Engine buildup for MPG?
 

Hey everyone, I've been wondering for a long time now. . . if I was to select a motor and then build it up the way I wanted to, what kind of things would I do for good gas mileage.

Engine selection:

Lets say I'm going to choose one of many different types of engines that a certain manufacturer made, lets say Honda. What are the important factors? Least displacement possible? DOHC or SOHC? In regards to bore x stroke, is undersquare or oversquare desirable? What else is important in the initial selection of a motor? Keep in mind this question is independent of engine management, just the physical properties of the engine itself.

Engine Build:

Now that I've decided on a motor what factors help with MPG? Within reason, aka 87 octane, is higher compression better or worse? Would there be any reason to increase displacement? What about ring gap? I would think that anything you could do to decrease rotating mass would be a good thing, aka pistons, rods, crank, flywheel, etc.? In regards to windage, the less friction the better, so perhaps a polished crank? Crank scraper? etc?

Ok I know alot of questions here, but if you were to build up a motor from the ground up how would you do it?

Why don't you follow the Honda example and do an OE rebuild on a d15z1?

There are ways to improve the engine, but from the factory, it is a *VERY* nice MPG engine.

Just a suggestion here. No use in re-engineering the wheel.

I'm not ACTUALLY building a Honda motor, I'm looking for some key principles for building up a motor for MPG. Perhaps there are some things Honda did with that engine specifically that we could apply to any engine build?

My personal recomendations are to go with a SOHC for reduced friction due to reduced surface area, although if you're building an engine from the ground up you may prefer DOHC so you have the ability to tune exhaust and intake cam timing independently. Long stroke, high compression, quench pads in the head, dual spark plugs (can fit better in a 2 valve head), polished crank, crank scraper, dry sump oiling system, light flywheel, direct injection (port injection is second best but more available). Ring gap doesn't matter as much, although some people love Total Seal rings.

Any ideas about tuning cam timing? What kind of overlap would make sense?

Why long stroke?

And what kind of compression are we talking about here?

Cam timing...factory is probably ok. If you are replacing the cam and having one custom made, look at the difference between the Geo Metro XFI cam and the standard Geo Metro cam for some guidance. Also there is an article on another MPG site which talks about an Oldsmobile which was designed for fuel efficiency back about '67 or so...they used a different cam, amongst other things to gain efficiency. I suspect a good machine shop would probably be able to help you here.

Long stroke = more torque @ lower RPM, lower RPM overall. This is one of the reasons why engines have generally gone to short stroke, big bore designs...higher RPMs, more power, high torque @ high RPM, but at a price (more fuel used)

I would say a highly undersquare SOHC engine with very mild cam, combined with a narrow, long runner intake manifold, and narrow diameter exhaust. You would want minimum overlap, because you don't want any of the fuel/air mix leaving the engine on the intake stroke. It's better to have some dead, burnt air still in the cylinder than have any fuel/air pass into the exhaust.
All of that would produce maximum low end torque, but would have a very weak, and narrow powerband. If you combine it with a nicely geared 6 speed transmission, you could probably have a very good MPG drivetrain, but incredibly weak in the higher revs, and rather underpowered.

You hit most of the important ones. Two other items are programable ECU, so you can fully map out the injectors and ignition timing, and also going higher compression. I would bump the compression to the max point that I could still use regular.

I'd install the higherst compression pistons I could find, and install an Atkinson cam from the Civic hybrid.

Use total seal rings or gap yours to the minimum tolerance. HP books “Engine Blueprinting" demonstrated a test on a Chevy 350 engine. Test 1 was with the gap set to the max. Test 2 was with the gap at a minimum. They gained 30 HP with them at a minimum because the engine was more efficient. This works with building power and economy. The best part is once it is built or paid for it is free forever.

Another option is to have a performance valve job on the head. If you upgrade to pocket port you will increase efficiency, which increases horsepower and economy. I have a Chevy truck that has a set of performance heads allowing me to run the smallest cam available and have more power than the stock configuration because the increased airflow from the heads equates to more power and the small cam provides better economy and off idle power. When I had this engine in my Z28 with a bigger cam it got 18MPG if I used the cruise. When it was stock it got 17! The cool part about it is that the fuel economy was the same if I was driving 55 or 85 probably because it had ample power.

Another option is to get a variable cam sprocket. When the cam is advanced you gain low-end power. Retard it and you gain high-end power. A typical engine runs 4 degrees of advance or retard. This way you can fine-tune cam timing up to 8 degrees to the optimum position at the speed you drive the most.

Engine blueprinting is expensive but can provide power and economy gains. By blueprinting you machine everything in the engine to perfect tolerance so it runs more efficient providing more power and economy.

Add VTEC to the list, or some sort of variable cam timing applied to the intake cam. To move the power from low rpm to hi, or vice versa, changing the intake cam timing has a larger effect than the exhaust cam.

Here's a great example of a long stroke engine https://people.bath.ac.uk/ccsshb/12cyl/ They wanted to make that engine as efficient as possible, like it says,"The cylinder bore is just under 38" and the stroke is just over 98". " Nearly 1:3 bore:stroke ratio.