Another engine theory Q - Blowers?

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Aviacs

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I'm aware of the energy value (efficiencies) of turbo prop systems. Especially for normalize HP maintenance high up.
This Q is not about that. :)

When converting small auto engines, we all tend to need high torque at lower rpm than original design rpm's for optimal HP range & peak torque in the original engine.

PSRU's are a common solution.
Especially if we hope for higher speeds yet good take-off and climb performance with longer props than the un-aided engine can swing.

So: now that small (relatively light) roots blowers are available somewhat economically,
Which is "harder" on a 4 stroke IC engine -
1.) gain HP & convert to torque by running the engine fast and reducing the output rpm with a PSRU?
or
2.) install a blower and run the engine at same rpm as above PSRU output, but with more available torque (More HP at same rpm)?

Both add heat to the base engine which has to be rejected.
Which system is theoretically (practical problems can be solved) "worse"?

smt
 

rv6ejguy

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The big problems with engine driven blowers is controlling boost efficiently and gearing them properly to work well over a wide range of altitudes, hence the popularity of turbos which makes that part easy.

Reg Clarke flew a DD turbo Sube for many years, worked well.

If you need a bearing housing to handle prop loads, that's halfway to a PSRU...
 

TFF

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Besides boost control being an issue, it’s not adding enough torque. You would want it to make double or triple torque of stock to run direct drive and a blower may add 15% more torque. Anyway, blower drive is essentially a light weight PSRU. You have to make a PSRU anyway, better to make it for the prop.
 

TiPi

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It is a bit more complicated than that :(
If you want to double the torque (power) at the same rpm, then you need to increase the intake air volume by a factor of 2 (boost pressure). If that is done at the same engine displacement, you will end up with roughly 2x the internal cylinder pressure during the combustion process. Lowering the CR is required to keep the dynamic CR (combustion space @ TDC / air volume in the cylinder at start of compression) within the fuel octane range. The standard head bolts & head gaskets will most likely not standup to this large pressure increase. Conrods, bearings, pistons, crankshaft etc might depending on the design criteria of the OEM (rpm, power levels).
An engine that is designed for X-hp at Y-rpm will still experience nearly 2x the combustion pressure to achieve that X-hp at 1/2 the Y-rpm.
The cooling side should be OK as the total hp is still the same and the internal efficiency is actually slightly better (same surface area, increased temps). Piston cooling might be an issue, though.
 

mcrae0104

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I'm aware of the energy value (efficiencies) of turbo prop systems. Especially for normalize HP maintenance high up.
This Q is not about that. :)

When converting small auto engines, we all tend to need high torque at lower rpm than original design rpm's for optimal HP range & peak torque in the original engine.

PSRU's are a common solution.
Especially if we hope for higher speeds yet good take-off and climb performance with longer props than the un-aided engine can swing.

So: now that small (relatively light) roots blowers are available somewhat economically,
Which is "harder" on a 4 stroke IC engine -
1.) gain HP & convert to torque by running the engine fast and reducing the output rpm with a PSRU?
or
2.) install a blower and run the engine at same rpm as above PSRU output, but with more available torque (More HP at same rpm)?

Both add heat to the base engine which has to be rejected.
Which system is theoretically (practical problems can be solved) "worse"?

smt
I can only answer your question theoretically, unlike @rv6ejguy & @cheapracer and others here, who have much more practical experience than I.

1) Running the engine at a higher RPM for higher power, but with an redrive for reasonable RPM, might be generally OK provided your engine has adequate heat rejection at that HP production rate. Take note of piston speed and other factors and the reasonable TBO may be marginally reduced vs. one spinning at a comparatively lower RPM.

2) Running the engine at airplane-firendly RPM, but increased MAP to produce more power, will increase internal cylinder pressure. The primary risk, then, is decreased detonation margin, and secondarily, reduced TBO.
 

Daleandee

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Years ago I read a very interesting article about HALT (Highly Accelerated Life Testing) of engines. The OP was asking about four stroke engine and the article referenced testing on two stroke engines but the answer in that scenario was that higher rpms and lower cylinder pressures would yeild much better longevity from an engine as compared to lower RPMs and higher cylinder pressures.

