Engine re-build, eco-style. Home project.

[caferacer51]

A hypothetical question...

Lets imagine an engine came up for grabs for my MX5/Miata cheap.

(1990, 1600cc, inline4, longtitudinally mounted, fuel injection)

Lets imagine I wanted to re-build the engine, replacing worn parts piston rings, bearings, gaskets etc...

So far all of the work could be done at home (apart from honing the bores).

But what modifications could I consider in the name of economy?

Bearing in mind that the lightening of any moving parts may require some specialist balancing...

Reducing friction seems logical, so bearings would come under scrutiny.

Also improving airflow.

What else could I consider...?

[Odin]

consider not using the 90-91 engines as the crankshaft is prone to breaking. It is rare in stock engines but shows up much more frequently once it has been taken apart they just don't like to stay together again.

[fjasper]

Maybe a cam grind? I think I heard the automatics had a different cam profile and timing that favored low-rpm torque (and maybe economy?) I think the automatic version had less overlap, but I'm barely literate in this area, so I could be totally wrong.

Maybe bump up the compression a little, if there's room? Hotter thermostat?

It would probably be a convenient time to switch in a manual rack if you want to, and delete all the air conditioning hardware if you have it and don't need it.

[fjasper]

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dkjones96
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Senior Member

Join Date: Nov 2007
Location: Albuquerque, NM
Posts: 1,350

-Polish only the exhaust port, do not port the head.
-Stock cams or cams that are ground to make power lower than stock.
-Polish valve faces, combustion chamber and piston crown to a mirror like finish.
-Exhaust header with straight though exhaust system (the exhaust system takes actual thinking and must be sized appropriately with the displacement).
-Compression should be around 10:1 for aluminum heads and ~9.2:1 for iron as these are the upper limits for pump regular gasoline (dependent on other factors but this is a general rule).
-Moly rings as stated above.
-Very tight clearances so you can run nice, thin 0w-20.

Break-in can make or break an engine with economy. Don't do the easy on it for 400 miles BS. After a couple of light warm up to operating temps and cool downs to ambient take it out and have a healthy run-in with a healthy combination of almost to red-line ~80% throttle runs and engine braking from the top of a gear (preferably 2nd) down to 1.5-2k. Make sure it's done in the first 20 miles.

If you do it right this will make for very nice economy. I built an engine following these guidelines and the engine runs powerful with good economy. I did a couple things differently that let me go with 11.2:1 on pump regular gas but it got direct injection and a slew of other items...
__________________
- Kyle
Last edited by dkjones96 : 05-09-2008 at 11:40 AM.

[discovery]

injectors

[roflwaffle]

I wouldn't bother rebuilding an engine just to get better mileage. All your major gains in terms of engine efficiency are going to be via swapping in taller gearing through a rear end and/or transmission swap. On the vehicle side you could go w/ LRR tires and aero-mods.

[elhigh]

I read when Hot VWs was building their "Mileage Motor," they were seeking to add rotating weight. I don't know why they would do that, but they were dead set on doing it, and they were in the mid-30s at 65mph on an otherwise stock VW when they were done. So maybe you don't need to search for the lightest weight on all your internals.

If anybody knows the rationale behind doing this, please elucidate. I'd love to know what they're on about, and why it worked.

[Ryland]

If you can, balance the moving parts, companies that sell race performance parts often sell tools for balancing as well, having your pistons and crank shafts all weight the same is going to make everything turn more smoothly.
from what I've read about intakes, a rough intake tends to help with the fuel/air mixing so that you get a more complete burn, so polishing the intake can be a bad idea, also take a look at tuning web sites that have exhaust calculators for sizing your exhaust system, larger is not better, you want your exhaust tuned for the engine size and RPM that you want it to perform best at.
If you can, it might be worth finding someone who does cryogenic treatments, when you properly cool metal down to -300F the structure changes on a microscopic level, I have to blades for a block plane for wood working that are of the same alloy of steel, one is cryo treated and the other is not, the differences in how they wear is amazing.
To me it seems like if you need to replace the engine that it would be well worth buying a new/used engine and rebuilding that, you would then keep the use of the car while doing the rebuild and would be starting out with an engine that might be in better shape, you might even check around and see what a brand new block would cost, for some vehicles they are amazingly cheap.

[Varn]

Get a VW diesel engine. Gear it up. I've been real impressed with the torque and economy of mine.

[fjasper]

Originally Posted by discovery

injectors

As in, smaller injectors plus higher fuel pressure for the same flow with better atomization?

It seems like reducing the reciprocating mass (pistons & rods) would be more important than reducing rotating mass. Maybe? And as far as manipulating rotating mass, I would think the flywheel is probably the easiest and most flexible way to do that.

Longer intake runners? Roller tappets? Mirror-polish the inside of the intake manifold or have a tube header made up custom. Set up a venturi in the exhaust to reduce crankcase pressure, improving ring sealing which allows you to leave out the oil-scraper ring, thereby reducing the sliding friction of the pistons? I'm not making this up.

Contract with a russian machine shop to make you a custom titanium crankshaft out of former Chernobyl cooling pipes. That way you don't have to preheat the motor in the morning. The radioactive decay will keep it at operating temperature all the time!

Ok, that's just silly. Everyone knows the contaminated cooling system parts aren't radioactive enough to generate that much heat.