On Mon, Jul 31, 2000 at 06:35:23PM -0400, WSpohn4@aol.com wrote:
> In a message dated 31/07/00 2:48:14 PM Pacific Daylight Time,
> owner-mgs-digest@autox.team.net writes:
>
> > There is one advantage to going to the lower (8.0:1) compression dished
> > pistons versus the higher (8.8:1) flat tops: A lot of performance
> > enhancements (cross flow head, Hans' supercharger) require the lower
> > compression in order to run the motor on 92 Octane gas.
>
> A couple of comments here. On an MGB (we are talking 'B' here, aren't we?),
> the flat top pistons give you more than 8.8 compression.
Yes, we are talking about a 'B'. I have not yet calculated the CR of
my car. Does anyone know the chamber volume of a stock head? The shop
manual claims that the High Compression motor is 8.8:1 and the low is 8.0:1.
>
> Second, while you are correct that any form of supercharging or turbocharging
> works well with a lower compression ratio, you are incorrect in attributing
> that same phenomenon to crossflow heads.
>
> In fact, the crossflow heads are alloy and can dissipate heat better than a
> cast iron head. They actually call for _more_, not less compression to
> function properly, and with the lack of hot spots will work well with 92
> octane gas. I have run 10.5 compression on 94 octane with no trouble. I don't
> imagine that flat top pistons and an alloy head would be any problem with 92
> octane.
When I was researching the crossflow heads I was told that due to a lower
chamber volume they would boost the compression of the engine, so that
the 8.0:1 low compression motor became about 9:1. It's been a while since
I researched this and I don't remember the numbers exactly.
Larry
--
I want to have profound thoughts and have other people implement them.- T. Bird
lrc@red4est.com http://www.red4est.com/lrc
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