I've searched for the article but cannot find it now. But the final analysis was that it was better to "let it spin" rather than put into a situation where the engine was highly loaded.

The other answer is the old rule of, "no replacement for displacement" but that was back in the days of big block engines & fast cars!

FWIW ...
 

Martin W

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The other answer is the old rule of, "no replacement for displacement" but that was back in the days of big block engines & fast cars!

FWIW ...

Agreed ... thanks ... but even in modern day Lycoming and Continental still use that proven approach .... "lots of cubic inches" ... to produce high power and torque at low rpm required for an efficient prop speed ...

Our biggest obstacle has never been a lack of power .... it has been prop speed .... if a prop was efficient at 8000 rpm we would all be flying lightweight 2 strokes with direct drive .

......................

Aviacs .... just a side note for you to consider .... a turbo does not really add power ... what it mostly does is remove power losses because the engine has to suck in huge amounts of air through small valves and manifolds .... that is why the intake system operates in a high vacuum .

The turbo harnesses wasted energy from the exhaust to pump intake air so in that sense it is "free power" but boost cannot be very high because of detonation and other issues .

It can be done (by turbo or supercharger) without detonation issues .... dragsters do it all the time and can produce 10,000 hp from an automotive V8 engine .... but to control combustion and cooling they literally pour straight alcohol into the engine .... so much so that it is still burning when piston is at the bottom .... that is why the exhaust sounds like shotguns ... the mixture is still burning at very high pressures.
 

cheapracer

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Which is "harder" on a 4 stroke IC engine -
1.) gain HP & convert to torque by running the engine fast and reducing the output rpm with a PSRU?
or
2.) install a blower and run the engine at same rpm as above PSRU output, but with more available torque (More HP at same rpm)?

3.) The concept of the Rohr 2-175 was of a ducted fan, designed so that a common car engine could happily run at 4000 to 4500 rpms direct drive.


www.youtube.com/watch?v=Yg_8hDMIJtA
 
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cheapracer

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In 1974, Rohr Chairman Burt Raynes resolved to move Rohr into the light airplane market. Challenging the marketing base of the Wichita Giants would be an extraordinary step in itself. In order to succeed, Rohr had to offer a product so undeniably superior to its competition that prospective buyers and dealers could not resist it. Raynes summoned Walt Mooney and told him to come up with a quantum leap in light aircraft technology. It must have better performance, greater safety, accessibility and comfort, greater economy and lower production cost than any competitor. It must also, in Mr. Raynes' own words, "drive Cessna nuts." Walt was given an adequate budget and promised the services of any Rohr employee he needed. Within budget and on time, Mooney et al. built three airframes (two flying prototypes and a static test article) and two scale models (1/10 and 1/2) for water landing/takeoff feasibility tests (!). By the time the project ended (for reasons having nothing to do with the merits of the airplane), one prototype had accumulated 23 hours in the air.

Walt took full advantage of Burt Raynes' license to grab the best people Rohr had to offer. The key players on the project team were:

  • Walt Mooney, Designer and Project Manager
  • Bill Chana, Engineer and Project Administrator
  • Mike Voydisch, Propeller and Duct Design
  • Don Westergren, Test Pilot
  • Bob Fronius, Shop Foreman

