Jackson:
I can't respond in a definitive way to the technical questions you ask, but
I can tell you that I've been running 10.5:1 CR in a street-legal vintage
race 1275 Midget on 93 PON pump gas. I've got 6.5 cc dished Powermax
pistons, stock valves, and "Vizard-recommended" de-shrouding and porting. If
you go to thew following site:
http://www.smokemup.com/auto_math/index.php
you will find a calculator to figure out your CR after plugging in all the
required measurements. Vizard and the Shop Manual give you the piston
dimensions, if they are stock. Determining the volume of your combustion
chamber is harder to explain than to do. You'll have to make some small
pieces of "equipment" out of a piece of plexiglas and come up with some sort
of graduated cylinder to measure liquid-- or a graduated syringe if you know
any junkies or lab techs. The best fluid to use, I've been told, is
antifreeze as it has less surface tension than light oil and gives you a
more accurate picture of volume. Look up "CC-ing cylinder head" in an
engine building book or on the internet.
Hope that is some help.
-JohnD
Date: Wed, 17 Mar 2004 11:59:48 -0500
From Jackson Zimmermann <JZIMMERM at albemarle.org>
Subject: RE: Compression Question & Calculating Camber Volume
I've got Vizard's book, V2 and V3, great reading as I pick it up about once
a week.
About the only issue that I have with either version of Vizard's book is the
compression stuff. In his table with compression and cam duration (I don't
have the page number handy), his reference with octane numbers does not
indicate if the base octane readings for gas is RON, MON or (MON+RON)/2.
There is no description of limited vs. unlimited valve sizes, exactly what
does unlimited constitute? Also, there is no mention of if this is
referring to street or race applications as typical rpm has an impact on
appropriate compression (he refers to this earlier in the chapter). Anyway,
I was hoping to find someone that was running 10:1, 11:1, or 12:1
compression on the street and see if they were happy. Vizard also never
mentions the impact on compression that heat barrier coatings may have.
Effective coatings should certainly allow for slightly higher compressions
to be used since they should inhibit transfer of heat to the intake charge
and minimize hot spots in the combustion chamber.
Maybe I should make compression rate a poll?
The primary difficulty that I have with the volume calculation pages is
that, under the influence of cough medicine last night, I couldn't quite
grasp it. I could probably figure it out on a better day, but after having
gotten the measurements last night and then taking them into the kitchen to
work out, I couldn't focus all that well. I also realized that I didn't
know what the base bore was to reference the chart listed on the volume
calculation pages.
Anyway, suggestions or help would be appreciated. My Sprite needs to get on
the road before Spring ends. Milling the head needs to take place before I
send everything off to be coated, so all this would add at least 3 weeks
before I can stick it back on the car and then start tuning the carb, etc...
Jackson Zimmermann
jzimmerm@albemarle.org
'64 Sprite (1275 & 5 speed)
|