I've tried lots of things in my Pitts and formed lots of opinions about stuff. These are not universally shared opinions and there are different mods with lots of ongoing discussion about their effectiveness. I've tried my best to capture an overview of all the mods I am aware of, their cost, and my impression of their cost/benefit ("ROI"). Please discuss your planned mods with as many people as possible. That's the beauty of owning a Pitts; there's someone out there who has tried everything! If you are an owner/pilot with an opposing opinion on anything here, please let me know so I can capture your comments.
Flat Bottom Fuselage
Reason: aestetic
Difficulty: 9
ROI: negligible
Some akro competitors make the argument that since the Pitts has no straight fuselage lines, it gets inherently lower grades. Thus was born the idea of straightening the lower fuselage ‘bathtub’ curve into a straight line. Unfortunately that requires creative re-routing of the elevator pushrod. Admittedly it looks cool, but I would only do it during new construction or a major rebuild.
Slave struts behind the I-struts
Reason: aerodynamics
Difficulty: 6
ROI: negligible
The advantages are that with the slave strut in trail with the I-strut, there is theoretically less drag. The disadvantage (other than the work required) is that on the rear wing spar there is a plywood doubler at the I-strut. In order for the slave strut and control horn to clear that doubler, the control horn needs to be shorter (which adds load to the slave strut) or the aileron chord needs to be shortened to accommodate it (reducing effectiveness). Tough call, I don’t know anyone who’s able to claim a significant speed advantage. On the other hand, I still take mine off each year when I go to Reno (and tape up the top ailerons).
Slave struts in the I-struts
Reason: aerodynamics (and let’s face it aestetics)
Difficulty: 9
ROI: negligible
OK, you’re a masochistic crazy person. Doing this requires the invention and construction of a set of bellcranks that would make a watch-maker jealous. Most of these attempts end up in sloppy controls due to the number of connections needed. Norman in Stockton is one person who has done it well. Falcon takes an intermediate approach where in-trail slave-struts are faired in behind the i-struts, giving the appearance of being inside. Again, the speed advantage is imperceptable.
Dihedral removal
Reason: Inverted stability
Difficulty: 7
ROI: unknown
This is pretty commonly found in “hot” Pitts. The idea is that if you remove all dihedral from the lower wing (3.5 degrees on the stock S1S) then it is more stable inverted. You might also put together an argument that bigger interplane-distance means less induced drag. It looks cooler also. You'll need new I struts and new landing wires, probably costing you $4K total. The disadvantage (other than cost) is that lowered ground clearance (especially with spades) can become an issue in botched or big-crosswind landings.
Extra flying/landing wire
Reason: Offsetting aileron torsional loads
Difficulty: 7
ROI: 7
This is a mod that is worth consideration for S1’s with large ailerons installed. The idea is that aileron deflection, particularly at high speeds is causing torsional loads on the wing that are absorbed by the wing structure. An added set of flying/;landing wires attached to the rear spar mitigate these stresses by transferring them to the fuselage. In some ways these wires can also help rigging and provide a redundant load path. Not cheap though, and it adds weight and drag. You make the call. I did not add them to my airplane, and I have not suffered any ill effects.
Drag/Anti-drag wire washer reinforcement
Reason: structural reinforcement
Difficulty: 2
ROI: 9
If you have your wings uncovered, it doesn’t hurt to remove your drag and anti-drag wires for close inspection of the blocks and washers. These are a frequent point of damage. If any of your washers are dished (rock them on a smooth surface to check), then you need something stronger. Get some 4130 steel plate and cut pieces to match the face of the blocks. Sand and revarnish the blocks flat if necessary, checking carefully for compression cracks. Drill, paint, rig, install, done.
