2.0 port size
 

Howdy all:
So after much cogitating and advice by this board, I am going to keep my 914-6 (69T) motor at 2L, but uprate to 'S' or 906 where useful and add MFI.

That said, the heads will get twin-plugged (I should finish the fixture this week over lunches). So the two and ten variants have 9.8:1 CRs. twin plugs rule of thumb seems to be another point so is 10.5:1 livable on 91 octane premium? The 'Porsche 911 Story' has power curves for the early 911S and it states that 98 ROZ fuel was used. I know RON and MON, but not ROZ. The car will be mostly a street driver, so I don't want to get into fuel additives.

This brings me to another question nexis:
The 2.0L Ts have 42 mm intakes and 38 mm exhausts, already 'early' (901-02) S spec. Is there a worth while gain to 'upgrade' to 906 spec (45/39 - more likely just the intake since the exhaust is only a single mm different). I realize that 901-10 S specs only picked up 10hp on top given the same cams/ports but I am curious as to the improvement in the mid range but haven't seen anything to answer this.

The big question is how bad the stock 32 mm ports are going to hurt. Both the 901 two and ten variants used 36/35 mm ports (the 906 was ginormous at 38/38). While I have them on the bridgeport I could use an indexable boring tool to open up the 'straight part of the port' then blend into the curves. It would be work, but what isnt.

Cams will most likely be S spec as 906 would be insane for street driving and I would like to keep the peak power around 7K for longevity.

Any thoughts or real world experience would be most appreciated for this newbie.

Cheers,

tadd

Interesting questions; I can't wait to hear from the experts. From my research, you should look at the "Mod-S" or "DC40" cams. They are a modern profile based on the original 'S' cam. Supposedly more robust all around than the original 'S'.

First off -- have you read this thread?

Here's an updated chart to give you a few ideas of what the flow numbers look like.
http://forums.pelicanparts.com/uploa...1139346959.jpg
(Disclaimer: Note that the numbers offset to the right are averages of the numbers on either side. I did this to smooth the chart and provide equal data points with the 2.4TK example.)

A couple of observations:
1) Valve size makes a big difference across the board, but especially at lower lifts. Note the flow numbers for the '66 style head.
2) Port diameter has a bigger influence at higher lifts. If you use an "S" or 906 camshaft in a T head, you'll get the peak RPM benefit and off-cam roughness of the extra duration, but not the extra flow from the higher lift.
3) You need to also keep your eye on the flow rate through the port. 911's seem to like a peak torque gas speed of about 70 meter/second in the intake ports, with most of the examples ranged between 55 m/s and 80 m/s. If the gas speed gets too high (over 100 m/s at peak HP RPM), you'll find that the engine will lose peak HP (compared to it's potential) as the intake flow is choked off at high RPMs. On the other hand if the ports are too small, the engine won't be as happy as it could be at low RPM's. Combine that with a radical cam (like a 906) and you'll have reversion issues, fuel dropping out of suspension and other issues.

All that being said, compared to their cylinder capacity, 2.0's seem to have fairly generous flow capacity. Porsche used a "T" engine with S cams in a 914-6 rally engine and it doesn't seem to have really chocked off the engine that much. The peak torque gas speed was 72 m/s and the peak HP gas speed was 94 m/s. The peak HP and peak torque engine speeds were pretty much the same as the '69 2.0S engine. The peak HP and torque numbers were slightly higher, but that may be also due to the use of a sport or rally muffler.

BTW: What's a "two and ten"???

lazy naming
 

jluetjen:
'two and ten' were/are 2 of the many variants of 911 motor. Bruce Anderson has a nice table of most of the 911 configs with a few specs. Its on page 100. He calls it a 'type#'.

I take your points on port size, but (and this maybe very wrong) MFI kinda screws up the typical picture. This is obvious stuff, but I need to visualize it so please bear with me...

Ignoring intake harmonics, eventually the air column in the intake since it has mass will be moving fast enough it won't care that intake valve has closed in its face. The air at the top of the intake will just keep rushing on in for a mild supercharging effect. Now the bigger the port, the faster the airspeed needs to be inorder for this to occur. You were kind enough to provide real world numbers, nice! Eventually flow goes to pot as the gas gets going so fast you have shear problems. This is a nice simple quandry. Now toss in the fuel droplets and trouble of keeping a suspension of a liquid fuel and air... hilarity ensues. This is one of the big bennifits of using propane for a fuel. It is already a gas, so we get to go back to the simple picture. This is where the assumption comes in: won't MFI let the intake behave almost like a pure gas system since it is injected almost on top of the intake valve? In addition the injector pressure is so huge the droplets are already tiny.

