Stuart, Thank you for your very helpful info.. In my case the compression ratio is increased from 7.5 to 9. I just heard from a fellow Spit owner who has done this and had a very noticeable increase in performance of his 1977 spit ( and also his 1991 Land Rover) He has of course switched to high octane fuel with relatively little change in timing to avoid detonation. He also had his EGR and engine rebuilt. He has past smog test twice since. Actually, I already have a 61 TR3A with a rebuilt TR4 engine and, of course, its performance is much better, but that Spit is such a blast to drive.
Hans Sent from my iPad On Aug 26, 2013, at 6:14 PM, Stuart Greenwood <[email protected]> wrote: > 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]
