Author Topic: thoughts on carbs?  (Read 3667 times)

Offline tweed

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Re: thoughts on carbs?
« Reply #10 on: 04 July 2009, 10:08 »
Carbs are good in fact great on the right car! Can be better than injection if you go for 2 twin barrels!
But on an already injected engine you have the problem of the ECU.
You either have to ditch this completely and go for some kind of electronic dizzy set up with a TDC sensor or find an ECU that works with carbs.

I'm going to have to argue about carbs ever being better than injection bodies, Len.
No matter what carb you use, you need the venturi effect to draw the fuel through, so you need a restriction in order to get it...with ITBs you don't.

Carbs can be set up to work when hoy, or when cold, but most often don't 'do' both well. I've had enough sets over the years to know what a pain they can be.

You simply cannot get precise metering of the fuel, using carbs and a stack of jets...precisely why everything of any importance now uses injection bodies, not carbs.

With an injection car, that has an ECU controlling the ignition and fuelling, you can normally just disconnect the feed to the injectors and run the ignition side as is, to take care of the spark.
But the fuel pump will give FAR too high a pressure for carbs...so that needs to be worked.


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Far better in my opinion to go for throttle bodies.

Absolutely.
And if the fitment is going to be on a car that's already injected, with all the sensors already fitted for engine management, I'd not even consider carbs...go straight to stand alone management and ITBs - be they bike ITBs of the likes of Jenvey.

Couldn't of put that any better. Totaly agree

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Offline DOA

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Re: thoughts on carbs?
« Reply #11 on: 04 July 2009, 10:19 »
Is this on an 8V or 16V engine as the ZX12R bodies are perfect for a 16V size wise and injector wise, even if you have to move the outer 2 bodies in by 3mm and do something similar with the fuel rail to make them fit with straight runners. Im in the middle of making a manifold up to do this for a MK1 owning friend and the biggest problem Ive had is getting the outlet of the bodies to fit to the inlet runners as the bore of the body is eccentric to the fixing boss (Ill get a picture once its a bit closer to being done at the end of this week coming hopefully lol).

However, if its for an 8V, ZX12R bodies are far too big (as said, about 46mm at the butterfly, 42-43mm at the outlet and about 48-49mm at the inlet) and the spacing will be way out due to the way the inlet ports enter the head. I want to do a crossflow head on injection in my own 8V Mk2 (Im a student at the mo though so I really dont have the money to do it lol) and Ive been looking at using early Suzuki GSXR600 bodies which are around 36mm bore which should be more than enough for the 8V's relatively rubbish breathing Whichever bodies you use, the spacing will be a nightmare due to the offset inlet ports.

Oh and as Ess Three said, dont bother with carbs, they may be different but they are not nice to set up and will give crap economy and emmisions (a big worry if you run a cat).
« Last Edit: 04 July 2009, 10:23 by DOA »

Offline Ess_Three

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Re: thoughts on carbs?
« Reply #12 on: 04 July 2009, 11:42 »
Is this on an 8V or 16V engine as the ZX12R bodies are perfect for a 16V size wise and injector wise, even if you have to move the outer 2 bodies in by 3mm and do something similar with the fuel rail to make them fit with straight runners.

Ahh...here speaks a man who must have a set in his sticky mits!  :smiley:


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Im in the middle of making a manifold up to do this for a MK1 owning friend and the biggest problem Ive had is getting the outlet of the bodies to fit to the inlet runners as the bore of the body is eccentric to the fixing boss (Ill get a picture once its a bit closer to being done at the end of this week coming hopefully lol).

Please do post a pic.

I did think after I posted that a 16v is by far the easier, and best suited to the ZX12R bodies - that's that I was doing.
I managed to work out the spacings issues with slightly offset alloy tubes welded to a milled down 16v manifold (just the bit that bolts to the head), so leaving the bike ITB spacing the same, and removing all problems of fuel rail spacing etc. The ZX12R bodies are out by approx 2-3mm on the outside bodies as you say.

Not perfect for sure as two runners would be fractionally longer than the others...but easier than re-spacing the bike ITBs and re-doing the fuel rail etc.


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However, if its for an 8V, ZX12R bodies are far too big (as said, about 46mm at the butterfly, 42-43mm at the outlet and about 48-49mm at the inlet)

Agreed...although, if you could get the inlet length long enough, you could claw back the torque you'd loose by having oversized ITBs (probably!) but there just isn't room on the 8v head if you stick to a standard head.


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and the spacing will be way out due to the way the inlet ports enter the head. I want to do a crossflow head on injection in my own 8V Mk2 (Im a student at the mo though so I really dont have the money to do it lol) and Ive been looking at using early Suzuki GSXR600 bodies which are around 36mm bore which should be more than enough for the 8V's relatively rubbish breathing Whichever bodies you use, the spacing will be a nightmare due to the offset inlet ports.

Very true...
I know a few rally lads who use GSXR600 ITBs on 150-160 BHP cars and they work well.
The early ones as you say...single butterfly again.

If it were me, I'd just swap the engine for a 16v and go from there though...more to start with and much easier to get ITBs on as there's much more room to work with.
Almost always the bike ITB set up ends up longer from head to trumpet, than a similar set of Jenveys etc...which may cause major clearance problems.


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Oh and as Ess Three said, dont bother with carbs, they may be different but they are not nice to set up and will give crap economy and emmisions (a big worry if you run a cat).

Amen!


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..but gaining motorcycles.

Offline DOA

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Re: thoughts on carbs?
« Reply #13 on: 21 July 2009, 21:41 »
Finally got round to taking some pics of the finished manifold.













Just needs a paint and its good to go. Fuel rail didnt need any mods after all  :smiley:.