3 liter MFI

[dpegan]

I have decided to build a 3 liter MFI engine with S cams. does anyone know what MFI pump was used with the 3 liter? I have been told a 2.7 liter RS spec pump will work. thanks for your help,

dan

[1972_911T]

The original 3.0 RSR's had there own space cam. These are very difficult to get hold of now. The 2.7 RS cam can be used on a 3.0. Im currently building a 3.0 and had my 2.4 pump built to RS spec by supertec.




Steve

[dpegan]

Steve,

Thanks,

dan

[Jeff Higgins]

I'm building a 3.0 liter on MFI as well. I've had a couple of threads going here and on the 911 tech board; it might be helpful to take a look at them. Lots of great input from some pretty knowledgable folks that have been helping me out with this.

One of the things I was most interested in, and sought some help with, was cam selection. Most folks will tell you that "S" cams are not the best choice in a 3.0. In my particular application, I felt it had too little lift, too much duration, and the lobe centers are too narrow. The lift will feed a 2.0 liter adequately, but not a motor half again as big. The duration and overlap are suited to high rpm power, and high static compression. If you want to spin up that 3.0 like it's a 2.0, the long duration and big overlap (narrow lobe centers) will be just fine, but you will still want more lift.

Get with John Dougherty of Dougherty Racing Cams. He is on here as "camgrinder". I literally just received my set of cams a few days ago; a set he ground specifically for my application. We talked about what I wanted, how I planned on using the motor, and all of that. He was a great help.

I'm going to start out with a freshly rebuilt 1972 "T" pump, the number 015. I say "freshly", but it's been on a shelf for years, after Gus at Pacific rebuilt it to "T" specs. I was going to send it to Henry at Supertec to have the space cam changed, but he (Henry) actually talked me out of it. He said just run it and see how it runs. If the low end and mid range mixtures are drastically off, then we can think about re-camming it. Sounds weird, but...

From what I understand, the clamshell counterweights that control space cam movement are at full deflection at 4,000 rpm. If I understand correctly, this means the space cam has no influence on rack position (and therefor mixture) over that rpm. I don't think it does anything at wide open throttle either, regardless of rpm. So it sounds to me like the space cam only influences part throttle, sub 4,000 rpm running. That, and all MFI pumps are capable of delivering the same maximum volume, which will be more than your 3.0 liter could use under any circumstances. So effectively the only differences are in low and mid range, part throttle delivery. If any of you MFI gurus are listening, please chime in - I'm not all that sure about this. Henry? Grady?

Anyway, my thoughts are that I'm building primarily a street engine that will see a dozen or so DE's a year. I don't want some 20 hour race motor time-bomb. So, rpm's will be kept down to maybe 6,800-7,000 max. It's going to have some pretty "radical" cams, so it won't idle so great and it just might pop and fart at lower rpm's. I'm not sure I will be able to tune that out even with the "proper" space cam in the pump, or even if I would want to. It will spend most of its life in that 4,000-7,000 rpm range, with virtually no stop-and-go city driving. It is a "toy car"; driven for enjoyment only. Civilised manners are not important. Hell, civilised manners would likely detract from its appeal.

I guess that's my way of saying you need to have a clear picture of what you want your 3.0 liter MFI to do for you. At one end of the spectrum, if you envision a civilised daily driver, I suspect you wouldn't be considering MFI in the first place. If you envision a fire-breathing monster, and low speed drivability isn't important, you can run your pump as-is. If you are somewhere inbetween, changing space cams may be the way to go. Get in touch with John about cams, and Henry about the pump, and tap their wisdom. They are both great guys to talk to anyway.

[1972_911T]

Id be very interested to here an expert chime in on this, ive heard of people using 2.4E and S pumps in a 2.7 but never a 3.0. Its my understanding that any MFI pump can deliver enough fuel to run a 3.0 or 3.6 for that matter simply by just turning up the mixture the problem comes when your not at full throttle and 6k rpm. The engine will run way too rich at idle and low RPM's which can result in washing your piston rings and diluting your oil with fuel. I may be wrong but thats what im lead to belive.

