Assembling new engine should I go MFI, Weber or PMO ?

[katigbr]

I'm in the process of assembling the pieces for a "hot rod" 2.7 engine for my 72T. Currently the motor is a 2.4 with 2.7 P&Cs with "S" cams and MFI. New pistons are ordered with 10.5:1 CR, heads are going to be sent out for porting and twin-plug'ing. Last decision to make is to either retain the MFI or go with either Weber or PMO carbs. Any advice from those of you that may have BTDT?

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Rick Katigbak
1972 911T
http://www.pelicanparts.com/gallery/katigbr

[Early_S_Man]

Rick,

The MFI system will make at least 10 hp more than Webers or PMO, assuming it is in good condition. Was the injection pump rebuilt to RS 2.7 specs when the 2.7 p&c set were initially fitted?

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[pwd72s]

Warren, an aside quesion here. Since the 2.7 RS and the 2.4 S used the same camshaft grind, are the MFI systems of both engines identical? I know the MFI 911T engines had smaller dia. stacks, different injection pump cams, than the S.

[katigbr]

Warren,
The PO had the MFI injection pump "recalibrated" by Stoddard. I don't know specifically what they did but it cost $325 in 1977! Is there a way to verify with a visual inspection? part numbers on the intake stacks, etc?

One of the reasons that I was looking at alternatives was the issue with aftermarket exhausts and the MFIs. The MFIs require a specific level of backpressure that is not provided by some aftermarket exhausts. My thought was that this indicated a loss of power potential. I thought that by going to carbs and aftermarket mufflers I'd pick up more power.
Rick

[Early_S_Man]

No, the RS injection stacks appear to be identical, but the idle air bypass passages were drilled slightly larger, from 3.3 mm to 4.0 mm. And, the distributor had a different advance curve, too!

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[Early_S_Man]

Rick,

I am quite sure that SSIs and the Dansk dual-in, dual-outlet sport muffler would make more power than Webers or PMOs and be perfectly compatible with MFI!

No, I don't think there is a way of externally verifying that an injection pump has the 2.7 space-cam in it, and the stacks are easily modified ... but, I don't think Stoddard would have cut any corners on a 2.7 conversion.

Finally, I believe the 'backpressure' requirement for MFI is simply a myth or urban legend ... Monza, Manta, and Bursch mufflers, to name three, do not provide a 'crossover connection' between the two banks exhaust pipes which the factory banana muffler does due to its' inherent design. It has been my experience that while being louder than stock, the Manta muffler causes a loss of power and torque, compared to the OEM muffler. I suspect that mechanics and garages came up with the 'backpressure' myth to explain away the loss of power!

The fact that factory sport mufflers fitted to rally cars did not suffer a power loss, and undoubtedly had significantly less backpressure than a stock OEM muffler ... tells me there is no 'backpressure issue' with MFI cars! If you look closely in Bruce Anderson's Porsche 911 Performance Handbook, on p. 145 of the 2nd edition ... you will see an example of a factory rally muffler that has extremely low backpressure!
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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[This message has been edited by Early_S_Man (edited 08-08-2001).]

[gaijinda]

The MFI system is part of the charm of the 911 from that era.

[zotman72]

Check the diameters of the stacks against the spec. MFI rules when it is operating as it was designed. You need the left heat exchanger to have the heat riser for the thermostat. I have a friend with 2.9 MFI
twin plug and that baby screams! Stoddard's calibration could have been the spacer cam for a RS spec engine.

Bill
2.4S MFI from 2.4T MFI.

[Early_S_Man]

Rick,

One thing that can be easily checked to see if that 'T' engine was properly modified when enlarged to 2.7 is the size of the ports in the cylinder heads, and the throttle bodies. The 'T' engines had 30 mm or 32 mm ports and the throttle bodies matched that port size. The RS 2.7 had ports of 35 mm or 36 mm, same as 'S' engines, and throttle bodies matched to the larger size. If your heads and throttle bodies are already enlarged, then you are all set for the rebuild!

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[T Bird]

Sorry to disagree with the popular wisdom but I vote carbs, particularly in a non-standard application, such as this one. Carbs are infinitely tunable and very DIY-friendly--MFI ain't.

[Early_S_Man]

bird,

The RS 2.7 engine wasn't non-standard ... it was in production for over 3 years!!! It just wasn't sold in the USA! And, I think you are missing the point that Rich already had a working MFI 2.7 and your suggestion would require spending an [b]EXTRA $2-3,000! That's over and above his other rebuild costs! The numbers just don't add up for this particular situation!

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

go with the MFI. It was the system of choice for the higher performance E and S ("standard production") and RS.

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Carlo
http://bigblue111c.tripod.com/dec2000/

[orbmedia]

Personnaly if I had a working and properly calibrated MFI I'd go with that. It's a beautiful system.

[T Bird]

Warren- Having converted from MFI to Webers 9 years ago, I appreciate the benefits that carbs offer over MFI. My MFI was impossible to keep in tune, did not provide proper mixture over the entire RPM band, and diluted the oil with gas. If building a replica motor is the object of the exercise, then by all means he should go with MFI. However, if driveability is the goal, I vote carbs.

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John
1972 911T Coupe
"S" pistons & Webers
PCA- Potomac Region

[RarlyL8]

If you already have the system, and plan to do all your mods at once, I would keep it.

HOWEVER, expect that it will need to be modified. I disagree with Warren on the backpressure issue. I had a '73T with MFI that worked perfectly - untill you uncorked the headers. It litterally would not pull itself out of the garage! Pop the stock muffler back on and you're back in business. I knew nothing of these antiquated systems and did not wish to have the system tuned to match my modifications. So I went to Webers and never looked back.
(Gave the system away to a fellow Pelicanhead).

[epbrown]

Question on the "cars v. MFI" issue: isn't there a performance difference as well? I've never worked on an early car, but I recall reading that the carbed cars offer better low-end torque than the MFI - something to consider for street driving.

Emanuel

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Everybody wants a normal life and a cool car;
most people settle for the car." Chris Titus
1966 Rolls-Royce Silver Shadow
1983 Porsche 911 SC Targa

[Early_S_Man]

Rarly,

Open heat exchangers lose a lot of bottom-end torque ... regardless of whether equipped with MFI or carbs! Put megaphones on ... and you get some back! Install a 2.25" crossover or 'H' pipe between the two banks and you get all of the torque back and then some. I have experimented with 20" megaphones without crossover, and 24" with and without crossover, and found the 24" with crossover to have better torque and responsiveness than stock muffler at autocrosses!

The point is, you can't really blame MFI ... when exhaust tuning is really the issue!

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[Oldporsche]

I have always like the MFI. It, along with the proper exhaust, has the performance. I have had personal experience with both carbs (2.7 euro piston, e-cam, webbers) and approximatey the same motor in MFI. As to the MFI warm up feature, it was replaced with a manual choke handle set up.

With the market for carbs being what it is you may want to invest in a set-up to try it out. It's not like you can't sell them later.

I just wouldn't dump the MFI right away. There are a lot of very successful setups out there.

Good luck,
David Duffield

[tmctguer]

in case you go the route of my friend RarLyl8

"So I went to Webers and never looked back.
(Gave the system away to a fellow Pelicanhead)"

please note that i would volunteer to be the recipient of your generosity....... ;-)

[clevy70911T]

"IF" you go the carbs route, spend the extra and get the PMO carbs. They are beautiful physically and mechanically. The only drawback is you might not want to hide them in the rear of your car