Gregg K

The most obvious gain would be due to the elimination of the intake tract. But why would anyone want to go from fuel injected to carb? I hate toilet bowls. In fact, I doubt my first sentence is correct, because one can just put throttle bodies on each cylinder and keep the injection system. Hmmmm. Still scratching my head.

Steve J.

"Project 928", pg 90 - "In 1972 as pre-testing was getting underway, Chassis Testing outfitted the first mobile test bed, "V1", a Mercedes 350SL with original engine. The first transaxle experiments were carried out on this car. Later, V1 was fitted with a 928 carburetor engine."

Don '85S3

How do you set a Shark back 10 years technically?, this would be it. I would have to agree with Greg and ask "why would one want to do this anyway?" Looks like a real freak show to me.

mark kibort

This is not nessesarily true. NASCAR still uses carbs and many ExTREMELY fast World challenge cars, were retro, forward , fitted with FI to meet World Challenge spec. (the Carb versons were faster and much stronger than the FI systems installed)

the only gains with a FI system is reliability, tuneability, simplicity. big carbs do a great job of keeping mixtures good, and flowing lots of air.
they dont start as easily, and have to be adjusted for large altitude changes. But make no mistake. you see a V8 with 4 dual webbers and WATCH OUT. that could make some scarry hp. (if it is cam'ed right)

the only real difference is in emissions. but if you are racing, who the heck cares about that!!!. (yeah, low emissions for FI, but I get 4 miles to the gallon at the track, but 22mpg on the way home!!)


MK

GMS

The only problem I see; there's nowhere to install an E-ram

mark kibort

simple, you need one of those mustang carburetor air boxes or MGB carburetor hoods for the dual twins with an inlet. the eRAMs would attach right to that!
ha ha

MK

GMS

Very funny...Happy New Year!!!

2V4V

Now c'mon Mark...

I know that we have our differences, but you can't really believe for a micro-second that a carb system is in any way better than an FI system, can you?

Carbs and other early technologies only exist in racing environments that require their continued existence. Please, call ANYBODY who builds NASCAR engines (or any other series that mandates carbs). They will tell you they'd switch to EFI (or hell, even CIS) in a heartbeat if the rules were changed, because injection makes more HP, more TQ, and get better MPG while doing it. Especially in a restrictor-plate environment. Last time I checked, fuel economy DOES matter in racing. And since you mentioned it, so does reliability, tunability, repeatability.

Sure, a really poorly designed/primitive FI system might be beaten by a really good carb setup, but even a mediocre FI system will beat a carb any time by any measure. Even a carb-per-cylinder rig.

I don't recall too many series mandating FI, rather, they allowed it and everybody immediately went to it - if they wanted a place on the podium.

Nobody argues that you can make a lot of power with a carb. You can even make a lot of HP with a magneto, a Roots blower, and pushrods actuating your valves. But you can take the same block and dramatically increase the output by installing EFI, CPP ignition, a twin-screw blower, and overhead cams.

Happy New Year,

Greg

mark kibort

I think we are getting into semantics here now. yes, FI is the lastest and greatest technology. no one is producing todays cars with carburetors, except retrofits on vintage racers! However, the gains in power are minimal over a carb by a FI system if you have a well tuned Carb. as long as the mixture is correct for most power, the effiency at full power will be only slightly better with a FI system, due to better atomization by the injectors. cruise economy will be much better with a FI system though.

Again, the main disadvantages of a carb system is that it is difficult to tune to optimize and tune. however, it surely can be better in areas of simplicity when things go wrong. (FI systems can require Computer specialists to diagn. issues!)

yes, FI is better than Carbs. but all things being equal, there wont be that much power difference if that is the only Metrix.

Mk

Bryan

I had a chance to look at a set of carbs off an early '90s Honda VFR motorcycle recently. They're really quite ingenious. It's a constant velocity design that uses a second vacuum actuated throttle plate in each barrel to keep the velocity of the air coming through the carb constant. This lets you have great low end performance (low volumes of air are still moving quickly for good fuel atomization) and screaming high rpm power too. And since the carb is a simple mechanism, you can have a very low restriction path for air to follow from the air filter to the intake valve. With fuel injection (excluding racing injection setups) you have to have a common manifold for all the cylinders to breathe from. This is because for injection, you have to measure the mass of the incoming air somehow - either through a CIS plate, a flapper-door and temp sender, hot wire air mass meter, or through speed-density calculation. Unless of course you want to put an air mass meter on each cylinder. The kinks and contortions of the intake tract are less optimized than having a dedicated throttle and velocity stack for each cylinder.

There's nothing inherently better about carbs other than they're simple and rather clever. Injection can also be set up to provide optimum power. It's all about the air/fuel mixture under load. Carbs can do it perfect, injection can do it perfect. But outside of racing injection, the complex curves and kinks in the intake tract of a fuel injected car with a common plenum/intake manifold give carbs a theoretical advantage (as long as they're jetted and synched right - a whole different story).

The ideal synthesis of the two, in terms of power, is racing injection. Frequently you will see pictures of race engines with velocity stacks but no carbs. In each stack is a little injector. All the injectors are plumbed back to what looks kinda like a diesel injection pump, usually driven by a cam. The system works by being engine speed dependent (the faster the cam turns, the faster the pump turns, the higher the fuel pressure at the injector) as well as having the throttle linkage connected to it to alter the fuel delivery based on throttle position. You can look down a velocity stack and see the tip of the injector, a throttle plate, then the intake valve. Talk about no restriction.

I believe the European 911S for one year back in the '70s had this setup. It's of course very hard to calibrate and set up, but once it's dialed it it's supposed to be awesome. This is what I would have done instead of carbs if someone held a gun to my head and made me remove my stock LH setup.

