Predict gas mileage-A formula??.

[1Gary]

Lets say you knew all the spec's on a engine you had hand built,you knew the wt of your van,the final gear ratio/tire size,what would be the formula(s) that you use to predict the city gas mileage and highway gas mileage??.I have been having a hard time finding one.The car manufactures have one it seems.

For me I can build towards a torque band/hp and have a general sense for the best result with a gear ratio etc,but I would like to vary the built in advance of the build plugging different variables of a formula to get the best results.

I do think it is a tough question and I am hoping you guys could set me straight how to do it.

[MountainManJoe]

Wow there's so many variables for that. I can't set you straight, but I may be able to get you started...

Lets use easy numbers to make it simple.
- 5.0L V8 Engine tuned for peak torque @ 3,000 RPM
- 4L60 transmission in 3rd gear: 1:1 ratio
- 4:1 rear end ratio
- 700mm diameter tires.

OK lets get started. On four-stroke engines, each cylinder is fired once for every two revolutions of the crankshaft.
3,000 RPM / 2 = 1,500 combustion cycles per minute
1,500 cycles x 5.0 L = 7,500 L of mixture per minute.

We know that the ideal (stoichiometric) air-to-fuel ratio for an internal combustion engine is 14.7:1 by weight.
Density of air (@ standard temp & pressure): approx 1.3 grams per litre
Density of gasoline: 720 g/L
For illustrative purposes, lets round these off to 15, 1, and 700 respectively.

7,500 L of mixture per min = 7,500 grams of air per min.
Burning 7,500 g air divided by our stoich ratio of 15 makes 500 g gas required.
500 g gas divided by 700 g/L = 0.71
We are burning approx 0.7L of gasoline per minute.

In standard units, fuel economy is expressed in terms of volume of gas per distance driven (litres per hundred kilometres), so we need to find out how far we've gone during that minute...
3,000 RPM (cankshaft) multiplied by 1 (transmission in 3rd) divided by 4 (rear end) = 750 RPM of wheels.
circumference of wheel = π x Diam = 3 x 700 mm = 2100 mm or 2.1 m
750 revolutions x 2.1 m = 1,575 m or 1.6 km

OK so we have both the quantities we need, so we just have to convert to standard units.
(0.7L gas/1.6km) x 100 = 44 litres gasoline per 100 km

This is obviously way too much. I have only begun basic calculations of an Otto cycle motor under ideal conditions, and I'm already way off. I haven't even factored in variables like transmission slip, pumping losses etc., aerodynamics, let alone the fact that gear ratios change as you drive. There's many more. Even if someone had done ALL the math, and boiled it down to a nice formula for you, I think the result would be a shot in the dark. I think you would be better off using common sense, general guidelines, experience, and finished product examples.

But, I could be totally wrong ...

[1Gary]

TB-I am trying to understand the process of a design team charged with coming up on paper a vehicle for a gas mileage.True they have a blank page to work from.If I knew how to apply those formulas,than I could tailor a build given what I have to work with.

[Kabey's Van]

Putting a V8 in your van kind of negates the want for better milage.
We all know that with a V8 in an astro/safari you will get about 20 miles to the gallon, more or less.
If you are planning on building a high performance motor then who cares about milage, thats not the goal, performance is.
Like TB said, "Their are so many variables involved with gas milage it's pretty much impossible to predetermine an mpg figure.
Altitude, lay of the land, driving habits and overall vehicle condition are some of these variables that will effect your milage, but there is no way to calculate these factors in to a predicted mpg.
The best advice I can give you with regards to better milage is to drive smoothly and don't over accelerate. This holds true no matter what your building.
What are you building anyway?

Peter

[1Gary]

Putting a V8 in your van kind of negates the want for better milage.
We all know that with a V8 in an astro/safari you will get about 20 miles to the gallon, more or less.
If you are planning on building a high performance motor then who cares about milage, thats not the goal, performance is.
Like TB said, "Their are so many variables involved with gas milage it's pretty much impossible to predetermine an mpg figure.
Altitude, lay of the land, driving habits and overall vehicle condition are some of these variables that will effect your milage, but there is no way to calculate these factors in to a predicted mpg.
The best advice I can give you with regards to better milage is to drive smoothly and don't over accelerate. This holds true no matter what your building.
What are you building anyway?

Peter

There have been here in the past been two members that found after the money was spent on their projects,got outstanding gas mileage and performance.They are SoCalli V8 with his 355 and can get 22mpg and Lockdoc with his crate 350 low rider which has since sold twice.Both has a pro tune on them.

My two projects are Ole Yellar and a swap for my M/H.Both have a wind resistance issue and I understand that at 70mph a generally accepted method is a 20% reduction of gas mileage.The other variables would have to be averages the same as the vehicle's manufactures must use in designing their product.

