Re: [Amc-list] LPG 258

[Greg Taylor <amundaza@xxxxxxxxx>]

Matt,
I think what is trying to be pointed out is that a motor may have an 11.0:1 compression ratio, due to the physical compression of air/fuel between BDC and TDC ... the dynamic compression starts when the intake valve actually closes, based on cam spec, so the piston is already heading upwards in the stroke, so a true 11.0:1 ratio is not obtained.  Dynamic compression ratio can vary widely depending on the camshaft duration and when the intake valve actually closes.
 
You can see how this is affected on my 330" stroker.   Since I am using variable duration lifters ...
 
At higher RPM's, my engine will look like this : http://i246.photobucket.com/albums/gg116/amundaza/AMC_330_Stroker_Comp_Ratios.jpg
 
At lower RPM's, my engine will look like this (due to the lifters) : 
http://i246.photobucket.com/albums/gg116/amundaza/AMC_330_Stroker_Comp_Ratios_low.jpg
 
In essence, with a milder cam, the E85 motor can run higher cylinder pressures due to the dynamic compression ratio.

Sincerely, 
Greg Taylor :) <>< 
Rochester Hills, MI
1989 Jeep Grand Wagoneer "TRMN8R2"

And the testimony is this, that God has given us eternal life, and this life is in His Son. He who has the Son has the life; he who does not have the Son of God does not have the life. - 1 John 5:11-12


--- On Tue, 10/7/08, Matt Haas <mhaas@xxxxxxx> wrote:

From: Matt Haas <mhaas@xxxxxxx>
Subject: Re: [Amc-list] LPG 258
To: "AMC/Rambler owners, drivers and fans." <amc-list@xxxxxxxxxxxx>
Date: Tuesday, October 7, 2008, 8:10 PM

So what you are saying is that the cylinder pressure is higher because 
you can get more air into the cylinder to compress, correct? I honestly 
do understand that and it's why turbo and super chargers make more power 
  but you still haven't altered the size of the cylinder so the 
compression ratio hasn't changed.

Matt

On 10/7/2008 11:24 AM, Jim Blair spouted this sage advice:
> 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

-- 
mhaas@xxxxxxx
Cincinnati, OH
http://www.mattsoldcars.com
1967 Rambler American wagon
1968 Rambler American sedan
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