Performance/drivability of MFI vs. Carbs for 2.0 race engine
 

I have been looking for a suitable 911 to build into a historics race car. The group I'm planning to run in spans 61-69. I'm looking for a 69 for its long wheel base.

The cars must run close to stock but engine internals are pretty free. The preferred car is a 69 911E so I can run MFI (without the S price premium) but even a decent E is proving hard to find. If I run a 69T it must use carbs.

QN1: What sort of a performance/drivability difference would I experience if I built a mild 2.0 race engine (10.5:1, GE80, big valve heads, megaphones etc) and ran it on 40 or 46 webers compared to MFI.

QN2: Could you realistically drive a 2.0 with 46 webers on the street? (This car would likely be driven to the track)

My rally car has a similar spec 2.7 with MFI which I love. I don't have a lot of carb experience but I was lucky enough to check out a 906 a few weeks ago and it certainly seemed to go OK on carbs...

The experts here in CT tell me that you can't tell the difference between a well set up MFI system and a set of PMO carbs.

Based on Porsche's experiences (and their published charts), a well set-up MFI will get you another 10 HP +/- over carbs at peak RPM's (all other things being equal). It will also make more torque at relatively lower rev's because it is less affected reversion. In between the two extremes carbs and MFI will get pretty much the same performance.

MFI also will get you better mileage.

tom - is this in a race only engine or a street/race?

john - i had heard something similar and my 2.7 runs far better at all points of the rev range with MFI compared to my previous carbs setup

any idea for QN2 - would 46 webers on a 2.0 litre be drivable?

I think you need to be careful of megapones on MFI (correct me if I'm wrong). Isn't MFI a little pissy if it doesn't have exhaust back pressure?

Actually it's a question of taking into account the change in the harmonics -- not back pressure. At about 4000 RPM with racing exhausts and cams with overlap, it's possible to reach a harmonic where there is actually a pressure wave at the exhaust port during overlap, which can allow the pressure pulse to go right through into the intake port. If you have carbs you'll have a reversion issue. If you have MFI, it will be more of a flat spot unless you adjust the map (ie: the 3D space cam) of the MFI pump to take the change in fueling requirements into account. The harmonic properties of Megaphones and their cone shape can make the affect even more pronounced then with a sport exhaust. The good news is that they also make extraction affect stronger when there is a negative pulse at the exhaust port which happens roughly 500 RPM away from the positive pulse.

If you can tweek the 3D cam (as Porsche did with their 2.8RSR engine), you'll most likely find that these changes in fueling requirements do not create a major driveability issue.

john: i agree - i have found my MFI to be primary length, not back pressure, sensitive. i have caps for the sport muffler on my 2.7 and switching from 1 out to 2 out (or even 3 out) does not cause any big tuning issues. i think this is because although the back pressure changes, the primary exhaust path length does not.

kenikh: i think a lot of stuff about MFI engines being back pressure sensitive comes from folk who don't fully understand how they work.

I think with modern PMO carbs you will find the results as good as MFI with about one tenth the effort in the tuning. I have MFI on my race car and love it but I'd trade it in for carbs if it wasn't already set up for my engine. I think the 10 HP difference was for cars with smaller webbers and restrictive venturis. I don't think you could find 10 HP anymore with the PMOs vs MFI.

-Andy

Isn't the MFI pump space cam set up for a specific fuel delivery for a specific factory engine? And if the engine isn't a factory setup (different cams, compression ratio, spark lead, enlarged ports, dual plugs, custom headers, etc.), what do you do? If there isn't an adequate answer, I'd go with the more tuneable PMOs or Webers.

Sherwood

"Isn't the MFI pump space cam set up for a specific fuel delivery for a specific factory engine?"

to an extent, but the adjustments you can make to the idle and main racks make them pretty flexible.

the 2.7RS space cam in my pump handles the mods (10.5:1, GE80, big valves, ported heads, 1 or 2 out sport muffler etc) to my motor no problem

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.

