IDI engines: fuel economy comparison

[jtanguay]

Another possibility is adding a belly pan to the underside of your engine bay to reduce drag. However that can be dangerous too as it can allow your car to "lift" like an airplane wing and lose traction at high speeds. But, apparently there are some worthwhile gains to be had there too.

hehehe... two less tires on the road = less rolling resistance  :lol:

[Ziptar]

However the newer cars also usually have alloy wheels instead of steel, and larger rim sizes as well which improve economy (less weight = less rotational intertia). So obviously the best combination is R15 alloy wheels on an older car.

Ok, This intrigued me but, I am still trying to get me head around it. Bear with me...

In the case of the Carat / Eco I am building, Stock tire size should be 185/60-14, I already have 14" Alloys. I am keeping the ASF Transmission from the Carat bolted up to the Eco Motor (I think):

Code 1st   2nd  3rd  4th   5th   R&P
ASF   3.45 1.94 1.37 1.03 0.75 3.67  

If I get 15" Alloys I'd have to use 195/50-15 tires to keep my speedo accurate according to http://www.miata.net/garage/tirecalc.html.

So if I plug that stuff into http://www.scirocco.org/gears/ at 70 MPH I get:
185/60-14 = 2544 RPM
195/50-15 = 2551 RPM

Forgive me if I don't sound bright but, I am not getting the Inertia part...  How would that play into it?

Would it be worth it in fuel savings over the life of the tires? Just for example comparison of prices / size from tire rack.
Michelin Pilot Exalto A/S 185/60HR14 $78.00
Michelin Pilot Exalto A/S 195/50VR15 $142.00
Fuzion HRi 185/60HR14 $41.00
Fuzion ZRi 195/50ZR15 $58.00

There is a much smaller price difference with the Fuzion but, does it save enough fuel over the life of the tire to justify the cost difference between 14" and 15"?

Or did I just completely over think the whole thing??

Or is the whole idea here to forget the speedo and get larger overall diameter for the tire and wheel? If I use a 205/60-15 tire on a 15" rim that gets the RPM down to 2343 at 70MPH. It also gets the tire price in line as well, as I can stay out of "V" and "Z" pricing.
Fuzion HRi 205/60HR15 $46.00
Michelin Pilot Exalto A/S 205/60HR15 $91.00

Based on that I can see where the fuel savings and higher MPG would come from lower RPM and probably pay for the tires easily.

I still don't get the inertia bit though  :?

[tylernt]

The heavier the wheel assembly, the more its inertia resists the engine trying to speed it up (it also resists the brakes trying to slow it down because once spinning, it wants to keep spinning).

The lighter wheels and reduced inertia improving economy thus only have to do with accelerating, so you will see the biggest gains in stop-and-go city traffic. You will get (nearly) no gain in steady-speed highway travel -- once the wheel is spinning down the highway, it wants to keep spinning so the weight doesn't matter much.

[Ziptar]

Ah Ok. Thanks, I get it now.

Ziptar <-- Physics Challenged

[Slave2School]

Narrower tires can assist you as well, but there are some disadvantages to that as well.

[AudiVWguy]

The 81 caddy with the Callaway conversion gets a combined average of only 38 to 40 mpg. I've played with the pump timing (.95 to 1.05) I've adjusted the fuel screw all over the place. I've done the gov mod and am using a green advance spring. This is all on a na pump. Has anyone  switched from na to turbo then switched to a turbo pump for an increase in fuel milage? I drive fairly conservativly to keep the EGT's in check. Its been very frustrating to do all this stuff to the motor and only get 38 to 40 mpg.

[tylernt]

Have you checked the pump's internal fuel PSI, like hagar recommends?

[jtanguay]

Narrower tires can assist you as well, but there are some disadvantages to that as well.

yea... the car will shimmy and drift at high speeds.  driving my dads '02 saturn on the highway was great.  mileage was probably better than my diesel right now.  it seemed to shimmy around on the highway though... which i don't really like.

[Jettadoor]

Regular figures for my 90 Jetta TD are 40mpg (imperial) winter and 45 in summer. Just changing back to summer tires is worth 2 mpg.

A mile or 2 to warm up followed by 20 or so on the highway and another couple to get to work. 2 or 3 times an evening I have to start up and do a 5-10 minute trip. 80km round trip.

Best mileage was on a 1200km round trip last summer with a fair bit of highway and some stiff hills; I turned in 50mpg even with a/c going occasionally.

The '84 Golf n/a I owned for a while got 50-55mpg in mostly city driving, back in the days when diesel was 49c a liter.

There are a couple of things I could do to improve economy, i'll be posting some questions about that elsewhere- hopefully in the right place!!