Nice call on the 914-6 rally car (type 901-26). I totally missed that one on the list. 180hp @ 6800 rpm same ports and valves as the T. Now how can that be when the 69 911S (901-10) only made 170hp with bigger valves-ports and the same cam/CR on carbs no less???

This is why I post. Books only go so far...

tadd

Re: lazy naming
 

I am preplexed by this as well. BAs book says the CR is .1 pts higher and the RPM peak was 200 revs higher...for 20HP over the carbed 'S' (160 HP). I do believe this car was running a lighter rotational mass (non-C/W crank) so there might have been some efficiencies gained there. But 20HP? Makes me wonder if that car wan't sneakily running some 10.3:1 pistons...

Interesting cam thread
 

Thanks for the link.

Tadd

Re: lazy naming
 

Would this help?
http://forums.pelicanparts.com/uploa...1139358432.jpg
Basically, it appears that MFI engine's perform better then carbs "off-cam" since they are less suseptable to reversion -- thus giving better flexibility. They also out-perform carbs at higher rev's due to the fact that they're not restricted by any venturis. In the middle of the rev range it appears that they perform pretty much the same.

Quote:

Nice call on the 914-6 rally car (type 901-26). I totally missed that one on the list. 180hp @ 6800 rpm same ports and valves as the T. Now how can that be when the 69 911S (901-10) only made 170hp with bigger valves-ports and the same cam/CR on carbs no less???

I suspect that it was a noisy 2-in-2-out exhaust. When combined with higher overlap cams, you can get significantly improved exhaust scavenging and a stronger tuning affect. Remember that the 911S was using a stock 2-in-1-out.

I don't like where this is going...
 

jluetjen:
Thanks for the graph. I was actually taking a hard look at that on another thread. The MFI 906 motor really caught my eye... That curve is darn flat 'down to' ~4.5k AND it is 20 units (psi?) above the 'S'. It would really have to drop like a rock, like 60(!) units, to fall below the 'T' which is still an 'acceptable' around town motor (heck, try a stock, older 912 - then you will know slow).

I had just assumed 906 cams would been a really dumb, dumb, stupid idea for a street motor, but depending on how fast that curve falls (grows?)... You don't HAVE to rev it out to 8k and whittle away its life, but it makes everything thing else on the graph look dumb. Becides, my numbers stop at 7K on my tach anyway!

Maybe Porsche just didn't care (about those values) cause it was a race car?

Any clue as to what happens below 4K for the 906 car??? If you don't know, I am damn sure gonna have to find out.

:D Thanks for making my life more difficult. :D


tadd

There are more modern cams than the 906. Ping Camgrinder.

Re: I don't like where this is going...
 

In the case of the carb'd 906, I don't think that they were able to get a clean full-throttle dyno run starting below 4500 RPM because of reversion issues. MFI is less prone to this since as you pointed out the fuel is injected at the back of the intake valve, so more if it makes it into the cylinder rather then spitting out the top of the stacks. So I suspect that is why reversion is less of an issue for MFI'd engines -- but it is still an issue since the charge will still be contaminated by exhaust gasses flowing back into the cylinder. I've got a guy near me with a factory ST which uses 906 cams and MFI and he's told me that at low rev's the motor runs fine , but it doesn't pull very hard until past ~4000 RPM. At partial throttle it's OK.

I agree, there are more "modern" cams then the 906, such as the GE or DC80. I believe that in general the GE80 profile opens and closes the valves faster, so it will act like a longer duration cam without necessarily increasing the overlap by much. The trade-off is that you need to use "competition" valve springs with higher pressures then the stock 911 valve springs which can be used with 906 cams. This increases the wear and stresses on the rockers and valve system as well as increases the overall friction of the motor.

But that same technology can be applied to more modestly profiled cams like the "Solex" or S camshafts, which brings us to profiles like the "Mod-S", GE40 and GE60 camshafts. Give "camgrinder" a call and I'm sure that he can walk you through the options.

Great info, thanks guys.

I have a 914-6 "mystery" engine that was built (by a pre-PO) with some S parts, but I don't know what if anything was done to the heads. Not actually positive about the barrels, pistons or cams either, except that it doesn't run like a "T", is jetted as an S and has an MFI drive on the cam.

