working out Inlet manifold size/length?

Competition engines and ancillaries - general discussion
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RedLexus
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working out Inlet manifold size/length?

Post by RedLexus »

Is there a formula or method for figuring out the best compromise of inlet manifold length runners? I'm fitting Motorbike Carbs to my Golf , 1825cc , 8v , Honda CBR 900 Carbs with a 38mm bore . The carbs are quite short , 2/3s of a Weber DCOE , approx . I have fitted the same carbs to a 2.0 Ford Zetec, with great results , so I know they flow well , and work well . The manifold will be custom made , so any suggestions for the correct volume or lenght is appreciated . I'll be using 38mm Inner Dia. Tubing , either Alu , Copper or Stainless Steel .
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torkil
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Re: working out Inlet manifold size/length?

Post by torkil »

Hi dont know what lengts to use my self ,using 40 mm KEI HIN 40 mm as throttlebodies ,I have my lenghts at 32 cm . from head flange to end off trumpet, dont know if it is right want powe from 4000-70000 , on my 2 l 16 V

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Guy Croft
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Re: working out Inlet manifold size/length?

Post by Guy Croft »

Actually a critical thing here is the header (ex manifold) configuration.

If the primary pipes, be it 4-1 or 4-2-1 are too short it won't matter what length the inlet tract is. Moreover the effective length of the inlet tract is that from valve head to rampipe exit ie: the open end where inlet waves see atmospheric pressure, be that in an air box or whatever.

Based on my experience I suggest that you go for a header like this:

1. 4-2-1 with primary (py) length 27" and secondary (sy) length 8"

or

2. Bit easier to make - a 4-1 with primary pipe length min 27" but ideally longer - say anything from 33-37".

Where as I hope you all know by now (it's been published here by me over and over) the primary pipe length is the pipe chord length (pipe axis) measured from head face to entry of collector on a 4-1 and a 4-2-1 the sy length is from py collector exit to sy collector entry.

If you have that in place you can run pretty-well any inlet manifold length you want and simply vary rampipe length to achieve an optimum torque for the kind of racing you want to do. A long pipe will pull the torque in lower down and vice versa. Matching pipe chords for length is VITAL. I see so many quite nicely made systems that are NOT and I reckon aNY guy making headers professionally should know that RULE and ADHERE to it. A few do.

I'd rather not be overly specific about the min py critical length - it applies primarily to 4-1. On 4-2-1 the old style was to run short py and long sy or equal length - that's really been overturned in recent years.

Hope that helps some. Can't be done by calculation, sorry.

G
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