Hi Guys
If I may be so bold as to put my .02's worth in here! the simplistic
mathematical equation to calculate a compression ratio will work fine in
theory, BUT, in practice so much depends upon the camshaft timing and lift
etc that it becomes very difficult to actually calculate an accurate figure
from the dimensional data given by the engine builder. The compression at
cranking speed may well be different than the compression at 6000
rpm..simply due to time etc
A great many other factors can cause pinking, such as the advance curves,
dwell, weak mixture, engine temp, fuel quality and altitude.
Engines will run at 10-10.5 :1 compression without a problem IF these other
factors are taken into account and adjusted accordingly.
-----Original Message-----
From: owner-mgs@autox.team.net [mailto:owner-mgs@autox.team.net]On
Behalf Of wizardz
Sent: Monday, August 14, 2000 10:20 AM
To: British Sportscar Center; mgs@autox.team.net
Subject: Re: Compression pressures ??? What?
I guess my quandary wasn't your original statement .....
... in the back of my mind I guess I'm questioning (or trying to visualize)
the scales and ranges.
I guess I'll have to go through all the calculations and compare
theoretical to actuals, to see for myself the how they interact and whether
or not
its a linear/non-linear curve. Hhmm... might make a nice web page chart
for the masses.
Paul Tegler wizardz@toad.net http://www.teglerizer.com
----- Original Message -----
From: "British Sportscar Center" <Lawrie@britcars.com>
To: "wizardz" <wizardz@maxinter.net>; <mgs@autox.team.net>
Sent: Monday, August 14, 2000 12:54 PM
Subject: Re: Compression pressures ??? What?
So, Paul, what's with your row of question marks?
Your stock B (by my ruff 'n' ready calculations) is at 9 to 1, your Spitfire
a tad over that and your Midget a tad under. You say they all run fine on 92
octane. I thought my note said that around 9 to1 was as high as one could go
on 92 octane fuel without pinging; your experience confirms this.
Lawrie
British Sportscar Center
----- Original Message -----
From: wizardz <wizardz@maxinter.net>
To: Lawrie Alexander <Lawrie@britcars.com>; <mgs@autox.team.net>
Sent: Monday, August 14, 2000 8:46 AM
Subject: Re: Compression pressures ??? What?
> ???? My stock '73 GBT 18V engine has roughly 160 +/- 5 psi across the
board.
> My totally stock Spit 1500 is running around 168
> and my VERY tired 1275cc Midget is at 153.
>
> Somehow I can't bring myself to believe any of these three engines are
> running high compressions. They all run just fine on 92 octane
> I've never ever had a pinging problem.
>
> Now my '78 Spit is a different story. It was running 185 psi dry 198
oiled.
> I was told (and calc'ed it all myself from the engine builders recorded
numbers)
> it was running roughly 9.5:1. I calc'ed it at 9.93:1 And yes, with
proper timing it
> would ping. I normally ran it with the timing backed off just a bit and
ran
> a lead substitute additive.
>
> Paul Tegler wizardz@toad.net http://www.teglerizer.com
>
>
>
> ----- Original Message -----
> From: "Lawrie Alexander" <Lawrie@britcars.com>
> To: <jon.arikstad@merkantildata.no>; <mgs@autox.team.net>
> Sent: Monday, August 14, 2000 11:01 AM
> Subject: Re: Compression preasures
>
>
> Jon,
>
> I have no mathematical formula, I'm afraid. However, experience has shown
> that 160 lbs cranking pressure is found in engines that have 9 to 1
> compression.
>
> Your 188 pounds cranking pressure suggests to me that your compression
ratio
> is around 10 to 1, certainly more than we could run on pump petrol in the
US
> but it should be OK for you if you can still get 5-star 100-octane petrol.
>
> I encountered an 18V engine last year that, with a single Z-S carb, ran
like
> a dual SU B. It was fast but I could not get it to stop pinging. Then I
did
> a compression check and found 185 lbs per cylinder. A careless combination
> of shallow-dish, oversize pistons and a head that had been shaved more
than
> it looked, I guess. Anyway, to get the engine to run without pinging I had
> to fit a second head gasket, which reduced the cranking compression to 155
> lbs. After that it ran just fine.
>
> Lawrie
> British Sportscar Center
>
> -----Original Message-----
> From: jon.arikstad@merkantildata.no <jon.arikstad@merkantildata.no>
> To: mgs@autox.team.net <mgs@autox.team.net>
> Date: Monday, August 14, 2000 2:33 AM
> Subject: RE: Compression preasures
>
>
> >
> >> But fear not - 150 is roughly equivalent to 8.8 to 1 so you will still
be
> >> able to run very nicely on 92 octane premium and you will notice an
> >> improvement in performance.
> >>
> >> Lawrie
> >> British Sportscar Center
> >>
> > How do you calculate this.
> >My 'compresson preasures' are 188 !
> >
> >Jon Arikstad
> >1970 MGB GT
> >
> >
>
>
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