Explain carbs to me please. :confused:

[touch]

So, im trying to learn the basics of mechanics. Thi is difficult however, as i dont own (or even have access to) a car. So everything i know is based on internet readings.

I like reading the project threads of people swapping engines and playing with welders etc

But im now confused about carburettors (sp?).
Link to a pic of a V8 running bike carbs so you can see what i'm talking about
So, it seems bike carbs are quite a common to do to improve the performance of the engine. (I think i understand why - i roughly know how all this works in theory).
My confusion is that there does not seem to be any sort of air filter? (I assume the the holes in the top are the air intakes?)
Is there a filter inside it?

Also, when i've read about people putting bike carbs on their engine, nobody seems to bother with any sort of cold air feed like a lot of injection engines have. Is air temperature less important for carb'd engines?

edit: oh, brilliant title . If any passing mod feels kind you could edit the title so i dont look completely stupid

 

[Dogbreath]

That engine is not exactly finished and ready to go though, hopefully the builder is planning on fitting filters. They are important for long term road road use, running without filters will significantly reduce the life of an engine.

Cold air is important for power whatever type of induction system you are using; carbs or injection, normally aspirated or forced induction. The reason why some people don't bother is that it's more difficult to do with bike carbs since you have multiple air inlets instead of the one you have with an OEM system. You'd need to add a plenum (which there may not be room for) and duct the air inlet to a suitable place.

 

[Deleted member 66701]

Pictures speak a thousand words.

My engine on bike throttle bodies without Air filter:-

With air filter:-

Re cold air feed - I have some trunking running from the nose cone up to the filter suppliying a cold air feed.

 

[Jonny69]

Yeah it's as said above, fitting in ducting is difficult in some applications and it doesn't actually make a huge difference. Not convinced it would on mine anyway.

Here's a pic of mine. Carburettors the old way, soaking wet, no filters:

So this is a pair of Weber DCOE carburettors like you'd find on an old race engine:

The idea here is to get one carburettor barrel for each cylinder so there's minimal restriction, so the engine doesn't have to suck too hard to get its air. The intake is at the trumpet end if you have a look down it you can see it's a pair of straight tubes with a slight throat where the petrol is jetted in and a throttle butterfly down at the bottom:

 

[Ricochet J]

If I'm not mistaken they fit directly on to the air intake manifold?

 

[kaiowas]

So the next big question, what exactly is the benefit of sticking with old technology like carbs? I see people throwing away a perfectly good EFI system to go back to carbs and can't see why you'd want to.

Is it just a case of luddites wanting the benefits of ITBs without whilst being 'scared' of the delving into the witchcraft that is EFI or do carbs themselves actually have any advantages over EFI?

 

[Dogbreath]

The advantage of carbs boils down to cost and simplicity. You can pick up a set of bike carbs considerably cheaper than fuel injection throttle bodies now that everyone wants them, you don't need an ECU with the associated wiring, you don't need a high pressure fuel system with a return. "Mapping" doesn't require a computer or special software, just a rolling road operator with some experience and a selection of jets (or jet drills).

They can also give better results (in terms of power) than OEM fuel injection systems which are designed for economy and maximum low/midrange torque.

 

[Jonny69]

So the next big question, what exactly is the benefit of sticking with old technology like carbs? I see people throwing away a perfectly good EFI system to go back to carbs and can't see why you'd want to.

Is it just a case of luddites wanting the benefits of ITBs without whilst being 'scared' of the delving into the witchcraft that is EFI or do carbs themselves actually have any advantages over EFI?

Well a pair of DCOE style throttle bodies will set you back the best part of £1K plus an ECU plus setting up while a pair of 45 DCOE carburettors are £500 new and less second hand, and they'll just work as soon as you bolt them on. Setting up a pair of Weber carbs is easy for me because they are basic but setting up an injection system would be well beyond me.

Swapping about injection systems isn't easy either and they are as much of a ballache as carbs can be. My dad runs a modified injection system on his V8 and it's nowhere near perfect despite being set up many times with many different fuel curves. It's always a compromise in my opinion, unless you're a manufacturer or race team who can justify throwing the $$$ and research into it.

 

[Deleted member 66701]

Setting up a pair of Weber carbs is easy for me because they are basic but setting up an injection system would be well beyond me.