Some of these things can improve the driveability of PMO's compared to Webers or Zeniths, but as far as peak HP -- a properly set-up MFI system (or any non-air-metered EFI system for that matter) will trump carbs for peak HP just because MFI doesn't have any venturi's -- which by definition restrict air flow. This is why carbs are NEVER used in top flight race cars any more unless they are required by the rules (as in NASCAR).

Is MFI used in "top flight race cars" these days? I also doubt that the MFI will make any more noticable horsepower than a set of PMO's. I am not an expert at all with MFI, but the expert I talked to seems to think they make about the same power on their engine dyno.....


Cheers,

MFI isn't used on ANY "top flight race cars" these days as far as I know, although diesels still use a varient of the technology (abiet with electronic computer control). I'm not sure if the Audi R8's direct injection use a varient of the technology as VAG's TDI diesels do.

Rather then using MFI, today's race cars use EFI (duh!):rolleyes: . HP is entirely about airflow -- and as far as the airflow is concerned, MFI and EFI look the same. The only restriction is the throttle.

Carbs on the other hand force the air through a restrictor (aka: Venturi) which limits the airflow, just like FIA mandated air restricters do. That's what limits the HP.

I understand how it all works. my question about the mfi on race cars was a response to you stating that carbs aren't used on any top flight race cars......:) The venturi is only a restriction if the cylinder can take more are than the venturi can flow, and I do not have the answer to whether or not this is the case as it would be different for every engine combo.

Cheers,

Let me turn the tables a bit...

Name me one "Top Flight" race car which uses carbs even though they are not mandated by the santioning body. We're talking professional race cars here -- not weekend racers. No professional race teams since the 60's have used carbs if they didn't have to.

I can't, nor could anybody else I guess. This was a discussion about MFI vs carbs and you made a point of saying that no top flight race cars use carbs. I assumed the only reason you said it, based on the discussion taking place, was to infer that becuase it is not being used it is not as good. My point in asking if any use MFI was to say the same argument would apply to it as well. That is all. If I misuderstood what you meant or the point of your statement, I apoligise. I never said MFI wouldn't make more power, I said that I doubt that it would. I am willing to be convinced, I come to this forum to learn.

Cheers,

The reason that no-one is using MFI anymore is because they are a 8itch to program. Getting out that grinding wheel and grinding the 3D-Space cam is really tough on the nerves. Pretty much anyone nowadays can change a "6" to an "8" (or something similar) on a computer screen though.

So basically, everything is done by computers nowadays. But the airflow management technology hasn't changed much since MFI and slide throttles. (The big exception is variable length intakes.)

The reason that I'm picking on you is because I purposely paranthetically added the note "or any non-air-metered EFI system for that matter" when I made the statement about MFI that you keep getting hung up on.

Now we know you are picking on me. :) I though we were only talking about and comparing MFI and carbs on a 2.0 race engine. Maybe it will make 10 more HP, only someone who has run both on the same engine really knows for sure.

Cheers

Here are the best examples that I know of and the source of the data:

1) Porsche 906's. I've compared the HP graphs of the carb'd engine to the MFI'd engine and posted the data in the past. The difference is as I described.

2) Jaguar Race Engines. I believe that there was a passage in "Classic Race Engines" that also said that there was a 10 HP difference. I'll try to find it tomorrow.

3) Coventry Climax Race Engines. I'm pretty sure there is some data in Des Hammill's book that also supports that there's about a 10 HP difference in peak HP.

I'll try to find the Jag and Climax reference tomorrow some time.

another issue to keep in mind is some of us race in historic events where EFI, computers etc are banned (some series won't even allow PMO's in place of Webers)

this is my situation and why i run MFI, why i retain CDI ignition and why my soon to be installed twin plug uses a bosch cap/rotor - plus dyno tuning carbs is a PITA (changing jets etc) compared to click click click on a MFI pump

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.