[Ziptar]

Ok,
One last look at this....

So if I already have 14" alloys, 15" alloys won't get me much.. In that case, I think I found the best way to go.

Going to a 195/70-14 tire would put my RPMs at 2337 @ 70MPH. Pretty good but, would make teh Speedo 8.8% too slow. I can live with that (or try a different speedo gear).

70s series tires are cheaper in comparison. Many brands can be had for less than $50 with higher tread life warranties. Closest Apple to Apples I can get..
Michelin Harmony 195/70SR14 Price: $86.00 (80,000 Miles Tread life Warranty)

Seems like cost, RPM, MPG, Tread life that would be the best mix.

I might be able to get a 205/70-14 on there and get the RPM down to 2286 but, I think that is pushing it.

[hotroddr]

Another possibility is adding a belly pan to the underside of your engine bay to reduce drag. However that can be dangerous too as it can allow your car to "lift" like an airplane wing and lose traction at high speeds. But, apparently there are some worthwhile gains to be had there too.

Im afraid this is exactly the opposite to what is true(the lift part that is).  Belly pans are usually used in conjunction with a diffuser to create downforce on race cars.  This works on the principle that as the velocity of air increases, the pressure decreases.  If you can "squeeze" air under the bottom of the car and keep the airflow smooth(this is where the belly pan comes in) The velocity will increase and the pressure will decrease which causes the car to be sucked down slightly.  There is a ton more to this but I wanted to clarify that a belly pan is pretty much a free reduction in drag with no bad consequence provided it is mounted securely.

This is something I will be doing to my 84 rabbit before too long in my quest for better mileage.

[tylernt]

Im afraid this is exactly the opposite to what is true(the lift part that is).  

http://www.hotrod.com/techarticles/body/hdrp_0609_aero_tricks_tips/index.html

Older cars with blunt front ends generated lift ... --air gets underneath and pitches the nose up. Pre-'68 Corvettes, old Cobras, and classic-era GT-40s were notorious for front-end lift at speed. But even modern, fuel-efficient, bubble-cars have lift, paradoxically caused by their very efficient wing-like shape.

Positive lift is bad; it makes a car float and hard to control, especially at speeds over 100 mph. It's far better to have downforce to push the car down against the pavement, increasing its traction. "It is important to be able to steer the car at the speed you want to run at,"says engineer Chuck Jenckes. "You don't want a buoyant car."

Ok, so we don't drive our cars 100MPH, but the possibility of excessive lift is there. I don't know how pronounced it is on our old VWs or if it's dangerous or not... just sayin' be careful, don't sue me if you crash.

[jtanguay]

lol... introducing the all new tailwing mod for your jetta! take corners at extreme speeds  :lol:

[hotroddr]

I may have misunderstood you the first time around.  I thought you were saying that the addition of a belly pan would produce lift.  I dont think this is true in any circumstance.  
I agree that most street cars, especially older ones are subject to a lot of lift at high speeds.  Air dams, splitters, bellypans and diffusers are all tools to reduce, if not eliminate this.  

From the testing I have done personally, I have found that there are a lot of gains to be had in gas mileage by reducing drag.  The first place I would start(since its the easiest) is to reduce the grill opening to the smallest size possible while still providing enough cooling.  Start by blocking a little at a time till the car starts to run warmer and then unblock a little from there.  Wide masking tape is your friend here.  Another place for improvement is to encourage the air that does have to go through the radiator to exit the wheel wells, or if you dont care about looks you could vent through the hood or behind the rear of the front wheelwells.  This can be combined with a belly pan to keep the air that does go through the radiator from going under the car and producing lift.  Care also needs to be taken on the front end of a car to make sure too much air is not getting under it.  Some air does need to get under the car but the ride height requirements of a street car will take care of this.  An air dam on a rounded front car and/or a splitter can make the bumper or air dam more efficient by keeping the air that hits the front of the car from spilling under the car.  It helps encourage it to go over or around the car where the panels are smoother.  Lowering the car can also help reduce frontal area.  These few things can go a long ways in increasing gas mileage.  I am out to see how much on my diesel rabbit.  It was good for over 10 mpg on the highway on a cavalier.

[Ziptar]

There is alot of good info about MPG related things on this site, granted it's devoted to Geo Metro type cars but, allot of theory and experiments with general ideas.
http://www.metrompg.com/

looks like a sedan has less drag than a hatch and changing the shape helps somewhat. http://www.metrompg.com/posts/boat-tail-prototype.htm



This makes me think about taking an old rabbit (golf 2might be better) and aero modding it. http://www.1989geometro.com/. Wonder what it would do with a 3cyl TDI in it.