Steve

[Jeff Higgins]

I've heard and read variations on that same theme, Steve. I plan on doing a very thorough shake-down on our local chassis dyno to make sure this is not happening. Henry did tell me, however (in one of our multiple phone conversations) that he runs some quite powerful race motors on "T" pumps.

I think the part I've heard about the clamshells being at full deflection at 4,000 rpms would preclude the specific problem you describe. If you are at part throttle at 6,000 rpms, you won't be getting full delivery because the rack will (of course) be at a part throttle position, and the space cam would have no affect at that rpm anyway.

My concern is below 4,000 rpm at part throttle. Again, my understanding is at above 4,000 rpm the space cam does nothing. I could very well be wrong, but that is what I think I have gleaned about all of this. Of course most of our "normal" street driving, and particularly cruising on the freeway, will be done at 4,000 or below. If steady state, partial throttle "cruise" settings prove to be too rich when I get it on the dyno, then clearly a space cam change is probably in order.

My current 2.4 "T" motor runs at a pretty optimal ~13:1 air/fuel at a steady state, part load 3,000-4,000 rpm (representative of freeway cruising). Full throttle at those rpms richens it considerably to about 10:1, and gradually leans out to about 12.5:1 when the rev limiter cuts in at 6,800 rpm. So there is a huge difference between the mixtures seen at full and part throttle, 3,000-4,000 rpm running. Part throttle, where the space cam is working, is leaner. As it should be; my right foot hasn't yanked the rack all the way open. The extremely rich condition at full throttle 3,000-4,000 rpm running leads me to believe the space cam has no infuence at full throttle, regardless of rpm. The pump is clearly delivering more fuel at those rpm's than the motor needs relative to the air it is taking in, and the ratio becomes balanced as the revs climb.

So it seems to me, when we consider the lower fuel delivery required of the "T" at part throttle cruise (compare to the "E" ans "S"), that the "T" would have the least "aggressive" space cam, or the lightest clamshells (if there is any difference in those; I don't think there is). So putting a "T" pump on a bigger motor should result in a lean condition when running on the space cam. Does that make sense, or is my logic flawed? Maybe the adjustment of the main rack to make up for that lean condition in the low and mid range results in a too-rich condition at high rpm?

[1972_911T]

The other big consideration here is not just displacement the cam and intake size also affects how much fuel is required at a particular RPM. Personnally I have no knowledge of what the cam does at any one particular RPM however my understanding of the space cam is that its the mechanical equivelant of a modern FI VE table (voumetric efficiency) hence not only will a larger engine need more fuel but a engine with a more aggresive cam ie a 2.4 S at 6k will breath more air than a T and hence will need more fuel but in a part throttle situation ie when crusing where the valve is easily opening enough to flow the air that the throttle plate is allowing to pass you would expect both engine been 2.4's to have similar fuel requirement and this is the job of the space cam to allow variations of fuel delivery dependant on RPM and load. By using a 2.4 T pump on a 3.0 with a wild cam not only are you increasing the displacement but also the caracteristics throughout the RPM.

Maybe the cam your running is just as crucial as the displacement??

When I spoke with henry before I first decided to send him my pump he explained that the 2.7rs cam is as close as he can get to a 3.0 and you can turn up the mixture to get the right mixture at full throttle but in doing this it will run rich at the low RPM's due to differences in the caracteristics so how much more so with A T pump??

Steve

[shbop]

signing-up

[Jeff Higgins]

I think you are pretty much spot-on, Steve. We can see the displacement/cam/port size relationship driving different space cams in the 2.4 liter series. Compression varies in this displacement as well, so we see a fourth factor entered into the equation. The only constant in the 2.4's are displacement and valve size. Yet we see three distinct space cams...

