Tracy Cook de-drive for sale cleaning out shop $2,500 obo.
Peter Morrison
2.85:1 or 2.17:1 ?
Paul Lamar
Being very much a beginner in this project, I have a couple of questions.
At the risk of demonstrating my ignorance:
1. Is 2:17 a good ratio for an RX-8 engine fit to an RV-4? Is this ratio
good for a 3-blade Catto prop?
2. Is this an early version of his reduction drive, and were there
significant changes to later ones?
3. Where is it located?
Thanks
David Walker
Ogden, UT
It depends on how much HP you expect to get from the rotary.
The amount of HP from the rotary increases directly with RPM.
The max RPM a rotary can generate depends on the gear ratio
of the gear box. A 2.17:1 gear box limits the max RPM to 6000 or 6500
depending on the diameter of the prop.
A 2.85 gets the RPM up to 7000 or 7500. The stock RX8 engine is
rated at 8500 RPM with a red-line at 9,000.
This is just for starters. There are lots of secondary factors as well
such as porting, intake runner length and prop pitch.
Paul Lamar
Not an expert, but here was my thinking when I decided to go with the 2.85-
My Viking had an IO 520 Cont with redline at 2950 swinging an 84 inch
McCauley. Take off performance was stellar, however you could hear me take
off from 2 miles away. I wanted the 2.85 so that I could get 7500 RPM for
takeoff and max HP, but keep the prop tip speeds lower for less noise. I
can always change the pitch of the prop, but it takes major changes to get
more HP from the engine. So with the 2.85 I start with max HP from the
engine, and a slower quieter prop.
As far as changes, Tracy did change the later units somewhat. He tells me
that the splines changed on the input shaft which means a different flywheel
adapter plate. The earlier adapter plates used neoprene bushings for
torsional dampening, the later plates do not have this feature and bolt
directly. The inside parts couldn't change much because the shafts are
splined to match the planet sets. He also tells me that the machine shop
that made the original input shafts is no longer in business, and that he
doesn't have the input shafts available as spare parts now.
Kevin Alderman
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