Playing with the size and shape of spades
Reason: control feel
Difficulty: 2
ROI: 7
If you have any sense that your control forces are too high or too low, then you may want to play with spades. Also, adjusting the spade mounts with washers are the easiest way to cure slight roll trim problems. Spades are a neat invention but aerodynamically inefficient. Any basic Aero course will tell you that a flat plate stalls at around 15 degrees, and induced drag just keeps climbing above that. So the average aileron fully-deflected at 23 degrees gives you a non-linear stick force and adds a good amount of drag. This is one reason that 30% hinge point ailerons (aka Super-Stinker) are now popular. Leo Loudenslager solved this problem on his Laser with a beautifully executed all-flying full-balanced carbon fibre spade (I recommend NOT experimenting with this due to flutter risk). You can just see it in this picture of Leo inverted. Another improvement is the delta-shape which can tolerate higher stall aoa. Also check out the Extra; the longitudinal bend adds rigidity, but also helps eliminate adverse yaw (oooh those Germans are sharp cookies!). Rigging spades is another place where the digital level comes in handy. Neutral rigging should be an angle between the chord line and parallel to the underlying wing surface.
Square Wingtips
Reason: aesthetic, simpler construction, roll rate, speed?
Difficulty: 6
ROI: 7
Most of the “hot” biplanes out there use square tips. They show better for competition. One argument is that with reduced (barely) span, and non-tapered ailerons result in noticable roll-rate improvements. There is also an issue of speed. Although elliptical wingtips are theoretically lower drag, I know of at least a couple airplanes that exhibited equal or better speed after square tip mods.
For new construction, there’s no question that square tips makes life easier. Aesthetically I think they make the aircraft look more modern. Folks like Sean Tucker certainly think so. It is fairly easy to modify existing wings into square tip wings be constructing a full size rib to replace the partial rib near the tip. Make sure the tip is structurally adequate or the fabric will bow the tip rib inward. Wider cap strips,a plywood doubler, and internal bracing are all recommended. And don’t forget you’ll have to update your ailerons too. Aha! There's your excuse to build those extended 360 deg-per-sec hot boards!
Truss Harmonic Protection
Reason: structural reinforcement
Difficulty: 5
ROI: 8
If you read your tech tips manuals, you came across a thread about broken fuselage diagonal members in the S1 Pitts. The tube underneath and behind the pilot is the longest tube in the airframe, and thus resonates at the lowest frequencies, often near the prop frequency. Hence the problem. To change the resonant frequency, some owners used Adel clamps to tie in a link to the longeron. Others changed tube diameter or welded a crossing diagonal member. I believe it’s only a problem with lighter weight S1C tubing, but you should read the article to be sure.
Spring Landing Gear
Reason: drag, weight, looks
Difficulty: 10
ROI: 7
The belief that a spring gear is lighter and less draggy is a bit of an unsubstantiated rumor. If done wrong, it can easily be heavier, more draggy, and cause structural problems vis-a-vis the conventional bungee gear. In terms of drag, propeller slipstream tends to cause airflow separation off the left gear leg that can carry down your fuselage and make your stabilizer look 3 inches thick. Mandatory modifications to accommodate a spring gear are: beefed up fuselage (longeron doubler, added truss member, radius plates), root cuffs, new brake lines, and possibly an H-stab strut brace. I think they look great, but don’t underestimate the work required. If you don’t do it now, you’ll do it later when your longerons crack or break.
Composite Cowl and Spinner
Reason: drag, looks
Difficulty: 7
ROI: 7
Aftermarket cowlings are
available from Doug Dodge (used frequently on the One Design) and from
Streamline Welding (aka Ultimate Aircraft). The ultimate stuff looks better with it’s 15 inch spinner, but requires that you change your firewall and the surrounding sheet metal. These are almost always heavier than sheet metal cowls, but they make up for weight with less drag. Ambitions ownsers may scratch build a custom cowl by packing the engine with 2-part foam and cutting it to shape to make a plug. Got 6 months?
