Page 85: It's (the Jag 3.8 race engine) best output was 307 bhp at 5500 rpm with fuel injection and 297 bhp at the same speed on Webers.

It's a great book by the way for people who like engines!

A few more data points...

1) BRM P56 1.5 Liter V8 F1 engine: Classic Race Engines, Page 145.
Looking at the Coventry FWMV engine data in Hemmill's book, there's a chart on page 119 with a back-to-back comparison of a "Short stroke" FWMV Mk III engine with IDF carbs versus Lucas injection. The Lucas version out performed the carbs from 6000 to 6500 RPM, and again from 7300 to 9100 RPM (with the exception of 8500 RPM where the carbs make 1 HP more). For some reason the Lucas line drops off at the last data point (9500 RPM) while the carb version doesn't. I suspect that the engine ran off of the injection map given the sudden drop.

I am still tempted to get a set of megaphones for the 911E specifically for use at Watkins Glen next year.

A MFI pump adjustment is definitely required, but with LM-1 technology there's no longer any guessing about that.

I saw Rick DeMan at Summit Point last weekend, he's got a dyno that attaches directly to the rear wheel hubs for no roller losses. If I get the megaphones that's where the tuning will occur.

All well and good. Back to Porsche engines.

Pg 108 of Bruce Andersons 911 Performance Handbook (2nd edition)

901/02- is a 67/68 911S engine with Weber carbs. 1991cc-42/38 valves-36/35 ports-9.8:1CR. It made 160hp at 6600 and 132lb/ft at 5200.

901/10 is a 69 911S engine with MFI. 1991cc-45/39 valves 36/35 ports-9.9:1CR. It made 170hp at 6500 and 134lb/ft at 5500.

Both are 1991cc with the same cams. The MFI motor makes 10 more HP! It also uses bigger valves and a very marginally higher CR. Not sure how much the valves and CR contribute, but they do for sure.

901/20 is a 906 racing engine with Webers and a 901/21 is a 906E engine with SLIDE VALVE MFI Both otherwise identical 2.0 liters. Make the same torque but the SLIDE VALVE engine makes 10 more HP (220 vs 210) How much does the slide valve contribute? Not sure, but some.

Interstingly the same page also lists 901/22 911r engine with Weber carbs. Comparing it to the slide vavle motor (901/21) shows same displacement, same cams, same valves, same ports, same CR and the same 220hp and same 152lb/ft!

The 901/25 shows the excact same specs again (914/6 race motor) 220hp and 152lb/ft. The 901/20,21,22,25 all show the following.

1991cc, 45/39 valves, 68/38 ports, same cams, 10.3CR. One has Slide Valve MFI, have carbs. One carbed on makes 10hp less and the other 2 make the same HP.

Also listed is 1991cc racing MFI motor (901/23) It does not show the valves, ports, cams or CR but it only makes 210hp.

Quoting text from the same book (page 61) "There were two versions of these engines, the Type 911/20 with mfi and the type 911/22 with 46IDA Weber carburetors. The power claimed for both versions of the engine was 230 DIN HP" However both of those were 2247cc.

I said earlier, more than once, that I doubted the motor would make 10 more hp with MFI vs carbs. I never said it would not. The motors I listed (except the last two in the text quote) are all Porsche flat 6 2.0 liters, which was the subject of the original question. Also these stats are with Webers not PMO's, I do not know if it makes any difference or not. I am still not convinced mfi would make claimed 10hp more that you are so sure it will. Like I said, it might, but it also might not.

Cheers,

Touche' SmileWavy

With all this discussion on MFI vs Webers...Shouldn't we be asking the question about the performance advantage of PMO's vs Webers.

Is there one and if so...Why???

I think John hit on the advantages when he said this:

"Actually, if properly set-up I seriously doubt that PMO's make any more HP then Webers -- or comparably sized Zeniths even (assuming that all 3 examples are 40 mm carbs). HP is a function of available airflow and the ability to get the main circuit to meter the appropriate amount of fuel. This is a function of the venturi size and jetting -- which would be pretty similar in all 3 cases. The big (and this is a BIG "big") benefit of the PMO's is in their user friendlyness. Just going off of memory...

- Sight glasses make setting the float levels a breeze. The same process is not so easy in the other carbs.
- A Better throttle shaft design that is more robust and reliable, and doesn't develop leaks as quickly.
- Improved orifice design/lay-out to correct some transition issues that Webers have.
- A better layout for serviceability in regards to the jets."

Not sure if there is any actual power increase at all.

Cheers

I would think that efficiency attributed to fuel atomization plays a part in extracting max power through better combustion. I had always thought that this was one of the main reasons FI produced more power than carbs.

Hmm... As far as max power is concered, there's probably not much in it. From BA's book:

901/20 - Carbs - 210hp
901/23 - MFI - 210hp
901/22 - Carbs - 220hp
901/21 - MFI - 220hp

Other specs (cams, ports, C/R etc) more or less the same.

Seems to me that if you already have an MFI setup and can deal with the space-cam voodoo, stick with it. If you're building something from scratch it would seem that throwing some $$$ at PMO would be the way to go. If you're constrained by rules & regulations then this thread is not even an issue is it?

