My 69 AMC 390 Engine Rebuild

By Dan Curtis

I recently had my AMC 390 engine rebuilt, mainly for street use. It had originally been rebuilt (and balanced/blueprinted) by the past owner, but the previous owner had 401 pistons installed, bringing the compression  ratio down to 8.5:1. Further down in this article you will find the specifications for the rebuild, the parts used, as well as the full results of the dyno runs.

When completed, this engine “dyno'ed” at 459 max hp with 439 average (456.4 and 437.5 with the alternative, smaller carb) and 448 max ft lbs of torque with 418.2 average (451.5 and 416.3 again with the alternative, smaller carb) which is quite an accomplishment for a moderately worked 390 with stock compression and no nitrous, stock but worked heads and a pretty standard carb-based fuel delivery system.

Reid's Automotive, the machine shop I used, builds a lot of racing engines and has a calculator that determines what an engine should do in the quarter mile. They determined that at a weight of 3250 pounds and with optimum gearing and traction, I should be able to turn 10.87 in the quarter mile. Pretty amazing! This engine is currently in my AMX.

All of the work and parts represented a total of about $6,500 (including a bunch of perfectly good used parts lost by DHL worth about $1500 to replace) and $3,500 in machine shop work. I also purchased a set of ported and polished 291C heads for $700 off eBay that were supposed to be ready to go from someone who had previously purchased them for $1100 but never used them. When they arrived at Reid's they determined that the heads had previously had one too many valve jobs which resulted in the edge of the valves having a “knife edge” which prevented them from being redone, as they were no longer in very good shape. Ready to go...yeah right! So now the heads have brand new stainless steel 202 and 168 valves and will be good for as long as I will have any use for them.

Regretfully, I later found another set of 291C heads that had been totally redone with brand new valves, studs, guide plates, high tension double valve springs, retainers and guides and although they were not ported and polished, they were cc'd and probably would have only had nominally lower volume flow than the other heads. I picked up that second set of heads for $550 which would have been a heck of a lot cheaper than what I have in total in the other heads that were supposed to ready to go! Initially I purchased an original group 19 AMC logo R4B and Holley 950 three barrel setup but in the end the I realized that I had put in enough performance goodies in the engine that the R4B was no longer sufficient enough for what the motor was capable of producing, thus the Performer RPM AirGap manifold. The Air Gap pulled strong right up to 6700 RPM on the dyno.

On the following page are the horsepower and torque results from the two dyno pulls. One had my trusty old 750 Holley on it that I have had since 1973. My brother and I rebuilt it in the early 80s with an 850 kit and pulled all of the choke mechanism out of it to give it maximum flow. That carb was again rebuilt but this time had the anti backfire power valve saver now installed. As you will seen the dyno results, it is essentially an 850 as it only had slightly lower volume on the dyno pulls than the 950 three barrel Holley that was also had rebuilt for the test.

Finally, the engine was “dialed in” on the dyno for advance curve and the carb was jetted for proper fuel burn ratio so as to not be dumping too much gas into the engine. With what I spent on this rebuild, I didn’t even want to think about trashing it by not having it set up correctly.

Below are the parts used and specs of the engine

PARTS:

Manley Perf. 11892 INTAKE VALVES - 8

Manley Perf. 11859 EXHAUST VALVES – 8

Comp Cams 10-00-9 A8 AMC V-8 ROLLER CAM 4975F/4876F-R110

Comp Cams 848-16 ROLLER LIFTER

Comp Cams 985-16 948-973

Comp Cams 740-16 1.437" RETAINER

Clevite SH671S CAM BEARINGS

Comp Cams 616-16 #.050 3/8 SUPER LOCK

Milloden K-85 OIL PUMP ADAPTER

Milloden 85-S PICK UP SCREEN

ELG PR537 PUSHRODS - 16

MSD 8519 AMC PRO BILLET DISTRIBUTOR

MSD 6AL IGNITION CONTROL BOX WITH ADJUSTABLE

REV LIMITER

MSD BLASTER 2 COIL

MSD 8006 BRONZE GEAR

AMC 291C PORTED AND POLISHED HEADS

EDL 7531 EDLEBROCK PERFORMER RPM AIRGAP MANIFOLD

CENTER FORCE II STREET AND STRIP CLUTCH, PRESSURE PLATE AND

THROW OUT BEARING

CAM SPECS: GRIND # 4875F/4876F R 110.0

VALUE ADJUSTMENT: INTAKE - .016 EXHAUST - .018

GROSS VALVE LIFT: .577 .583

DURATION @ 0.15 TAPPET LIFT: 285 291

VALVE TIMING @ .050 OPEN 14 BTDC 57 BBDC

CLOSE 54 ABDC 17 ATDC

FOR CAM INSTALLED @ 110.0 INTAKE CENTER LINE

DURATION @ .050 248 254

LOBE LIFT . 3850 3890

LOBE SEPARATION 110.0

Run #1

85% efficiency,  29.92 Barometric Pressure @ 70F, Fuel .699,  Time 12.6,  Holley 950 3 barrel, vacuum secondaries, 4100 - 6700 RPM, advanced ignition timing to 38 BTDC, Headman Headers

RPM                 Corrected Torque            Corrected HP

4100                 425.6                            325.5

4600                 447.7                            392.1

5100                 443.4                            441.1

5600                 423.4                            451.5

6100                 395.2                            459.0

6500                 367.3                            454.5

Average             418.3                            439.3

Run #2

85% efficiency 29.92 Barometric Pressure @ 70F, Fuel .699, Time 12.6,  Holley 750/850 4 bl, mechanical secondaries, 3000 - 6600 RPM ignition timing 38 BTDC Headman Headers

RPM                 Corrected Torque             Corrected HP

4100                 432.7                            337.8

4600                 443.8                            388.7

5100                 439.4                            426.7

5600                 420.3                            448.5

6100                 392.7                            456.1

6500                 363.3                            452.8

Average 416.3                            437.1