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Sarah Forst posted on January 24, 2011 19:38
Ask Sarah: What's Your Intake?
By Sarah Forst
I'm debating going carbs or ITB's with EFI. What is the advantage to using carbs over EFI? How do ITB's perform over a single TB?
J.T.
I love it- a question (and name) full of initials. Thanks for getting right to the point! Simply put, carbs are simple. Modern fuel injected cars use a throttle body and fuel injectors to control the proper air/fuel ratio. A throttle body uses a butterfly valve to regulate airflow into the intake manifold and combustion chamber. There can be a single throttle body or individual throttle bodies (ITB), which is a system using one throttle body per cylinder. A manifold absolute pressure (MAP) sensor or mass air flow (MAF) sensor is located in the intake to let the computer know how much air is flowing into the engine. Other sensors help the computer measure engine load, engine temperature, and calculate the required air/fuel ratio to control how much fuel- and when- is delivered by the injectors.
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| Bisi Ezerioha discovered that carbs can make more power than EFI under certain conditions and are particularly well sorted for all motor drag racing. Bisi uses sidedraft Weber DCOE carburetors on his Drag Honda Insight. |
Carbs handle the job of both the throttle body and fuel injectors by delivering both air and fuel. The carb directs air through a venturi. The air pressure differential creates a suction which emulsifies the air with the fuel. An increase in air mass creates a greater pressure differential and supplies more fuel. Since it's an all in one package, it's usually a cheaper and more simplistic system to employ- no fancy electronics or sensors. This primitiveness is also its downfall.
Modifications to carbureted systems were necessary in order for the engines to comply with the stricter exhaust emission regulations passed in the 1970's and 1980's. Manufacturers started moving to fuel injection systems because it was easier to pass these regulations. Early fuel injection systems may have been less reliable than carbs because they are more complex systems and rely on many other components to function properly. There are fewer failure points on a carb system. But nowadays, OBDII monitoring basically spoon feeds a diagnosing tech feedback from all the sensors in order to pinpoint any issues. With modern reliable electronics, a wideband O2 sensor and proper fuel management system, EFI is now more reliable and adaptable than carbs ever were.
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| A suck through carbed turbo system, the carb can function normally but the fuel distribution is pretty poor and it is difficult to eliminate bog on tip in. It is also hard to get a carb to fuel properly over such a wide operational range. Boosted operation requires a very large fuel flow, big needle and seat, big main jets. It is difficult to calibrate the low speed jetting on many domestic type carburetors. |
Carbs are a constant fuel situation. They can be dialed in for a small rpm and air density range and run less optimally at other engine speeds. This means if they're tuned for lots of power (more top end), they can be boggy and lose some low end torque. Though they are easier to tune (basically trial and error just changing the jet sizes and emulsion tubes), they require more tuning adjustments for changes in altitude or temperature or for different power goals.
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| A blow through turbo system, this works for low boost levels and has better distribution, however if boost pressure is very high at all, the fuel won't flow into the engine as carbs work at low pressures. It is important to make sure the carbs float bowls are pressurized to boost pressure. The carb will still have some problems carburating cleanly across such a wide operating range. |
Modern day electronic fuel injection systems are able to quickly adapt to varying conditions automatically. When tuned properly, they can provide the right amount of fuel for the entire rpm range. This also helps with fuel efficiency and they'll idle more smoothly. Carbs on the other hand, require a sufficient supply of fuel for all conditions, meaning some cylinders may get too much fuel at some times (in order to supply enough fuel for all times), but fuel injection can adjust to provide proper air/fuel ratios in all conditions. It's like having somebody change the jetting and emulsion tubes continuously. Carbs can offer a lot of peak power but at the expense of the rest of the powerband. EFI systems are capable of high horsepower applications without sacrificing low to mid range torque or driveability.
