I'm looking at alternatives for the standard Ford V6 setup, and looking at the standard manifold you can see the the individual runners are quite long, and I understand that this is important, but when cars are converted to ITBs it doesn't seem to matter or maybe it does matter but people don't do anything about it anyway.

Can anyone explain that? I know that it's a lot to do with getting the gas speeds up at low throttle openings, to get good torque, but if anyone has any more info I would be grateful

It is important but the gains in going ITB are normally relatively large anyhow so it normally gets overlooked.

There is a formula to calculate the optimal length of inlet, but I don't know it off the top of my head - something to do with a standing wave and harmonics IIRC....

If any of you remember the huge "flowerpot" intakes that was popular on caterhams a few years ago, this was what drove it....

Copy paste from Jenvey FAQ

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.
Any air feed system to an airbox or filter can have a large effect on the power curve and must be considered carefully - particularly if the airbox is small.
The induction system is part of a resonant whole - from air inlet or trumpet to exhaust outlet - and the ideal length is heavily influenced by the other components.

I am looking at putting TBs on my Duratec V6 by utilising the existing manifold and converting it to take primary butterflies alongside the std secondary ones - 12 butterflies in all!!

I am also anxious to get trumpet length right so I asked Jenvey about their recommendations and they said [paraphrasing here] "as long as possible" which didn't help very much. When I pushed them a little further they sent me their TB kit for the Jag V6 which is very similar...


Jag V6 ITBs

(sorry about the size )

As you can see, the trumpet length looks to be about 75mm by my reckoning, but they use a manifold to change the wierd obround ports to a conventional shape, making the whole TB setup a bit longer.
I reckon about 100mm trumpets will do as a start and from what I understand, the power will mostly be at the top end so it should make the V6 sing at the top end of the rev range (7500 rpm in my case)

That Jaguar set up is interesting, more appropriate for a mid engined car though. The injectors seem a long way from the heads too.

I guess for a front engined car I would need to be thinking along the lines of a swan neck arrangement, with the runners curving back over the tops of the cylinder heads, as this would not be a race engine and I'd be looking for a good midrange. Maybe it would be better for me to stick the the donor manifold.

Those were my thoughts too - Jenvey must move the original injectors into the TBs but my idea keeps them in the original position. Is your engine a Duratec V6 or Essex? If I can get my idea off the ground, maybe we can combine resources - they should be a lot lower than the Jenvey / Jag setup

[Edited on 30-8-10 by RazMan]

I'm guessing you're planning on an aftermarket ECU.
Any other changes, such as porting or cams?
There isn't a simple calc to work out the lengths, it's pretty much an extension of acustic harmonics, with a staning wave being generated by the inlet valve.
The cam timing has an effect, as does the dia of the runners, hence there not being an easy solution. The best bet is to try and keep them the same as std, maybe a bit shorter if you like to rev it a bit more.

quote:

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Originally posted by RazMan
Those were my thoughts too - Jenvey must move the original injectors into the TBs but my idea keeps them in the original position. Is your engine a Duratec V6 or Essex? If I can get my idea off the ground, maybe we can combine resources - they should be a lot lower than the Jenvey / Jag setup

[Edited on 30-8-10 by RazMan]

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It might be a Duratec ST220 which is pretty similar to the Jag, although I think the latter has variable inlet cam timing and completely different combustion chamber design, making a bit more power.
I'll swap if you like