PMO carbs or slide valve injection for 83 SC ?

[911pcars]

Not sure of the fascination with MFI other than a nostalgic look, especially compared with EFI. EFI is modern, it provides a more ideal A/F mixture throughout the rpm/load range and thus should provide more useable power, torque and fuel mileage. And given an affordable throttle body assembly, it should also be easier to install and tune.

Perhaps an EFI box and injectors can be adapted onto vintage or vintage-looking MFI stacks for the best of both worlds.

Sherwood

[RoninLB]

screw MFI. It's a carb eat MFI world imo.

[randywebb]

"Not sure of the fascination with MFI..."

- People keep giving anecdotes that it has incredible throttle response -- at least when properly set up. I've never seen a direct comparison with carbs tho. I've only driven one MFI car ('73 S). It was nice but didn't seem to have super response. The pump was noisy too.

[Tspringer]

My '69 S has a Supertec built 2.5L with MFI. I am a believer! The throttle response of a well built and properly setup MFI engine is better than anything else I have ever driven. You get on the throttle and its like telepathic... BANG your getting power and engine is screaming toward the redline.

The MFI injectors fire at something like 230psi. The fuel not only atomizes extremely well, but it is shot into the cylinder. The result is throttle response that is tough to beat.

[randywebb]

Terry - do you have comparable experience with carbs? did you try the two side-by-side?

[Tspringer]

I do. I used to own a '69 911S that had 40 webers on it. It was a very strong 2.0 liter. Excellent and smooth power delivery. But it did not have the throttle response of the MFI.

My Ferrari has 6 webers. The webers have a very good system of acceleration pumps that inject fuel down the stacks upon throttle application. But, the velocity of this injected fuel into the intake manifold is nowhere near as high as the MFI injectors. The throttle response is good... but not like the 911S!

[Walko]

Having had both carbs and MFI I willalways go for MFI.

Throttle response and no hesitation

Michael

[randywebb]

OK, sounds like we have two people who believe the MFI throttle response is better than carbs, based on earlier experience but not on a simultaneous comparison. That's worthwhile.

[TonyG]

Wouldn't you have to have the MFI setup that's properly sized (example diameter of the throttle blades & velocity stack dimensions & pump calibration) for a 3.0 engine? And what about a modified 3.0 engine?

I don't think you can just slap on any old MFI setup and expect it to work properly.

I recall a few threads here where people had problems with the MFI setup with resepect to flat spots with nothing more than a non-stock muffler. What then?

I would have loved to install a Tec3 and FI setup, but it just wasn't worth the money on an SC (even a very very nice SC) and the differences in the end for the N/A setup would be fairly small with respect to performance. Plus... setting up a stand-alone engine management system is not a walk-in-the-park. It requires a considerable amount of time (even if you have done it before), and expense (dyno time) to get it 100% correct.

Dollar for dollar, on a N/A car that's not a collector car, the PMO's are hard to beat.

TonyG

[dd74]

What's always intrigued me is the one-time use of Kugelfischer fuel injection on the 911 engine. Correct me if I'm wrong, but wasn't that injection used only on the 3.0 RSR engines? Anyone have experience with that, or know about that injection system?

[Purkinjefibre]

I didn't think carbs could ever have the response of MFI. If only porsche still made MFI, we'd probably all have it.
EFI is a good middle of the road choice. Also aren't carbs well passed their "tech by date". I mean they are soooooo old fashioned. I guess you should keep tha vacuum advance on the dizzy too, for that antique feel...

[Purkinjefibre]

Weren't Kugelfischers used on all the 935 etc.s

[911pcars]

"Not sure of the fascination with MFI other than a nostalgic look, especially compared with EFI."

I meant to compare MFI with EFI not necessarily carbs. I have no doubt MFI has better throttle response than carbs (typically +10 HP as well), but for overall operation, tuning and maintenance, I'd have to go with EFI.

Can an MFI system produce an ideal A/F mixture throughout the rpm range? I mean; stoichiometric when needed for cruise, correct richness for power and optimum fuel mileage. So far, MFI hasn't shown to be a flexible system. That is, the ability to change the A/F mixture characteristics when installing a different muffler or cam, for example.

If MFI was demonstrably better, you'd see them in modern cars, race-type or otherwise.

There might be a point in which fussing with a balky MFI system might not be worth the trouble and expense as performed by an experienced tech.

