Re: [Amc-list] 71 Gremlin 232

[Frank Swygert <farna@xxxxxxx>]

84-87 258s had 9.2:1 compression, 82-83 8.6:1, 78-81 8.3:1, 71-77 8.0:1. 
The higher compression may help some, but the 4.0L heads hands down are the best flowing. Even the Renix head outflows a 258 head on the intake side. There's not as much difference on the exhaust side. The intake ports were raised about 1/8" higher on the Renix than the 258. What that did is increase the radius of the turn down into the cylinder, allowing the air/fuel charge to enter the combustion chamber at a higher velocity. Even though the ports are smaller, the added velocity increases flow significantly. In 91 the H.O. came out. The intake ports were raised an additional 1/8", giving an even better travel radius for the intake charge and further increasing velocity. There's is a 13 hp difference between the last Renix and first H.O. It's not all in the velocity increase though -- the computer programming changed slightly and the cam degreeing was changed (I forget if it was advanced or retarded 2-3 degrees, but I think it was advanced?). This was done more to decrease p
 ollutants than increase power. The knock sensor and EGR were eliminated, the EGR mainly due to the cam timing and velocity change. 

I don't think the extra compression will make the late 258 head perform any better than the 4.0L head. Furthermore, I think you'll have about the same compression. The combustion chamber in the head didn't change from 82-87 as far as I can tell from the TSMs (I only have through 83 -- earlier chambers were a bit larger @ around 67-70 cc, 82-87 were 65-67 cc) and the "Jeep Engines" book by Mopar Performance. That means the piston dish had to change to go from 8.6:1 to 9.2:1 -- unless someone has better combustion chamber data for the 84-87 heads. 


--------------------
Date: Thu, 21 Feb 2008 12:25:49 -0800 (PST)
From: Greg Taylor <amundaza@xxxxxxxxx>

I would recommend getting the later 1980's 258 cylinder head, intake and exhaust manifold.  Those 258's had 9.2:1 compression.
   
  Then you could do the Chevy 1.94/1.50 valve conversion, using the undercut stems for more low/mid range air flow.  Picks up an easy 30 HP in breathing on a stock 258 in my '83 spirit GT.  FYI, on a stock bore engine, I had the exhausts cut down to 1.45" diameter.

-- 
Frank Swygert
Publisher, "American Motors Cars" 
Magazine (AMC)
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