2-175.gif

At first glance, the Rohr Two-175 looks like an exercise in novelty for its own sake. It is a low wing Delta of stressed skin, fiber-reinforced plastic (FRP) construction, propelled by a buried pusher engine driving a shrouded propeller. Its nosewheel fairing doubles as a "rhino rudder." Both the wing and the vertical tail fold for transport and storage. Seating is side by side; access to the cockpit is through huge polycarbonate (!!) panels that open in gullwing fashion. The single stick is mounted centrally, accessible to both pilot and co-pilot. The landing gear is fixed and rigid...no springs. Oh yes: the vertical tail is attached to the top of the propeller shroud. It is very hard to find a single conventional feature in the aircraft. In context, the bizarre features listed above seem not only sensible but ingenious. Take the Delta wing: the whole machine was intended to fit a standard FHA single-car garage, which limited span to 20 feet. Keeping the wing loading and structural loads within reason led to a deep root chord and sharp taper, and voilà! A Delta with folding outer panels. The vertical tail alternatives were a fuselage-mounted tail which would have to be huge to compensate for its shorter moment arm, or a small-area, high AR tail mounted on the prop shroud. The second choice reduced wetted area, so that's what Two-l75 got...with a hinge so it would fit that low-ceilinged FHA automobile hangar. But why composite construction? Surely that was a big technical risk in 1974. The point of that choice of manufacturing technique was to reduce the parts count and the number of manufacturing operations, which it did. The combination of adhesive bonding and molding major sub-assemblies in one piece kept parts count down to a miniscule fraction of that of a two-seat metal airplane, even counting each ply of laminate as a separate piece. Wing cores for the prototype were cut by the hot wire method familiar to builders of modern FRP homebuilt airplanes, but a faster and more elegant method had been worked out for production. The wing, which was the airplane's primary structure, would be built in clamshell molds. First, foam "sugar" would be placed in the mold. Live steam would then be injected to expand the polystyrene beads to the mold contour. The mold would be opened, the core temporarily withdrawn, and "B" stage fiberglass prepreg would be laid up on the inside surfaces of the mold. The core would then be reinserted and the mold would be closed, compressing the core slightly and ensuring even pressure on the laminate. Heat would then be applied in the usual manner to cure the assembly. The use of prepreg would avoid the weight gain problem of a wet layup and would prevent the laminate from becoming resin-starved. The process as a whole is simple, repeatable and cheap. Polycarbonate is both stronger and harder than Plexiglass, both very desirable features considering the huge "glass" area in the cockpit, but the conventional wisdom said it couldn't be molded. Bob Fronius and his team worked out a way and molded three sets. The landing gear needed only to be long enough for the 'plane to rotate without dragging its tail; prop clearance was not a problem. So why not have a fixed, unsprung gear, with the mains faired directly into the bottom wing surface and the fin-like nosegear fairing attached to the steerable nosewheel strut? Conventional wisdom said it wouldn't work. Of course it did.
Deltas need high static thrust for takeoff, when their high induced drag puts them at a disadvantage vis-à-vis conventional planes. Getting the necessary thrust with reasonable horsepower and prop diameter called for a shrouded prop turning at 4400 rpm, faster than any conventional reciprocating airplane engine's output. Fate intervened: Lycoming had two special high speed engines which had been built for a contract that didn't quite pan out; Lyc let Rohr have them cheap. These gave 150 horsepower at 4400 rpm. They had special cam profiles, but were otherwise conventional. That stroke of luck left the problem of cooling a buried aircooled engine for long periods at zero forward speed. A generously-sized dorsal scoop and an exhaust-driven ejector nozzle did the trick. In typical 108°F weather at the test site, the engine never overheated. The cooling system was in fact a bit too effective, and the inlet area would likely have been reduced or louvered if the program had continued. A short extension shaft drove a four bladed propeller. Mike Voydisch's prop, shroud and six-bladed stator gave a 1280 fpm climb with one man on board. Stator and blade profiles and pitch distributions are critical. The wing and fuselage underbody formed a "chassis" which was the major structure of the airplane; the rest (except for the duct and tail) was a fairing. Plugs for non-structural panels were carved out of body clay, Detroit style. Even the engine mount, though otherwise conventional, attached to the structural "pan" instead of the firewall bulkhead. Fatigue tests on the FRP structure showed that a structure that could resist a specified static load had an indefinite life under alternating loads. NASA spent a lot of money some years later to find out the same thing.

A few details of wing design: the outboard leading-edge droop/extension (Walt Mooney calls it a "dog tooth") was added to correct a pitch-up tendency at high angles of attack. The airfoil section is a symmetrical french curve special laid out by Walt according to exacting scientific criteria: if it looks right, use it. There is no spar, and the structural joint between the center-section and the outboard panel is made up of two FRP piano hinges adhesive-bonded to the stressed skin of the wing. The FRP hinges were developed for the Two-175 because of questions about the reliability of adhesive bonds to metal. Torsional loads were transmitted by trunconic bosses molded into the root of the outboard panel near the leading and trailing edges; these fit neatly into recesses in the center-section. Folding the wing was a matter of removing the lower hinge pin.

