big_wasa
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| posted on 27/9/08 at 07:16 AM |
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Tb's how big is to big ?
There are a few people that have now gone over to bike Tb's.
So I am wondering, has any one had problems with them being to big for the engine ?
What do you feel the max size is on say a 200bhp 2L engine, and still get a good idle ?
just pondering 
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Myke 2463
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| posted on 27/9/08 at 07:21 AM |
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Mornimg Wasa.
According tp PPC GSXR 600 at 38mm id with std injectors will make 200bhp.
Mike.
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big_wasa
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| posted on 27/9/08 at 07:26 AM |
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Is that this months Mike ?
A set of zx12r's that I just hapen to have kicking about would almost bolt direct to the head. They are 43mm 
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zetec
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| posted on 27/9/08 at 07:37 AM |
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I've got 45mm Jenveys on a 170 bhp 2ltr zetec and all, ticks over just like a normal tin top. Also not going too mad with the injector flow rate
makes a difference.
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rusty nuts
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| posted on 27/9/08 at 07:49 AM |
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Don't know if it's any help but when I converted my x flow to injection I looked at Dave Andrews??( IIRC ) website on tuning Webers that
has a graph that gives an approximate choke size for engine size/expected power/revs etc . Worked out what size chokes I needed and found some T/Bs
with that bore. . Suspect too big would cause low engine speed problems that may not cause poor idle?
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bimbleuk
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| posted on 27/9/08 at 08:27 AM |
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The toyota production 4AGE 20V engines of 1587cc capacity had two sizes of TBs. The early engines were 45mm and later 48mm. Both were tapered designs.
They had to meet the emmission and noise tests of the time they were produced.
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BenB
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| posted on 27/9/08 at 08:33 AM |
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The throttle body bore is a lot less important with TBs compared to carbs. The main disadvantage of a huge TBs would be difficulty controlling partial
throttle opening. However some TBs have cam'd crank arms to overcome this....
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MkIndy7
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| posted on 27/9/08 at 09:54 AM |
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I'd echo the above statement,
We've got 2002 R1 ITB's and i'd say there too big, infact we've restricted them down now.
Unlike carbs you can get them to idle perfectly, but its the part throttle opening where things get a little mad.
I think as there's so much air rushing in for only a slight opening you have to add far too much fuel to it to make the mixture correct, making
ours quite un-nesacerily thirsty.
At low revs and acceleration away anything over about 20% on the throttle peddle is like full throttle meaning you've only got that 20% to tune
within so things change very rapidly.
The last 20% of the throttle pedal travel probably does nothing as the engine is sucking in as much air as possible anyway.
I've heard that car Tb's can/are so bigger because the throttles work differently, they have a much more progressive opening than bike
Tb's helping the part throttle running, rather than just a sharp edged blade (butterfly) instantly opeing.
[Edited on 27/9/08 by MkIndy7]
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clairetoo
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| posted on 27/9/08 at 11:42 AM |
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quote:
Originally posted by MkIndy7
Unlike carbs you can get them to idle perfectly, but its the part throttle opening where things get a little mad.
I think as there's so much air rushing in for only a slight opening you have to add far too much fuel to it to make the mixture correct, making
ours quite un-nesacerily thirsty.
At low revs and acceleration away anything over about 20% on the throttle peddle is like full throttle meaning you've only got that 20% to tune
within so things change very rapidly.
The last 20% of the throttle pedal travel probably does nothing as the engine is sucking in as much air as possible anyway.
[Edited on 27/9/08 by MkIndy7]
That describes perfectly the problems I am having - I've got 6 40mm TB's on a 1.8 . But rather than find smaller bodies , I'm hoping
to sort it by fitting a bigger engine
Its cuz I is blond , innit
Claire xx
Will weld for food......
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big_wasa
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| posted on 27/9/08 at 11:59 AM |
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^^^ I like your style ^^^
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Richard Quinn
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| posted on 27/9/08 at 02:49 PM |
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Ive got a choice of either 52mm or 57mm for my Aprilia engine
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martyn_16v
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| posted on 27/9/08 at 03:49 PM |
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quote:
Originally posted by MkIndy7
I've heard that car Tb's can/are so bigger because the throttles work differently, they have a much more progressive opening than bike
Tb's helping the part throttle running, rather than just a sharp edged blade (butterfly) instantly opeing.
Bike throttles tend to have a higher 'closed angle' (for want of a better term) than car ones. In other words, the butterfly is at say
82deg to the port wall at idle on a bike throttle, whereas a car one will close down to 85 or so deg. This means the same small change in throttle
angle off-idle will produce a much bigger change on throttle opening on a bike throttle compared to a car one, making the bike throttle more on-offy
(I need a thesaurus ).
The clever ones amongst you could probably modify/make new butterflies that are less oval than the originals so they can close closer to perpendicular
to the port walls, but you'd have to be very precise to avoid the risk of them jamming or spinning all the way past idle.
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stevebubs
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| posted on 27/9/08 at 07:25 PM |
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quote:
Originally posted by clairetoo
quote:
Originally posted by MkIndy7
Unlike carbs you can get them to idle perfectly, but its the part throttle opening where things get a little mad.
I think as there's so much air rushing in for only a slight opening you have to add far too much fuel to it to make the mixture correct, making
ours quite un-nesacerily thirsty.
At low revs and acceleration away anything over about 20% on the throttle peddle is like full throttle meaning you've only got that 20% to tune
within so things change very rapidly.
The last 20% of the throttle pedal travel probably does nothing as the engine is sucking in as much air as possible anyway.
[Edited on 27/9/08 by MkIndy7]
That describes perfectly the problems I am having - I've got 6 40mm TB's on a 1.8 . But rather than find smaller bodies , I'm hoping
to sort it by fitting a bigger engine
I have similar issues with my GSX750 bodies. however the way I was going to tackle it was by putting the throttle cable on a cam to improve resolution
at the bottom end of the scale.
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MkIndy7
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| posted on 27/9/08 at 09:25 PM |
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I'm glad i'm not barking up the wrong tree then!
They were my observations after seeing a Vauxhall Xe engine single throttle body,that has both a cam and a delayed opening method.
The throttle operates on a cam that opens a smaller air bypass butterly and then as the throttle is pressed significantly more it opens a second much
larger throttle.
This larger throttle has got what can simply be described as a segment or chocolate orange screwed to it, which has the effect of slowing down the
rate that it opens and has moved almost completely out of the way at high throttle opening.
Keeps it nice can smooth but you can really tell when you gun it enough for the large throttle to open!
Keep it on the little throttle at about 68mph on the motorway and its pretty frugral too 
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Jenko
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| posted on 29/9/08 at 02:34 PM |
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Back to the idle thing.....Also worth pointing out that the injectors can have a marked difference......
To big, and they deliver too much fuel / pulse to get a nice idle...........The 600cc bikes tend to be around 230cc/min, which is perfect for around
170-190 bhp.......
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