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:

• LPG fuel system (M&M X450 carburetor, M&M converter)
• 28 gal. LPG tank (24 gal. fuel capacity). The gasoline tank will be removed and the rear hatch area cut to fit.
• 2.53:1 axle ratio (a "model 15" light duty axle from a 1978 Concord).
• 215-R75/15 tires on 15x6 wheels.
• Engine modifications (see below)

The engine is stock except for the following:

• Higher compression (via milling head and block; using stock pistons)
• Custom camshaft from Crane ("RV" cam timing; the specs are at the engine builders, more later).
• Larger valves (Martin Wells Stellite-lipped, stainless steel, small-block chevrolet, undersides are swirl polished), 1.94" intakes, 1.55" exhaust. Because this engine will be low-revving there was no great performance reason for the change, but (1) it won't hurt! and (2) fancy Chevy valves were cheaper than stock OEM replacement AMC valves, though there is extra machining involved [nd (3) Stellite-lipped valves aren't available for AMC V8's.
• Edelbrock two-plane manifold, with M&M X-450 LPG mixer on a Holley 4-bbl throttle body.
• All engine internals plus flexplate balanced.
• "Second generation" closed-loop real-time LPG mixture controller ("computer") from Autotronics, electronics and oxygen sensor in the tailpipe.
• Mechanical cooling fan removed, replaced with electric (later this summer).
• Synthetic oil, likely AMSoil, 20,000/annual change interval.

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.

((((more later))))