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In a message dated 9/15/2003 9:34:53 AM Eastern Daylight Time,
13brv3@bellsouth.net writes:
> I'm starting to believe that I could just retard the timing
> a few degrees, and make sure I'm slightly on the rich side with fuel, and
> let-er rip to 6-7 psi. Maybe I'm making too much of this 2-3 psi that
Bruce
> suggested as a maximum. Gotta think about this some more.
>
As I said before I no not about turbos. I have been up to my eyes in
Mazdas all day, as this is the first day of the SCCA national championship race
week. The fastest Mazda is 8th on the grid so far. We get three more qualifying
sessions before the race. My car was 20th of 29 cars today. The Toyotas have
more power and are lighter. I got to talk to Daryl Drummond the Mazda engine
builder for a while. Got the word on sagging oil pressure. I feel much better.
Some observations about turbo engines. The intake and exhaust manifolds are
not works of art. Nothing you could get away with on a NA engine. So some value
of boost (intake manifold above ambient in pounds) is used just to get back
to what you would have had from the same size NA engine. Turbo engines have
lower compression rotors than NA engines. So here again, some of the boost is
used to make up for the lost compression. So, before the boost comes in, the
turbo engine is a dog.
Here is why they work.
When we raced a Fiat, I angle cut the heads until I was touching both valve
seats with the cutter. Then I relieved the head so that the pistons just
cleared with the head gasket missing. So, we would have about 245 pounds of
compression while
cranking. Now nobody should run that much compression and expect to make one
lap. But it runs just fine, because static or calculated compression ratio
doesn't mean squat. One dare not load the engine at low speed at all. Once
moving, you don't dare lug the engine below say 6,000 RPM. So it is a pain to move
the car to the false grid, and to get it going for the parade lap. But once up
to speed and the revs kept in the power band, there is music. So the 15:1
compression must run no advance at all right? No, 35 degrees all the time. Crank
triggered for accuracy. A similar pickup in the distributor with the advance
weight range widened to go from 5 degrees at idle to 35 degrees wide open.
The answer is that the effective compression goes down as revs increase,
because cylinder filling time is reduced per cycle. So at speed we don't have 16:1
we have maybe 10:1 depending on how much inlet tract restriction the rules
stipulate. So what is the effective compression ratio of my competition with the
same engine but who installs 10:1 pistons? Maybe 8:1?
So to start with, the turbo starts with lower compression so that you can
turn the screw in and make boost without hurting the engine. Then you turn the
screw in some more to make up for the intake with no tuned lengths. Then you
turn the screw in some more to make up for the dreadful restrictive exhaust
header and turbine. Then you turn the screw in some more because this pig is
supposed to be stronger than the NA engine. Then you turn the screw in just a bit
more to account for the extra 47 pounds the turbo adds to the plane. So how much
boost is that exactly?
I have no Idea. A guess might be 8 to 10 pounds. I would watch the EGT and
keep it rich enough to always be 100 degrees rich of peak EGT. Add a knock
sensor. I would split the timing and add a retard system for the highest boost. I
would over cool the oil. I would buy the ceramic seals for boost over 10
pounds. I would hire a turbo expert before I melted something expensive.
So the answer is cylinder filling at high cyclic rates. The cylinder filling
of a well done NA engine at sea level can be replicated at any RPM and any
altitude with the turbo.
Lynn E. Hanover
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