I'm not in a hurry to tear it down to find out what it really is or make any changes, but if there is a way to determine more of the specs without a teardown that would be fantastic. If I do pull the heads I'll post what was done just for another data point.

Progress!
 

Well, even though I have no decison on ports, I am sure enough that I am going to leave the valves the way they are and I am going to go shopping for a 906 cam.

Yes, I know the magic rule of cams is to undercam, but... its a 'fun motor' that I drive to work and run around town in. I figure I have three things going for me. First, CR - that will be as much as I can get on premium which would seem to be around 10.5(3?):1. With twin-plugs and the physically small combustion chamber (I have read that the 2.2 heads are a big improbment in chamber angle to resist detonation, but I have never seen anyone say the 2.2 was better then the 2.0 heads once twin-plugged). Second, I can always tweek the cam timing if need be to push the torque curve down a bit more. Finally, the smaller valves will also push the torque curve down a bit. I think what really nailed trying the 906 cam was jluetjen's last post about the 911ST. I mean, whats the worst that can happen? I have to take a long weekend, pop the motor from the car, and drop back to S cams.

I am going to shy away from 'modern' profiles for now, just because I like the 'softer' design of the older cams from a longevity aspect.

The only thing left in up in the air is port size. I am really leaning towards opening them up to the full 38/38 of the 906, but the more consertive 36/35 of the 901-10 might be a nicer choice.

For the MFI people: how is the pump attached to the case? Are there special bungs cast in or is there a mount that picks up stock points? I am really wanting to put the pump somewhere else other than the stock location. My current favorite right now is just to the right of the fan where the factory put the air conditioning pump. That way you can get to the darn thing. I figure all it will take is a mounting plate and a 2:1 pully set. There are enough accessories that use toothed (gilmer) belts that I should be able to find something that I can bolt to the crank nose and use with the stock MFI pulley.

I am going to dump the heat requirement of the cold start circuit and go with either a cable or an electric switch. That should shorten things a bit. I haven't reached a decision about dumping the 'stop solenoid. My 76 scout II has 250k miles on it and it only needs work because a few of the springs in the hydrolic lifters have finally given up the ghost, not because of a manual enrichment circuit. Becides the heat in the car sucks (considering how long it takes to warm up - I figure I will just go with this: http://www.warmseats.com/) and I may dump the heater boxed exhaust for headers anyway. Speaking of heat, has anyone put a heater coil (like a block heater) in their oil tank? I saw this on a 908 in 2001 at an HSR race meet to bring the bulk of the oil up to temp before they fired the engine. I remembered it this morning (cause it was damn cold, 14*F) and it seems like a cheap wall timer and an extension cord might be worth the time savings of having to baby the engine for 10 or so minutes.

Ok, coredump over. Cheers, all.

tadd

Last but

Todd;
Much as I love MFI, considering your application -- why go to the trouble? You'll be able to get comparable performance just by using a mapped EFI system. It would most likely be a lot easier to install and tune then trying to come up with a custom location for the MFI pump as you're describing.

I have to admit that moving the MFI pump and going to all of the trouble to engineer that solution seems like an excercise in foolishness. If your rationale is to save yourself time in the future by making the MFI pump easier to get to, are you REALLY saving any net time when it is going to take you gobs of effort to machine the specialized adaptors to make this set up work? Why would you do this to yourself? I agree that if you aren't looking for originality, EFI is cheaper, simpler and downright better.

Relocated MFI
 

I have a mega-squirt Version II board that has taken me almost two years to complete and it is headed towards my RZ350.

As for the MFI and the relocation... well ultimately I would like to use MFI for liquid propane injection - 104 octane with wicked charge cooling from the latent heat of vaporization, you get money back as a tax credit, I don't support the middle east, and maybe I get to drive my car for reasonable money in a few years when gas is really expensive. I 'think' it can be done since the injection pressure of 250 psi is well over what the natural vapor pressure would be for 'most' temperatures (110*F is 197psi). So as long as the injection pressure is greater than the vapor pressure you know that you are injecting only liquid and fueling is golden. This can be done with EFI, but it takes special injectors that are hard, if not impossible, to find. The folks playing with liquid EFI are typically using EFI diesel injectors or are companies making their own special injectors. When liquid propane EFI is done it usually just requires smaller injectors to correct for the density difference (plus what ever remapping is required). SO.... this, in my mind means being able to make your own 'space cams' (corrector bobbins??). I have also toyed with adding a rpm controlled valve to change from pumping injectors in the head to injectors in the top of the venturis. As well as fun things I haven't even thought about yet.