Bryan

Jim bailey - 928 International

the 1980-1984 USA fuel injection was designed with one primary objective to meet USA smog laws ! remember this was when speedometers were only legal to read 85 MPH . Performance was a bad word , horsepower levels of all cars was way down ........... it is not that hard to improve on a "bad " thing .

2V4V

I don't really think its a semantics thing. The fact of the matter is, sure you can get HP out of a carb. We have no disagreement on that one. But, a perfectly (at least for that exact temp, air density, humidity, engine condition, ad nauseum) carb will not produce the final HP/TQ of an equally prepared FI system. The (even average) FI system will auto correct for all the variables above, and then some. When any of the air temp/density/humidity variables change on a carb car, get ready to re-jet. And by the time you got it set up "right" again, the conditions have changed again and so it goes. The FI motor will just keep outproducing the carb motor with no operator input - self correcting thousands of times a second. Even in an engine dyno room.

I guess you could make a case that a carb is "simpler" to fix, but a properly executed FI system is pretty much self diagnostic these days, I have yet to see a carb that tells me whats wrong with it.

Of course, we can only have this "maybe I can get within 10% of an FI system with a perfect carb at top revs" discussion in the limited realm of NA cars. The minute you force-feed an engine, the carb gets pulled off and melted back down to make more Milwaukee's Best 40 oz. cans. Or your powerplant will. All the inherent limitations of this piece of history render it all but useless. Yes, I remember guys used to rig carbs to "make do" on forced-charge scenarios. It never worked even half right.

Even EFI, at least as most of the world knows and sees it, is fading into the engineering rearview mirror as well - being replaced with even more advanced stuff.


Greg

Bryan

Ah yes, the late '70s and early '80s. Our pennace for the crazy hp of the muscle car era.

I don't understand why Porsche switched the 928 from CIS to L-jet. Lots of other cars (Audis and VWs mainly) stayed with CIS through to the early '90s, and it met the ever tightening emissions standards without a problem. I'd love to know the technical reasons for the switch.

The 85mph speedometer was so ridiculous. I can only imagine the eye rolling that happened at Porsche when yet one more goofy piece of equipment was mandated for the US versions. The idea was that it is "disturbing" to the driver to drive with the speedo pegged, so people will slow down, preferably to the big red circled "55" on the instrument. The motivation behind the Cannonball Run is pretty apparent in historical context.

If carbs had received the same couple decades of technical development that EFI has, I bet they'd be pretty sophisticated today too. I do understand the advantages to EFI for maintaining consistent air/fuel ratios especially when used with a closed-loop control system. But I also admire the simplicity and elegance of the constant velocity carb too. The Honda VFR starts perfectly and runs perfectly in hot weather, cold weather, wet weather, dry weather, you name it. So it's hard to trash the carb setup too badly.

The EFI setup on the Euro 928 ('84 to '86) is an interesting variation. It has all the advantages of EFI - precise (via hot wire air mass meter) incoming air measurement for precise fueling, but is somewhat carb-like in that it's an open loop system. You can set your air/fuel ratio via an adjustable fuel pressure regulator and you actually get altered fueling across all operating conditions. There's no O2 sensor circuit overriding everything and forcing a specific air/fuel ratio. Again, with closed-loop systems there's good and bad here. The good is that the engine will run stoichiometrically all the time and get good mileage and low emissions and can compensate to a degree for engine wear and vacuum leaks. The bad is if you want to set your a/f ratio to 12:1 instead of 14:1, you really can't unless you're at full throttle (when the system goes open loop).

Bryan

2V4V

Bryan,

Sure, bike carbs aren't bad - but once you've ridden a properly FI'd bike, you learn what's missing. The only reason they still use them on bikes, is MONEY - the tooling is paid for, the R&D is in the can, they can still squeak by on emissions. Every new sport bike that comes off the paper, is injected, because that's when it gets worked into the budget. To make a carb work even close to right is far more complicated than an FI set-up, and wait till those fancy CV Keihins have got 20K miles and a couple of years on them, you might be far less enamored of them.

If you ever have a few days to waste, rent a dyno room, buy a motor, hire the best and brightest race carb doctor there is to come in an "set-up" the carb system. Dyno for a whole day.

Tear off the carbs, put on a decent EFI system, have an average race FI tech tune it, dyno for however long it takes for you to read that all the output numbers are bigger (except fuel consumption and emissions), and then take the carbs to a museum or the aluminum recycling center where they belong.

Carbs have had 120+ years of development. They're still a stone axe. Since the day of their creation, they were a stepping stone only here to be stepped over. They have lousy mixture control, lousy repeatablility, tiny little passages and jets that are negatively impacted by wear incredibly quickly.

I find it quite easy to trash them- because at their very best, they don't work very well.

Greg

Jim bailey - 928 International

Greg...."Tear off the carbs, put on a decent EFI system" I think that is a very accurate statement .....The initial question however was why put carbs on a 1984 USA 928 ? I think .... Tear off the L-jetronic , put on a decent set of carbs is equally accurate , especially with bigger cams and headers . The biggest problem people have with the carburetors is excessive heat boiling the fuel after shut down since the manifolds are so short and the carbs so close to the heads . No argument here ....Modern EFI is much superior to carbs ......point is L-jetronic 1980-1984 is far from modern or decent but does a good job making 200-230 HP ....... Robby Gordon's offroad Baja trophy truck has a unlimited NASCAR Roush Racing engine FUEL INJECTED and was said to make 950 HP ; easy 100 more than the normal carburated.....his race shop is one block down Mira Loma ave .