View Full Version : 390 rebuild

08-15-2006, 02:02 PM
So, after trying for Reno and overheating several times, and smelling the crankcase thru the window I have decided to go through the motor and paint the engine comp this winter. Seems I can only find pistons that take a longer rod, like a 401 rod. I would like to do somwething with the heads also.
Question is....does anyone know where to get parts to fit a 68 390 or do I have to upgrade the block. Seems like if I do that then nothing will fit anymore like my manifold for one.
Suggestions? Thanks

Big Bad AMX
08-16-2006, 05:43 PM
You might want to read through this thread: http://www.amx-perience.com/AmericanMotorsForum/showthread.php?t=98 (http://showthread.php?t=98)
There's some pretty good info/ideas on rebuilding your engine.

Here's another on painting your engine:

I wasn't aware of piston shortages for 390s. There's limited choices but you should still be able to find them.


08-17-2006, 06:07 PM

check the comp ratio....your thoughts?

Big Bad AMX
08-23-2006, 11:55 AM
I read the article and to quote "Brandes says AMC engines are not compression-sensitive, and he has run stock cast-iron heads at 11.25:1 without spark-knock on premium pump gas." is a generalized statement that in my experiences simply isn't true. It would be interesting to see a follow-up article after that engine has 50,000 and then 100,000 miles on it. I have busted 10.0:1 pistons in my '70 AMX from detonation and I seriously doubt an AMC engine with 11.25:1 comp on pump gas would get close to those kind of miles on it before suffering major damage. It's your engine and you're certainly entitled to build it how ever you like but if you want longevity and reliability I would not even consider that high of compression for street use on pump gas for an AMC V-8. And keep in mind the engineers who designed these engines stated that 100 octane MINIMUM is required. You may get by with a few points less but not dropping 7 to 9 points, at least not for long. But, I did want to get another opinion so I asked Larry Mitchell what his thoughts were on the article and this topic. He replied with:

I wrote the AMC Illustrated Buyer's Guide (published 1995) and The Color AMC Musclecar History (published in 1999, both by Motorbooks, Int.) I also started the AMC national hobby in 1974 and wrote and published the award winning club newsletter, The AM-Xtra for (originally) The Classic AMX Club, International and AMC World Clubs and for 30 continuous years. I could go on, but --.

In the last 15 years, I have built over 25 AMC 390 and 401 V-8s for my self and my AMC/Jeep and collectible passenger car friends. I ran Pikes Peak in 1987 with my 401-powered 1969 AMX I hand built the motor for and also three runs in the early days of the Silver State Classic Challenge Nevada Open Road Race. Best run was just under 160 mph top speed on radar and 130 mph average for 92 miles. My fun driver today is my 1979 Pacer Limited wagon in solid black and an AMC 9.25 to 1 compression 401 soon with Edelbrock EFI.

I disagree on the article on AMC V-8s written by Mr. Brandes alluding to a "fact/opinion" that AMC V-8s can resist detonation with up to 11.5 to 1 slugs and other mechanical factors he listed and run pump gas. I am not closed minded in the fact that the mechanical things he mentioned CAN HELP resist detonation, but I am real sure major engine failure is just around the corner for all who think as he does. I have never found an AMC V-8 more defiant in the laws of physics than other V-8s of that time period. I have been witness to and informed of LOTS of AMC street/off-road V-8s that have suffered failure (most, but not all with new, stock-type cast and forged pistons). I could tell you stories.