Don Westergren, the project's test pilot, noted that despite the machine's unorthodox layout it had a very normal feel. He got used to the machine quickly and felt comfortable with it, despite the fact that the wing was completely outside his field of vision, requiring the use of the instrument panel as an attitude reference. Only in landing did the Delta's special character demand special technique: approaches must be flown at constant attitude and the machine allowed to flare itself in ground effect; hauling back on the stick C-l50 style would have "buried" the tail. Don had a chance to test engine-out performance somewhat earlier than intended when a driveshaft coupling failed shortly after takeoff. Don 180'd and put the 'plane down on the runway without damage. The pivot point of the nosewheel fairing/rudder had to be moved forward early on. Otherwise the tests were uneventful and there was little down-time for modifications.

Early in the program somebody realized that the plane was capable of floating on its sealed, foam-filled wing. Tests with scale models showed that the airplane could take-off and land on water with the help of hydro-skis. This opened up the possibility of snow landings as well. With retractable skis, the Two-l75 would certainly have had the cleanest seaplane configuration ever seen.

So what happened? Rohr got into financial trouble with other projects and the Two was a victim of the ensuing corporate belt-tightening. Reusable equipment was salvaged from the airframes and they were destroyed, as was every speck of technical documentation. The plane lives on only in Walt's collection of color alides, a few 3-views and memory.
 

wsimpso1

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WOW. Some serious, if very interesting thread drift above.

Several things to remember about engine driven vs exhaust turbine driven superchargers:
  • The engine driven supercharger has to make all the power you want for output plus all of the power the compressor uses;
  • The exhaust turbine driven supercharger only has to make the output you want - the turbine runs on otherwise wasted exhaust gas stream;
  • Engine driven superchargers are either:
    • Fixed displacement and move some multiple of engine displacement at all speeds, thus at close to fixed pressure ratio all the time or;
    • Turbomachinery running at some multiple of engine speed all the time, and thus on a schedule of pressure ratio tied to rpm squared;
    • The result is that the targeted performance is generally available over relatively narrow ranges of rpm and atmospheric pressure.
  • Exhaust turbine driven superchargers run turbine and compressor on the same shaft, and the combined rpm floats to give manifold pressure you want. Properly size the compressor, make the turbine somewhat oversize, and run a wastegate to regulate boost and you can get whatever boost you want over a much wider range of engine rpm and ambient atmospheric pressure.
  • Engine driven superchargers have a few other issues:
    • They need to have a torque flow from the engine to the compressor, which is bulky and only has a couple ways it can work;
    • In airplanes they tend to need charge air cooling for durability - detonation must be kept at bay, and combustion/exhaust gas temps must stay in range;
    • Try to fit both under your cowl simultaneously.
  • Exhaust gas driven turbochargers can be run anyplace you can route both exhaust gas, charge air, and a pair of oil lines. Still bulky, but way more doable in most packages than a Roots supercharger.
Now these may just sound like details, but turbocharging OWNS on-road and off-road diesels that work for a living, marine applications from pleasure craft to some pretty serious working equipment, most high altitude and racing airplanes, and most types of auto racing. Yes, 2-stroke marine diesels still have a Roots supercharger, but that is for starting and low power operation, they usually turbocharge for boost.

So that is the short trip through why we see mostly exhaust turbine driven supercharging and not much engine driven supercharging. Now if you want to run nitromethane at near hydro-lock in engines and can live with power-on engine lives in the seconds, engine driven superchargers without air charge cooling is pretty much it.

Billski
 
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wsimpso1

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Besides boost control being an issue, it’s not adding enough torque. You would want it to make double or triple torque of stock to run direct drive and a blower may add 15% more torque. Anyway, blower drive is essentially a light weight PSRU. You have to make a PSRU anyway, better to make it for the prop.

