Does anyone know of any books or links on fuel injection vs carbs in a general sense?

Any help would be greatly apreciated.

Thanks!

Andrew

FI blows but carbs suck... what else?




try www.howstuffworks.com

"Scientific Design of Exhaust and Intake Systems" covers some basics.
the Robert Bosch Fuel Injection book is quite comprehensive.

carburetors must be vented to the atmosphere - it's how they work.
they are therefore incompatible with contemporary emissions control requirements.

with today's increasingly volatile fuels, float bowl evaporation is significant.

carburetors must restrict intake airflow - they work by creating a pressure differential across a venturii. FI doesn't have venturiis; slidevalve throttle bodies don't present any restriction to the airflow at WOT - not even the edge of the throttle plate and the shaft that carries it.

QUOTE (tat2dphreak @ Oct 6 2005, 06:44 AM)

FI blows but carbs suck... what else?

Carbs. are compatable with vapor emission standards, My triumph Carb float bowl is vented to the vapor recovery can, and this car runs very very clean per dyno smog tests, it even bet some new cars, and it is 25 years old

A good carb book would be one of the Haynes series. They have a book on Webers for instance, and it goes into a lot of detial on theroy and how the carb works. Even my old 1961 VW Bug shop manual has nice cut away drawings of the carb, and how it functions in different operating modes (idle, accel, power etc....) So check out an old VW bently manual (Heck get the one that covers the last Bugs, cause it will have both carb and FI info)(admitatly early FI system on the Bug)

There are also good basic books on FI, again I would check the Haynes series of automotive book (look on ebay, sometimes you can find them there for cheap)


You should also be able to find stuff on the internet (but I usually prefer books) also this site is a great resource, we have folks that know carb, FI and both, so ask any questions you have.


good luck

QUOTE (dmenche914 @ Oct 6 2005, 08:41 AM)

Carbs. are compatable with vapor emission standards, My triumph Carb float bowl is vented to the vapor recovery can, and this car runs very very clean per dyno smog tests, it even bet some new cars, and it is 25 years old A good carb book would be one of the Haynes series. They have a book on Webers for instance, and it goes into a lot of detial on theroy and how the carb works. Even my old 1961 VW Bug shop manual has nice cut away drawings of the carb, and how it functions in different operating modes (idle, accel, power etc....) So check out an old VW bently manual (Heck get the one that covers the last Bugs, cause it will have both carb and FI info)(admitatly early FI system on the Bug) There are also good basic books on FI, again I would check the Haynes series of automotive book (look on ebay, sometimes you can find them there for cheap) You should also be able to find stuff on the internet (but I usually prefer books) also this site is a great resource, we have folks that know carb, FI and both, so ask any questions you have. good luck

Charles Probst's Bosch EFI book also covers the topic decently, and is a useful book in general.

I'll agree that carbs CAN be made to work reasonably well with emissions, up to a certain level. Venting the float bowls in to a charcoal can, and fuel cutoff solenoids, can solve some of the problems. CV carbs, like SUs and Strombergs, also tend to meter fuel better, and thus generally run cleaner. However, no carb could approach the LEV standards, let along the ULEV, SULEV, or PZEV standards that EFI cars are now passing. This is partly because to reach SULEV and PZEV levels, you have to run very lean at cruise, yet still be able to run rich enough at WOT to have some acceleration. This is hard to achieve with an all-mechanical carb. I've seen some setups where a separate WOT valve will kick in to supply extra fuel, sometimes based entirely on manifold pressure. It's still not all that well controlled.

One must also remember that the regular smog checks, even the dyno tests, are vastly oversimplified from the EPA tests that set the standards. Just because it does well on the smog check doesn't mean it would do well on the EPA tests. I expect your 25-year old Trumpet would do fairly badly on the complete tests.

That said, new catalytic convertors are amazing compared to what was produced in the 70s. I've owned 70s cars with 70s emissions gear but a new cat that could barely be measured on smog check equipment, and would pass late 90s standards.