I am a Certified Classic Car Appraiser based here in Colorado. Though not a scientist with a physics education, I am no dummy on the subject of the octane needs of AMC and other older carbureted V-8s. I can't tell you guys how many owners of older V-8s come to my office for an appraisal and tell me their (mostly stock) 10.0 to 1 any brand V-8 runs "just fine" on 91 octane. Yet my tuned ear heard detonation when they pulled up my inclined driveway. Many owners don't really know detonation (especially light called "trace detonation") when they hear it. They think it is a belt or a pulley or a water pump or something. Sad, but true. Others have exhaust so loud you can't hear anything anyway--"I don't hear it--it ain't there!" Or engine noise like solid lifters or gear drives. I used to take these people out for a short ride around my neighborhood where lugging the engine at 15 mph in a low gear and light throttle makes a light, but very audible rattling reflected by the 6-foot high, wooden privacy fence along the sidewalk. No wooden fence--no noise! WOT--no noise that can be heard due to engine noise! They are amazed. But I was wasting my time as most owners "know better than I" (called attitude) or are mechanically "uninformed". And "out of earshot--is out of mind" as "I drive this classic car or modified Jeep to look cool and to have fun and you want to rain on my little personal parade." Ignorance is bliss!

What varies octane requirements for any combustion engine is certainly what Mr. Brandes said as design, EFI and computers allow the new ZO6 Corvette to run 11 to 1 on 91 to 93 pump gas. But, also seriously affecting octane requirements on the older carbureted engines are: Altitude (less octane needed as you go up in altitude), Temperature (the hotter the ambient air is, the more octane is needed) and Humidity ( the higher the humidity, the less octane needed). And, as an added bonus from my experience and education-- MOST store-bough octane boosters that boost with alcohol or aromatics like naphtha are not good for the older vehicles and their engines. Pre 1974 vehicles are vented-to-the-outside fuel tanks and systems and the aromatic chemicals simply float up and out of the tank and into the clouds in a matter of a few days. Gone. And alcohol is NOT what these engines were designed to run on and corrosion and deterioration can result from alcohol in older gas tanks, fuel lines, older fuel pumps and carbs. Should I mention what alcohol in gas from the pump and through aftermarket additives does to help cause Vapor Lock? And how that 17 year-old kid in the rusted and lifted K-5 Blazer behind you caved in your ride's behind at taillight level because vapor lock caused your motor to flame out and he already dropped the clutch 3 seconds before the light changed?

I disagree very much with Mr. Brande's opinion on octane needs for AMC V-8 (and I-6) engines for most of us who drive stock-to-mild street-driven AMC passenger cars and Jeeps. (I drove a 1976 CJ-7 with a 401 AMC V-8 for from 1987 to 1999 daily and as Trail Leader for the Red Rock 4-Wheelers on Kane Creek on Fridays during Safari during that same time period.) They are fine motors and all I have had in daily drivers since I got deeply involved with AMCs in 1968. (My current ride is a 2001 TJ with the "AMC" 4.0 liter I bought new.) Readers who trust what he says and do what he says is "OK" could find major engine failure in a few thousand miles--or much less. And they might want to stone him Monty Python-style for leading them to believe what I say is not the whole picture or fact. For the engine they have--stock or modified. And how they use it. And how long they would like to have the engine last for. And we all know, engines never "let loose" and drop rod or a piston at idle. It is always at high rpm--which trashes the whole motor in milliseconds. The ultimate lesson here is my opinion for the "average" guy or gal. Don't build a stock or mildly modified AMC engine with more than 9.25 to 1 compression and 10-degree advance at the crank with a quick curve in the distributor (all in at 3,000 rpm or less and 32 degrees overall) and run 91 to 93 octane pump gas. For a highly modified engine--you are on your own to believe who you want. But don't get too far from home on a hot and dry summer's day, in my opinion! And for the stock-to-mild type of AMC V-8, you can't beat the Sig Erson AMC TQ-20 cam and springs. For the same in an AMC I-6, the new Crane torquer cams are the cat's meow!
Larry G. Mitchell


08-23-2006, 08:16 PM
Note Updated 08/24/06: I did not mention about Holley's Power Valve. If not setup properly with the proper pressure rating... your primaries and or secondaries will be lean. Let say you want to keep economy but need some performance... You may be able to get away with 65 or less primary jets, and open up the power valve bleeds a little and use a power valve that will open sooner as pressure drops. This take small and careful adjustments to get it right, since every engine setup is differnent.

************************************************** ********

I agree with Larry... but have a few things to say.

