Gremlin motor build

I'm having the engine from my 1975 AMC Gremlin X rebuilt; it's a 304ci V8. It had only 65,000 miles on it, but it leaked a lot of oil from the usual places (pan, valley gasket, valve covers). I suspected the car had sat for a while, wich was confirmed by the fine black corrosion on the bottom-end bearings probably from condensation-contaminated oil. It wasn't fatal, the car ran OK.

The car came with a 904 Torqueflight, 3.15:1 axle ratio, and stock 14" wheels. I'm converting it to propane, using the experience of 10+ years of LPG on my 1963 Classic Cross Country (232ci I6). My goal is absolute maximum fuel economy, period. The changes to the car to do this include:

With this setup the engine will be doing about 1950rpm at 60 mph, or 2300rpm at 70mph.

The engine is stock except for the following:

Higher compression
The goal is a modest 10:1, fine for AMC engines on crappy gasoline, and a decent improvement for use with LPG, an excellent fuel, knock-wise. We had a hell of a time finding pistons for this engine. Ultimately I wimped out ($$$) and went stock, Nyland or Silvalite or something. Basically, few make high compression pistons, and we couldn't find a set less than $700 and custom ordered. 199/232/258 pistons are the same as the 304 V8, and I have used 199 pistons in my high-compression 232; the 232, 258 and 304 pistons are
dished, with valve-clearance eyebrows; the 199's are flat-topped, with valve clearance eyebrows. What with the higher-lift cam, it was going to require much custom fitting of each cylinder to make sure the pistons had enough clearance for the valves, so I gave up for money reasons.

We found out later that 290 pistons, flat-topped but with valve clearance eyebrows, would have raised compression without the machining, but none of the builder's catalogs even listed a 290 piston.

.005" was milled from the block, and .026" from the heads (or do I have that reversed!), total of .031" (obviously), guesstimates are 9.5:1; heads will get cc'ed later and I'll get firmer numbers. The limit here was dropping the intake ports too low such that they won't line up with the intake manifold.

An annoying, last-minute problem was that we assumed that OEM-thickness (.025") headgaskets would be available, instead of the more common .040" composite gaskets. The builder was reluctant to take the additional metal off the block to compensate; in hindsight, I should have insisted on this, or researched better how much it's safe to remove from the top of the block.

Another approach might have been to keep the cam lift stock, and used the 199 pistons, the logic being that for my application (under 3000 rpm!) the compression would have done me better than valve lift. Oh well.

AND IT TURNS OUT, with the above combination, we have slightly over .100" valve-to-piston clearance, so we could have done a lot more machining for compression.

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