Custom 4-2-1 Exhaust Manifold

Discussion in '16-valve' started by theboysmiffy, Jan 2, 2020.

  1. theboysmiffy

    theboysmiffy Forum Member

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    Hi all. I am going to get a custom manifold made. The engine is 9a 16v, shed head, Schrick 276, Dta ignition, twin 45, dyno at 215bhp@7100rpm. Currently running an Ashley 4-1 and Jetex 2.5” system. I was thinking of going to 4-2-1. Am I right in thinking that the longer the primaries are the better/more top end? Does anyone have any advice regards best design for this setup? Any help would be great. Thanks
     
  2. Tristan Forum Junkie

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    Actually, shorter primaries are usually more suited to high rpm power, same as shorter intake runners. Obviously there are many other factors and all engines behave differently.
     
  3. theboysmiffy

    theboysmiffy Forum Member

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    Is there anyway to determine what might be best suited to this particular engine? I have dyno plots if that helps.
     
  4. Tristan Forum Junkie

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    One for Hugo/HPR , RobT, Jon Olds or someone like that.

    Why don't you want to go with an off the shelf option?
     
  5. theboysmiffy

    theboysmiffy Forum Member

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    A friend of mine has just started a business making exhausts/race prep and he is keen to do one, thought I would get the best advice from the knowledgable folk on here first. The Ashley that is on is 4-1 and I was told by the mapper that a 4-2-1 might fill the dip in the mid range.
     
    Last edited: Jan 3, 2020
  6. Tristan Forum Junkie

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  7. Toyotec

    Toyotec CGTI Committee - Happy helper at large Admin

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    Road cars engines with up to mild tunes and maybe some hardware hardware, with the majority of torque between 2 to 6K rpm, required a tri-y or 4-2-1 headers.
    The length determines the where the strongest exhaust scavenging will occur and this is influenced by valve timing events.

    For an ABF road/track car with some work done to head, cams and compression, I use the latest mainland Europe OE tubular component, a component I know was used in GrpA MK3 220ps engines.
     
  8. theboysmiffy

    theboysmiffy Forum Member

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    Thanks for the replies guys. Is there a good way (easy) to determine primary length. What are the lengths of the primaries on the OE tubular manifold you use?
     
  9. Toyotec

    Toyotec CGTI Committee - Happy helper at large Admin

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    I will attempt to dig out my spare 4-2-1 out of the shed when I am out of the office. You can use that as a guide.
     
  10. HPR

    HPR Administrator Admin

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    fsi mani20090710_02.JPG

    To solve the dip in the curve play with airhorns lenght > longer / shorter
     
  11. theboysmiffy

    theboysmiffy Forum Member

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    I don’t have much scope for airhorn tweaking unless I do a long low custom rad on the front crossmember. Is that a custom manifold?
     
  12. Brookster

    Brookster Paid Member Paid Member

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    i used to run both 4 -1 ashley and 4 -2-1 supersprint

    The 4-2-1 was great for when it was used on the road , but the Ashley 4-1 was better for top end and racing .

    In a MK1 Golf and Rocco the issue in space behind the engine
     
  13. theboysmiffy

    theboysmiffy Forum Member

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    Thanks Brookster. What would be the benefit of more space behind the engine, longer primaries, different route? I remember Paul Bargate’s Scirocco having an unusual manifold design, not sure why tho. C2148D16-2055-4881-B5FA-829A40F0E47E.jpeg
     
  14. Tristan Forum Junkie

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    More space would make routing it easier.
    Bargates was like that to get extra length in the primaries iirc.
     
  15. Toyotec

    Toyotec CGTI Committee - Happy helper at large Admin

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    As I had quite a lot of experience with this topic, I have found my old notes and pictures and written a thread on the topic. Click -> here.

    Feel free to read it and I hope it helps with your decision.

    Ed.
     
  16. theboysmiffy

    theboysmiffy Forum Member

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    Thank you for that write up, very useful. I am definitely going the 4-2-1 route and will try and use the Golf 3 kit car 4-2-1 as a design guide.
     
  17. Toyotec

    Toyotec CGTI Committee - Happy helper at large Admin

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    Assuming you engineered your engine with the correct camshafts, compression and rev range, matching a gearbox, not kit car, unless you are building a + 250 ps engine.

    Just the 4-2-1 LHD production manifold with a tuned downpipe.
     
  18. mr hillclimber Club GTI Supporter and Sponsor

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    Comparing 4-1 & 4-2-1 manifolds is like comparing apples and oranges....similar shape, edible but totally different taste.

    The old tales of 4-2-1 being better mid range than a 4-1 blah blah doesn't take into account bore size, primary length, merge configuration etc. For example, a reasonable 4-1 will out-power a badly designed and made 4-2-1 right through the rev range...so that blows the midrange myth out the window to start with.

    A 4-2-1 won't always cure a dip in the power curve...been there tried that. In fact, on a carb engine it can actually make things worse.

    To cut a very long story short, if you want to try a 4-2-1 I'd start with....26-28 inch primaries @ 1.75 o/d, 3-4 inch merge collector, 8-10 inch secondaries @ 2inch o/d into the final merge @ 3-4 inches out into a 2.5 inch tailpipe. Set the first silencer 22-26 inches away from the last collector.

    Depending on the curve you want a smaller bore may help midrange but cap the top end a bit...or maybe it won't....it all depends on the rest of the engine set up.

    I did a similar spec to yours last year on a 4-1 with a modded collector...it was pants as it came....no dip in the midrange from what I recall.

    Your comment re the radiator location hints at a cranked up inlet manifold...poor for flow and mixture distribution....I'd look there first before the exhaust manifold. Use 60mm trumpets if it's a short manifold or you replace it with a better flowing short one, or 40mm trumpets with a long manifold. Either way the radiator will be in the way if you have a good straight inlet.
     
  19. theboysmiffy

    theboysmiffy Forum Member

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    Thanks Hillclimber.

    Are those exhaust manifold specs what you would recommend for my engine spec? I have to do the manifold as my Ashley is on it’s last legs!

    With regards the inlet, it isn’t particularly cranked up or short but I’m sure you will know better than me. The only pic I have of my manifold and carbs is on the previous owners car, I now have longer trumpets on: F4046947-3C9B-44A6-8D10-867AEA626C61.jpeg

    Adam
     
  20. Toyotec

    Toyotec CGTI Committee - Happy helper at large Admin

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    What Jason is saying is true in principle, especially when you do not have a reference component as a baseline for your specific build.
    He has given dimensions that should work on one of his example cars, which may or may not work in your case.
    My post shows directional trends, with all components remaining consistent. I demonstrate the direction to results took. But that was a stock engine.

    Just simply quoting the maximum peak bhp, says very little for the calibre of folks in this discussion. A full power and torque curve is really required when choosing an exhaust system to match the cams you are running and the very short open inlet.

    Dips in "power" are based on torque and cylinder filling and the tuning effect of cylinder scavenging and mild supercharging from the inlet tract.
    In your case, it can be made worse by fitting a mismatched exhaust system, to an engine that might have the cams not dialed in or a pretty short intake tract.

    Stock compression also?

    With questions like these, subjectivity over a peak dyno reading needs to be negated, so only then can any of the replies be tailored to assist remotely.

    Wake up a picture of your 215 bhp dyno plot showing torque/power against engine speed and that would help.
     

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