258 info thread

[BrockGrimes]

I've been looking over the stock power spec's of the AMC 258 i6 / 4.2l from the factory.
Someone please explain why each makes the power it does, especially the ones making AMC 150 hp and 240 tq.

 

[CJ]

In 72 they changed from gross to net HP. Hence the perceived drop in ratings.

SAE gross horsepower

Prior to the 1972 model year, American automakers rated and advertised their engines in brake horsepower (bhp), frequently referred to as SAE gross horsepower, because it was measured in accord with the protocols defined in SAE standards J245 and J1995. As with other brake horsepower test protocols, SAE gross hp was measured using a stock test engine, generally running with few belt-driven accessories and sometimes fitted with long tube (test headers) in lieu of the OEM exhaust manifolds. The atmospheric correction standards for barometric pressure, humidity and temperature for testing were relatively idealistic.

SAE net horsepower

In the United States the term "bhp" fell into disuse in 1971-72, as automakers began to quote power in terms of SAE net horsepower in accord with SAE standard J1349. Like SAE gross and other brake horsepower protocols, SAE Net hp is measured at the engine's crankshaft, and so does not account for Transmission losses. However, the SAE net hp testing protocol calls for standard production-type belt-driven accessories, air cleaner, emission controls, exhaust system, and other power-consuming accessories. This produces ratings in closer alignment with the power produced by the engine as it is actually configured and sold. The change to net hp effectively deflated power ratings to assuage the auto insurance industry and environmental and safety lobbies.

Horsepower - Wikipedia, the free encyclopedia

 

[BrockGrimes]

Ok that I get but are there factory differences?
I notice compression changed a few times.
BTW which spec goes with what year, that chart is kinda misleading?
8.0:1 at 110 195
9.2:1 at 115 210

I figured a compression ratio change like that would have produced more power.
Maybe it's just a thing with inline motors to make more torque, with improvements.

 

[CJ]

Whew... Where to begin...

I see where you are talking about - in '82 the compression ratio went from 8.3 to 9.2. I'm positive that was because of the addition of fuel injection and computers.
I'm also positive that the addition of more EPA requirements caused just a small gain in torque while the HP dropped 5 (the torque RPM changed dramatically).
I bet that the runner size of the intake was decreased or the cam was changed increasing torque yet net HP decreased due to the lower RPM efficiency of the engine.

Could I be wrong - damn right I could... That's just a best guess I have... I don't have access to all the cool fluid dynamics software we have at work here at my house... I guess if I could get models of the old AMC 258 i6 / 4.2l engines I could work up a better hypotheses for ya...

LOL - I don't know jack about designing engines... What I do know is that as the computer systems evolve and the EPA systems become more stringent, advances are made and HP / Torque seem to advance and receded as stipulations are created (EPA) and technology advances...

 

[cdlongjr]

Musta been the Holley!

 

[CJ]

Musta been the Holley!

:laugh: Damn straight! :laugh: A 750 double pumper solves everything!

[geek mode off]Holy , I just read my last post and my fingers were off on their own... So, a condensed version... As engines in the late 70's and early 80's were introduced, HP and torque increased and decreased as different EPA mandates and new computers became available... Hence the up and nature of HP and torque between the years.[/geek mode on]

 

[BrockGrimes]

I'm wondering if mixing parts might make for mor epower on the cheap, if they are already lying about.
If they made a cam change or intake change with the FI setups, it might be worth checking out when I swap to a TBI setup.

I've been thinking about getting a AMC 258 i6 / 4.2l head worked over on the cheap, just because.
I'm not wanting to do the 4.0 swap yet but maybe. I'm always up for free or cheap HP.

Hell been thinking if I had a 8.0:1 motor I'd consider a JY turbo setup.
But I think my '79 is 9.2? or 8.3?

 

[CJ]

I'll have to differ to a better educated person then I on the AMC 258 i6 / 4.2l 's... I couldn't tell you what year intakes / exhausts / cams were better then others.

According to all the info I can dig up, a 79 is 8.3 - just perfect for a turbo swap!

 

[dalefan88]

the bad thing is my 03 honda accord with a 2.4l 4 banger puts out more hp andi think torque then my jeeps AMC 258 i6 / 4.2l but the jeeps torque is where you need it.