destroking and boosting for mpg

[Frank Lee]

Originally Posted by ecomodded

So much drama Frank, come on,lets keep things in perspective,your imagination has brought my idea to life and blown it up.
If you made it yes it would fail. If i made it yes it would succeed, you have giving up before you started. It can be done, if you can machine the block down it can be done, and would in fact be half way done at that point, buy shorter connecting rods, Boom, its damn near done.
Is it something i want to do ? no.

There's no drama; cutting rods and cranks in that manner is simply a bad idea that will almost certainly lead to disaster.

And what the hell is this: "If you made it yes it would fail. If i made it yes it would succeed"- it doesn't matter who makes it, it's gonna break.

That's all I was referring to before; the rods and crank. Though destroking is eminently doable (NOT the way as has been described already) there's probably a dozen other good reasons why it's not the best solution here.

I got the impression OP was talking about a real, live project; not a sky-high "what if". So throwing out a bunch of uninformed speculation really doesn't fit in that well there (but by all means, throw just about anything out there if it is clearly labelled as such).

[cabman73]

it is a project mainly in theory unless someone had an idea that would be reliable and cost effective. after reading everything the only sound way to do it would be a custom crank and custom rods. a persom would never see the payoff on that. idea scrapped. wonder what sub liter engines might make a good transplant. gas or diesel

[ksa8907]

kubota makes a 500cc diesel 2 cylinder. IIRC it's only 18hp so that might not work out.

[cabman73]

just out of curiosity and pulling some numbers from thin air about the smallest you could make the g10 by offset grinding and resleaving would be 904 cc. that is assuming you could offset grind the crank 1.5mm and if you went to a 2mm smaller bore.

[metromizer]

I have a few comments for the original poster:

Crankshaft- when destroking an engine, you usually purchase a destroked crankshaft, or modify the one you already have by a couple of methods.
a) no one makes aftermarket crankshafts for Geo engines
b) the traditional offset grinding you read about requires a different, smaller journal rod and some research.More than a couple mm is not usually possible without grinding into the oil passeges. Finding a rod with a smaller journal than a Geo my be tough, they are already tiny. Research, research, trail and error.
c) Weld offset grinding, by a process called submerged arc welding, can only be done on a steel crankshaft, and the first several cranks are research projects. The 4cyl metro crank on my bench is cast iron, I'd guess the 3 cyl crank is too. so weld stroking is out.
d) custom cranshaft machined from billet. I've personally ordered semi-custom cranks (custom stroke, different main journal and rod journal sizes, etc) for commonly modified engines. It's expensive and often takes months. But their isn't much call for semi-custom Geo cranks. Fully custom odd-ball engine custom cranks have been built, but it's painful, $$$, time, begging and pleading to stay on schedule...

Rods -you never modify rods to make them shorter, or longer. For commonly modified engines, different length rod options are available. The metro engine does not fall into that category. However, with a big budget, semi-custom connecting rods can be ordered for just about any project from places like Pauter Machine in Chula Vista, CA, Cunningham in Denver, Crower, etc. for about $250-$500/rod. The last semi-custom set I bought from Pauter, 2" chevy journal, 356 Porsche length, 22mm wrist pins, light weight chromoly race rods cost ~$800/4 rods

-pistons with custom placed wrist pins, higher for larger stroke, lower in the bore for a shorter stroke, is a common way to account for changed stroke and not modifying the block. The dimension you'd change is called 'compression height' of the piston.

So in a nut shell, you might be able to shorten the stroke by a few mm's, if you can find the right rod journal diameter, rod, and custom pistons.

I hope that is helpful.

[metromizer]

Maybe it's already been mentioned, but what about converting a 4 cyl Geo engine to 2 cylinders, by simply removing 2 rods and pistons, and their cam lobes, for a 650cc engine?

Then turbocharge that?

[ecomodded]

Originally Posted by Frank Lee

There's no drama; cutting rods and cranks in that manner is simply a bad idea that will almost certainly lead to disaster.

What part of theory on paper do you not understand.
You are grand standing , being a fool on purpose.
It was a idea , that was there to think about not for you to shove back down my throat.
You know what ? your being a jackass. Plain and simple.

[cabman73]

the 4 cylinder idea might work amd wouldnt be to expensive to try. only problem may be the computer but one could maybe usd a stand alone ecm or fall back to a carb. worst case you putthe pistons and rods back in and install a new cam.

[cabman73]

a metro gti/2

[metromizer]

Originally Posted by cabman73

the 4 cylinder idea might work amd wouldnt be to expensive to try. only problem may be the computer but one could maybe usd a stand alone ecm or fall back to a carb. worst case you putthe pistons and rods back in and install a new cam.

With the 4cyl Metro engine being throttle body injected, one TB common to all 4 cylinders, the 4-to-2cyl intake side fuel injection mod might not be a very big challenge.

Modifying the distributor may not be a big deal, either, but I'd have to look at on I have on the shelf. Maybe as simple as grinding 2 of the 4 tangs off the reluctor wheel and modifying the distributor cap. At least you have wrecking yard options, as the earlier sold 84-89 or so Samurai uses essentially the same 1300cc 4 cyl as the Metro.

At least the mechanics/machine work of converting a 4cyl to 2cyl isn't a huge and costly hurdle, nor rocket science... the older Bonneville Salt Flats racers used to do this all of the time to move down an engine displacement class... you've gotta love racers, kings of improvisation/innovation.