Whoever has seen Sean Tucker's cowl will have noticed the annular inlet cowl design. This is a custom job that has been applied to experimental Pitts' for several years by the infamous builder Steve Wolf in Oregon. A very clean design with a nice plenum and side exhaust venting.
Canopies
Reason: drag, looks, egress
Difficulty: 4-7
ROI: 4
Owners almost always toy with the idea of modifying the canopy arrangement. Changes can vary from adding a latch to full blown single piece composite-framed canopies. My problem initially was that everytime started a dogfight or akro sequence, my stock canopy would start to creep backwards backwards. I'd be holding the canopy with one hand, the stick with the other, pushin the throttle with my knee, etc. So eventually I tossed out all that cable mechanism for a simple overcenter latch in the center of the canopy. It is good to think about how someone on the outside (eg firefighter) might get in if it's locked. I cut off the screw heads on my overcenter latch and put a stiff clevis wire through that could be pulled out. In terms of composite framed canopies, the Aerophile guys have a good blow-by-blow on how to make a composite framed canopy. The trick with the Pitts is how do you get it open?? The top wing blocks a lot of conventional designs. So composite framed Pitts canopies generally come completely off (lay it on the ground to get in).
Changing seat angle
Reason: comfort
Difficulty: 8
ROI: 9
For those of you having a hard time with the uncomfortable upright position of the S1C type fuselage, there are ways to solve the problem if you’re willing to do welding. And you shouldn't have to stretch the fuselage. The general approach is to replace the crossmember holding the top of your seat with something that arc’s back a few inches. C&H Aircraft Restoration (479)848-3991 in Arkansas has done this if you want external help. Another way to provide more room is to make sure your seat-pan is all the way down to the torque tube. I have a nice contoured wood seat that gets my butt right down there. If you do this, there is typically a CG impact that should be looked at before making the change. Also, be cognizant of Reno Race rules calling for a maximum seat setback of 20 degrees.
Sloping Turtledeck
Reason: Dogfighting visibility, looks
Difficulty: 5
ROI: 6
The sloping turtledeck 'leans' the face of the turtledeck rearward so that you can look over your shoulder easier. Provides slight visibility improvement and typically accompanies new canopy designs.
Squaring the tailfeathers
Reason: looks
Difficulty: 7
ROI: subjective
Although I have no experience with it, there is plenty of precedent for providing a more squared-off look to the tailfeathers, especially if a "large rudder" has been added. Sean Tucker's plane (careful, that's no longer a Pitts), and Jim Leroy's Bulldog are good examples. IMHO that mod joins the category of "only worth doing while recovering".
Turtledeck Window
Reason: Tail inspection
Difficulty: 2
ROI: 10
Not enough owners check their tailcone for debris on a regular basis. The window sure helps. Cut a triangular or round shape piece od 1/8" lexan and attach it with 1 bid fiberglass or screws.
Propellers
Reason: performance, crank longevity, CG effects
Difficulty: 2
ROI: ?
ROI is hard to figure because of the cost involved. You might be able to pay $800 for a used prop that gives you 10 mph ar Reno. On the other hand you can pay $15K for a constant speed that does nothing but screw up your CG, increase empty weight, and crack your crank. Actually, going from fixed pitch to constant speed can be great, but note most airplanes must be engineered from scratch to accommodate it due to the potentially significant changes in CG. New CS props such as the Whirlwind 200c and Hartzell 'Claw' are getting lighter and lighter, so who knows what may happen. Other options are wood and composite. Wood props are fine on 200HP motors and greatly reduce gyroscopic stresses. I personally know of Warnke and Hoffman fixed pitch props flying on S1's in unlimited IAC. Carbon is awesome in it's ability to provide metal prop performance with wood-type weight. However, carbon requires expensive mold work up front, so it's been prohibitively expensive. Steve Hill at Owens Composites in NM is working on it though!