On the research paper, I have no idea how extensive this needs to be, but "fuel injection" covers a pretty wide ground. There are basically only two classes of carbs (fixed and variable venturi, the latter also called constant velocity, or CV carbs), while there are several classes of fuel injection systems.

Direct (straight into the combustion chamber) v. indirect (into the manifold, before the valve(s)) injection, mechanical v. electronic. Mechanical direct injection predates all other forms, originally invented for the Diesel, and applied to spark-ignition engines just prior to WWII. It fell out of favor due to high costs, but is now returning to production use. The newest VW and Audi cars have direct injection. The advantage is the fuel can be injected (at high pressure and thus high speeds) after all the valves have closed, so wild cam timing doesn't allow any unburnt mixture to escape out of the exhaust valve. The fuel can be sprayed directly at the plug, as well. Combined with the better mixing from the vaporization caused by the high injection pressure, you get much better combustion, so both power and economy improve.

While most production EFI systems were employed for improved emissions, with better power just being a side-effect, it should be noted that in F1 of the early 1960s, both carbs and FI (usually mechanical) were employed, and the FI cars generally made 10-20% more power. The FI was more expensive, and F1 was a cheaper sport then, so not everyone used them. Privateer teams and junior drivers tended to get carbs, where senior drivers got the FI. The FI cars also had better drivability, since the carbs used on the engines of the period (2.5L engines of 250-300hp) used 50-52mm Webers, not the wimpy little 44mm jobs seen on 2.3-2.5L street Type IVs. Drivabilility with those carbs was terrible, however, with nearly zero power below 70% of redline. FI doesn't rely on throat size to meter fuel, however, so a 50mm throttle body still gave decent drivability, and there was no venturi, which is where the extra power mostly came from.

Thanks all! everything helps!

Ranting and raving would be great for "quotes"... lol

I appreciate your help

Andrew

QUOTE (Andyrew @ Oct 6 2005, 01:03 PM)

Thanks all! everything helps! Ranting and raving would be great for "quotes"... lol I appreciate your help Andrew

Andrew,

I'd look at Hot Rod and Car Craft, I know they do comparisons all the time between the 2 types of fuel control.

From one test I remember reading the motor was like a 700hp big block, the carb put out like 5hp more at redline, yet the torque was better with the FI (huge 4 barrel carb, large 4 barrel throttle body mounted to manifold)

another test on a mild small block, the FI was about 10% less in peak HP, but the torque curve was much, much better with the FI....in this test, they had a 4 barrel carb but installed a stock tuned port FI which is not really a fair comparison in my book.

Both mags should have on-line archives of these articles.

In my opinion ....

The real advantage of carbs is that (if you ignore what goes on inside them) they are simple. If the carb isn't completely gummed up inside, anybody can make an carbed engine run. And it's not too hard for someone that actually knows what they are doing to tune it so that it runs well. But basically, its a mechanical system with only a few parameters that can be adjusted, so you have to hope that the carb in question was well matched to your particular engine, including aftermarket mods.


Modern fuel injection systems are electronic, and as you might have noticed, electronics have improved somewhat in the past few decades. The actual guts of an electronic FI system are dirt cheap (check out MegaSquirt, and try pricing the chips without the circuit board; circuit boards approach $0 in cost when mass produced.) So basically, an EFI system can deliver exactly the amount of fuel your engine needs under any circumstance. The only issue to resolve is : how much fuel does your engine need ? This is resolved by a number of sensors that report : intake air temperature cylinder head (or coolant) temperature, throttle position (and hence changes of throttle position), air pressure inside the manifold (on newer systems, this is absolute air pressure, not just relative or "gauge" pressure, engine RPM, and the level of oxygen (O2) in the exhaust.. So I think its pretty much a consensus that if all this information is properly sensed and processed electronically, then extremely well-tuned fuel delivery is possible, even commonplace. Also, the O2 sensor provides something that is (sort of) lacking in carbed or older FI systems : feedback. If the engine is running too lean, there will be O2 in the exhaust and this is reported back to the EFI controller; same for running rich. The downside is all that complexity. If you're not a total geek, the number an complexity of all the parts and wiring can be intimidating, and paying for someone to diagnose the system (usually by someone who isn't a total geek and is also intimidated by all the complexity) can be $$$. Another limitation of stock EFI systems is that they are well-tuned for the car AS IT WAS SOLD. If you put in a hot cam, add a sporty exhaust, do head work, etc., you have a very different engine with very different needs. Some of this is adjusted for automatically, but some of it is not. Fortunately, for this problem there is a cure : programmable EFI (PEFI). These systems allow you to specify for a given RPM and a given manifold pressure the amount of fuel needed. This amount is tuned by the owner, or by a tuning shop, to maximize power and/or emissions performance.