When viewed as a percentage of total rack movement (idle to wide open), I wonder how much the space cam effects rack position as the clamshells go from zero to full deflection. It does not appear, percentage-wise, that it is much. So I wonder... if the optimum space cam is providing an economical 13:1 A/F at part-throttle 3,000 rpm, what would the "worst" space cam provide? Certainly nothing as far off as, say, 20:1 or 5:1. I bet it is closer to just a point or two off; i.e. 14:1 down to 12:1. Maybe a little more; I just don't know. I would love to find out (thinly veiled plea to the more knowledgable to save us from ourselves...)

So didn't the early race only pumps (906, 908, 917) run with no space cam at all? I think the space cam was a concession to drive-ability and fuel milage on the street cars. The race pumps ran purely on throttle position, with no allowance for rpm. "Load" was indirectly measured via the position of the driver's right foot. Full throttle; full fuel whether at 3,000 rpm or 8,000. I think our pumps do the same over 4,000, if I understand this correctly. Where ours differ is that at partial load, under 4,000 rpm, engine speed is accounted for by the space cam regulating mixture instead of simply by rack position. My A/F ratios on the dyno seem to bear this out.

Anyway, back to the displacement/cam/port/compression factors. My motor will be a 3.0 with 9.5:1 compression, 36mm "S" throttle bodies, 37mm intake ports, and Dougherty GT2 turbo cams cut on 102 degree centers. These are high lift, short duration cams pretty similar to GE or DC 60's but with shorter duration. I'm looking for good power up to 6,800 rpm, where the traditional longer duration 60's make their power about 800-1,000 rpm higher. So I'm in a weird no-man's land on the pump.

It would be a slam dunk to build an 8,000 rpm RSR spec 3.0 liter on MFI. Maybe not monetarily, but the combination - if you can scrounge the parts - is proven. Mine isn't. There is no "cookboook" to follow that includes a proven space cam selection. So, the plan is to run it and see. The "T" cam may be optimal; it may not work at all. When I have some numbers to peruse, I'll call Henry and talk about them. Until then I'm jut guessing.

In an effort to not thoroughly hijack this thread with my project, where does all of this leave a 3.0 on "S" cams, as far as a space cam? It is still not a "standard" set-up. RSR sprint cams, with a red top pump harbouring an RSR space cam, with high butterflies or slide valves would be "standard" for a 3.0. Anything else would not have a specific space cam optimized for it. A 2.4 "S" or 2.7 RS, like the common wisdom already suggests, will be very good.

But now we get back to running those "S" cams in a 3.0 liter. I think Dan would realize a great deal more power from his project if he went with a more modern profile, with enough lift to feed a 3.0 liter. Now he is wading into the deep end with me...

So Dan, we have been pretty busy here while you have been away. I guess other than using your original question as a springboard, we really know little about what you are up to. Do you have the MFI set-up yet? What, if anything, do you already have towards this project?

[kenikh]

FWIW, I seem to recall reading that some have used an MB space cam for 3 liter 911 motors. Not even sure if it makes sense, but...

[dpegan]

Hi,

Great suggestions, thanks.

Sorry I was out most of the day working on getting a 3.6 into my 82. This is my track car. I also went to see a guy to buy what was suppose to be a MFI pump recammed to RS spec, but upon testing the rack motion, well there was no motion. So what is a MFI pump worth that need to be rebuilt at a cost of $1,000? Back to the original question.

I bought a 74 911 roll a while back and have plans to make it a 74 RS with a 3.0 MFI. Sort of a RGruppe car. The car will be more of a street car with a few track days a year when the 82 with the 3.6 is being worked on.