Mark

914/6 2.7L with good ole' Webers :-)

I have 46mm PMO's on my race motor.... Big improvement over the webers in cornering and throttle response. PMO's are $hitload easier to adjust than MFI or webers.
My vote...PMO's.... less headaches all around.

MFI systems are about throttle response not just horse power.
Most of the specs quoted for 2.0 liter 911 race engines with 46 mm Webers are set up with 42 mm venturis.
For those of us who have driven this set up, drivablity is an issue.
Richie and Milt could drive these engines but mere mortals struggle to make it work.
Hi g force, long corners cause carburetors to starve for fuel. Hard cornering acceleration is often interrupted by a bobble.
When properly set up the MFI system doesn't make more horse power it's just more drivable.
With carburetors port size is for more critical, because port size effects port velocity and with MFI port velocity seems to be less critical.

PS: How many of you sync carbs at multiple positions through out the race weekend.
If you don't, half your engine could be running stronger than the other.

Urban legend? Jarvis Tech claims to have a 2.5 liter twin Porsche engine that makes 325 with PMO carbs. If that is true, PMOs are king.

We had the bobble with the Webers in high-G corners. No bobbles at all with the PMO's. Thats our experience with (3)- 2.0L 906 spec race motors with 46mm PMO's, and one 2.5L with 46mm PMO's. Our expereince with the Webers and PMO's was night and day. I only know of one guy off the top of my head that ran MFI with VARA. He no longer races. All the fast guys/gals run PMO's. I'm not saying that MFI is bad, we just do not see them at the track that much anymore.

Henry, what do you think of running PMO 46 carbs on a twin plug 3.4 with 10.5:1 and a modified S cam? How much more power would I get going with ITB (jenveys) and say a DTA or Haltech?

Cheers

Quote

Originally posted by Plavan "I only know of one guy off the top of my head that ran MFI with VARA. He no longer races. All the fast guys/gals run PMO's".

Welcome to tunnel vision.
Vara is a very small world with limited exposure.
Porsche raced carburetors generally when homologating for production classes could not be achieved.
Let's take a walk down memory lane to a time when 2.5 liter Porsches were raced by the pros for money not checkered flags.
I can't remember one front running 2.5 in IMSA that ran a set of carbs.
Few cars runs MFI now, but performance is not the reason.
Most clubs like SCCA outlawed MFI as a competitive advantage and by definition if clubs outlawed MFI years ago then it follows that vintage race car would be run sans MFI.
BTW: Most vintage clubs have outlawed MFI as a competitive advantage (you can only run the cars as raced in their day)

The last race car that Porsche built that ran carbs was the 2.0 and real factory race cars ran MFI until rule changes made fuel mileage an issue. At that time we note that they didn't go back to carbs.
2.0 Twin cam, 908, 910, late 906, *917*, 911ST, RSR, 935, 936, RSR 2.1 Turbo, Baby 935.......

One last thought: How many more guys would still run Webers if you could buy them new at the same price as PMOs?
Don't get me wrong I like Richards' product but let's use some logic when accessing uses of a particular product.
"IF THERE IS ONLY ONE PRODUCT AVAILABLE, EVERYONE USES IT."

ALL THE FAST GUYS IN NASCAR RUN GOODYEAR TIRES, WOW THEY MUST BE THE BEST

I would guess 7 to 12 %. I build an engine very similar to what you describe (46 Webers on a racing 3.4) and we should have a dyno sheet next week. I'll post the results as soon as I have them.

I love this discussion. My dad and i are planning a 964 3.6 for his car right now and ive been wondering about the cost difference between mfi and carbs. Personally i think mfi is the sexier setup so im leaning towards that. Of course crankfired mfi is the ultimate but this isnt a track machine.

Henry, didnt the factory run mfi in limited number on some of the 956/962 variants?

From reading and research carbs are the more adjustable alternative versus mfi in which you need a 3d space cam.

Henry, what would you charge to setup a custom pump with a bespoke throttle body setup? I would just need the pump all fixed up and a custom cam thats all, oh and your headstuds and slick fuel lines.

God I hate porsche sometimes, i see my cousins stock bottom end supra pumping close to 900bhp and i shake my head.

ryder

Henry,

Can you elaborate?

Should I be doing a synchrometer check on the MFI stacks, also?

Henry,
How many of your motors are running in VARA with MFI? How many of your motors are running with HSR West with MFI?

I only know of one Supertech motor that runs with VARA, and it has carbs..... Why? "Tunnel Vision" ????

Quote "Let's take a walk down memory lane to a time when 2.5 liter Porsches were raced by the pros for money not checkered flags.
I can't remember one front running 2.5 in IMSA that ran a set of carbs."

"BTW: Most vintage clubs have outlawed MFI as a competitive advantage (you can only run the cars as raced in their day)"

Maybe I don't understand but the statement above seems like a contradiction? Why wouldn't a club allow a 2.5L with MFI if it was raced that way?

I would imagine that if you owned an Ex IMSA car you could run it in some class in vintage.

I don't believe that there were many 2.0 911 IMSA car's but if you had one they wouldn't fit in most of the rules set by vintage racing clubs.

As an example, CP race cars have a 7" wheel limit and GTU cars ran 14" rear wheels.