If you're really into this (and if not, you probably would have chosen a different topic...) check out the websites of various aftermarket EFI vendors. I know of
sdsefi.com and msefi.com. The latter one, MegaSquirt, is pretty cool : it is an open design that you can build from a kit, modify, and extend. And its the cheapest thing going. If I didn't already own an SDS system, I'd be all over it.

Good luck and have fun.

QUOTE (jsteele22 @ Oct 6 2005, 01:22 PM)

... Also, the O2 sensor provides something that is (sort of) lacking in carbed or older FI systems : feedback.

Actually, there are closed-loop carb systems. Calling them "complex" would be a pretty vast understatement, though... And they don't meter fuel as accurately as any current-generation FI system does.

BTW, some carbs even without O2 feedback are still hellishly complex. For instance, if your early-80s Honda has carb problems, it is usually cheaper to buy another car than to have the carb rebuilt...

--DD (Whose folks had an 82 Accord for many years.)

QUOTE (ArtechnikA @ Oct 6 2005, 07:13 AM)

carburetors must be vented to the atmosphere - it's how they work. they are therefore incompatible with contemporary emissions control requirements. with today's increasingly volatile fuels, float bowl evaporation is significant. carburetors must restrict intake airflow - they work by creating a pressure differential across a venturii. FI doesn't have venturiis; slidevalve throttle bodies don't present any restriction to the airflow at WOT - not even the edge of the throttle plate and the shaft that carries it.

Nit-pick (apologies in advance), and a little input..........

Pump and diaphram carbs do not vent to atmosphere, outside of the discharge jets (nozzles).

Carb jets respond to the velocity of the air passing across their orfice. Venturi's are used to boost (or increase) that velocity. A carb can be configured without a venturi. Look at a flat slide dirt bike carb (though you could argue that they have a defacto venturi created by the valve itself, though only at less than WOT).

Andys

QUOTE (andys @ Oct 6 2005, 02:52 PM)

QUOTE (ArtechnikA @ Oct 6 2005, 07:13 AM) carburetors must be vented to the atmosphere - it's how they work. they are therefore incompatible with contemporary emissions control requirements. with today's increasingly volatile fuels, float bowl evaporation is significant. carburetors must restrict intake airflow - they work by creating a pressure differential across a venturii.  FI doesn't have venturiis; slidevalve throttle bodies don't present any restriction to the airflow at WOT - not even the edge of the throttle plate and the shaft that carries it. Nit-pick (apologies in advance), and a little input.......... Pump and diaphram carbs do not vent to atmosphere, outside of the discharge jets (nozzles). Carb jets respond to the velocity of the air passing across their orfice. Venturi's are used to boost (or increase) that velocity. A carb can be configured without a venturi. Look at a flat slide dirt bike carb (though you could argue that they have a defacto venturi created by the valve itself, though only at less than WOT). Andys

Even at WOT, the slide hangs down a little. Also, the needle usually helps a bit in creating the necessary vacuum. The carb throat upstream of the slide is generally never completely smooth, either, but forms something of a venturi shape. Some slide-throttle carbs (certain Dell Ortos spring to mind here) even have a small air dam just upstream of the jet to create extra vacua above the jet.

