Predicting efficiency w/ Desktop Dyno

[strotter]

For any of the internal combustion/compu-gurus out there: I'm building a '72 with a 327/T5 as a high-performance commuter/touring car. So I'm looking for not just good power output, but (relatively) good mileage as well - as good as I can get. I have seen over the years that engine buildups with roughly the same power output vary widely in how much fuel they consume. I picked up Desktop Dyno to help me ballpark part combinations and horsepower numbers, but it's mostly aimed at straight power numbers, and doesn't (at least directly) calculate fuel burn/hp/hr, or thermodynamic (Carnot?) efficency.

 

So my question is: with the numbers DD2k turns out, is there any way for me to predict how much fuel I'm going to burn, at least as a comparison? The program outputs a table for the engine setup with the following numbers at 500 RPM intervals: HP, Torque, Intake Manifold Pressure, Volumetric Efficiency %, IMEP (Indicated Mean Effective Pressure), FMEP (Friction Mean Effective Pressure), and BMEP (Brake Mean Effective Pressure).

 

What I'd *like* to do is create a spreadsheet to calculate fuel burn/hp/hr at different RPM's and compare different setups: my build VS. LT1 sort of thing. Though the numbers will be for WFO, I think it would be reasonable to draw qualitative conclusions for other driving regimes (such as "My motor has a similar burn/hp/hr to an LT1 at 2500 RPM at WFO, therefore I will get similar mileage at normal cruising" kind of thing).

 

And yes, I emailed Motion Software with the question some time in June, but I don't think they're going to get back to me.

[Kevin Shasteen]

Scott,

 

This is an an issue anyone wanting to Dyno their engine should be aware of: "Brake Specific Fuel Consumption". Unfortunately DD2k doesnt address the issue.

 

In John Lawler's book, "Auto Math Handbook" by HP Books w/copywrite date 1991, he address BSFC in Chaptor 4.

 

He says when you get the Print Out of your Dyno-it should have the BSFC on it; it the vendor providing the services of their Dyno doesnt include this factor-then go somewhere else whose Dyno does include this info.

 

He further explains-once you have your Dyno w/the BSFC column included; it will indicate the "Fuel-Flow Rate" in [lbs/hr] at any given RPM.

 

Once you know the "BSFC" in "Lbs/Hr" at any one "Rpm" you can determine that engine's MPG.

 

His example is as follows:

 

A 350 SBC that indicates 144 lbs/hr at the rpm where it develops 300 brake hp at the [Peak Torque] rpm, simply work the math like so:

 

BSFC = 144/300

 

Lawlor claims the BSFC will usually be anywhere from .48; generally less than .50; again this will be at the Peak Torque RPM-even tho the formula calls for the use of the BHP figure. At lower rpms the airflow through the intake is slower than at peak torque and, at higher arpm, there isnt time to maintain the same airflow as there is at peak torque. So at engine speeds below or above peak torque, airflow will not be as efficient and, as a result, the BSFC will be greater.

 

That is basically all it says.

 

Since you dont have access to the BSFC, then the question begging to be asked would be, "What is considered normal Lbs/Hr for any given engine?"

 

Since we are guessing: "As We Have No Real Engine To Put on a Dyno" we could use the "CFM" formula to get our theoretical "Lbs/Hr" formula and then plug that theoretical number into the "BSFC" formula.

 

The formulas or is that formulii(?) are as follows:

 

1) AirCFM = (Displacement x RPM) / 3456) x VE%)))

2) Lbs/HR = (AirCFM x 4.38 x .08) /6Lbs per Gallaon))

3) BSFC = "Lbs/Hour" / Brake Hp

 

*NOTE=the Lbs/Hr formula was something I found in a Fuel Injection book, which addresses Duty Cycle of an Injector, nonetheless, it will give you the theoretical fuel requirements in "Lbs/Hr" for any one rpm.

 

Keep in mind; these formulas are merely "Educated Guesses" and should be looked upon as "Baseline" focal points....not Absolutes. When dealing with the Fuel Delivery System I dont think the formulas can possibly factor in "Efficiency, Intensity" nor the Application of Efficiency and Intensity; if that even made any sence to you at all(?). Nor will these formulas take into account the Parasitic Losses to the "Electrical" pump during acceleration at any one given point.

 

What I'm getting at is this; how can the formulas possibly take into account the effects of a 100 Gallon/Hr fuel pump with a Fuel Pressure Regulator set to "X" -vs- a fuel pump whose output registers 80 Gallons/Hr with the same Fuel Pressure Regulator set to "X" during cruise/acceleration. Then factor in a 750cfm carb -vs- a 600cfm carb on an engine when the CFM formula only calls for 450cfm: to go even further which jets, power valves and accelertor pump springs/cams are to be used?

 

Hope that all made sence; without an actual Dyno print out, which includes the BSFC taken directly from the engine..., it is all just one big guess.

 

I would say this though; even DD2k uses formulas to make their "Guess"; so guessing isnt bad-it just isnt shouldnt be looked upon as an absolute.

 

Kevin,

(Yea,Still an Inliner)