I am not getting the limit on torque increase. You can get whatever volume ratio and pressure ratio you want. Just make the compressor bigger and/or drive it at higher speed ratio. The big limitation is how much additional torque and power the engine will take and how long it will run there. Some engines have sturdier bottom ends and better cooled engines and heads and will run fine at more boost than others.

Getting further into how long the engine will run fine at boost - this can be extended by cooling the charge air. The lower the temps at the intake valve, the lower the temps at the piston, exhaust valves, exhaust system, and turbine, and the longer they too will last. Subaru EJ257 is 150in^3 making 300 hp as a production road vehicle. Lots of other examples of durable high power per cubic inch road vehicles with turbos and charge cooling. Air-to-air coolers are most common...

Billski
 

TFF

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A 6-71 blower on my Triumph Spitfire is a pretty big mismatch. Usually questions like this means they want to get a blower off a Buick or Tbird coupe and slap it on a Honda and make magic. When the blower exceeds the prop drive in weight and size, it’s not a good match. Yes you can design it all to work, but really this is an off the shelf question. The power to drive a 6-71 on my Spitfire would probably net no more power as the engine is struggling to turn that big pump.
 

dwalker

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It seems like very linear thinking going on. My personal bend is always turbocharging as it gives the best performance for the weight, simplicity, and of course, efficiency.
However, if I were looking to supercharge an aircraft engine I would likely use a Miller Cycle supercharged engine.


 

rv6ejguy

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It is a bit more complicated than that :(
If you want to double the torque (power) at the same rpm, then you need to increase the intake air volume by a factor of 2 (boost pressure). If that is done at the same engine displacement, you will end up with roughly 2x the internal cylinder pressure during the combustion process. Lowering the CR is required to keep the dynamic CR (combustion space @ TDC / air volume in the cylinder at start of compression) within the fuel octane range. The standard head bolts & head gaskets will most likely not standup to this large pressure increase. Conrods, bearings, pistons, crankshaft etc might depending on the design criteria of the OEM (rpm, power levels).
An engine that is designed for X-hp at Y-rpm will still experience nearly 2x the combustion pressure to achieve that X-hp at 1/2 the Y-rpm.
The cooling side should be OK as the total hp is still the same and the internal efficiency is actually slightly better (same surface area, increased temps). Piston cooling might be an issue, though.

I've done it multiple times with dozens of turbo engines. Fuel octane and CR may be a concern in some cases but we can mitigate that to some degree with ignition retard through the ECU running through torque peak. Certainly running an auto engine on 100LL gives you a lot of latitude here.

I've increased output of some engines 5X the original output with stock block, crank, rods, head bolts, main cap bolts etc. for road racing engines which have to last many hours of flogging (the whole race season). Case in point was the 1702cc Toyota 2TC engines (1970s era 2 valve pushrod engine) I developed which won 4 championships here. Developed 360hp at 7700 rpm on only 15 psi boost running M85 fuel. Some engines are stronger than others from the factory of course. We identify weak areas and modify them if needed.

Look at any modern OEM turbo SI engine, most develop peak torque in the 1500-1750 rpm range and hold that value to as high as 5500 rpm. Some of these are running on crap 87 octane fuel too.
 
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Aviacs

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Most of you went off on tangents of your own interest, as tends to be typical for forums, & told me a lot of stuff that is not new info for me, though perhaps reminders are useful. (eidted: while slow-typing this, some addtional posts have swerved back more toward the original Q)

Only Daleandee answered the Q, and with reference to an actual study: "A revved engine is more durable than one pressurised to the same HP at a lower rpm." At least for 2 strokes in one study, but that seems useful. The answer is not intuitive. For AC the platforms do have to be same weight as well.

As noted, i'm not interested in turbo and the questions don't have to do with thermodynamic efficiency.
A direct drive flat 4 VW engine can only swing small diameter propellers at typical cruise rpms of 3300 to 3600 rpm.
Dropping cruise rpm to 3000 would allow usefully more efficient propeller length for better TO & initial climb, but the HP drops some 20% for cruise at that rpm (2900 - 3,000). Stock/non-boosted torque does not drop much in that range, so another (modest) 15 - 25% from boost could be impressive.