As far as parts go I now have the 3 liter with Webers (these are for sale), a working (I had Gus test it) MFI pump (013) and throttle bodies rebuild to Spec for the 3 liter. I got the MFI pump and Tbodies from a guy that had a shop and was getting out of the business. I need stacks and may bore out a set of Mag stacks or have some fabbed up plastic ones. Sounds Like I could just try the S pump and see how it works.

dan

[1972_911T]

I think when mine is up and running I will be investing in a wide band O2 sensor and display which will help massively with tuning to get the best compromise. Im on the otherside of the fence to you in that my car will do quite alot of low RPM town driving so low end driveability will be a bit more important than all out top end power.

Steve

[1972_911T]

Originally Posted by Jeff Higgins

In an effort to not thoroughly hijack this thread with my project, where does all of this leave a 3.0 on "S" cams, as far as a space cam? It is still not a "standard" set-up. RSR sprint cams, with a red top pump harbouring an RSR space cam, with high butterflies or slide valves would be "standard" for a 3.0. Anything else would not have a specific space cam optimized for it. A 2.4 "S" or 2.7 RS, like the common wisdom already suggests, will be very good.

I may be wrong but I carnt help thinking that cam selection would be far more critical than displacement. Surely if the displacement is increased by 10% that will be across the range so by increasing the main rack 10% that should be about right? with S cams and an S/RS pump?

Originally Posted by Jeff Higgins

But now we get back to running those "S" cams in a 3.0 liter. I think Dan would realize a great deal more power from his project if he went with a more modern profile, with enough lift to feed a 3.0 liter. Now he is wading into the deep end with me...

As for cams if you want good street performance many people go with the MOD S profile (DC40's) as I will be doing with mine. Or possibly DC 60's? for a bit more aggression

Steve

[304065]

An S pump runs lean at high RPM under load even on a 2,2S.

Whole lotta trouble for aesthetics if you ask me, you'd get more power, better fuel economy and better high-rpm safety with a set of PMO's new throttle bodies and an appropriate EFI box.

I am a huge fan of MFI but if the Club Racing rules allowed me to ditch it in favor of EFI with no class penalty I'd be on the phone with PMO so fast it would burn the numbers off a credit card.

[1972_911T]

Originally Posted by john_cramer

Whole lotta trouble for aesthetics if you ask me, you'd get more power, better fuel economy and better high-rpm safety with a set of PMO's new throttle bodies and an appropriate EFI box.

Do you really think PMO's will give better fuel economy than properly setup MFI?

Steve

[304065]

Steve, I know we are "two nations separated by a common language" but I was trying to be as precise as possible when I said "PMO's new throttle bodies and an appropriate EFI box," not PMO carburetors.

[Jeff Higgins]

Originally Posted by john_cramer

Whole lotta trouble for aesthetics if you ask me, ...

Well... of course it is. There are any number of more modern systems that will beat MFI in any measure of performance. I think we all know that. That's not the point here; the point is to experience something from the past. Warts and all. Just for fun.

The only 2.2 S I have seen on a dyno, under load, with an exhaust gas analyzer plugged in was rswannabe's a couple of years ago. It did not lean out at high rpm under load. As a matter of fact, his A/F ratio was more consistant accross the rpm range than mine was on my 2.4 T. The S did not go nearly as rich in the mid range as my T did to maintain a safe high rpm mixture. So, even if you had one that was leaning out on top, it would be a simple matter of backing out the main rack adjustment until the ratio is correct. It would fatten up the mid range some, but it appears there is more "wiggle room" there than on my T.

I've been told that the 2.4 T, being a U.S. emissions expedient, was rushed into production and was not as well refined as the E and S set-ups. This was borne out on the dyno that day, and on the road any other day. Every 2.4 T still running MFI I have ever seen stinks like hell, going very rich under load in the mid range. The E's and S's don't have that problem.

And yes, Steve, just about anything will get better milage than MFI. While I can pull 18-20 mpg on a very disciplined, very steady 60-70 mph cruise, my daily average is more like 13-14 mpg. On DE days it drops to 9-10 mpg on the track. It simply pours in more fuel than the motor needs most of the time. Carbs will do a little better, and any EFI will do much better. Oh, and I think John is refering to PMO throttle bodies to be used in an EFI set-up, not their carbs. Edit - John beat me to it - see above.

