MFI injector placement
 

Why are MFI injectors mounted in the cylinder heads? Why not put them higher on the intake like TWM does with their EFI throttle bodies? Would there be any benefit to having the injectors sitting higher above the intake valves?

TIA

Sherman,
I think they took some of the design cues , from Desiel engines.
I agree it would burn better , maybe if placed further up the intake.
But the injection for EFI are on all the time also, well till 2005 .

Derek

Sherman,

Check this out:
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=10156&item=4533077 806&rd=1

specifically "Intake given high butterfly affect with the us of 94 Turbo stand-offs, raising feul injecters up from the intake valves, gives much better atomisation of the intake mixture Air/Fuel"

whoops - just saw you has already noticed that...

Yep, thats what why I posted this.

Here's my take on it from what I've read in various (non-Porsche) sources.

1) Injectors close to the intake valves are better for "low RPMs" since they don't give the fuel much chance to drop out of suspension prior to entering the combustion chamber.

2) Injectors further upstream work better at "high RPMs" since they give the fuel a better chance to do a couple of things prior to entering the combustion chamber.
a) Build up momentum thus providing more inertia to the intake charge which aids using the tuning of an engine with cam overlap at high rpms.
b) Gives the fuel a better chance to mix with the air prior to entering the combustion chamber.

Note that I've put "high RPM" and "low RPM" in quotes since these are very relative terms. In general use, extended running at 3000 RPM or less is considered "low RPM" and extended running at 6000 RPM or more is considered "high RPM", so most 911 use is "low RPM". Race engines often use high injector positions or even place injectors above the intake trumpets. The most sophisticated engines (F1 for example) often use multiple injectors to improve the engine's torque band across a wide rev range. At low RPM's they use the lower injectors and at higher injectors either augement or replace the lower injectors with the high injectors.

So if you put an intake system with high injectors on a 911 T for example, you might find that the engine doesn't like cold mornings as much as a stock configuration, or possibly the mileage suffers more on cold mornings then it would with the same engine using low injectors.

Also consider that the standard 911 intake track (MFI or carbs for example) holds more volume then some of the smaller engines (2.2 or 2.0) have in one cylinder's swept capacity. What does this mean? It means that if you were to mount an injector too high in the stacks that the intake valve would close and open once prior to that fuel entering the combustion chamber. Not a problem at 7000 RPM's where that may be 0.00011 seconds, but at idle it might result in a lean condition for a period 7x longer if you suddenly open the throttle, contributing to a stumble or throttle response reminicent of a CIS system.
:(

out of curiousity - does anyone have a part number for these "94 Turbo stand-offs"?

Let me show you guys where I’m going with this.

These are TWM 3003 throttle bodies mounted on Weber manifolds, roughly $1700-$1800 worth of parts with linkage. Readily available stuff.

http://forums.pelicanparts.com/uploa...1082103248.jpg

MFI High butterflies. Probably worth more than my car. Not so readily available.

http://forums.pelicanparts.com/uploa...1110247620.jpg


Looks like the injector bungs for the TWM's could possibly be bored and tapped for MFI injectors. Installing the injectors here would also eliminate the need for extra drilling and tapping MFI injector bungs on 3.0, 3.2, and 3.6 heads (with different manifolds, of course). The butterflies would be higher like on the 3.0 RS (early 2.8 RSR also, I think).

EFI is pulsed. So is MFI, albeit at much higher pressures. So what would be the effect of having MFI injectors this high above the intake valves? One side of my brain says that if this was a good idea, Porsche engineers have done it on the high butterflies. The other side is telling me that I’m looking at 30 years of engineering advances with the TWM setup.

I'm not forgetting the challenge of fabricating custom primary hard fuel lines or the cold start system. That doesn’t sound too scary.

What do you MFI guys think? If custom hard lines could be built would this work? Not period correct looking but it certainly would be cheaper and allow for more air to be pumped into the combustion chamber than bored out MFI stacks & throttle bodies.

Anybody dealt with the TWM’s and MFI that can offer some guidance on this? TimT, SteveW, or Henry? Grady? I know you guys are probably busy building real engines instead of dreaming up hair brained schemes for reinventing a 30 yr old technology, but any help you can give is appreciated.

re: "challenge of fabricating custom primary hard fuel lines"

i would not think that this is necessary - i've seen a few mfi systems that use flexible lines, you just need to specify the pressure requirement and adapt suitable end fittings

I always think of the ht. of the intake tubes as being proportional to the peakiness of the engine...

