less air you are getting from your warm air source.

For example, if your hot intake sends 120F air to the engine and a ducted intake would bring in 80F air, the difference would be 40F. Find 40 on the left axis and move right to the sloping line. Drop an imaginary line straight down and read 0.925. That means you get 7.5% (1-.925 = 0.075 or 7.5%) fewer air molecules from your hot air intake than from your cold air intake.

Here it is in Farenheit, and in Celsius for you Canadians and Norwegians.

So vtec-e, if you increased your diesel's air intake temperature by 19F (~10C) you will have lost about 3.2% of your available power. That will have lowered your mileage since you would have compensated by increased throttle.

What is really interesting is that in stop and go traffic my engine compartment air is 48F hotter than outside air! That would cost me a whopping 8% in power, and a corresponding loss in fuel economy.

Note that this direct application of Boyle's Law v. power only applies to diesels and not to gas engines which have a much more complex response to air inlet temperature.

It may not be the grill block its self but where you blocked the grill.
I blocked the passenger side of my upper grill. What I got was cooling problems and I looking at it and said WTF. I only blocked 25% of the total area for air intake. So I changed it to the upper half of the grill still only 25% block and the cooling problem was corrected. Also although I'm not positive but pretty confident that blocking the upper half did improve my fuel mileage over the side block.

Very interesting. :thumbup:

I have only diesels and I have just blocked the lower grill of my car. From the dash board the temperature is greater, but lower than when I'm at highway speeds.

I don't have any instrumentation :( so I'll take your informations into account when I have the results for my current tank.

Denis.

Remember, it's intake air temperature and not engine temperature that is important. I measured my intake air temperature with a houshold digital thermometer. I put its remote sensor at the inlet to the air intake so it measured the temperature of the inlet air. I could read it from inside the car.