???? 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
>
>
|