What makes a engine efficient???
 

I am completely lost, but I am happy it is better.

Also on the I am very happy side is my truck with 10.5-1 compression now runs great on 89 octane. Testing with 87 will happen when I get around to it as I am burnt out on carbs right now.

My rcsb has 4.88 gears, a TH700, and 30x9.5x15 AT tires.

Previous motor was a 383 LT1 with 500 lift 206 duration 116 lsa cam. Rough specs as I do not have the cam card.

New 350 LT1 has a comp 306 cam 510/540 lift 230/244 duration 112 lsa.

They share the same 1 3/4 headers, GMPP LT1 4 barrel intake, same 750 VS Holley, and transmission with 2500rpm stall converter.

So the 383 got 6-8 mpg city, and 18-19 mpg highway at 75. Pretty routine as I drive the engine for a few months.

After I figured where I went wrong on the Holley. My 350 does 10+mpg city, and 18 mpg highway at 75. Preliminary results based off a some quick testing, but I drove 30 miles city, and a quick blast to Apple Valley to drop something off for my BIL.

So anyone got an idea why my mpg went up with a wilder cam in the city?

Was it some cam magic inside specs that I am completely missing?

Re: What makes a engine efficient???
 

Maybe the new combo puts your engine in the peak efficiency zone when cruising around the city while the old combo just wasn't quite there at peak efficiency?

Re: What makes a engine efficient???
 

Not sure about your particular setup but what makes a motor efficient is mostly airflow and airflow through the heads mostly. Sure there is more to it but airflow is the biggest part, that is why most new motors have big heads with multiple valves.

Re: What makes a engine efficient???
 

I took a class in college about vehicle efficiency. Although I have forgotten the vast majority of the information from that class, I remember the professor saying that any given engine is the most efficient at full throttle at the RPM where it produces peak torque. That isn't practical for large powerful engines though because they will accelerate away from that point. That's one of the reasons a tiny little engine is more fuel efficient. It can operate closer to that peak torque point without accelerating the vehicle. This is why engines that can disable some of the cylinders are more efficient. So I guess the point is if your goal is fuel efficiency, then go with a smaller engine. You also need the gearing to be optimized for the most efficient RPM at cruising speed.

Having optimum air/fuel mixture at all different driving modes improves fuel efficiency too, thus fuel injection. You need the air/fuel mixture to burn completely, so adequate spark, quench and good flow through the heads, as already mentioned, will all have an impact on fuel efficiency.

Re: What makes a engine efficient???
 

Most likely:
Did the season change?
Maybe the carb changes that you focused on

Less likely, but possible, and only an engine geek would appreciate. ;-)
Wilder cam = later intake valve closing.
With that later valve closing pushing more mixture back into the intake, you need to put your foot into it a little more deeply to maintain speed.
You wind up with lower throttling or pumping losses (keywords: Miller and Atkinson, throttling losses, pumping losses).

Or maybe the earlier intake opening gives you a little more exhaust gas puffed back into the mix. (keywords: residual EGR, throttling losses, pumping losses) And you have to put your foot a little deeper into it. And wind up with those decreases in throttling or pumping losses.

Other engine geeks might challenge those comments. We could have lots of good discussion and the only way to settle it would be a really well instrumented engine and careful experiments.

Re: What makes a engine efficient???
 

Unless you are racing around, you aren't using all the power the engine makes. So first possibility:
The 350 easily makes enough power at part throttle just like the 383 did, except its moving 33 cubes less air to make that power. That makes sense for the city mileage, but not the freeway staying the same.
Second possibility and more likely: carb tuning.
Third possibility: that LSA. It sounds like your 383 had a stock LT cam in it, and the LSA is wide. The more narrow LSA of the aftermarket cam might make it work better in the rpm range your running around town.