This is really another post... I haven't really held one in my hand yet, so this is all speculation from staring a pictures, but because of where the feeler wheel is located (and how its attached determines its direction of motion) the 'correction data' should be 'encoded' on the outside of the bobbin (since the bobbin is on the shaft, I assumes it spins at pump speed). One can also assume that since the correction cam adjusts the mixture for all the pumps, the information can only be encoded along the rotational axis. In other words, the correction cam is not elipsoidal (egg shaped). The encoding is mearly changes in radius on a cylinder with the feeler wheel rolling along the axis responding to the radii changes as it travels. This would only encode throttle angle as rpm is encoded by the centripital regulator. Two degrees of freedom provide the third (pump adjustment) giving a 3D fuel map like on on page 37 of the "Porsche 911 Story". Or thats my best guess until I dig into the S MFI pump I just bought...

tadd

This would make it a breeze to create a cam with a simple 2D lathe.

So if you know your current AF ratio, and you know the shape of the current bobbin along with how much the pump stroke is corrected you know how to shape the new bobbin.

On a side note, you are putting an MS-II on a 2 stroke motorcycle? NICE. I am planning a similar system on my CB450.

motorcycle EFI
 

Kenikh:

Its really the only system that I could find that does an actual 12k at the crank. Most EFI are designed for cam speed and for a popcorn popper, 8k at the crank just doesn't cut it.

tadd

Re: Relocated MFI
 

http://www.pelicanparts.com/support/smileys/shake.gif Right.

Bring it on!
 

jluetjen:
The delay on the mega-squirt was giving it to a buddy who offered to put it together for me, that was one year. Then I got knee deep in dissertation writing while working as a post-doc, and well, nothing much happened. Oh, yea there was a house bought in there too... I am better equipped for time now. *snicker*

As for making my own space cams, I humbly ask, am I wrong as to the operation of the metering unit? As I stated, I am just guessing till I get inside one and see how it moves.

If I am out in la la land, I am nothing if not willing to learn! Ignorance is only cured when those who know share.

Not to put too fine a point on it, but there really seems to be only one running adjustment - the main rack. It is my current understanding that, in a nutshell, every corretive system just pushes or pulls on the rack adding or deleting fuel by shorting or lenghting the pump stroke. Fuel mapped to rpm is adjusted by the centrifgual govenor while the space cam adjusts the pump output for throttle position. So if you know how much the rack is moved though the feeler arm, then you can add or delete fuel as desired by chainging the thickness of the cylinder at 'X' lenght units (throttle positon) from the 'starting' position of the feeler arm.

Color me naive, but if you have an alpha-N map of your current lambda (I would assume you can get this from normal driving by data logging an oxygen sensor then dumping the data into a matrix of throttle position and rpm - rpm is logged from the tac signal while a pot would be needed for throttle position), and you know how much pump stroke is added or subtracted for a given space cam radius, then you can make a good guess as to what a new space cam should look like.

Somebody had to build the thing in the first place AND they did it in the 60s, slide rules and all.

tadd

Actually Tadd;
My roll-eyes had more to do with your extravigant plans to modify MFI to run your car on propane. The challenges are formidable. But you seem to be smart enough to figure them out between your post-doc work, fitting out your house and everything else going on. It's better you then I!

hobbies?!
 

jluetjen:
The propane thing is a pretty long term goal and as for now is only a guess as to if it will work. Propane just has so many advantages it's siren call is hard to ignore...

Propane was used years ago as an industrial refridgerant and those compressors were the older piston style so that classic 'lubricty' argument is bogus in my mind.

I have a buddy back east that does the 60-70s american car thing and he says everyone is switching there big block monsters to propane so they can actually run the CR that the cars came with orginally AND they get to be waved thru emissions inspections (the 70s folks).

Hopefully this summer I will be able to start on a 'racer for the street' roller to put my 'creations' into. I do like the look of the early 70s RS and RSRs.

Everyone has to have a hobby, right?

tadd

Breakfast MFI-cam timing tought
 

So another question... One day I may run out . :)

Does anyone know how may degrees of rotation a single MFI pump squirts over?

