'79 304 vs. '73 304

Does anyone know the specific causes of the power drop in the 304 in 1979? I have a freshly rebuilt 304 and the valve cover tag is saying it's a 79 (its out of a 1972) that I just put a ****-load of time into and I have a dirty, not-so-fresh but definitely useable 304 still sitting in my other jeep that I'm using for parts and its valve cover tag says its a '73.

So my 2 questions are:

What are the specific (like exactly what part) causes of the power drop in 1979?

and

Is it worth me tearing down the '79 I just built and putting all the new parts in the '73?

 

I honestly don't know for sure but I could give you a couple of educated guesses and assumptions. I am not sure when the manufacturers went from SAE gross HP numbers to SAE net numbers but I am pretty sure they swiched over in the early 70's. This can make a big difference between posted performance numbers on essentially the same engine. I would have to say there is very little differences in the longblocks of both engines. If anything there might have been a compression change between years and possible differences in camshaft grind specs. I wouldn't be surprised if there were some changes in external components such as intake, carburation, ignition, exhaust and such but if you are building the engine I would think you will end up with a very similar product when you are done. Are you planning any performance upgrades? If you are adding a cam, intake, carb and ignition system there will likely be no difference between the two engines when you are done. I am sure there is someone here that might have some more specifics but I am pretty sure you are not going to have to worry about ending up with a turd of a motor because you chose the later one. Hope that helps you out some.
Greg

 

I have an Edelbrock Performer intake, Holley Pro-Jection TBI, an Advance Distributers DUI, and Blackjack in-frame headers. Thats an interesting point about the change in the rating system.

 

Ratings change happened between '71 and '72 as I recall. A '73 and a '79 should both be rated with the same method (net hp).

Internally, the main change I see is a different camshaft. The pistons are the same for all 304s. The cylinder heads were revised in '78 and in '79, but I somehow don't think they did that to change the compression ratio. More likely there are more or different tapped holes for accessories, or something like that.

I'd guess the cam change and more exhaust gas recirculated with a leaner mixture accounts for a lot of the difference. The 304 got really anemic before it was discontinued in the CJ for '82. By that time, HP and torque of the 258 was nearly equal to the 304, so there was not much point in continuing to offer it. I expect that AMC was having a tough time keeping the 304 emissions compliant by 1980.

With your induction and exhaust, I expect there would be no real difference if you started with the '73 or '79 engine. You still have the '79 camshaft? Instead of thinking about switching to the '73 block, I'd consider adding an aftermarket camshaft.

 

recommendations on an aftermarket camshaft?

 

I put a 304 together for a friend a number of years ago. We used a performer intake, quadrajet carb, and fenderwell headers. The camshaft we chose was the Edelbrock performer camshaft.#2132

ENGINE: AMC 290-401 V8
RPM RANGE: Idle-5500

CAUTION: Use only Sure Seat Valve Springs, #5832 or 5932 Use stock ratio rocker arms only

Duration at 0.006" Lift: Intake: 278° Exhaust: 288° Duration at 0.050" Lift: Intake: 204° Exhaust: 214° Lift at Cam: Intake: 0.280" Exhaust: 0.295" Lift at Valve: Intake: 0.448" Exhaust: 0.472" Timing at 0.050" lift: Open Close Intake: 3° ATDC 27° ABDC Exhaust: 42° BBDC 8° BTDC Centerlines: Lobe Separation - 110° Intake Centerline - 105° That engine ran great and was extremely strong. I thought it was surprisingly so. One other reason we went with the Edelbrock grind was it was really reasonable cost. Summit has the cam and lifter set for cheaper than just the cam for a Comp grind. Summit price for the Edelbrock 2132 is $149.00. http://www.summitracing.com/search/?keyword=edelbrock 2132&dds=1.
Another good friend of mine ran a 304 with the Comp Cams 260H grind but he had stock 2 barrel intake and carb with headers and it ran pretty good too but not on par with the other. I think I would also try the Comp cams XE 262H10 PN 10-215-5 or a Comp cams 268H10 PN 10-202-4. IN the Comp cams grinds the XE or extreme energy grinds are the most advanced lobe profiles and hence the cost. Any of those cam grinds or something with similar specs should work well. One other grind that would work well on a budget is to go with the Summit 8600 http://www.summitracing.com/parts/SUM-8600/ , it is only $59 without lifters. Go for the larger cam if you have steep axle gears and you don't mind buzzing a little more RPM such as getting into a bit of mud. If you are really wanting the max low end torque shoot for the smaller grind. The most torque possible will likely be the Comp XE 268H grind. That said my buddies engine with the Performer cam had good all around power with 35s and 4.56 gears.
Greg