>Im getting an urge to fiddle with my carbs, but have been advised it may
stunt my pistons.
>
>Specifically, @ 12:1, I have heard that there seems to be great risk of
erring on the lean side
>and perforating same.
>
>On the shallow face of it, it seems that an Oxygen sensor would be a
foolproof method of getting
>the mixture right, presumably one for each carb/exhaust combo would be
spiffing. However
>these sensors seem to be big and cannot stand leaded racing gas for long
and so would only be
>useful for an occasional test and tune session.
>
>On the other hand, fitting exhaust gas sensors could also be useful in
setting the mixture, with
> the advantage that the thermocouple is small and can be left in place and
you can keep your eye
>on it while racing. Presumably somewhere there is a good correlation chart
from temps to
>mixture strength to help me get it right.
>
>Your thoughts, agreements and denials, gentlemen please.
>(By the way, these are or course vintage oxygen sensors and vintage exhaust
gas temperature
>thermocouples I am referring to ;-).
>--
>Malcolm Cox, Napa, CA malcox@napanet.net
>Vintage Race MGA 1960, #80
>
Malcolm,
I used an exhaust temp gauge on my Formula Ford for several years. I first
put it on when I was running solo, and had a modified engine--12.5
compression and dual 48 mm Dellorto side drafts.
The EGT is good for tuning, and I would say that there is no "magic" number.
You need to start well on one side of the ideal mixture--prefereably rich!
Then as you lean it down, the EGT will go up. If it gets too lean, the EGT
will again go down.
With good forged pistons, you can run about 1400 deg. F. With the cast
pistons in the FF I used about 1250. It wasn't legal in FF to use Thermal
Barrier Coated (TBC) pistons, and it may not be within the spirit of vintage
(you decide), but TBC will definitely give you more margin for error. There
is some performance gain (more heat kept within the cylinder for expansion),
but I would consider it primarily for reliability. With that in mind, I
wouldn't object if someone used it in vintage, but I'm sure there are others
that are more purist in this regard. I have never used TBC, but as an
engineer, I consider piston crowns to be a marvelous application.
The best tuning I did with the Dellortos was in dedicated testing sessions
with a G-Analyst. It records to .01 G. Many people would probably tell you
that you can't run 48 mm dual carbs on a 1.6 liter engine. It can be done
and it can run fantastically well, but it takes work. I was fiddling with
chokes size (venturi), e-tubes, idle fuel jets, main fuel jets, main air
jets. The G-Analyst could detect small changes which I could not feel by
the seat of my pants.
But many small changes add up to a significant improvement. The end result
was very noticeable, but you don't get there in a quantum leap (at least I
didn't).
If you want best performance, you will be on the verge of running too lean.
Not there, but you are at risk if you run at different elevations, or the
barometric pressure changes. That means you must be willing to change
jetting almost every time you run. Once you get the jetting right, use an
air density gauge every time you run. Then makeing adjustments isn't by
guess work. The results can be VERY rewarding.
In vintage racing my FF, I never ran against anybody who seemed to have a
stronger engine. I built the short block myself, and reconditioned a used
Loyning head. I did takes pains to do the block right, and even bought a
deck plate. Local machine shops had them for Chevies but obviously weren't
willing to buy one for a one-off deal. I had all the bores sleeved so that
I could use standard size pistons (uprated FFs can't use oversize). My
engine was absolutely legal but ran with the best of them. I am convinced
it is because I worked at keeping it in ideal tune (at least within my
capabilities). By the way, I also used Total Seal two piece rings. They
are perfectly legal in FF. In an engine with little power like the FF,
every little bit helps.
Along those same lines, I used high octane racing fuel even though I was
limited to 9.3:1 compression. It allowed me to run more timing advance.
Perfectly legal, and again it increases the risk of meltdown. Most people
don't push the limits. If you are willing, you can have a legal, competitive
advantage.
Off my soapbox. My wantdering probably went in directions which weren't of
interest. My recommendation is to go for it! You may have read "The
Unfair Advantage" by Mark Donohue. The bottom line is that the unfair
advantage often goes to those who are willing to work for it.
Earl Zwickey
Amarillo, TX
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