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

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...?

http://i519.photobucket.com/albums/u...d/IMAG0660.jpg

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.

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.

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dkjones96
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Join Date: Nov 2007
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-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.

injectors

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.

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.

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.

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

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.

Go super high compression but don't ever race it, hard on head gaskets A heavier flywheel will make the engine run smoother.

To save me thanking you all individually, please accept my thanks on a whole to all those who've posted!

It seems like there are certain paterns building up;

Keep intake stock (ie not polished)
Keep the exhaust smooth (but dont f*ck about with it)

I can get the polishing mops on the combustion facing internals

Heavier flywheel, but lighter rods/pistons

Plus I've been reading up on the 14 degree timming advance alteration too.

With regards upping the compression, is that as simple as a head skim, or are shaped pistons or longer rods...?

If your going to change the weight of pistons, rods, and fly wheel you better have the engine balanced.

I'd look into titanium valves too. Saving valve train weight will reduce losses overcoming the inertia of the valves. This will help most at higher rpm.

I don't know if this is still the case, but a decade ago titanium valves were a Very Bad Idea on anything but race motors built for very limited duration (like 3000 miles or less). Titanium loves to gall on just about everything, and the valve keepers were real trouble sites.

You would want to deal with airflow management inside the engine as well. Those pistons moving up and down make a good bit of air move around inside the case, and the crank and rods churn it all around as well. So doing stuff like "knife-edging" the crank, and making sure the air has good ways to flow around things, will help cut down on power lost to moving the air around. Drawing a vacuum on the case helps that as well, but it costs you power to pull the vacuum.

The oiling system in the stock motor is likely intended to provide more capacity than a low-RPM eco-driven motor would need. So downsizing on the pump can help reduce the power used to drive it, without risking the engine. Especially if you make sure that the oil passages are port-matched to each other. (We have seen significant mis-matches in the aircooled VW world, and matching the passages in the pump and the case to each other has helped.)

Coatings can help as well. I am told that an oil-shedding coating on the parts of the bottom end that don't need oil on them helps reduce the power lost to flinging oil around inside the sump. Friction-reducing coatings can be used in some areas as well.

Most of the changes I am talking about are going to give very very small results, likely less than 1% improvement each. They are likely past the point of diminishing financial returns until fuel costs get into the truly absurd levels. But they are interesting as an exercise.

-soD

i'd say try for max torque at as low a rpm as possible and then get higher gearing with it. I've also heard that cryogenically treating your engine could increase your FE by 50%. I'm pretty sceptical about that but everything i've read said that crogenicly treated parts have 50% or less friction then they had before, so maybe it well cause the engines supposidly lose 30% of the energy to friction.

I have read many times years ago about using ceramics on the head and piston tops to help reduce heat loss in the combustion chamber during the combustion stroke helps to improve mileage. I have not read much about it lately. It does make sense, if you can keep a little more heat during the power phase of the cycle you get a slightly better improvement in power and mileage. Maybe studies have shown that it isnt durable or worth the investment, I have not heard. Maybe something to look in to.

I've personally rebuilt several engines. Were I to do this, I would look at pretty much a stock rebuild... unless I was going all-out, then I'd consider destroking first, deboring second, or maybe even cylinder deletes. I would not spend more to get light rods as that mainly comes into play at high rpms. I would have the head trued for a nice straight gasket surface and teensy compression ratio bump. I'd have a nice multi-angle valve and seat job done. I'd port match head to intake if needed. I put a windage tray in. I would consider a heavier flywheel if my driving style would be to short shift a lot; it would be especially useful paired up with a higher trans regearing. I would re-cam with a grind that's optimized for lower rpms, and also play with cam timing if that's what the cam grinder suggests. I haven't been ambitious enough yet to try to fab up some nice small diameter long tube headers(cuz the commercially available ones are all about high rpms) but that might help too. Coatings and such... who knows, I've never had that done, but if I did I'd probably do the piston skirts first- actually some of the new hypereutectic pistons come with it.