Looking at it from the other end, a PSRU to go from 3600 cruise rpm engine to 3000 rpm prop seems like weight and complexity for the gain. Also, only Marcotte, of unknown continued availability puts the propshaft aproximately inline so engine location need not be compromised significantly in terms of streamlining.

I like the reported cruise of my Sonerai 2 project in former owners' & guest pilots' hands.
I don't like the 52" prop and the "long" TO runs on hard pavement.
The engine has the HP & torque to do better; if the system were better.

IOW, choosing one-
controllable prop
PSRU
Mechanical boost - I don't see turbo adding much to TO without controllable prop.

smt
 
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rv6ejguy

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Years ago I read a very interesting article about HALT (Highly Accelerated Life Testing) of engines. The OP was asking about four stroke engine and the article referenced testing on two stroke engines but the answer in that scenario was that higher rpms and lower cylinder pressures would yeild much better longevity from an engine as compared to lower RPMs and higher cylinder pressures.

I've searched for the article but cannot find it now. But the final analysis was that it was better to "let it spin" rather than put into a situation where the engine was highly loaded.

The other answer is the old rule of, "no replacement for displacement" but that was back in the days of big block engines & fast cars!

FWIW ...

For sure on 4 stroke engines, higher MAP and lower rpm increases longevity.
 

rv6ejguy

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......................

Aviacs .... just a side note for you to consider .... a turbo does not really add power ... what it mostly does is remove power losses because the engine has to suck in huge amounts of air through small valves and manifolds .... that is why the intake system operates in a high vacuum .

The turbo harnesses wasted energy from the exhaust to pump intake air so in that sense it is "free power" but boost cannot be very high because of detonation and other issues .

Sorry, this is just wrong on every level. Are you saying that that turbocharging didn't add power to the BMW M12 F1 engines way back in the '80s which produced nearly 1000hp/L in qualifying trim? It did a lot more than relieve some breathing losses I think...

Some SI turbo engines are running up to 80 psi boost in drag racing today. Detonation doesn't seem to be an issue or they'd disintegrate every pass.
 

dwalker

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I've done it multiple times with dozens of turbo engines. Fuel octane and CR may be a concern in some cases but we can mitigate that to some degree with ignition retard through the ECU running through torque peak. Certainly running an auto engine on 100LL gives you a lot of latitude here

I've increased output of some engines 5X the original output with stock block, crank, rods, head bolts, main cap bolts etc. for road racing engines which have to last many hours of flogging (the whole race season). Case in point was the 1702cc Toyota 2TC engines I developed which won 4 championships here. Developed 360hp at 7700 rpm on only 15 psi boost running M85 fuel. Some engines are stronger than others from the factory of course. We identify weak areas and modify them if needed.

Look at any modern OEM turbo SI engine, most develop peak torque in the 1500-1750 rpm range and hold that value to as high as 5500 rpm. Some of these are running on crap 87 octane fuel too.
My experiences mirrors this. My personal street car with a 2.0 litre 4G63 made 500whp on pump gas (91 US octane). My 130hp 2.5 MZR made 600whp on e85 with stock engine, just the balance shaft assembly deleted, for a season of drifting. At the end of the year I replaced the rods, pistons, and camshafts and left the rest. Made the same 600whp with a broader powerband.

Once upon a time I actually hated turbocharging because the fuel and spark control was horrible. I remember tuning 930's to perfection on the CIS and a week later they would burn holes in pistons. That all changed in about 2001. We got away from Electramotive and Haltec and went to Motec, and I went to Motec USA to learn about EFI.Had some very fortunate opportunities to work with a variety of EMS, including GEMS, DTA, etc. In about 2003 I did a supercharged NSX with an AEM ECU and it was enough like the GEMS thatI picked it up pretty quickly but hated the fact that like many other PC tuned ECUs, the EEPROMs were vulnerable to various Microsoft quirks. Once they ironed that out, my shop put them in all sorts of things. I did a custom 5cyl Audi turbo with an AEM race ecu, did the beta work on several projects including most of the Series 2boxes and my Evo9 maps were and likely still are the basemaps you download from AEM.
I honestly cannot consider any reason to not use EFI on an aircraft,, especially a boosted aircraft.
 
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