Advantages of a step in the exhaust flange

[Sam_Q]

In the current book "four stroke performance tuning" I am ready it says that there should be a step between the head and the exhaust flange. As in the hole and pipe size of the headers should be larger than the port in the head. This is to prevent reversion and the author claims that on the dyno he would lose out every single time when there was no step.

Anyone ever had experience with this?

[abently]

Well if you run Camshafts with a lot of Overlap then reversion is a big issue. It depends on the Head design as well. There is definitely no dis-advantage to running the step regardless if it's N/A or Forced induction, so its worthwhile doing it just knowing if there was HP to be gained, you did so.

[Screamn_Sleeka]

Well it definitely should not be the other way around.

This is the same kind of theory they put behind porting intake manifolds, but its slightly reversed, they say the ports should be larger. This makes sense in both cases and the flow should be into a larger diameter.

[The Real Roadrunner]

its alot like putting anti reversion cones into headers but alot less work.
having a 1/8 step on each side can give phenominal increases to midrange.
done a bit of testing but ill have to see if i can dig up the graphs and logs for it.

cheers
linden

[Hurricane]

yeah i make all my exhuast flanges and pipe larger than the port, cant say ive tryed it with out doing it, but ill allways been told to do it to stop the zorst gases from goin back into the chamber on over lap or some thing like that? cant remeber what it was, but yeah its not hard to do it and it make sence to me. better flow if it can run with out hitting any thing.

On an inlet you dont want i A/F mix hitting any walls, like from the inlet manifold to the port cos it swirls around in that spot and makes the flow size were the wall is smaller.



you can see when the air hits the wall it makes a swirl pocket, that then reduces the air flow hole down smaller than the port size, slowing flow

[big_zop]

This step up in size is also recommended for turbo dumps as it helps increase flow by supposedly promoting the change in the spinning exhaust flow (from the turbing) to straight flow.

On another note, i have gone through some of that book in detail and found it to be really good. I know you dont have turbo, but his book 'forced induction performance tuning' is quite good too.

Having said that, i havent tested but it is something i may consider...

having a 1/8 step on each side can give phenominal increases to midrange.

Could you explai this a little more and are you referring to turbo manifolds or jsut NA ones? also, do you make many NA manifolds, or is it mostly custom work?

[The Real Roadrunner]

havnt tried it on turbo stuff. i normally use the same size as the ports, cause most street turbos are restrictive anyway.
i think 205hpatw out of a blacktop 20v is reasonable, so yes it works.

cheers
linden

[big_zop]

i think 205hpatw out of a blacktop 20v is reasonable, so yes it works.

nice!!! lookin forward to any dyno comparisons you may have...

[Sam_Q]

its alot like putting anti reversion cones into headers but alot less work.
having a 1/8 step on each side can give phenominal increases to midrange.
done a bit of testing but ill have to see if i can dig up the graphs and logs for it.

cheers
linden

do you have a picture of one of these anti-reversion cones? In this book he says that instead of having a step either side like 1/8 its better have have the floor level and have 1/4 just ontop.

Also those graphs and any pics would be aweome



So heres a question to you guys: what if my ideal pipe size is the same size as the port? do I have it flared at one end?

[The Real Roadrunner]

what engine size? it all depends on cyl displacement and what rpm range you want to use.

ill have a look for the graphs over the next couple of days, im just busy at the moment.

cheers
linden

[Sam_Q]

4A 1587cc

any tips are appreciated, so far I have worked out 1.5 to 1.625 inch O.D is a good size.

[Sam_Q]

forgot to add the rpm range, I want it from 6000 to 8500 maybe? either way a harmonic resonance at 7000 would be good.

Oh and also take all the time you need

[The Real Roadrunner]

im allowed to show the graphs but the tuner wont let me show the logging (hes a weird one).

ill get them mon for you.

sizes id rather not hand out to much but send me a pm and il give you some sizes/lengths that should work for your application.

cheers
linden

[Sam_Q]

that would be great thanks