Amx 390


New member
So after years of rumors about 13:1 compression, huge cam, .060 over and on and on... we find:

.030 overbore with what might be 401 (lower compression) flat top pistons
Basically stock or slightly more than stock cam
1968/69 Heads, which are outflowed by the 1970+ heads by 20-50% from what i've read. Why the 1970 heads are missing nobody knows...

So it seems when the original owner asked for "More power" out of the Calgary machine shop, they gave him a boring camshaft and less compression... and possibly someone switched heads on him while he wasn't looking. Pretty dissapointing in my eyes!

So, i am seeking the advice of Jaffray Bob, Creston Bob and whoever else might have some valuable information as to what to do next. I have a mid 70's 401 sitting here that i could use the heads from, as they are better flowing "Dog Leg" versions that were designed for unleaded gas. I could find some Edelbrock aluminum heads, and either bolt them on or have some mild porting done as recommended by some guys online. Once deciding that, i'll need to figure out what pistons to use and head gaskets. Like any daily street driver, i would like maximum streetable (and budgetable) power and the ability to run regular gas with the option of premium or better.



New member
I'm sorta new at forums and am a bit lost at the context here. You bought or were given what you were told was a 'built up' high performance 1970 390. You opened it up and found you had been lied to. Now you are wondering what to do next. Am i correct so far? You seem to be asking about heads, an existing set of pre 70's that were on the engine, (forget them), a set of mid 70's 401 heads you have available, (forget them) or purchasing a pair of Edelbrock aluminums, (buy them) your other choices would be to try to find a pair of reasonably priced 1970 stock heads, (not going to happen) or a set of reasonably priced early 1971 stock heads (may happen). Out of all the options your best bet is still the Edelbrocks so buy them. Invest some bucks in some Harland Sharp roller rockers 6:1 ratio. They should fit under the stock valve covers I think. You really won't need them but I like them and they like me or buy a set of affordable roller tips from Summit racing. (they are all you really need.) Use stock valve covers if you can as its design actually does have a purpose. Sell me your old ones for cheap if you don't use them.)
Hard to tell from the picture but if the pistons and block are happy with each other use them.(they look like old TRW's and will give you 9.5 -10.2 with stock.045 head gaskets)
As a first choice use an Edelbrock RPM Air Gap manifold if you have something else after market kicking around for free you might consider it. Avoid the single plane Torker manifolds that are very common to find. Nothing really wrong with them if you are building a track car but not so great on the street.
VERY IMPORTANT. Oiling is an AMC's worst problem so google that. There are a couple of simple oil pump mods (don't get carried away) and a range of mods for the lifter valley. Always put in a Cloyes double roller timing chain. A quick call to Summit racing again when you have all your bits and bobs in a pile tell them what you have (rear end ratio, tranny, stall if automatic, manifold etc.) and have them recommend a cam, Lunati, Comp, Isky. Don't be afraid of a solid lifter cam either AMC's like them. As for a carb I llke Holleys but an Edelbrock 650cfm will probably do you just fine and they are less troublesome. If you don't want that darned old 390 call me. Good luck!


New member
Thank you for replying first of all. I was beginning to think nobody would. I guess I didn't clarify the story too well, I got the car from the original owner and it came with the 390. When we opened it up, we found what I consider to be suspicious but I'm glad it's not .060 over like the rumor told. I appreciate your advice, I'll take anything constructive at this point. This car deserves to have the 390 back under the hood again and the 390 has to finally do what they're known for.
Nothing wrong with the "Nash's", they will make very good power when done properly,
I would suggest they are NOT just a "run of the mill" machining endeavor, and DO have their own quirks, likes and dislikes, especially if you intend on driving it like you are mad at it occassionally. The problem is compounded by the lack of different bearing eccentricity availability, NOT that the "P" designation bearings won't work..they will, just that attention to sizings is important.
IMO, they ARE a different animal.
1.) while some of the online information is nice, Oil Pump Mods, etc., etc., and very well intended, unfortunately, much of it misses the mark by attempting to overcome internal problems dealing with "symptoms", rather than tackling the root "causes" internal of the AMC Engines when machining.
2.) Throw the Factory Spec book out the window at the start.

Just my opinion, no wars wanted
Bob @ rmp Engines in Jaffray

here's an AMC Stroker we built last year by offset grinding the AMC Crank and applying Chevy Rods, we still get a few in from time to time.

Pump Gas, Street/Strip, Flat Tappet Cam, our Stage 2 "sreet" porting on Eddy Heads, NOT radical
NOT saying this is what you should do, more just as an example of what "can" be done. Note the Oil pressure.
570hp @ 6200 rpm/531Ft lbs torque
This is the final tune "backed down" as it left to the Customer. We had more power, but as a "real" street deal, we usually default back a bit, especially since pump gas fuel qualities can vary, and an extra 8-10 hp ain't worth it over time
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New member
Thats pretty awesome alright... more than i intend to do to this engine for sure. I did get your phone message today on my house phone and i'm looking fwd to seeing the other dyno sheet you were talking about with the dogleg heads.
As soon as I find that sheet I'll get it up, waaaay back 15 years ago.
It was a .030" over 390
We used the BB Mopar 2.08" / 1.74" Valve Combo in the Doglegs / Stage 1 Porting
Hydraulic Flat Tappet Cam
Stock Crank & Rods W/ARP Bolts
Some TRW Forged Pistons we worked
9/32" Oiling and Crossovers / Cut Bearings.

The biggest surprise I remember, is that the Herman Lewis Intake Manifold made no more power, than the Torker Intake we tried on it next ?
It was right around the 530's hp 500+ somewhere Torque ?
Went 11.80 in the 1/4 on street tires like spinning on banana peels, at something stupid like 123mph !


New member
Sounds very close to what i think will be happening to my engine.. if we have to smooth it out a little to make it run well on pump gas then so be it (hoping premium 94 will be an option not manditory as it will get somewhat daily driven). Wasn't shooting for the 5 mark... but i wouldn't complain ;)