The reason I ask, to tie back to the start of the thread, is assuming the pump injection requires less degrees of rotation than the intake is open, one could 'slot' the attachment holes of the pully on the pump (like the aftermarket cam pullies on a ricer) so that the pump would only inject after the exhaust has closed or is darn close to closed (stealing a page from the two-stroke diesel people). This would eleminate fuel shortcutting the exhaust or reversion, thereby increasing gas mileage, power, and emissions all at once.

I guess what I am getting at is that it would let you get away with a much more overlap than carbs or even EFI would.

While searching about non-counterweighted cranks, I found a very appro thread entitled '911T head flow rates' (funny the stuff you find when not looking for it). jluetjen made a really nice post of flow vs lift for several early heads (they take out the pics at work along with other things) and I just wanted to revisit the valve size question. jluetjen made mention that valve size is more important at lower lifts, but boy does that graph drive it home. I was thinking about leaving the stock 42/38 combo and spend my time enlarging the ports, but since the 906 cam does so well on top, it seems like anything I can do to 'fill in' below 4k is a good thing to do (assuming the above question about the MFI pump is true and I don't dump half my fuel out the exhaust).

Which leads me to assume that upping the intake to the full 45 mm of the 906 would be an excellent thing to do. I would think that the 1mm increase in the exhaust would be just regrinding of the seat. I would assume that going beyond that to the 46/40 combo of the 2.2-2.7 heads would not even fit? I am having a hard time even seeing a downside to bigger valves especially with the softer profile of porsches cams.

tadd

I don't know the answer to your MFI question. But people have played around with the MFI injection time (by indexing the pump by a tooth on the belt) and apparently it made little if any difference in the performance of the engine.

More subtle effects?
 

Man you are fast!

I figured adjusting the MFI pump timing wouldn't yield direct HP, but I was more concerned with avoiding what creates the 'holes' from using long duration and carbs in the first place.

I just figure these engines have been around so long that there really isn't anything new under the sun and all you have to do is ask!

tadd

Reversion has less to do with the injection timing and everything to do with overlap. Basically, as you rev the engine it goes through phases of being "in tune" and "out of tune". "In Tune" means that there's a low pressure wave at the exhaust port during overlap that pulls the intake charge into the cylinder. "Out of tune" means that there's a positive pressure pulse at the exhaust port during overlap that does a few things:
1) It pushes exhaust gasses back into the chamber and even the intake port -- thus contaminating the incoming charge
2) The pressure pulses in the intake mess up the carb since the vacuum that it depends on to operate fluctuates. In extreme cases the fuel gets ejected out of the carb mouths.

The difference between these two phases is about 1100-1200 RPM (to give you a sense of magnitude. The actual number can vary with intake and exhaust designs.)

Once the engine is "on-cam", it doesn't really matter since the air's momentum in the intake track overruns any reversion issues. But just below when an engine comes "on cam" -- say around 4000-4500 RPM, the pulse affect is strongest and the inertia isn't strong enough to overcome it. This is when you'll have problems with a 906 cam and carbs. MFI helps by ensureing that the fuel doesn't fly back out of the carbs.

ugh...
 

jluetjen:

I understand what you have said, but I don't follow the disconnect between reversion and injector timing - color me slow.

If I inject the fuel after the exhaust valve has closed and only while the intake is open, how can there be reversion?

My understanding (which may be broken):

With only 'air' in the system during overlap there isn't any fuel to blow out the intake stacks during overlap (since we havent put it in yet). I just assumed that once the exhaust valve closes that reversion is not possible since the off time pressure wave from the exhaust just smacks into the closed valve. I can see having some dilution 'off cam' of the 'air' by exhaust though. I would guess since the cam and the pump are 1:1 ratio pullies, that moving the timing a whole cog (the pullies are small) that is too big of a timing change. What are there about 8-12 teeth?

tadd

Reversion can happen even if fuel isn't being spit back from the carbs. That's just the most extreme case when that happens.

1) Even if the exhaust valve is closed, reversion may have contaminated the cylinder charge with exhaust prior to it's closing.

2) Reversion also may have pressurized the chamber prior to the exhaust valve closing, so no air (and the fuel that it's carrying in suspension) may be flowing into the cylinder from the intake track until the crank has rotated X number of degrees. The inertia of the charge will be lost. So even if the injector is spraying, the air it is spraying into may not be going anywhere.

