No...yes...I don't know.

First, you keep mentioning this question you have and nowhere in your post is there a question.

Now, I see you have a 4-speed so it may be different. I'm thinking automatic here and when you take off nice and easy the transmission will shift at a much lower RPM than if you've got your foot to the floor. My guess is that at part throttle the power curve is shifted downward some, but that's just a guess.

When you're cruising along at 75 mph, the engine is really only using a small amount of HP. Which means it's nowhere near peak torque at that point. If you mean you're going to build the engine to cruise along at peak TQ (in this case 3500 rpm), don't count on getting good gas milage. It might not be terrible, but deffinately won't be killer. Go with a smallish duration cam and highway gears if you want killer gas milage, just don't expect it to be a 1/4 mile hero.

 

Sorry, question is in the title. Now at the risk of sounding like a rookie, doesent torque keep the car moving at speed?

 

I have noticed over the years that the bigger the engine,the less throttle is required to maintain a certain speed,if that makes any sense.

 

Your statement definitely supports my theory.
Also the statements by JimM make perfect sense, lower rpm better milage. I had no idea that it only takes apprx. 30 lbft to maintain speed,
but, I am not talking about building an engine to operate at torque peak output @ 3500, merely the rpm at which peak torque occurs at any given throttle angle, is this peak at the same (approx.) RPM whether WOT or
5% throttle opening?

 

of course it does... but steady state cruising requires only the teenciest fraction of an engine available torque.

To rephrase... if you're cruising along at 70mph, and it takes 30 ft-lbs of torque to overcome wind resistance and parasitic drag, your engine's torque peak rpm has NO bearing on this condition. You are not operating at peak torque, except possibly if you're driving a Prius.

If you build your engine to get peak torque at 3500 rpm, and gear it to run 3500 rpm at 75 mph... and check the mileage... and then change the gears to run 75 @ 2500, the second case will get better mileage... If you change the gears again to run 75 @ 1500 rpm, you will get better mileage yet.

My 93 vette is a good example of this. It's LT1 put's out 300 HP @ 5500 rpm, and peak torque is right at your 3500 rpm. With an overdrive trans and a 2.53:1 rear, it really is taching right at 1500 rpm @ 75. And, it will and does get 31 MPG doing this.

The key to mileage with a big engine is to run the lowest rpm possible, and to cam the motor to run efficiently at that rpm.

 

Any time you are running at part throttle, you are not getting the full benefit of your compression, since the cylinder is not full of air. For it to run most efficiently, you want it running at max compression, like a diesel. Much of the diesel's efficiency comes from max compression at low RPM. Take a lesson from this. You want a small engine, with high gears, loaded down so you are running near full throttle. (Go back to JimM's Vette. 2.53 gears load the engine, thus raising it's usuable compression and efficiency.)

As soon as you get that running, you will find that it doesn't last very long because it is not physically large enough to hold up well. Back to diesels. That is another reason why diesels are built so heavy. They run full throttle all the time and consequently, put out a lot of heat. So, you will need a large radiator for you small engine running fully loaded down all the time. Are you beginning to see a trend here?

Large diesels can reach over 50% efficiency because of the WAY they produce power. If you know anything about pellet guns, it can teach you about efficiency. The pump gun is like a spark fired engine. You get power from expanding air. As the air expands, each time it doubles in volume, it has only half the pressure pushing against the pellet, and the expanding air also cools off, shrinking it's volume. On a spring propelled gun, the spring compresses the air as the pellet is fired. The continual pressure on the air keeps the air hot, not allowing it expand and cool the way the pump gun does.

A diesel works in a similar fashion, fuel starts burning at the top of the stroke and continues to burn for a longer period of time than in a spark fired engine. Think about an engine with 22:1 compression. When the piston is half way down in the stroke, compression is still 11:1. That kind of slow expansion pulls a lots more work out of the same amount of fuel.

 

Think about an engine with 22:1 compression. When the piston is half way down in the stroke, compression is still 11:1. That kind of slow expansion pulls a lots more work out of the same amount of fuel.

Compression ratio is the proportion of volume above the piston when the piston is at TDC vs. BDC. Put another way, it's the ratio of combustion chamber volume (piston at TDC) vs. combustion chamber + cylinder volume (piston at BDC)

If you double the effective volume of the combustion chamber by moving the piston downward; you cut the compression in half. Doubling the volume would not require a lot of piston movement.

Moving the piston halfway down the cylinder is going to add WAY more volume than just doubling the combustion chamber, and the "compression ratio" is going to be MUCH lower than 1/2 of the advertised static ratio.

 

I dont know that anyone can answer your question for sure. Maybe some of the real world dyno operators will chime in, but I don't know that they do dyno runs to simulate a part throttle test.

Instead of trying to re-invent the wheel here, why not just ask, how should I build my engine to get good low end torque and gas milage?

 

Torque and HP are interrelated. See:

http://en.wikipedia.org/wiki/Torque

The answer to your question is yes they are related for a specific load condition (so I just put another variable in the question). There are many more variables to this question like atmospheric pressure, humidity, rpm, etc. If it were only as simple as throttle position all would be easy!

Ron

 

Thanks, everyone, for your input.
I guess I won't know the answer to my question until I build the engine and actually put it on the road.

 

Yes tq curve is related to % TPS opening.

If you open your blades 2% it will produce for example 40ft/lbs. But if you open it 100% it will be more like 400. So yes, more you open it its a steeper curve.

But if you have it at 10% and thats sufficent air to do full power thru 2000-4000 and the tq curve will max out at the same rpm as the 100% throttle pull will. But if it dont have enough air. Youll drop.

Small throttle blades that needs to be open alot, small runners, small heads, little bore, little stroke, and a ECU that cuts 4 of the cylinders on part trottle so youll need 100% trottle opening, gear it for max tq readings and it will get best milage at that speed.

 

so, building a small inch big block(402+.030), with heads that flow (GMPP 22363390),
dual plane intake(GMPP can't remember P/N but it is factory port matched to the heads),
a holley style carb(around 800 CFM), 10.25 : 1 squeeze, and a roller cam to make everything work at apprx. 3500 RPM, it may be possible to get 15 MPG w/3.73's and a 4spd. Or am I totally wrong?

 

Just as important: the diesel doesn't have to fight to pull air past the throttle. Imagine how much power is wasted when the crank has to yank a piston downward--at high speed--when there's ~15--20 inches of vacuum on the top of the piston.

 

I am bumping an old thread, but better late than never.

So, the smaller the throttle opening angle, the earlier torque peak comes, and the lower the output.

A picture says a thousand words:

https://s3.amazonaws.com/static.optimumg.com/wp-content/uploads/2012/05/throttle_map.png

 

Do you have a reference source for those charts? I'm interested in learning more.

 

Not much usefull information here, but I read an article once where an engine was dyno'd with varying throttle opening. It was for a circle track application, something about coming off the corners drive-ability