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| The 150 horsepower Chevy Corvair Spyder was one of the first production turbocharged cars. It used a draw through turbo setup and was known for its high operating temperatures leading to small oil leaks from failing o-rings. The oil would burn on the hot exhaust manifolds or occasionally generate a lot of smoke or a putrid oil smell. |
Carbs aren't exactly ideal in turbocharged applications either. On a blow through system, the turbo delivers the pressurized air to the carb and the carb goes "WTF?" because it doesn't know what to do with the pressure. Remember that a carb functions (provides fuel) due to a pressure drop across the venturi and will supply the fuel for that pressure drop regardless of the boost pressure. Without additional fuel enrichment, there could be a lean condition at higher boost pressures and issues with fuel flow. When blowing through carbs, you also don't want to leak the pressurized air/fuel mix so you should seal off the carb and pressurize the fuel bowls to equalize the boost pressure in order for the jets to deliver the proper amount of fuel.
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| A pressurized box for a a carburetor with a blow though system, This works at higher boost pressures. |
On a draw through system (the simplest way to turbo a carbureted engine), the fuel and air are mixed together first before passing through the turbo. But you can't employ intercooling because if you draw this air/fuel mixture through, the fuel vapors (which are denser than air) can separate and become concentrated, possibly igniting if the engine backfires; flambé anyone? Water injection is the only option for cooling. Since the turbo also has to pump this heavier mixture, it won't be as efficient trying to increase pressure in an area where the carb has already created a pressure drop. Lower turbo efficiency translates to a hotter air/fuel mix- not helpful for making power.
Basically, carbs are cheaper and simpler to tune but fuel injection is more adaptable, more fuel efficient (depending on your driving habits and what driving conditions the carb is tuned for), better for forced induction, and provides more driveability throughout the entire powerband.
Monday, January 24, 2011 11:48 PM
That V12 is serious eye candy. Was it anodized that color?
Tuesday, January 25, 2011 5:47 AM
There have been a number of comparos in HOTROD and other similar magazines on this subject. And the above is pretty much what they found. Muscle car guys usually go with carbs. Not because they want more peak power, but because they can't tune on a computer. I also heard somewhere that somebody has developed a carbeurator that uses electricity or electromagnets to improve atomization. This supposedly improves torque and mileage considerably. Wish I remember where I heard this...
Tuesday, January 25, 2011 6:40 AM
FAST and several other companies make fuel injector kits for muscle cars that automatically tune themselves...
Tuesday, January 25, 2011 10:04 AM
Bisi ran carbs for a long time, but has since switched over to AEM EFI. http://www.bisimoto.com/ There is no denying that a carb works. It even works well, but it is not as refined as EFI for part throttle and cruising.
Tuesday, January 25, 2011 10:38 AM
Something like this is more of a compromise between the two. http://bisimoto.com/store/images/B-seriesITBsJpeg.JPG By putting the injectors further from the intake valves, it gives the fuel more time to atomize and gives you a little more power by being able to run a little leaner. The advantage I see to carbs is the atomization. Most fuel injected motors put the injectors near the intake valve and don't allow enough time for the air and fuel to mix fully. And ITBs do sound DOPE Sarah!
Tuesday, January 25, 2011 10:44 AM
I talked for a little while with someone from Nissan about the RB26 and ITB's. The answer I got, why Nissan used ITB's was that in 1987,1988 when Nissan was developing the RB26, the ECU's available for the mass market were not that sophisticated. To make the HP goal, Nissan needed fairly large cams to be homolgated for Group A racing. In order to get the car to idle better they ran an ITB setup. The RB has no real plenum. If the plenum is between the throttle body and the head, then its only about an inch long. There is a collector, but its on the opposite side of the throttle body. The problem with an ITB is the thickness of the throttle plate, and boundary layer of air on each side of each throttle plate. It causes turbulent/no flow areas that actually end up being a restriction to flow at high power levels. This is one reason why you see a switch to a single throttle body. Less turbulent airflow with one big blade. Once you put the throttle body there, now you gain some volume for a plenum.
Tuesday, January 25, 2011 1:47 PM
The plenum is the "airbox", if you will, before the manifold runners, or with ITBs, before the TBs. So, the RB26 does indeed have a plenum, just a small one. Throttle plates and the shaft they attach to will always cause issues, but no more so than with carbs such as the Webber DCOE carbs in the article. They too have individual throttle plates. The comment about slide valves regarding the Toy 152E engine is misleading. Slide valves are simply the throttle plates and have nothing to do with fuel injection per se. Slide valve throttle plates replace the butterfly valve throttle plates. At WOT the slide valves slide totally out of the intake air path to present zero restriction of turbulence. There are slide valve throttle bodies and slide valve carbs as well. They have nothing to do with fuel metering.
