MSefi.com read only forum (Jan.15,2018)

Sort of off topic here, but I figure many people here are in tune with this sort of thing. I would like to build up a pickup with X engine/manual transmission/rearend package. I have always heard people talk about vehicles having a "sweet spot" for mileage going down the highway. How would one look at a dyno sheet and fuel map to determine the RPM that the engine would work well......mainly from a highway mileage perspective? The way I would like to look at this is with constant tire size, the axle ratio options, and several transmissions with known gear ratios. I'd target a speed like 75mph for example and try to hit "good" rpms with different drivetrain combinations.

Any input?

Thanks!

I'll open this debate...

The answer to that question is most likely not very straight forward.

Here's what I would consider a rather basic approach. Generally, the lower the RPM, the lower your fuel consumption. Eg, I found its more economical to drive up a hill with foot flat than in lower gear with higher rpm but with a lighter foot.

To move your car along the highway at 75mph will require a certain amount of BHP / KW.

Draw a line over your dyno maps that produces that power.
Now find the lowest RPM, with the lowest manifold pressure at which that power is produced. That should produce the least # of injections and at the same time the smallest injection pulsewidths, which should also produce the lowest fuel consumption. However, it might be that the required power is produced at 1000rpm at full throttle, which wont be feasible either, so you have to find the lowest PRACTICAL rpm that will leave some power reserve for the uphills and acceptable acceleration.

Not so?

Generally, the lower the RPM, the lower your fuel consumption

I'd agree with that. However, on vehicles I've owned, I notice that at certain speeds up to a point, mileage does not vary much, but when that threshold is crossed it may fall off in a hurry. I wonder if there is a"characteristic" about a torque curve that may help explain this....

Thanks!!

My mental musings:

At a certain speed you will need X amount of force to over come the friction of driving. (Tf) Tire to road friction + (Df) Drive train friction + (Wr) Wind friction (resistance)

I would postulate that Tf and Df don't increases significantly with speed. Wr (Wind resistance) does!

If you produce more force than Tf + Df + Wr your vehicle will accelerate. The more force over this (Tf + Df + Wr) the faster your vehicle will accelerate in a inverse relational to the vehicles weight (mass). If you produce less force then (Tf + Df + Wr) your vehicle will decelerate. Theoretically it takes no more force to keep a 5000 lb car moving at a constant speed than it does a 1500lb car. At a constant speed force needed would be zero if there where no friction. A 5000lb car just has a tendency to have more friction from one of these three areas.

Why do 4 wheel drive vehicles get noticeably less gas mileage than their 2wd brothers? I believe this has more to do with the extra gearing in a 4wd system adding to Df (Drive train friction) than the extra weight causing extra Tf (Tire Friction).

In order to calculate what you are asking I believe you are going to need to know Tf, Df and Wr for your specific vehicle. You are then going to need to know at what RPM your engine is most efficient at producing that amount of force. I don't know how to relate force generating efficiency to a torque or hp curve? It seems like you would need a curve plotting the amount of fuel needed to produce a particular force at a particular RPM. The standard curves showing Max Hp and Torque at a particular RPM don't seem like they would be of much use to me?

Just my thoughts.