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 <sagreenwood@att.net> 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: "spitfires-request@autox.team.net" <spitfires-request@autox.team.net>
> To: spitfires@autox.team.net
> Sent: Monday, August 26, 2013 11:00 AM
> Subject: Spitfires Digest, Vol 7, Issue 48
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> 1. High compression pistons and smog test (Hans)
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> ----------------------------------------------------------------------
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> Message: 1
> Date: Sun, 25 Aug 2013 17:14:21 -0700
> From: Hans <sukaspit@gmail.com>
> To: spitfires@autox.team.net
> Subject: [Spits] High compression pistons and smog test
> Message-ID: <356B6E8E-7020-4D5B-A0DF-9188CA7C22E8@gmail.com>
> 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.
>
>
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