Sherwood

[TonyG]

>>>I have no doubt MFI has better throttle response than carbs (typically +10 HP as well), <<<

While I did go the PMO route on my 911SC project (and I'm glad I did), I'm not the worlds biggest carb fan.

However would enjoy someone articulating how a MFI setup would produce 10hp over that of a properly setup/tuned carb. I don't see how that could be (throttle response... perhaps, but I would doubt that also...). If the a/f ratio is perfect, where's the additional power coming from? The carb itself isn't a much of a restriction with respect to air flow.

So are you saying that my PMO'ed SC which makes 200rwhp would gain 10Hp by dumping the PMO's and going with MFI?

I will have to say that if you've (not you.. .but anybody in general) never compared a PMO to a Weber side-by-side, then you can't make the carb to FI comparision objectively.

The PMO's are truely a work of art and really don't have much in common to the Weber. The quaility and tolerances are night-and-day different.

Disclaimer: I'm not in any way affiliated with PMO, nor do I sell PMO carbs. Just a surprised and satisfied customer.


TonyG

[Tspringer]

IS MFI as good as a modern EFI system in a car that is primarily to be driven on the street? I would think not.

Where the MFI is really going to fail when compared to EFI on a street car is in fuel mileage. My 911 MFI system is setup extremely well. Franz Blam did it and he did a fantastic job. Even after 3K miles, most of it on the track, since he set it up the tail pipes remain a nice even medium grey color inside. The car does not smoke at all and it runs like a scalded dog!

But, I get perhaps 12mpg driving around town and something around 4mpg on the track. The MFI system is NOT designed to conserve fuel!

I would imagine you would also have a hard time getting a modern car to pass emissions with it.

So, if emission and fuel economy are issues you must deal with relative to your P-car I would expect EFI to be a better option. It will also be easier to tune and far cheaper in the long run.

But if your a track hound and you love the older cars, you cannot beat it! The induction noise and throttle response are addictive.

[911pcars]

"However would enjoy someone articulating how a MFI setup would produce 10hp over that of a properly setup/tuned carb."

TonyG,
Okay, maybe I'm giving EFI too much credit over carbs (being a carb guy myself), but I was comparing two early S engine specs off the top of my head.

Upon further research:
67-68 911S w/Webers: 160 HP @ 6600 rpm, 132 ft.lb torque @ 5200 rpm
69 911S w/ MFI: 170 HP @ 6500 rpm, 134 ft.lb torque @ 4200 rpm

In all other respects, these engines are the same. Minor differences include:
67-68 Intake valve size is 42 mm, Ex is 38mm, C.R. = 9.8:1
69 In. valve is 45mm, Ex. is 39mm, C.R. = 9.9:1

Valve timing is the same. Looks like the 10 hp could be attributed to valve size and slight C.R. increase.

However, another comparison:
'65 2 liter 906 race engine w/46 Webers, 210 HP @8000rpm, 152 ft.lb torque @6200
'66-67 2 liter 906 race engine w/slide valve MFI, 220 HP @8000rpm, torque is the same. All other specs are the same*. Note: Slide valve MFI is less restrictive than throttle valve MFI.

Sherwood

* Bruce Anderson, 911 Perf. Handbook, page 108

[Tspringer]

The guys in HSR vintage racing who WIN, in the 2.0 liter catagory all the way up to the 3.4L and bigger are mostly running MFI.

Dave White Racing builds racing engines for many teams running in various HSR classes. His engines run at the front of the pack and are often winners. He builds some engines using PMOs and others using MFI. But HIS car that he races.... MFI. The other cars directly run by his team that frequent the front of the pack: MFI.

It may be that peak HP between the two is the same. But HP is not everything. If the MFI gives a better throttle response this may yield a tiny advantage coming out of corners. Perhaps this tiny advantage equals 2-3 seconds of time over the coarse of a 30 minute race. That may end up making the difference between first place and LOSING.

[911pcars]

"....But HP is not everything. If the MFI gives a better throttle response...."

Agreed. Throttle response.

Sherwood

[randywebb]

So, this means that those of us who want top notch throttle response had better limit our engine sizes to 3.0L. I thought about trying to put MFI on my 3.2L but went with carbs because I had no idea how you'd find a space cam for it. The RSRs went to 3L and those are spendy enough... Or did I make some mistake, not being "tuned in" to all the fine things available from all the specialty shops in California?

[kenikh]

My understanding is that the big MFI tuners have the capability to custom build space cams per application. Am I wrong here? I have seen threads on 3.6L cars w/ MFI and can't imagine they use anything but a custom space cam.