H-stab carrythrough
Reason: tail integrity
Difficulty: 2
ROI: 9
Verify that the horizontal carrythrough on your airplane meets the latest Aviat specs. Tech-tips manuals has an article on this. I believe there was an AD many years ago that was a result of Hstab cracking at the carrythrough. To some extend, the Hstab strut reduces the loading here as well. Aviat carries this part.
Hooker Harness
Reason: more accurate competition maneuvers
Difficulty: 2
ROI: 7
If you fly at any level above sportsman, a ratchet is the way to go. Although Hooker is the original developer of the idea, you can also get them from my friend Alan Silver. The idea is to eliminate all ass-play during akro. Ass-play results in either poor control or bloody hip-bones from moving around against your belt under negative G. Also, try tightening a conventional belt while inverted someday.
Wood or composite leading edges
Reason: stiffer wing structure
Difficulty: 8
ROI: 6
After I experienced airframe flutter, wood leading edges was one of the things we did to change the flexibility of the wings. By converting the leading edge bay into a structural D-section, you substantially stiffen the wing. Other benefits include a lack of nails to pull out and no metal seam at the center section to cause the fabric to bunch up. On the flip side, One credible expert I talked to thought that solid leading edges transfers load to the ribs and contributes to rib shear. In other words, more load is carried to the ribs behind the spar causing broken ribs (traditionally a sparcraft problem). So far so good. Construction is typically aircraft plywood glued to the nose ribs using T88 or similar. Dummy nose ribs are also not a bad idea to prevent any visible scalloping. Roger used solid foam, hotwired it to shape, then added a kevlar wrap. I don't think there's a way to make a structural D-section (wood or composite) that's lighter than 0.016 sheet metal. You might have a shot with pre-fabbed carbon skins but I don't know anyone who's done that.
Seat construction
Reason: headroom
Difficulty: ?
ROI: ?
There are ways to redesign your seat to get your butt closer to the torque tube, thus improving headroom. they way they did it in my airplane sure looks like a pain in the ass (so to speak).
Extended Ailerons
Reason: roll performance
Difficulty: 7
ROI: 7
This is one of the most popular mods for unlimited-level Pitts. It involves all-new ailerons, hinges, and typically slave struts. Not to mention the fact that it's hard to do without really cutting into wing fabric. Clearly, however, you can roughly double your roll-rate to around 360 deg/second. Prevailing consensus is that the new super-stinker style ailerons with 30% hinge points are the way to go since they minimize the need for spade.
To my knowledge, noone today is shipping aileron kits for the S1 so you may be on your own for construction. But there are plenty of comparable references out there: Eagle plans, Kinner guys, etc. Hold the press, The Devin York (Devin.york@sympatico.ca) who is taking over the Ultimate business says this:
"we can sell you a set of plans for the entire Utimate Wing Mod for the S-1. The plans also show the wrap around spar fittings which allow the wing to go to +8.5 Gs plus the Ailerons and aluminum "machined" hinge blocks which are a lot nicer than the Falcon wings even. The ailerons in the wing are FULL span and all layouts are given. The true fully symetrical ribs dwgs are given also plus a really cool dwg of the whole assembly in 3 dim. Roll rate with these wings goes to 360 deg/sec for the S-1 where a typical S-1S is around 180 deg/sec."
Square trailing edge
Reason: Control response
Difficulty: 5
ROI: 4
Normally this accompanies the construction of extended ailerons. To uncover a wing/aileron just to add square edges is sheer insanity. The best way to describe their effect is that they make the aileron feel like it has a bit more chord. That's because the last little bit of a pointy aileron provides very little marginal force due to airflow separation. The square edge eliminates separation. Because the 'added chord' is right at the trailing edge, the secondary effect is a significant increase in stick pressure, which is usually met with larger spades (and more drag). Regarding the TE on the rest of the wing, I personally am convinced that it is a waste of time to mess with it.