We have a known factor in the case of the Astro vans are heavy and need torque to move them.I can build torque until I would melt your eyes to the back of your head,but the flip side of that is a limited return where one more foot pound of torque costs more in gas mileage than it saves.I am not talking about burn-out driving either.Affordable parts that are made today,if properly applied,almost anyone can build a really fast van.That is one side of the ledger.The tricky line comes about with that knowledge and applying it to a gas mileage goal as well in advance of spending any money on parts.Tailor made build based on changing say cam profiles,heads,tire sizes,gear ratio's,without the trial and error.Plugging it those variables to see what inpact they have and adjusting them for a goal result.The general knowledge of 3.72 rear end gears is ok,but to have a ballpark result in a formula would be better.I am very sure I can build a torque curve that would start at 2,200rpm to 2,500 rpm and set up a gear ratio that at 65 mph would run at just the start of that torque curve and give me after testing on the real street a given mpg.That is applying the generally known knowledge.But it is after the fact of spending thousands of dollars and a open question of what if..........I had done........a taller gear and got further into the torque curve to find a even better mpg.

My M/H is wet 8,000lbs.It was mis-built by GM/Winnie with a 350/400 tranny and has only 165hp and only 230lbs of torque and 4.10 rear end gears and a 16.5 tire.It is a 20' M/H.
People say that with 8 to 6mpg,that M/H's are known for really bad mpg.A lot of that is the truth.

I have been researching two possibilities to resolve the gas mileage issue.

One is a tall deck mod'ed 427 BBC and would be the easiest to do because this M/H came with a BBC option.

The other which "could be made to work" is a 5.7 Cummings diesel with a Dodge 47 tranny.The sweet yrs for that are from 1994 to 1998.5 mechanical fuel injection engines.

I know that I could build a 427 to give me almost the same torque figures as the 5.7 diesel and I know that the Dodge truck's wt can be in the range of 7,500lbs with mpg in the range of 18 to 22.If I wanted to compare the two options,I would need the formula I am trying to find.

Sorry for such a long winded reply.I needed to try to explain more fully.Please do read and comprehend what I posted.

[MountainManJoe]

Show us what you have, Gary ... these formulas you say you're trying to understand.

From what you just posted, I still can't tell what you have in mind for your Astro. I do know that modern engines are much better than what they put in the Astros. For the same displacement, you can get far more power now (around 300hp if I'm not mistaken). They are also beginning to adapt them for things like cam phasing, advanced fuel management and so on.

[1Gary]

Show us what you have, Gary ... these formulas you say you're trying to understand.

From what you just posted, I still can't tell what you have in mind for your Astro. I do know that modern engines are much better than what they put in the Astros. For the same displacement, you can get far more power now (around 300hp if I'm not mistaken). They are also beginning to adapt them for things like cam phasing, advanced fuel management and so on.

The Ole Yellar build is a 383 9.0 compression ratio roller cam under a 500 lift with a 72cc head and a 180cc intake runner/TBI and cast iron exhaust manifolds,3.70's Ford 9" rear end gears,likely 28 or 29" tires.But I didn't want to narrow this thread to that alone and yet want to apply a gas mileage formula to it.I have on file the auto converters formulas.

http://www.automotiveconverters.com

[MountainManJoe]

I can't access your hard drive (C:\), Gary

Try to attach it.

[1Gary]

I fixed that link.

Here is a link that I looked at recently.It does say testing tries to simulate road conditions be increasing the wheel rear dyno resistance.

http://www.mathworks.com/products/fuzzy ... sdemo.html


A little off topic here.The M/H is certainly usable as it is with a 350 of only having confirmed miles of the mid 40,000 miles on it.It will be taken down to the shop for renovations as it gets closer to spring.The life expectancy of the older M/H O.E.M. engines is usually around 80,000 miles because of the wt of the M/H and the rear end gear ratios.

[MountainManJoe]

http://www.automotiveconverters.com

This link isn't working for me.

[MountainManJoe]

http://www.mathworks.com/products/fuzzy ... sdemo.html

Gary, this is part of an advanced field called "modeling", and involves regression techniques that no unaided human could do. This is done on computers using licensed (not cheap) software. This is isn't something you can do with a pencil and a piece of paper.

[Kabey's Van]

I hate to say it, but I totally agree with TB.
The information you seek Gary, is something that automotive engineers have been developing for decades and to try to figure out these slight differences between diesel, gas, cams, plenums, cr's, t/c stall speeds, gear ratios, tire sizes, wind resistance. You'll never figure it out accurately.
I'm sure an automotive engineer only has a window of mpg to aim for when designing a vehicle.
For someone to say here is a vehicle, yet to be built, and this is exactly the mpg it will get would be laughed at by his engineering colleagues.
Even two identical motors don't produce the same horse power, there close but not exactly the same. The differences you seek to know about are so finite it doesn't matter.
If you are truely after the best milage for the h/p and torque given, drop the old bbc or out dated diesel technology and buy a modern engine.

Peter

[MountainManJoe]

Maybe a nice LS series motor would suit you. Anybody know how much they go for?

[Kabey's Van]

You can buy a brand new LS3, 515 h/p for $7300.
You would be hard pressed to build a 427 TD that could produce those numbers for any less.
LQ9 at 310 h/p is also a good motor that you could pick up at a wrecker, in decent condition, for just a couple grand, tops.

Peter

[astrozam]

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