I'm not familiar with the terms "diaphram carbs" or "pump carbs". Any carb with a float bowl, however, needs to vent the float bowl to the atmosphere. It's not the low pressure in the venturi that "sucks" the fuel, it's the higher atmospheric pressure above the fuel in the float bowl the "pushes" it up through the jet and into the carb throat.

If by "diaphram" carb you're referring to carbs that have a slide held by a diaphram, then that's a CV carb, and they most certainly have vented float bowls. CV carbs are used on most street bikes now, and have been used on English cars for decades (the SU and the Stromberg).

QUOTE (lapuwali @ Oct 6 2005, 04:40 PM)

(the SU and the Stromberg).

They are called "constant depression" carburetors.

QUOTE (lapuwali @ Oct 6 2005, 03:40 PM)

Even at WOT, the slide hangs down a little. Also, the needle usually helps a bit in creating the necessary vacuum. The carb throat upstream of the slide is generally never completely smooth, either, but forms something of a venturi shape. Some slide-throttle carbs (certain Dell Ortos spring to mind here) even have a small air dam just upstream of the jet to create extra vacua above the jet. I'm not familiar with the terms "diaphram carbs" or "pump carbs". Any carb with a float bowl, however, needs to vent the float bowl to the atmosphere. It's not the low pressure in the venturi that "sucks" the fuel, isn't the higher atmospheric pressure above the fuel in the float bowl the "pushes" it up through the jet and into the carb throat. If by "diaphram" carb you're referring to carbs that have a slide held by a diaphram, then that's a CV carb, and they most certainly have vented float bowls. CV carbs are used on most street bikes now, and have been used on English cars for decades (the SU and the Stromberg).

There are plenty of racing flat slide MC carbs where the slide moves completely from the airstream. There are also many that have a completely smooth bore. Most protrusions around the discharge areas, AFAIK, are mostly there to create turbulence to avoid the fuel from clinging to the carb bore although there are designs that use no protrusions as well. Yes, fuel can, and does travel up the needle in a needle type slide cab. I believe Edelbrock has a Harley carb where the needle has a flat on it's back side to allow the fuel to climb....or at least they used to.

A diaphram carb (with or without an integral pump) can be found on things like outboards, chainsaws, racing karts, ultralites, etc. where inverted positions and G forces do not effect their performance. I have designed and built a couple of carbs about 10 years ago. Also dry-sumped some carbs. Fun stuff, but lots of testing and development.

Andys

QUOTE (andys @ Oct 6 2005, 04:10 PM)

A diaphram carb (with or without an integral pump) can be found on things like outboards, chainsaws, racing karts, ultralites, etc. where inverted positions and G forces do not effect their performance. I have designed and built a couple of carbs about 10 years ago. Also dry-sumped some carbs. Fun stuff, but lots of testing and development. Andys

OK, do you mean the diaphram is basically in the float bowl, separating the fuel from the atmosphere? If so, that would neatly solve the problem. I suppose a traditional float simply isn't used, but rather the diaphram is thick enough to set the fuel level...

QUOTE (lapuwali @ Oct 6 2005, 04:43 PM)

OK, do you mean the diaphram is basically in the float bowl, separating the fuel from the atmosphere? If so, that would neatly solve the problem. I suppose a traditional float simply isn't used, but rather the diaphram is thick enough to set the fuel level...

Basically yes, that's the general idea. Very insightful, as I presume you're not familiar with them. The "wet" side of the diaphram contains the fuel and the passages to low, transitional, and high speed discharge ciruits. As the fuel is consumed, the diaphram trips a needle valve allowing fuel to replenish the chamber. Sort of like a pressure regulator, but the fuel is not forced into the throat. The "dry" side of the diaphram, though usually covered for protection, is vented to atmosphere. They are really pretty neat when understood, but can be tricky to get right. Fuel delivery systems take an awful lot of development to get right, and the downside consequences can be $$$. I had thoughts of converting a standard Holley 4 barrel carb as a possible off-road racing conversion product, but EFI pretty much takes care of such severe applications these days. Before effective EFI, they used to dry-sump foam-filled float bowls.

Andys

damn, after all this, my explaination is looking good