PA-38 Tomahawk O-320 Installation
This is the one I did the engine change.
I shouldn't of sold it, it was so much
fun to fly
I bought my first Tomahawk, used, in 1981 for one of my boys to learn to fly. By 1984,
it was at TBO so while the O-235 was out for rebuild, I hung an O-320-E2D on it from a
1976 Cessna 172M. Along with a 74DM6-0-58 prop from a Cherokee 140. Had to change
the prop flange nuts as Cessna and Piper use different size bolts. And a 13" spinner/back
plate from a Warrior 151. The accessory case was changed for a fuel pump. The Oil Sump
was changed to an -E2G type as the Carburetor mount is at the back of the pan so the
PA-38 Air Box fits to the cowl without any modification.
If you can start with an -E2G from a Grumman Traveler, it will be much simpler but the
-E2D was readily available to me.
I used the same size Lord mounts as the O-235. I figured they would handle the extra
weight of 37 lbs. You can save about 10 lbs with one of the new light weight starters.
I used the O-235 exhaust pipes and muffler/heat exchanger. I think I'll use O-320 size mufflers
the next time. The O-235 set may of caused some much back pressure. I'll test it with both and
put the results here. Maybe I'll look into the new Tuned Pipes available for the C-172. A 13 to
15% performance improvement is claimed.
A set of upper air baffles from a C-172 can be cut down so there is very little fabrication needed.
Mostly to the Tomahawk front intake baffels to fit the larger cylinders. No changes to the Cowl.
The O-235 Oil Cooler is to small. I mounted a larger one in the same location but next time, I'll
mount it up front in the cowl as a C-172. The installation still ran hotter then I liked so a pair
of 4"x6" louvered vent plates were cut into the bottom of the cowl. This helped.
The engine I used was rather high time, 2100 hrs, with compression still in the mid to high 70's.
Static run up was 2350 rpm.
My DER told me to double the inboard flap hinges. I bought 'em but never got them installed.
Climb out of Van Nuys, 800'MSL, at 1530 lbs, 80 degree day, was 1100' FPM as compared to
620 FPM for the O-235 equipped plane. Cruise was 117 kts against 103 kts standard. VNO,
Max Structural Cruise is listed at 108 KCAS on the FAA Type Certification Data Sheet.
The lower compression O-320, 7:1, uses 80 octane. The O-235 is 8.5:1 and needs the plug fouling
100LL. I did about 100 hrs testing with 92 octane Mogas. All fuel hoses were changed to automotive
type to handle the MTBE additive used here in NV in the winter. I flew it at all altitudes up to
12,500 and in temperatures up to 115 (on the ground) with no problems.
The added 37# on the nose moved the empty CG to 72.4, right up to the front of the envelope and
stabilized the roll around the yaw axis.
I did all of this under an STC Application but stopped when the FAA told me I had to do spin testing.
I wanted to placard it 'No Spins' but that was not acceptable to them. I would of had to mount a
spin chute and a quick release door for their pilot to test fly it and I didn't want to go thru all of that.
So I filed a Form 337 to cover the changes and dropped the STC. I never did spin it but I'm sure it
would of had a much better recovery.
(More to be added)