Tuesday, January 25, 2011 5:52 PM
Nowhere does it say that slide valves have anything to do with metering.
Tuesday, January 25, 2011 6:00 PM
Let me rephrase.... Misleading is probably the wrong word. Substitute confusing. "It has ITB's not with electronic fuel injection but slide valve mechanical constant flow fuel injection." Slide valve mechanical flow fuel injection? It appears to suggest that the slide valves have something to do with the fuel injection.
Tuesday, January 25, 2011 8:34 PM
Not to me.
Tuesday, January 25, 2011 8:42 PM
Sean, does the R32 use batch fire or simultaneous injection control?
Tuesday, January 25, 2011 9:23 PM
R32 is SFI. Coil on plug , etc, etc. Although it shares a ton of things and ideas with the Z32.
Wednesday, January 26, 2011 2:49 AM
That's because you know what slide valves are Mike. To someone who has never heard of them, that sentence would appear to suggest it does have something to do with the injection system.
Wednesday, January 26, 2011 7:30 AM
The Z32 is sequential fire. I think Nissan switched around the late 80's early 80's.
Wednesday, January 26, 2011 8:26 AM
I've heard the difference between carbs and fuel injection explained this way: Fuel injection gives an engine what it should have. Carburetor lets an engine take what it wants. Phil
Wednesday, January 26, 2011 9:59 AM
Alot of the muscle car guys prefer carbs not only because of the metering efficiency or otherwise but because of the torque curve the Manifold styles create, HotRod recently did a carb VS Factory EFI comparo (with tuning) on an ??LS3?? iirc, (both maintained coil near plug ignition) The difference in torque curve was pronounced, I'm working on a 540cu in BBC that i'm gonna install injectors near port (hidden) and gut a holley to make it a glorified TPS and use MSII to tune and get the best of both worlds, As for the slide carb confusion, I'm not sure any more info on the slide valve setup would make it any clearer, mechanical fuel injection in and of itself is a very confusing ordeal,
Wednesday, January 26, 2011 2:16 PM
Oh and it looks as iff the block and heads have been chemfilmed AKA (mil-dtl-5541) or alodined, it is a corrosion inhibitor with almost no build up and almost zero abrasion resistance aswell...lots of aerospace stuff gets it,
Wednesday, January 26, 2011 6:52 PM
I saw that Hot Rod article. I need to look at it again. They have done a few tests over the years. I am still thinking about big cams and reversion with the inline 6. Thinking about ITB throttle response. I can see that if the ITB's were carbs, that the response would be quicker as the distance the fuel has to travel is shorter, however with ITB's and fuel injection, the distance from the injector to the valve is pretty short. I would like to see some data on response on a carb ITB vs single carb, and with ITB's fuel injection, and single TB fuel injection.
Wednesday, January 26, 2011 8:49 PM
It's been a while, but someone makes a ITB EFI set-up that bolts to a DCOE pattern AND is essentially the same sized body.
Thursday, January 27, 2011 1:44 PM
best atomization with ITB's and EFI is firing the injectors upstream of the TB's (usually shower type iirc) or against the air flow,
Friday, January 28, 2011 8:07 AM
I believe current practice in F1 is a down-firing injector, centered over the slide valve (TB). I don't think this practice has changed. http://blog.gtroc.com/dino/fer12.jpg
Friday, January 28, 2011 5:01 PM
hence the very specific wording used....Upstream, OR against the air flow, Ferrari is using a shower type injector most probably a magneti marelli unit
Friday, January 28, 2011 8:11 PM
Actually, it's WITH the airflow, not against it. I tried to look up what they are doing in Superbike and Moto GP, but had a harder time finding images. Those two are even more advanced than F1 usually. Man, if you want to follow state of the art in engines, those two series are amazing.
Sunday, January 30, 2011 11:32 PM
Is the shower injector better because of charge cooling? I wonder if throttle response is worse? Ever see the reversion cloud hovering around the stacks on sidedrafts on the dyno? I wonder what happens with shower injectors.
Monday, January 31, 2011 7:08 PM
I wonder what happens at 19,000 rpms ?
 
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