Re: [Amc-list] LPG 258

[Matt Haas <mhaas@xxxxxxx>]

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

On 10/5/2008 10:31 PM, Jim Blair spouted this sage advice:
> 
> Static compression ratio is different from running ratio. Why do you think motors that breath better make more power? The "cork" was removed from the flow of air in the 4.2L when they went to the 4.0L head by raising the intake ports so there is a straighter shot at the valves by the air fuel mix. Why do you think people swap the 4.0l head on if the chambers were the same otherwise?
> 
> Jim Blair, Lynnwood, WA '87 Comanche, '83 Jeep J10, '84 Jeep J10
> From: Matt Haas <mhaas@xxxxxxx>
> Subject: Re: [Amc-list] LPG 258
> To: "AMC/Rambler owners, drivers and fans." <amc-list@xxxxxxxxxxxx>
> Message-ID: <48E950B2.2010202@xxxxxxx>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>  
> Since when did VE have anything to do with compression ratio? If the 4.0 
> head has a smaller combustion chamber, than compression goes up but how 
> well the head flows doesn't magically change the physical properties of 
> the motor.
>  
> Matt
>  
> On 10/5/2008 1:59 AM, Jim Blair spouted this sage advice:
>> Actually, since the VE (volumetric efficiency) goes up with the 4.0L head (even more with the HO head) the compression will go up slightly.
>>
>> Jim Blair, Lynnwood, WA '87 Comanche, '83 Jeep J10, '84 Jeep J10
>>
>> From: Frank Swygert 
>> Subject: Re: [Amc-list] LPG 258
>> To: amc-list@xxxxxxxxxxxx
>> Message-ID: 
>> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>>
>> Look for different pistons -- you can only go up maybe one full point by shaving the head alone. Compression doesn't go up with the 4.0L head on a 258 -- only 8.7:1 (stock 4.0L). The combustion chamber sizes are the same on the 258 and 4.0L.
>>
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-- 
mhaas@xxxxxxx
Cincinnati, OH
http://www.mattsoldcars.com
1967 Rambler American wagon
1968 Rambler American sedan
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