NOx production in the combustion chamber is dependant on flame temperature and combustion pressure. Increasing the compression ratio will have little effect on the flame temperature but it will ncrease the combustion pressure which will increase NOx. I have no experience with reciprocating engines but in a gas turbine engine the NOx increases by the square root of the pressure. Assuming that this is the case in a recip, then going from ,say 9 :1 compression ratio ro 10 :1 will increase the Nox by 5.5% The EGR valve system is calibrated to admit sufficient exhaust gas into the combustion chamber to reduce the NOx to whatever the regulations require. This calibration is done with the standard compression ratio for the engine. If you increase the compression ratio the calibration will not be correct. despite the above I think that since the California smog test is done at very low power settings I would be surprised if increasing the compression ratio had any significant effect on NOx readings. However when you raise the compression ratio you will get detonation unless you retard the ignition, because the best fuel you can get in Ca is 91 Octane, and this will reduce power. So don't think that you can get the engine back to MkIII power levels. I suggest if you want more power sell the Spit and buy a TR !
Stuart A Greenwood 71 MK I Stag 71 MK IV Spitfire ________________________________ From: "[email protected]" <[email protected]> To: [email protected] Sent: Monday, August 26, 2013 11:00 AM Subject: Spitfires Digest, Vol 7, Issue 48 Send Spitfires mailing list submissions to [email protected] To subscribe or unsubscribe via the World Wide Web, visit http://autox.team.net/mailman/listinfo/spitfires or, via email, send a message with subject or body 'help' to [email protected] You can reach the person managing the list at [email protected] When replying, please edit your Subject line so it is more specific than "Re: Contents of Spitfires digest..." Today's Topics: 1. High compression pistons and smog test (Hans) ---------------------------------------------------------------------- Message: 1 Date: Sun, 25 Aug 2013 17:14:21 -0700 From: Hans <[email protected]> To: [email protected] Subject: [Spits] High compression pistons and smog test Message-ID: <[email protected]> Content-Type: text/plain; charset=us-ascii I am rebuilding my California 1976 spitfire's engine and considering using Moss's HC flat top pistons instead of the LC (dished) ones. I have been told that I risk failing the NOx smog test max requirement. I understand that this is due to the combustion chamber's temperature potentially exceeding 2500 degrees , which generates NOx. Assuming the EGR to function perfectly and the fuel mixture to a bit rich, is this still potentially a high risk problem? There must be a few a guys out there that have been tempted to do this to get the nearly 20 extra hp. That is so badly needed. ------------------------------ _______________________________________________ Spitfires mailing list [email protected] http://autox.team.net/mailman/listinfo/spitfires End of Spitfires Digest, Vol 7, Issue 48 **************************************** _______________________________________________ [email protected] Donate: http://www.team.net/donate.html Suggested annual donation $11.47 Archive: http://www.team.net/archive Forums: http://www.team.net/forums Unsubscribe/Manage: http://autox.team.net/mailman/options/spitfires/[email protected]