I never have ran an AMC with 11.0:1 compression... Did meet up and take a few test runs with a fellow AMCer that claimed his ran on 93 octane pump gas and was very impressed, since his car was a real racer and above 11.0:1.

But I did have a 65 Buick Riviera GS that had 10.5:1 and was able to run it on premium 92 gas without any issues.

Cast pistons and around 90,000 miles... The original owner had kept a diary since purchase up until 8 years before his death. The engine was tight and clean for the cars age at the time. To say the least it could burn the tread down to the cords, non-stop through all gears WOT.

My theory... based on carb tuning, is that many do not adjust their carbs to allow for proper fuel richness. Most of the time they are lean on the primaries, causing detonation before secondaries kick in. This may be what Larry picked up when the car was driven up his driveway (not very many cars need to kick in the secondarys pulling in to the garage). Second vacuum leaks or lack of when using big cams can cause advance issues with timing (Using a distributor with vac advance), and primary leanout with big carbs at low to mid RPMs.

Also to properly quench the combustion chamber, some reworking needs to be done around the valves. All in all AMC has one advantage over the others is that you dont need to add hardend seats unless you are moving the valves a bit.

By the way a Nail head is like the Bumble Bee... it is big and a freak'n wonder why it flies. GMs first big block at 401 cu to blast over 475 ft LBS of torque and break TH400's in testing with turbo's. By the way the three they tested were the only Buick engines that had forged rods and crank.

Sorry to carry on... I just like interesting engines.

Good ol late 40's early 50's Cadi flat head (Tank Engine)
391 hemi
AMC 390 - 401
Buick 401 - 425

If your wondering... they are all exellent mid range torquers for there size.

08-25-2006, 08:37 AM
Good info from some good sources, thats what I love about the internet.,
As far as the article about rebuild I really dont think the engine is expected to run high mileage. In my opinion compression, or an overall rebuild, has much to do with driving attitude. Some peeps can afford to build a fire breather and if it goes 15000 miles they call it an awsome experience.
In my case, 15k would be about 5 years. I built my first engine when I was 13
, a vw 1200 that belonged to my dad. It scattered like leaves in the wind on its first trip to SF. Pops said "ok, yer gonna do this till ya get it right". That was 42 years ago. Since then I have built 8 second quarter milers, outlaw engines, all the way to 39 buick straight 8.
I have never used an additive as I think they are unreliable at best. High octane fuel is more expensive but I can pump it right across town at 3.95 a gallon.(last year, havent had a need lately)
My point is....it seems to my, as you said, you can build your engine any way you like. You must be informed of the pro and con. You can choose to smoke also, but ya know where that gets you.
I am going to run probably 10:1 max like my 69 Mustang and if I get 25k out of it, it will be 2011 or so, I'll be happy.
At the same time, I have already picked up 2 other engines for this build. I never ruin the original as I might need it back sooner than I planned on.

09-01-2006, 04:01 PM
I have built some AMC motors and have follow Larry Mitchell's advice on them. I have had the priviledge of having Larry build my first AMC motor a few years back and can attest to what he is saying about the pinging problems of these motors. Larry rebuilt a 1970 390 V-8 and actually followed his own recommendations. As most know, the 70 390 is the most powerful of all of the 390 built. To this day, this motor is reliable and powerful. Being in a 70 AMX, it can light up the tires in just about any gear. I followed his recommendations when myself and a friend built my next motor, a 401. Got the same result and the motor found its home in a 71 Javelin SST. Although I do have to use a octane booster in my 70 AMX, I buy the stuff from AMX Enterprises and the stuff really works. The octane booster contains a certain percentage of good old fashion lead and depending upon how many ounces you use, you can adjust your octane levels by using this stuff. I currently run on premium gas (91 octane) and somewhere between a half quart to a full quart of the booster. Stuff aint cheap, but it is real good insurance from premature wear and tear. My 401 is ran on premium pump gas with about half of quart of the stuff. Both engines are running fine and I have not any problems with them in the last 5 years of running. I do not race and that was not intention when the motors were built. Just some decent street engines with an occassional woop *** on a few Mustangs from light to light.