Hehe - I'm the opposite - hand me a laptop and aftermarket ecu and I'm away. Start talking about carb jet sizes, venturis and float chambers and I'm totally lost.

**BIG GENERALISATION WARNING**

Carbs and throttle bodies (we're talking 1 intake per cylinder here) of the same size should produce the same maximum power and torque. There is no power advantage to either but both will be a substantial increase over OEM systems (I'm not talking about exotica here - M series engines, RB26's etc).

However, EFI systems (throttle bodies etc) generally offer better drivability, warm up charateristics, ability to deal with altitude changes and poor fuel etc.

The downside is obviously the perceived complication of an EFI system - you need a lot of sensors giving the ecu info, you need to be very precies with your wiring and then you need to know how to use often complicated (needlessly so IMO) tuning programs to set all the correct parameters - and a lot of them have to be "just right" on an EFI system otherwise the engine wont run at all, whereas with acarb set up, you can often get a rough running engine and then some magic with a screw driver improve it within 20 mins.

Hope that helps!

Any questions about EFI systems, please ask - I've done a lot of installs now - ranging form budget Megasquirt + bike TB installs to V12 Jenvey + Motronic set ups.

 

[saitrix]

Out of interest, what do you do for a living then amigafan?

 

[Deleted member 66701]

Siebel/Cobol engineer with XML/XSL and web design thrown in. (Computer Programmer in common langauge )

BUT I have a keen interest in cars - always have - currently doing a HND vehicle control systems at the local college Tuesday nights.

 

[GeX]

Hehe - I'm the opposite - hand me a laptop and aftermarket ecu and I'm away. Start talking about carb jet sizes, venturis and float chambers and I'm totally lost.

ditto. carbs, dizzy etc = old, comprimised tech

**BIG GENERALISATION WARNING**

Carbs and throttle bodies (we're talking 1 intake per cylinder here) of the same size should produce the same maximum power and torque. There is no power advantage to either but both will be a substantial increase over OEM systems (I'm not talking about exotica here - M series engines, RB26's etc).

i would disagree with you there. a carb'd engine will generally have a dizzy on it as well, meaning no real control over the ignition advance. (spinning weights, vacuum advnace units etc are not precise control..) running an aftermarket ECU, you're going to have that control, and can make more accurate ignition and fueling inputs.

 

[Dogbreath]

i would disagree with you there. a carb'd engine will generally have a dizzy on it as well, meaning no real control over the ignition advance. (spinning weights, vacuum advnace units etc are not precise control..) running an aftermarket ECU, you're going to have that control, and can make more accurate ignition and fueling inputs.

A lot of people using bike carbs on the LocostBuilders forum use Megajolt for ignition.

I agree that carbs are always a compromise, but the CV type carbs used by bikes are better than most, and it makes a very compact and low cost fueling solution if you are on a tight budget.

 

[GeX]

the thing is, if you've gone to effort of installing a trigger wheel for megajolt, then for not much money extra you can add fuel injection - then you can properly control fueling accross the engines whole operating range.

 

[Deleted member 66701]

i would disagree with you there. a carb'd engine will generally have a dizzy on it as well, meaning no real control over the ignition advance.

BUT you can still accurately control max advance at max throttle opening/rpms with a dizzy - so the max power output of carbs + dizzy and throttle bodies + mappable ignition is still the same for both.

Mappable ignition only affects driveability and smoothing the torque curve out - it doesn't let the engine make more power.

 

[Dogbreath]

the thing is, if you've gone to effort of installing a trigger wheel for megajolt, then for not much money extra you can add fuel injection - then you can properly control fueling accross the engines whole operating range.

I suspect money is not the main problem, (though the popular bike throttle bodies are commanding silly prices these days), but rather the perceived extra effort and knowledge required to get a FI system running.

 

[GeX]

BUT you can still accurately control max advance at max throttle opening/rpms with a dizzy - so the max power output of carbs + dizzy and throttle bodies + mappable ignition is still the same for both.

Mappable ignition only affects driveability and smoothing the torque curve out - it doesn't let the engine make more power.

being picky now, but say the engine made max torque at 4500rpm with 30 deg, but made max power at 6500rpm with 40 deg, to accurately control that you'd be better off with EFI, yes it is coming down to driveability, but everyone knows torque is more important then max power.. but like you said, it was a large generalisastion to begin with!

but rather the perceived extra effort and knowledge required to get a FI system running.

nail, head, hit.