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

[obrut]

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...

[tom1394racing]

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.

[jluetjen]

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.

[obrut]

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?

[kenikh]

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?

[jluetjen]

Originally posted by kenikh
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.

[obrut]

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.

[Eagledriver]

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

[911pcars]

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

[obrut]

"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

[jluetjen]

Originally posted by Eagledriver
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

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).

[Jeff Alton]

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,

[jluetjen]

Originally posted by catca
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.....

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!) . 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.

[Jeff Alton]

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,

[jluetjen]

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.

[Jeff Alton]

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,

[jluetjen]

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.

[Jeff Alton]

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

[jluetjen]

Originally posted by catca
Maybe it will make 10 more HP, only someone who has run both on the same engine really knows for sure.

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.

[obrut]

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