New wheelpants
Reason: weight, aerodynamics
Difficulty: 3-8 depending if you build/buy
ROI: 6
The wheelpants you have on your airplane now may be heavy and draggy. A nice vacuum bagged pant will save a couple pounts in total weight. Also, attention to airfoil will make a drag difference. If you fly off grass/dirt you'll have other considerations, but people who care about speed should care about pants. Anecdotally, I've heard that a good set of pants can add 10+ mph in certain situations. Make them yourself using a plug/mold technique like I did or buy a set pre-made. You'll spend several hundred dollars for top notch carbon pants. Here's my story on building a set from scratch...
Lamb tires
Reason: weight, aerodynamics
Difficulty: 1
ROI: 6
Lambs (now sold under the Cheng Shin brand) seemed a little squirelly to me at first. but jillions of high speed landings by Lancairs and Vari-ezes have proven their durability and worth. You can buy these things for $30, save a pound on each side, and mount them on standard Clevelands 5" wheels. Considerations to think about are
a) They lower your 3-point attitude a bit
b) They use 60+ psi so a bit of shock absorbtion is removed.
c) You might need new wheelpants to maintain ground clearance.
d) Average FBOs will not carry spares, so think about carrying around a spare tube on cross-countries.
e) Better not do any rough field stuff.
10:1 Pistons
Reason: HP
Difficulty: 1
ROI: 8
Get these pistons from Lycon for better HP. Highly thought of by the people I know who use them. Listed as low difficulty, because you'll probably just have Lycon or your mechanic take care of it (all it takes is $$ bub!).
Exhaust
Reason: HP
Difficulty: 3
ROI: 8
Different exhaust schemes provide arguable performance improvements. Clearly there is a very old understanding that tuning for peak HP/torque involves optimization of the exhaust system. The white paper written by the CAFE Foundation is the best reference doc for 0-360 exhaust performance.
Cold Air Induction
Reason: HP
Difficulty: 6
ROI: 4
"Cold air induction" systems replace the conventional intake spider arrangement in fuel-injected Lycomings with a system that keeps the spider outside the sump. This minimizes heat transfer to the spider from the oil-sump. Sky Dynamics has the only off-the-shelf system that I'm aware of although several enterprising owners have built their own from scratch. The Skydynamics includes sump and intake runners. One gripe is that incoming airflow is required to make a >90 degree turn from the injector body up to the cylinders, thus thought to create turbulence.
Lightweight Alternator
Reason: weight
Difficulty: 5
ROI: 8
Bill Bainbridge at B&C has a cool alternator (a dynamo actually) that provides just enough juice for charging the battery and running a radio/transponder. Perfect for the S1! Well thought of, reliable. Mounts on the vacuum pad, thus requires the use of a Lycoming adapter. B&C can supply this, plus a version with an inverted oil pickup. Upgrade your stock alternator with their SD-8 8A alternator. If you really care about weight, you'll run with no alternator and charge between flights.
For true minimalist power generation, it occurred to me that a good setup would be to use a Peltier-effect thermoelectric generator. If you could figure out a way to mount the device, it could be good for 1-4 amps! Let me know if you figured it out because I want to do it.
Lightweight starter
Reason: Weight
Difficulty: 3
ROI: 7
A couple of vendors make lightweight experimental starters (B&C) and Skytech among them. These are contilevered gear reduction starters with good torque-per-amp and are derived from automotive starters. They bolt right on to the usual case mounting. Just remember to count the teeth on your ring gear before placing your order.
Electronic Ignition
Reason: HP, efficiency
Difficulty: 5
ROI: 7
Thanks to the homebuilders industry for driving development of things like digital avionics and now electronic ignition. These systems continually adjust ignition timing for peak power and/or efficiency while eliminating the risk of detonation. Finally we catch up to the automotive world. Klaus Savier's Lightspeed Engineering has one setup, but there are others. Definitely improves performance, but they ain't cheap. Now legal for Reno Biplane class.
