I wrote this over 20 years ago when I had a Precision STC'd Cheetah, and keep that time in mind regarding the prices:
If you want a Traveler or Cheetah with a 15% increase in climb rate, 1000-1500 foot higher service ceiling, and noticeably improved acceleration to take-off and climbout speeds, i.e., better short-field performance, I recommend the high compression (so-called “160 HP”) modification, which is basically just a piston change.
Now, speed is not significantly changed. If you run the same RPM as you did before in cruise, you'll have the same speed but 2-3% lower fuel consumption. These numbers are based on performance calculations and validated by five years of flight operations with a converted engine. Useful load is unchanged -- there is no gross weight increase, but there is no empty weight change, either. There are two reasons the gross weight doesn't change.
First, although the engine is putting out more power on takeoff and in climbs, there is no increase in official rated horsepower. If there were, the aircraft would have to go through the full certification performance tests, and the STC developer didn't want to go through the trouble and expense. The deal is that an engine must put out no less than rated power at rated RPM, and no more than 5% over rated power at rated RPM. The engine modification is merely a change in compression ratio from 7:1 to 8.5:1. If it were run at 2700 RPM, it would put out 160HP. At 2650 RPM, it puts out 157 HP, which is within 5% over 150 HP, so it's rated at 150 HP and no additional flight testing is required. Basically, at 2650 and below, you're getting the same power at the same RPM regardless of which engine you are using. As a result, you get identical performance at the same RPM in part-throttle operations (e.g., normal cruise) although you’ll be using slightly less fuel. The reason you get more performance out of the HC engine on takeoff and in climbs is that you’ll be turning about 100 RPM more than you used to in any given full-throttle situation (e.g., 2400 vice 2300 in climb at Vy – giving you about 7% more HP). However, the good news is that the Precision conversion is still rated at 150HP, and so is exempt from the recurring hollow crankshaft AD inspections.
David Fletcher also points out that: “If you compare a Tiger vs a Cheetah POH performance, look at the takeoff roll chart, the higher you go the bigger the spread you will see over 50% improvement in ground roll with only 20% more HP @ 8000ft 40 c. Its my experience that the 150 cheetah and even more so the older Travelers that won't turn a 59' prop over 2500 rpm quit climbing at 9000 ft when heavy, the HC cheetahs keep on going 13,000'. So at those altitudes there may be several 100% more climb. There is also the altitude fuel increase in efficiency, less weather, fewer fuel stops and in my opinion more then 15% improvement in X-country times.”
Second, the gross weight on the Cheetah is limited by spar strength, not performance. The Tiger has a slightly beefier center spar section, allowing the higher 2400 lb gross weight over the Cheetah/Traveler's 2200 lb. Further, any attempt to increase gross weight on the HC-modified Cheetah would also require the full set of flight performance testing -- which would entail a much higher cost to the STC holder in both dollars and effort -- and a reduced maneuver envelope, such as eliminating utility category operations.
The HC STC’d installation can be run on autogas, but only 91 octane, and it requires an STC available from Petersen (Route 1, Box 18, Minden NE 68959) for $160. If you previously had a Petersen Mogas STC and install the Bill Scott High Compression STC, the old 87 octane autogas STC is invalidated, and you’ll have to get the 91 octane STC from Petersen for the full price (no “upgrade” of the existing autogas STC) before you run autogas through the upgraded engine. Mr. Petersen says that, to the best of his knowledge, few FBOs who pump auto fuel offer 91 octane. Even so, he told me that if keep an eye on the ratios, a mix of 1/3 100LL avgas and 2/3 87 unleaded autogas yields an acceptable octane for the HC-modified engine.
It can be performed by changing the pistons and wrist pins, or as part of a top or major overhaul. I'm told a piston swap costs like $1500-2000 for parts and labor unless the cylinders have the original factory stock cylindrical bore, in which case the reboring turns it into a top overhaul (with associated cost increase). When done as part of an overhaul, there is no cost delta over the overhaul price beyond the $500 price for the STC paperwork (parts cost the same). I most strongly recommend this to anyone with a Traveler/Cheetah if they're getting either a top or major overhaul. The cost delta is insignificant when you're spending $10-12,000 for a major or $5-6,000 for a top. The difference in cost between getting staying with the stock 7:1 compression setup and upgrading to the 8.5:1 high compression system is $500 for the STC from either Fletchair (1-800-FA-WINGS/www.fletchair.com) or Bill Scott (Precision Engines, Owensboro KY, 502-684-1083, precisionengine@mindspring.com). There is no cost difference on cylinder exchange or overhaul going from the existing cylindrical bore to the HC bore. Also, if you're interested in either an overhaul exchange or sending your engine out for overhaul, Precision has a very high reputation amongst Grumman owners, but you pay for what you get -- his prices are a little higher than most. You might also ask him about oil system flow improvements; he has some interesting ideas, but they cost extra.
Finally, Fletchair now has available a new Sensenich prop that allows you to take better advantage of the HC power. It increases pitch, getting you more speed, but does so without losing any of your newly-gained climb performance. The prop runs about $2700 and comes from Fletchair with the STC to mount it. Your old McCauley prop is likely worth something on the homebuilders market.
