[Fot] SU oil/refurbishing race suit?

[Duncan Charlton]

The piston not only controls velocity by restricting the size of the
opening below; it also controls mixture by raising the tapered needle.
Moving a car to a higher elevation suggests that the engine would be
happier with a leaner mixture to run properly and I can see how removing
the damping effect could improve that.  Would lowering the needle jets have
done the same thing?  How about using a softer spring above the piston to
counter the lower atmospheric pressure?  There's another factor on racing
engines, too, and I'm not sure how to estimate its effects -- a long
duration cam would raise manifold pressure (or reduce manifold vacuum; pick
your nomenclature).

I recall reading a tip in one of Kas' tuning books to remove the pistons
(or the rings from the pistons, I can't recall which).  Since reading that
I've wondered how they tuned the carbs to cope with coming out of slow
corners that required the engine to be in the low end of the rev range,
below the fat part of the torque curve.  Did they run a very rich needle
and jet combination?  Did they strictly use OD gearboxes to keep the revs
up in those situations?

The following may be the opposite case to a naturally aspirated race engine
run at high elevation: I had a supercharged 1275cc Mini (supercharger was
downstream of an SU HIF44 carb) that bogged significantly when the throttle
was snapped open (I'm at 300' elevation).  The PO had been dutifully using
SU brand dashpot oil and that oil appears to have low viscosity, so it's
perhaps 10 weight.  After experimentation I found that 40 weight ND engine
oil really woke up the engine.

Duncan

On Tue, Nov 24, 2020 at 6:01 AM John Styduhar via Fot <fot at autox.team.net>
wrote:

> I found the opposite response with my CD175 Strombergs.  If the piston
> rises too quickly the venturi effect is reduced and adequate fuel is not
> drawn through the jet causing a lean mixture.  I use marvel mystery oil.
>
>
> On Tue, Nov 24, 2020 at 3:19 AM Rocky Entriken via Fot <fot at autox.team.net>
> wrote:
>
>> Re Oil:
>> I once did a race at Woody Creek Raceway, Aspen, CO (8000 feet) and could
>> not get the car to rev over 5000 rpm. A friend diagnosed the problem ...
>> got out his wire cutters and snipped the posts off the rods in the SU caps.
>> Never had a problem again!
>>
>> He about gave me a heart attack when he did it. Thing is, the purpose of
>> the oil in the SUs is to retard the piston going up and down, mostly up. It
>> was explained to me, you don’t WANT to retard the carb piston on a race
>> engine. You hit the gas you want it WOT. RIGHT NOW! Street car, sure. Race
>> car, no.
>>
>> But a tip: Clean the inside of the carb, piston & cylinder, regularly.
>> Before each race. Brakleen. Fuel residue will gum it up if you don’t.
>>
>> --Rocky
>>
>> *From:* Henry Giffin via Fot
>> *Sent:* Tuesday, November 24, 2020 12:58 AM
>> *To:* fot at autox.team.net ; fot-bounces at autox.team.net
>> *Subject:* [Fot] SU oil/refurbishing race suit?
>>
>>
>> FOTs, I have two questions for the knowledgeable ones.
>>
>>     Do you use light oil in your SUs on your racing engines?  I have
>> heard both sides of this.  I have not been using any oil, but have heard
>> from friends I should be.
>>
>>      A number of years ago when my son-in-law started racing we sent my
>> old racing suit to someone to be refurbished.  It came back in great
>> condition.  My current suit could use some work.  We lost the name of the
>> company who did the work.  Can anyone recommend a source for refurbishing
>> my suit?
>>
>>      Thanks, Hank
>>
>>
>>
>> Hank Giffin
>>
>> 757-375-1491
>>
>>
>>
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