"Marginal" (MFI) injectors

[Matt Holcomb]

I had my MFI pump rebuilt by Gus Pfister last year. I had to send my injectors with the pump so Gus could test them. Gus' rather elegant condition report for the injectors was this: "marginal."

I should have bought new injectors as soon as I read Gus' condition report. But alas, I did not.

As some of you know, I've done a couple of club sprint events since the pump was rebuilt. As evidenced by the lap times, the car is pretty fast. But on the street it feels -- and sounds -- restricted. I have an OEM, single-outlet muffler on the car at the moment, and while the engine feels like it's got more torque, it's now more reluctant when my right foot demands a visit to redline -- it doesn't whip to 7,300rpm as quickly as it did before the pump was rebuilt, and before I swapped the 3" Monty muffler for a factory one.

I suspect that the muffler is too restrictive given the specs of the engine (11.3:1 C/R; GE-60 cams). I'll be replacing the OEM one soon with a modified Monty. It seems as if I've traded top-end power for more torque, which isn't a bad thing. But I want torque and top-end power!

What are the symptoms of "marginal" injectors with MFI beyond greater fuel consumption?


Matt Holcomb
MFIWDP
1974 911 Carrera 2.7 (RoW)
Dedicated Homepage
Engine Rebuild Homepage
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[Adam]

I don't know if it relates, as MFI injectors are different beasts, but my CIS injectors, (which were probably the factory-fitted original injectors) were so bad that they were delivering fuel in spray, drip and dribble patterns when bench tested. Not one of them was even considered to be running at 50% flow.

They were dumping so much raw fuel, that the engine oil was contaminated and the bores were literally being "washed" of their oil. With "dry" rings and a couple of too-tight exhaust valves, a compression test showed 165-170 on four cylinders and barely 95-100 on the remaining two cylinders!

Once the new injectors were fitted and the valves adjusted again, compression was restored and performance was better than ever.

[Tyson Schmidt]

I doubt the injectors are the culprit. The muffler is the problem, in my opinion.

The best muffler for MFI is a factory sport muffler. You can modify your existing stock muffler to make it a sport muffler. It will definitely open up the top end and sound amazing to boot.

[Matt Holcomb]

Adam,

I had no idea injectors could cause problems that severe!


Tyson,

I was hoping you'd reply! Should I modify the OEM muffler, or should I modify the 3", single-outlet Monty sitting in my old man's garage? This muffler had some of its baffles removed before it was fitted to a 3.2 club car many moons ago.


Matt Holcomb
MFIWDP
1974 911 Carrera 2.7 (RoW)
Dedicated Homepage
Engine Rebuild Homepage
Pelican Gallery Profile

[CamB]

Are you sure the retarded timing to make it work on 98RON isn't a contributor (I seem to remember you are running 28 deg not 32-33)? Can you get hold of any decent (as in 11.3:1 worthy) gas - be it avgas or race gas - and put the timing where it belongs to check?

The reason I say this is that I have an OEM muffler (off SSIs) on the 3.2 with S cams (running 98RON) and it doesn't feel restricted at the top end (dyno in the link):

http://forums.pelicanparts.com/uploads/3_2_dyno_270hp_hp_only.jpg

You might need to revert to Gus on what "marginal" means for injectors. If "worn out" you could have an expensive problem. If "dirty" they are cheap to have ultrasonically cleaned.

[Matt Holcomb]

The retarded timing is definitely a contributing factor. I'm hoping that a sport muffler will compensate for the loss of top-end power; I'm not sure if the retarded timing -- and an OEM muffler -- is the best combination.

I must admit, I am concerned that we might have done the wrong thing by retaining the stock ignition system and adding high compression JEs.


Matt Holcomb
MFIWDP
1974 911 Carrera 2.7 (RoW)
Dedicated Homepage
Engine Rebuild Homepage
Pelican Gallery Profile

[Jay Auskin]

What does a MFI injector consist of?? Is it just a precision machined nozzle assembly, or does it have moving parts in it. I pulled mine out of the 2.4L engine I'm rebuilding and threw them in a jar of Berryman's Chemtool. Will I ruin them after a period of time, or does more=better ??? Thanks!

[Tyson Schmidt]

Originally posted by Matt Holcomb
Adam,

I had no idea injectors could cause problems that severe!


Tyson,

I was hoping you'd reply! Should I modify the OEM muffler, or should I modify the 3", single-outlet Monty sitting in my old man's garage? This muffler had some of its baffles removed before it was fitted to a 3.2 club car many moons ago.


Matt Holcomb
MFIWDP
1974 911 Carrera 2.7 (RoW)
Dedicated Homepage
Engine Rebuild Homepage
Pelican Gallery Profile

Matt, you should modify the stock muffler. It really does open up the top end quite nicely. I just recently reinstalled MFI on my car. I got it dialed in, then swapped in a sport muffler that I made from a factory muffler. It required some adjustment of the rack to compensate for the backpressure change, and really opened up the top of the rev range. And there is simply no better sounding exhaust out there than the factory sport. It's perfect IMO.

Trading a little timing advance for some compression is not a bad trade. The 2.0 S's ran pretty well with high compression and backed off timing from the factory. 28degrees is an acceptable amount of total advance and I think you'll be fine.



Jay, an MFI injector has a valve inside that opens at something like 240 psi. it's supposed to hold a certain pressure without leaking, open at another pressure, and have a nice spray pattern. Any one of these three could have been the reason for the "marginal' rating that Gus gave.

[Matt Holcomb]

Tyson,

Thanks for putting my mind at ease. Did you retain the corner tailpipe, or did you cut if off before adding the two straight pipes?

Also, who should I talk to about modifying an OEM muffler? Should I call my mechanic, or should I contact an exhaust specialist?

Thanks in advance.


Matt Holcomb
MFIWDP
1974 911 Carrera 2.7 (RoW)
Dedicated Homepage
Engine Rebuild Homepage
Pelican Gallery Profile