Intake length?

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Not-a-Number
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Intake length?

Post by Not-a-Number » Wed Apr 16, 2014 2:27 pm

Looking at mounting my induction stuff now. I havnt decided if I will put my webers on (with some injectors) or put on my current throttle bodies. Either way the mounting plate im making will be the same.

My question is what affect does the intake tract length from the butterfly have on things? Basically I want to make a 12mm plate that bolts to the head and then I bolt the carbs directly to the plate. With my webers there will pretty much be no length between the butterfly and the head!
With the TBs there will be a bit but not much.

Ive read a bit about tract length but I dont know how much it affects things in real life. There would be a lower volume of vacuum so may give me a less stable MAP reading?

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Re: Intake length?

Post by Rookie » Wed Apr 16, 2014 2:37 pm

Also you will get less bottom end as there won't be as good mixing of the intake charge.

And piss the map sensor off and run alpha n.
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Re: Intake length?

Post by Not-a-Number » Wed Apr 16, 2014 3:00 pm

If I ran upstream injectors would I get that mixing time back? Or does that just open up a heap more problems with wetting walls and crap idle?

Im thinking of trying alpha n.

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Re: Intake length?

Post by Rookie » Wed Apr 16, 2014 3:08 pm

Alpha n was designed because ITB map is shit, you would be a fool not to use it.

And no, you would get it back but you will improve the top end by using outboard injectors as it again increases mixing, but this time when the air is moving really fast hahah. Basically you need some manifold for the air stream to smooth it self out after you spray the fuel in, otherwise you will end up with heavily turbulent flow going through the valves which will cost you a bit.

Basically you will get little gains pissing around with this, so you are much better off just to make the manifold fit well and the add cams.
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Not-a-Number
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Re: Intake length?

Post by Not-a-Number » Wed Apr 16, 2014 3:50 pm

I actually have a stable map but it does vary with air temp and altitude obviously and I havnt properly tuned in the compensations.

If I use my current TBs I'll have about 70mm from the injector to the head. At the moment I would be about 150.

What sort of power decrease are we actually talking though. Are we talking 1/4 of a hp or are we talking 10hp?

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Re: Intake length?

Post by Rookie » Wed Apr 16, 2014 4:13 pm

Having a stable map is of little consequence when you have the same map reading from 1/3 throttle -> WOT though hahaha, you will probably find that your car is running on one or two rows in the fuel map.

Probably more than one but less than 10, what you are talking about is 10/10ths stuff.
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Re: Intake length?

Post by bzrse7en » Wed Apr 16, 2014 5:12 pm

From butterfly to valve isn't a biggy, but from valve to the open bell mouth is more important-ish.
I reckon BS is as good a start point as any mathematical theory. If you want to go the theoretical way, you'll need to know what RPM you want it to be most efficient first. If too low RPM you'll end up with too long a tracts that won't fit under your bonnet. Measure up some 20v 4age ones from valve to bell mouth opening, that will be close.

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Re: Intake length?

Post by Not-a-Number » Wed Apr 16, 2014 6:12 pm

Thats really all i was after. I need to get it as close to the head as possible and I didnt want to be losing a noticeable amount of power. If its going to do jack all like I thought then Ill go with it.

I dont know if it will clear my pedal box though so I may actually need to make a manifold. Zetec weber manifolds are pretty $$!

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Re: Intake length?

Post by Rookie » Thu Apr 17, 2014 8:56 am

You should go have a look at speedtalk, they will have the answers.
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Re: Intake length?

Post by Not-a-Number » Fri Apr 18, 2014 8:01 pm

How would this setup go?
Far too upstream?

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