A bit OT, but Frank, I had this silly idea that since most headers are designed for high rpm, high load applications, using a header designed for a gas engine on my n/a diesel (which is always at WOT, remember), would yield some results.

I'm gonna try it, anyway, with a full 2" exhaust...if it doesn't work out, I can always go turbo later. ;)

Back on topic, the OP should consider diesel engine swaps which normally would bolt to the transmission, such as the 2.0 diesels found in escorts and lynx models. The time/money you spends rebuilding a gas engine could easily build mount adapters, linkages, and the simple wiring necessary.

a process developed similiar to shot peening, but at a much smaller level, reduces friction and is safe enough for all parts of the engine, including bearings.
WPC - Metal Surface Treatment / Micro Shot Peening

an article discussing WPC vs. Shotpeening
Shot Peening & WPC Treatment - Metal Improvement Company - Turbo & High-Tech Performance Magazine

On thr rotational mass issue, I have been thinking of adding flywheel mass to make the engine smoother at low RPM when used with my torque converter lockup mod, it is a real limitation at the moment, the engine is very rough below 2000 RPM, I want to be able to run it down to 1000 which would mean I could use 1 gear higher around town.
I have heard of people removing the inside valve spring to reduce friction, if you goind all out for economy you will not be using higher RPM where valve float will happen. the Toyota Prius uses a "Atkinson cycle", where the inlet valves are kept open for much longer than normal so the air that is sucked in on the inlet stroke is partially pushed back out on the compression stroke. this reduces the pumping loss, the vacuum the engine has to suck against. in this way the engine behaves like a smaller engine for inlet conditions but a bigger one for the power stroke, it still uses the full power stroke.

the Prius also has narrower main and big end bearings to reduce friction, you could remove some of the bearing white metal from the outside edges. also a dry sump conversion would help.

^Good, forgot about things like that! I know the trend in racing has been to downsize rod bearing diameters, I don't know about mains too but for the rods at least they claim a noticeable friction reduction. I'd think diameter reduction to be more effective than width reduction but of course more expensive to accomplish.

The valve spring idea is good too, if you know it'll not ever get run into the valve float zone. Realistically valve springs return all the energy it took to compress 'em minus internal spring friction and system rotating friction... so the return may be minimal... but if it hurts nothing and costs nothing why not go for it.

I'd go on a campaign to reduce engine driven accessories too.

I would not downsize bearings, I would run wider bearings with thinner oil. If you need 5w30 stock then go to 0w20 and keep an eye on your oil pressure gauge

ICBW, but it makes sense that smaller diameter bearings would reduce drag. It seems like the drag of an oil bearing would be of a fluid-drag sort, and increase disproportionately with the speed of the surfaces, maybe with the cube of the speed?

So a bearing with half the diameter (probably unrealistic, but makes the math easy) and twice the width would have the same area, but less drag. If everything else stayed the same, it seems like it should be (1/2)^3, or 1/8 the fluid drag of the larger bearing. The "Unrealistic Claims" detector just went off, let me go ahead and reset that. It's a big percentage drop, but I figure the drag is very low to begin with, so the actual change would be very small. Apparently windage in the form of the crank and rods hitting oil that's flying around in the crankcase is a much bigger deal. (For airplanes, I know that "spray impingement" is a big factor increasing takeoff distance on a wet runway-the tires kick up water which hits the airframe, making it harder to accelerate.)

There are probably other factors involved, though. It might be that slowing down the relative speed of the surfaces affects the ability of the oil to keep the surfaces from touching, or the smaller crankshaft might not be strong enough, or the smaller bearing might not handle detonation forces, or who knows what else.

I'd suggest roller-bearing mains, but the only engine I ever had with those in it needed oil changes every 1500 miles, which made a mockery of whatever fuel savings it produced.

Would it be worth making a sticky somewhere with things that should theoretically improve engine efficiency, along with the status of each (testing, independent documentation, etc.)?

I.e.,
hotter thermostat, yes,
thinner oil, yes,
HHO, no,
smaller injectors+higher fuel pressure, unknown,
crankcase vacuum, unknown,
thermostatically-controlled oil cooler, unknown,
etc.