Shuie; The reason that Porsche didn't use high mounted injectors from the get-go may have a lot to do with other issues such as...

1) Emissions
2) Space utilization within the engine bay. Remember they most likely wanted to keep the intake shroud to manage noise and ensure proper filter performance.
3) Engine performance in the range that most people drive their 911's.

I wouldn't dismiss the engineering know-how of the late 60's too quickly. It would have been fairly easy for them to try what you are describing. Most of the technological improvements of the last 40 years have been computer or electronics based or enabled. The technology of fuel injection systems goes back at least through the second world war. The big change has been computer control of the injection systems -- not the mechanical hardware.

As far as the path that you are taking, I'm sure it will be fine for a play engine that doesn't have to pass emissions, and will most likely spend more time above 3000 RPM then most 911 engines.

somewhat related question ... no , not really , but it's also about intake , and alternatives from the past...so yes , maybe a bit.

what's up with them slide valves , vs butterfly valves... is there a performance advantage, any disadvantages??

the slide valves seem simpler in design to me
are they any better?

Do a search on "slide throttle". The big improvement from slide valves is that at WOT there is absolutely nothing in the intake's path. When you have butterflies, the shaft (as well as the edge-on throttle plate) is still in the intake path.

The problem is that slide throttles can be less reliable in dirty environments. The failure mode is also not particularly pretty since the slides will jam open.

High-mounted intake trumpet injectors were used from day 1 on the pure racing MFI engines using the 'tossel-head' plastic injection lines ... look at the mid-late '60s pics of the MFI 771 [flat-8] engines, 906E, 910, 907, 908, & 917 engines!

Production 911 MFI and early 2.8 RSR engines didn't seem to lose anything from having the in-head injector mounting, and certainly the low & part-throttle responsiveness of those MFI engines isn't matched by anything in production since!

search on "slide throttle" at RennList also - Steve Wiener corrected a misperception I had that they were not good for street cars because of dust, etc. clogging the mechanism.

Obviously the hole will not be round at low throttles (like the pupil of a cat's eye...) - dunno how "bad" that will be. Someone crazy enough could always design a diaphragm shutter mechanism - like in our own eyes or in a camera lens.

Is it round when you use butterflies????

Just illustrating what Warren has already said:

http://forums.pelicanparts.com/uploa...1110318478.jpg
Picture from: (C) cyberspeed, http://962.com/historic_porsches/porsche_917_01_021/

Also, to comment on the slide throttles, these are extensively, almost exclusively used in the motorcycle/snowmobile world. I can't think of any high performance off-road motorcycles that use butterflies. And while tubular throttle slides were (and still are to an extent) popular, now "flatslides" have become very popular, as well as "oval" looking slides. I realize many of these applications are carburated, and that fuel is metered at the base of the "cats eye" refered to above, but I don't think the "cats eye" shape is harmful to engine performance, after all, the throttles are just choking the engine to begin with. While the layout and architecture is different than on a 911 engine, I have not heard of any problems with slides "sticking," on vehicles that operate is far worse, dirtier conditions. I'm not saying it hasn't/doesn't happen, but it must be uncommon. Obviously the slide throttles had something to offer b/c the factory used them.

Randy,
I am not one to argue with Steve Weiner and I say this not having recently read what he wrote on Rennlist. I do recall that he mentioned as I did Sam Shalala making some trick slides out of his shop Protechnik which is in my backyard. Sam has told me that these things need to be torn down after every race. The concern of dust and dirt clogging the system is real, I believe.

I actually thought SW had told me that at one time. I'm sure I saw it somewhere (and I usually mentally separate info from good sources from that from unknown rumor, drivel, etc.). Certainly there is more friction area to have problems with.

Anyway, I am happy with my primitive carburetors -- for now. I am able to burn up plenty of $$ on the paint & interior for the present. Then, I can see if the Wevo brakes system is offerred; tear out the newly updated susp. rubber for polybronze; and get the tranny rebuilt/hotrodded....