2.0 port size

[tadd]

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

[tadd]

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

[jluetjen]

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.

[tadd]

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

[jluetjen]

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.

[tadd]

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

[jluetjen]

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.

[tadd]

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

[tadd]

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.

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.

[kenikh]

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!

[tadd]

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

tadd

[jluetjen]

Originally posted by tadd
Do you know if the 906 used iron cylinders?

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

[304065]

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).

[tadd]

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

tadd

[tadd]

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

[kenikh]

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.

[tadd]

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

[kenikh]

Ah yes; water cooling. That makes sense.