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.
- LPG fuel system (M&M X450 carburetor, M&M
- 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
- 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
- Synthetic oil, likely AMSoil, 20,000/annual change
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
.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.