Hey Guys,

Does anyone know the stock compression ratio of a 1998 360 magnum from a Jeep Cherokee?

Thanks!

9:1 (9.1:9 for a factory fresh engine)

Thanks!!!!

Shave the heads .030, run a felpro 1008 and you'll be close to 9.5:1
The factory gaskets were .054" thick. Felpros are .039"

Starting with the 1993 FSM both the 5.2 and 5.9 V8s are listed as 9.1 to 1 static compression

I have measured the 1995 dishes to be 11 cc

Around 1996 Chrysler began advertising the 5.9V8 as 8.9 to 1 static compression. This was about the same time there were many pinging complaints by owners that eventially resulted in the infamous "Death Flash" for the JTEC pcm.

I suspect that Chrysler made the OBD-II 5.9 piston crown dish a little bigger, maybe 16 cc

With the piston typically
0.055 in the hole,
a 0.0475 factory gasket and
a little bigger than stock spec
62 cc chamber in the Magnum cylinder head,
the true static compression ratio is in the mid 8s,
which is close to the
8.4 that Chrysler listed in the FSM for the 8L iron V8

Willem Weertman wrote that the original 8L iron V10 used the stock Magnum 5.9L V8 pistons "for cost saving reasons"

I have come to feel that the 5.9 V8 would have benefitted from a flat top piston crown like the 5.2
and the cylinder head cc for a 360 should have been increased by 11 cc by making the exhaust valve side of the chamber bigger.

I also don t understand why the piston crown to block deck height was not zero decked,
when such a thick 0.0475 head gasket was used.

I bought this engine as a rebuilt engine. I was mainly asking so I knew what octane to run. It's slightly peppy, I doubt they messed with the heads or compression. It is running a carb, electronic distributor and no computer.

Thanks for the info!

On 95 deg F and dry summer days with the Mopar Performance PCM at least one of the cylinders on my 360 Magnum will ping at full throttle above 3400 rpm even with Sunoco 94 octane. The MP PCM maximum ignition timing seems to be 34 degrees btdc

With the Factory Stock PCM computer on the same hot dry day the engine will not ping on Sunoco 89,
but will ping on Regular 87 octane.
The stock PCM seems to have max timing of 28 degrees btdc at full throttle.

With your engine and the older style adjustable distributor i think you can easily run whatever octane you want to adjust for, with 26 deg btdc mechanical advance a good starting point for regular 87 octane

At 60 mph cruise on the highway with about 13 inches Hg of vacuum at 1550 rpm the vacuum advance addition sends total ignition advance as high at 40 degrees btdc

Stock Intake?

Have you checked the plenum gasket and ensured it's not leaking. Many Dakota guys have complained about terrible mileage, misfires, and pinging, then claim "my truck runs so much better on premium"

then they check and fix the plenum gasket and realize they were wasting gas on premium fuel when they didn't need it.