Turbo
Reason: HP
Difficulty: 10
ROI: 3
Good ole Jack Steinfelt used to compete in Unlimited with a real unique S1. He'd drop into the box with that thing spooled up and draw a 5000 ft vertical it seemed like. Anyway, an immensely complex project for an already crowded cowl. You don't own a Pitts for FL200 cross country. A turbo would do fun things but ultimately just cause your engine to fall apart (IMHO). Stay away. ugh.
Advanced Mag Timing
Reason: HP
Difficulty: 2
ROI: 5
Theoretically, 'bumping' your timing forward allows you to increase power at higher RPM under *certain* conditions (e.g. racing at Reno where the air is thin and hot). Try this at sea level and you might fry your engine. Try this on a high-compression motor and it might melt down. Use caution, or use an electronic ignition system that does this without the risk.
Intake Dongle Thingy
Reason: HP
Difficulty: 3
ROI: 7
A fairly simple way to boost your manifold pressure just a bit is to extend your intake out to near the props trailing edge. Even Hartzell got in to the act with a certificated CS prop with a trailing edge tab (heck it must work!). I'm pretty sure this only applies to fuel injected engines with the forward facing injector body. generally used for racing but not for everyday akro and fun flying. Maybe it's to make up for something else that's too small, I dunno. Anyways, I made one up out of carbon fibre for the 2003 races. It impressed everyone, but did not measurably improve RPM. If I put in my manifold pressure sensor on my Rocky Mtn, I'll let you know if it makes a measurable MP difference.
Smoke System
Reason: Fun factor
Difficulty: 6
ROI: 7
Yeah it's fun to borrow someone's plane with smoke and go make clouds, then hand the plane back and have the owner clean it out. If you are a competition. I welded a couple tabs in my fuselage to accommodate a smoke setup, and I'd still like to have the option, but I've never gotten around to it ($$$). Generally, the components are: pump, switch, check valve, manifold, hoses, and injectors. Selecting the right pump and getting the perfect injectors seems to be an ongoing argument. Aftermarket automotive fuel pumps (Holly, Mallory, Walbro) seem to work fine. The injector is generally a stainless fitting welded or hose-clamped onto your header about 4 inches from the head. Although 2 pipes will do the trip, getting oil to all four is probably recommended.
Fuel Header Tank
Reason: weight
Difficulty: 7
ROI: 6
The basic benefit of adding a header tank under your main tank is that it allows you to fly akro on less gas. That is to say, the engine won't starve when you're flopping around. These are usually 3.5 gallon tanks, so you do get a little extra capacity. Haigh made one, Ultimate's tank had a built in header, I'm sure Al Evans in AZ could weld you one to your specs (great outfit BTW). Even though I had one of the Ultimate tanks, I switched back to stock setup. Here's the questions youz needs to asks yourself. Am I really flying such hard akro that I need to fly on 4 gallons of gas? Have I ever heard of anyone having problems with the stock tank and floppy? Do I really want to add to the empty weight of the airplane? Do I really want to add to the complexity of the cockpit? Is there something else I'd rather spend $1000 on?
Internal Antennas
Reason: looks
Difficulty: 4
ROI: 7
You can't put an antenna in the fuselage cage, but if you have a wood turtledeck, you can put it there. I used Jim Wier's copper foil antenna taped to my turtledeck. It works great. My Pitts also used to have a dipole in the wing (attached to the rear face of the front spar). I never did use it, but it should have worked ok. The drag wires would have created some interference, but not much. Since comm sould be vertically polarized, that would have been an issue. Putting GPS antennas in turtledeck or in the top wing is a great approach. No, as far as research has shown, the silver (aluminum) base coat in your dope does not appear to affect reception.