Anyway, for those of us who choose to run this funky old system, it is a lot of fun. As far as your observation on the importance of cam selection vs. displacement, I think we can look to the S series of motors for an answer. It ran the same cams in three displacements; 2.0, 2.2, 2.4. We know the external controls changed somewhat, but how about the space cam? I know they don't interchange, but are they the same profile?

[304065]

Originally Posted by Jeff Higgins

Well... of course it is. There are any number of more modern systems that will beat MFI in any measure of performance. I think we all know that. That's not the point here; the point is to experience something from the past. Warts and all. Just for fun.

The only 2.2 S I have seen on a dyno, under load, with an exhaust gas analyzer plugged in was rswannabe's a couple of years ago. It did not lean out at high rpm under load. As a matter of fact, his A/F ratio was more consistant accross the rpm range than mine was on my 2.4 T. The S did not go nearly as rich in the mid range as my T did to maintain a safe high rpm mixture. So, even if you had one that was leaning out on top, it would be a simple matter of backing out the main rack adjustment until the ratio is correct. It would fatten up the mid range some, but it appears there is more "wiggle room" there than on my T.

I've been told that the 2.4 T, being a U.S. emissions expedient, was rushed into production and was not as well refined as the E and S set-ups. This was borne out on the dyno that day, and on the road any other day. Every 2.4 T still running MFI I have ever seen stinks like hell, going very rich under load in the mid range. The E's and S's don't have that problem.

Hmm, was the S you looked at using a sport muffler or megaphones? To be true, the example I cited was using megaphones, obviously race-only.

Hey, don't get me wrong, I'm a big fan of MFI but when you get out beyond the 2,7 space cam and "S" cam combination you're a test pilot, and the fact that the fuel-rpm-throttle position relationships are set in steel can make things difficult to get right without multiple trials.

[Jeff Higgins]

Originally Posted by john_cramer

Hmm, was the S you looked at using a sport muffler or megaphones? To be true, the example I cited was using megaphones, obviously race-only.

Hey, don't get me wrong, I'm a big fan of MFI but when you get out beyond the 2,7 space cam and "S" cam combination you're a test pilot, and the fact that the fuel-rpm-throttle position relationships are set in steel can make things difficult to get right without multiple trials.

It actually ran on four different combinations that day. He had a stock air cleaner and rainshields, then a stock single out muffler and a sport muffler. He ran it stock air cleaner / single out, rainshields / single out, stock air cleaner / sport muffler, and finally rainshields / sport muffler. From what I recall, none of the combinations had very much affect on mixture. The stock air cleaner / sport muffler made the most power, but not by much at all. I think total spread between the "best" and "worst" combination was less than five horsepower.

"Test pilot." Very well put. I'm certainly into uncharted territory on mine. If I can get it to work, I'll be very happy. If not, there are always Webers or PMO's (carbs for me). They are certainly move tune-able to run on odd combinations. I think MFI is adaptable (to a degree) as well, if you are willing to accept more compromises. I am, as long as they don't involve risk to the motor, like a drastically over lean or over rich condition somewhere. I don't think that will be the case. I think it will be more of a drive-ability issue at lower rpm's, and I can live with that.

[1972_911T]

Originally Posted by john_cramer

Steve, I know we are "two nations separated by a common language" but I was trying to be as precise as possible when I said "PMO's new throttle bodies and an appropriate EFI box," not PMO carburetors.

Sorry John didnt realise PMO made ITB's, originally I was looking into doing an ITB megasquirt setup but again this option dosnt come without alot of hasssel and some compromises from the numerous threads ive read on the subject, not to mention A couple of people who have jacked it in after 1000'$ and lots of time invested and just gone with a set of PMO's (carbs) So if all else fails thats my backup or possibly EFI if I see a few more success stories but for now MFI for me.

Steve