DIY ITB - 42mm vs. 45mm for 2L
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DIY ITB - 42mm vs. 45mm for 2L
From what I've researched 42mm is believed to give a lot more midrange and possibly less peak than a 62-64mm TB..
Are 45mm DIY ITB's the happy medium for gaining power everywhere across the powerband?
Are 45mm DIY ITB's the happy medium for gaining power everywhere across the powerband?
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Joined: Dec 2001
Posts: 3,643
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From: assville, MI, USA
Re: (b20mike)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote »</TD></TR><TR><TD CLASS="quote">
A. What is the best throttle body diameter?
Factors influencing size are: Power output, rpm, cylinder head design, cylinder capacity, position of the throttle body in the inlet tract and position of the injector.
Choice of bore size is a balanced compromise resulting from the following:
1) A larger bore leads to lower flow resistance, but obeying the laws of diminishing returns.
2) A smaller bore leads to better throttle control and response (never underestimate) and improved fuel mixing.
3) The system should be considered in total - from (at least) trumpet flange to cylinder and proportioned accordingly.
Basic references for BHP per cylinder, assuming ca.(ca.= approximately) 120mm from butterfly to valve head and a max of 9,000 rpm are:
Up to 30 BHP/CYLINDER - 30mm
up to 33 BHP/CYLINDER - 32mm
up to 39 BHP/CYLINDER - 35mm
up to 46 BHP/CYLINDER - 38mm
up to 51 BHP/CYLINDER - 40mm
up to 56 BHP/CYLINDER - 42mm
Up to 65 BHP/CYLINDER - 45mm
up to 74 BHP/CYLINDER - 48mm
up to 80 BHP/CYLINDER - 50mm
up to 87 BHP/CYLINDER - 52mm
up to 93 BHP/CYLINDER - 54mm.
These power figures may be increased by up to 10% in a purpose - designed, well proportioned system.
As butterfly to valve distance increases, butterfly size will need to increase in proportion to system taper and vice versa.
Lower revving engines and those with injectors placed before the butterfly will generally accept a larger body.
B. What is the correct overall system length?
Induction length is one of the most important aspects of fuelling performance engines.
In our experience an under-length system is the greatest cause of disapointment, with loss of up to 1/3 of power potential. There are a number of good books on the subject and the serious developer is referred to these and, in particular, dyno trials.
A guide figure, from the face of the trumpet to the centre of the valve head is 350mm for a 9,000 RPM engine. Other RPM are proportional, i.e. for 18,000 RPM the figure is ca. 175mm.
The induction system is part of a resonant whole - from trumpet to exhaust outlet - and the ideal length can be heavily influenced by the other components.
</TD></TR></TABLE>
A. What is the best throttle body diameter?
Factors influencing size are: Power output, rpm, cylinder head design, cylinder capacity, position of the throttle body in the inlet tract and position of the injector.
Choice of bore size is a balanced compromise resulting from the following:
1) A larger bore leads to lower flow resistance, but obeying the laws of diminishing returns.
2) A smaller bore leads to better throttle control and response (never underestimate) and improved fuel mixing.
3) The system should be considered in total - from (at least) trumpet flange to cylinder and proportioned accordingly.
Basic references for BHP per cylinder, assuming ca.(ca.= approximately) 120mm from butterfly to valve head and a max of 9,000 rpm are:
Up to 30 BHP/CYLINDER - 30mm
up to 33 BHP/CYLINDER - 32mm
up to 39 BHP/CYLINDER - 35mm
up to 46 BHP/CYLINDER - 38mm
up to 51 BHP/CYLINDER - 40mm
up to 56 BHP/CYLINDER - 42mm
Up to 65 BHP/CYLINDER - 45mm
up to 74 BHP/CYLINDER - 48mm
up to 80 BHP/CYLINDER - 50mm
up to 87 BHP/CYLINDER - 52mm
up to 93 BHP/CYLINDER - 54mm.
These power figures may be increased by up to 10% in a purpose - designed, well proportioned system.
As butterfly to valve distance increases, butterfly size will need to increase in proportion to system taper and vice versa.
Lower revving engines and those with injectors placed before the butterfly will generally accept a larger body.
B. What is the correct overall system length?
Induction length is one of the most important aspects of fuelling performance engines.
In our experience an under-length system is the greatest cause of disapointment, with loss of up to 1/3 of power potential. There are a number of good books on the subject and the serious developer is referred to these and, in particular, dyno trials.
A guide figure, from the face of the trumpet to the centre of the valve head is 350mm for a 9,000 RPM engine. Other RPM are proportional, i.e. for 18,000 RPM the figure is ca. 175mm.
The induction system is part of a resonant whole - from trumpet to exhaust outlet - and the ideal length can be heavily influenced by the other components.
</TD></TR></TABLE>
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Joined: Feb 2003
Posts: 2,475
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From: owen sound,ON, canada
ive always thought 50mm was way to big for a street motor...but i have no proof of that ..just what i see in my head and cant seem to put into words...very frustrateing
im going to see hoe the 42 mm works on my f20 b ...
im going to see hoe the 42 mm works on my f20 b ...
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Joined: Apr 2002
Posts: 390
Likes: 1
From: Lake Tahoe, Nv
Re: (machine4321)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by machine4321 »</TD></TR><TR><TD CLASS="quote">ive always thought 50mm was way to big for a street motor...but i have no proof of that ..just what i see in my head and cant seem to put into words...very frustrateing
im going to see hoe the 42 mm works on my f20 b ...</TD></TR></TABLE>
I've always thought the same thing... that's where the myths about them not being streetable come from IMO... people running too large of diameters...
I'm trying some 40mm's on a street DOHC 2 liter non vtec... I don't expect to get over 200 hp at any point so it should be fine...if not, then I'll drop the $200+ and have them bored out...But I've heard all the meatheads telling me it's too small of a diameter but I actually read and absorbed alot of the available information... including this little tidbit...<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by StyleTEG »</TD></TR><TR><TD CLASS="quote">2) A smaller bore leads to better throttle control and response (never underestimate) and improved fuel mixing.</TD></TR></TABLE>
IMO runner length & expected rpm/power ranges have just as much to do with what diameter you choose...
But just speaking in vague generalities, IMO I'd follow Jenvey and say that 43mm is the best all around diameter...
im going to see hoe the 42 mm works on my f20 b ...</TD></TR></TABLE>
I've always thought the same thing... that's where the myths about them not being streetable come from IMO... people running too large of diameters...
I'm trying some 40mm's on a street DOHC 2 liter non vtec... I don't expect to get over 200 hp at any point so it should be fine...if not, then I'll drop the $200+ and have them bored out...But I've heard all the meatheads telling me it's too small of a diameter but I actually read and absorbed alot of the available information... including this little tidbit...<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by StyleTEG »</TD></TR><TR><TD CLASS="quote">2) A smaller bore leads to better throttle control and response (never underestimate) and improved fuel mixing.</TD></TR></TABLE>
IMO runner length & expected rpm/power ranges have just as much to do with what diameter you choose...
But just speaking in vague generalities, IMO I'd follow Jenvey and say that 43mm is the best all around diameter...
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