Like I said, MFI is significantly less prone to reversion issues then carbs, but it is not immune. There will still be a bit of a flat spot at that point of the rev range where reversion is occuring rather then an all-out hole like you'd have with carbs.

got it.
 

Ok, got it. I think we were on the same page but that I am not communicating correctly. My bad.

Do you know if the 906 used iron cylinders? JE makes a nice looking 906 piston clone that I think I am going to go with and I am curious if I should dig way deep in the wallet for LN engineerings 'nickies'. I may just be missing the data, but the two porsche books I have, "the 911 story" and BAs book don't spell that out.

tadd

MFI porn
 

Ok, if I upload a pic to the pelican site what is the address that I need to put within the image tag? I cant seem to find that little tidbit. THe help says it should just show up. No dice for me. :mad:

Well, I was kinda bored tonight, so I popped the end off of the 2.0L S MFI pump I got off of evilbay. So I was wrong about how the space cam works. It is egg shaped and it does not spin at the pump crank speed. The feeler wheel is circled in yellow. It is linked to the red screw which is the end of the main rack that controls the pump stoke. The space cam is 'circled in green while the centripital governer feeler is in blue. The big dudes on the right side are the flyweights.



So I was right about two systems acting on the main rack independantly. The spacecam has a spring behind it so when the flyweights move they allow the centipital feeler to back away from the spacecam. This lets the spacecam move along the axis of rotation for the flyweights and hence 'read' rpm (N) while rotation of the throttle rotates the cam (white arrow) around the spinning axis. This gives the the feeler an XY translation over the surface of the spacecam (bobbin). Unlike my previous assessment, this would be a tricky part to make in my limited (read manual) mill-lathe experience. I am going to have to ping a few machinists to see how this would have been made. The surface finish is too nice to have been cast.

You can upload the pic directly into the site and it will do it for you.

BTW, the 2.0S pump is the best made. Good choice!

posting pics
 

Kenikh:
I tried that. Must be something with safari. Said it downloaded just fine. Oh, well. I'll toss it on another server.

tadd

Re: got it.
 

I'm pretty sure that they were aluminum with a special dimpled surface on the bores. on the bores.

http://k43.pbase.com/u21/9146gt/larg...0mmPhoto10.jpg

http://forums.pelicanparts.com/uploa...1139949481.jpg

http://forums.pelicanparts.com/uploa...1139949666.jpg

Download the "MFI Repair Manual" from the MFI section of the Pelican Web site, it is an invaluable tool for the MFI hacker (and will help you get that S pump back together).

Already there!
 

John:
Downloaded that before I even bought a pump. I just wish it was of better quality. The images are horrible.

tadd

Dynamic CR and detoniation relationship
 

So Waynes book arrived from amazon today and I got to read thru the performace options section.

He makes reference that 911s are more prone to detoniation at lower revs and therefore with long duration cams you can get away with a higher CR since the overlap 'vents' some of that at low speeds. Makes sense.

What I don't follow is why the engine should be less prone to ping at higher engine speeds. The only thing I can think of, is even with the 911s 'lazy' lack of swirl heads, things are happening at such speed at 6-8k that you get pretty good turbulance even if the head isn't set up for such.

tadd

I had thought on the way to work today. Why not essentially engineer reversion out of the equation? Since propane is 104 octane, you don't need twin plugs...thus why not use the extra spot on the head for a direct injection port for liquid propane? You could inject the fuel directly into the combustion chamber at the exact instant you need it instead of worrying about those pesky valves affecting your mixture.

Thats what the Orbital folks do.
 

Kenikh:
The Orbital company was working on a direct inject two stroke a few years back. Havent seen anything lately. They like Bimota (on the tesi) put there injectors in the cylinder wall so that they avoid directly exposing the injector to the combustion process.

Detroit on there two-stoke diesels inject directly into the combustion chamber (no pre-chamber) and there injectors (becides being both the pump and injector combined) are directly cooled by the cooling water. There is a thin metal 'thimble' that is pressed into the head that the injector fits into so that it can dump as much heat as possible.

Being air cooled, I just don't see that being feasable. Even if you put the injector in the cylinder wall (pointing upwards) I would guess it would be too hot. I would also guess that you would need pressures far greater than 250 psi to keep the liquid propane from vaporizing in the lines at those kinds of temperatures.

Nice out of the box thinking.


tadd

Ah yes; water cooling. That makes sense.