Breakaway Pitot Tube
Reason: reduce damage risk
Difficulty: 4
ROI: 7
Next time you recover your wing, think about relocating your pitot tube from the stock leading-edge location to an under-wing location. This reduces the chance of hangar rash. The next trick (which I'm proud of) is that I mounted a nylaflow fitting flush with the lower wing surface. The pitot tube is an aluminum tube bent 90 degrees that screws right into the fitting. If you knock it from the side, it will give, but airflow won't budge it. Keep in mind that the pitot system works on stagnation point static pressure. No air flows through the system, so the pitot port diameter doesn't need to be a certain size (3/16" OD aluminum tube is fine).
Synthetic Oil
Reason: engine longevity, reduced temps
Difficulty: 1
ROI: 7
In general, synthetic oil is great stuff. Much better shear film strength,
better resistance to breakdown, ability to handle higher temps, etc. I would recommend Synthetic, for example, in airplanes that regularly bust the traiditional 240 deg oil temp redline. Synthetics are perfectly happy well above those temperatures. You may end up with CHT problems, but at least the oil is happy.
When I use Redline, I can count on a 5-10 degree immediate reduction in oil temps. In fact, sometimes it can be difficult getting the synthetic oil up to appropriate temperatures. However, there are a lot of independent variables that tweak the results. For example, I tried using the same viscosity (20W-50) of a different brand and CHTs went right to redline before I made pattern altitude. This was probably a function of oil viscosity and pressure (see my discussion of oil temp/press), but the point is you should consider yourself a test pilot.
Another characteristic of synthetics is that they do not keep ferrous engine components nicely coated with slime when the airplane sits. Synthetics all run off and into the sump, leading to the risk of rust and premature wear. Although I wasn't there, I understand that was a big factor in well publicized aviation synthetics a few years back. Now if you couple that with the oil's tendency not to get above 212 deg where water/etc is burned off, you're asking for engine damage. Therefore, do not use synthetics if you fly infrequently and/or in humid climates.
If you talk to the well respected Lycon guys, they say they're not convinced and have seen issues. So the jury is out. Because of it's cost (~$8/quart), I generally run Aeroshell during the year (especially since they give us a free case at Reno). Read TBO advisor article on oils and you will opt for straight 50W.
Oil Cooler changes
Reason: reduced oil temps
Difficulty: 4
ROI: 6
High oil temps are a common problem. Root causes may be related to engine clearances, bad regulator/vernitherms, leaky cooler bypass valve, high oil pressure, poor cooler airflow, etc. You should experiment with the cheap/free variables before spending on a new cooler and installation. See my discussion on the inverse relationship between pressure and temperature.
I went to more rows on my cooler and saw a marginal reduction in temps. Some have added second coolers (in series, NOT parallel). Since I saw someone blow-up an Earl's cooler at Reno, I've got second thoughts about automotive racing types. On the other hand, they do claim to be pressure tested so perhaps that was an anomaly. Regarding mounting location, putting it on the baffle behind #4 seems to be simplest. Urban legend has it that that air will be preheated due to running over the cylinders but I proved that hypothesis false by doing some digital thermometer testing (I guarantee that air at the #4 baffle is equal to ambient temp). The next most common location is to take air off a front baffle and duct it to a remote cooler, or even mount the cooler up there directly. Mounting the cooler in the cowl (eg Christen Eagle) is a nice arrangement, but cowl installation becomes more difficult, and you'll probably have to think about structural implications. NACA scoops are cool, but they are finicky about local pressure gradient (I'd rather take it off the #4 baffle). Other interesting approaches are to put coolers in the wing or fuselage. That may simplify your engine compartment, but forces you to run long pressure lines through the cockpit, plus the obvious wing construction work.
Teflon hoses
Reason: fuel system integrity
Difficulty: 2
ROI: 9
I took my aeroquip lines off the airplane for a couple months during maintenance and I couldn't get them back on. "My airplane stretched 2 inches!" I cried. Those rubber hoses frag with age, get brittle, and shrink. Teflon has much better heat resistance, and stands up to avgas much better than stock hoses. You can make them yourself using Earls stuff. WM Engineering is a good place to shop online.