Re: [Amc-list] LPG 258

[Jim Blair <carnuck@xxxxxxxxxxx>]

While you aren't changing the static compression, you are increasing the efficiency of the motor to draw that in which is what increases it's potential and at the same time, if you pumped the motor before and after a 4.0L head swap, the compression would be slightly higher. The port and intake restriction of the 4.2L creates a blockage.

Jim Blair, Lynnwood, WA '87 Comanche, '83 Jeep J10, '84 Jeep J10
From: Matt Haas 
Subject: Re: [Amc-list] LPG 258
To: "AMC/Rambler owners, drivers and fans." 
Message-ID: 
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
 
I understand the gains you get from better flow. An engine is basically 
an air pump so the more air you can move through it, the more potential 
for power you have. I'm not arguing that the 4.0 head isn't a good swap, 
just that without changing the cylinder volume, you are not altering the 
engine's compression ratio.
 
Compression ratio is a measurement comparing the cylinder volume with 
the piston at BDC vs TDC. The only way to change that is to physically 
alter the size of the cylinder.
 
Matt
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