Whats the deal with it, as I know turbos, but not n/a at all.

Was talking to someone with a 968 yesterday and he was talking about doing a few mods (they already 240bhp from a 3litre 16v 4cyl n/a) and though he was on about some posh Porker tuners that charge a fortune, I didnt even know what sort of exhaust size would be any good TBH.

If all the boxes were straight thru, so no restriction there, how do you work out size (obviously its not an exact thing, lots of variables, but roughly)?
Is just it on bhp? Or capacity too?

What would be a good size for a 240bhp 3litre?

Teach me!

 

It's a hugely complex subject. It's like tuning a fine instrument. Performance Trends do some software that you could play around with to get an idea of what works and what doesnt.. (in theory)..

 

Yeah I know its massivley complicated, with counltess variables, but like anything, is there a general rule of thumb?

 

not sure about size, but i think there are general rules of thumb re the design of the manifold.. i.e. 4-1 or 4-2-1. (iirc the latter is better for peak power but the 4-1 is better for low down/midrange - don't quote me on it though!)

when i phoned simpsons re an exhaust for my escort they started asking me all kinds of questions that i wouldn't have a clue abotu the answers! then suggested i get in touch with my engine builder and ask them for the relevant info. as said.. massively complicated!

I did have a link to a site that calculated primary lengths and bores based on some basic info though.. will see if i can find it.

 

here you go..
http://www.mez.co.uk/mezporting/exhaust_length.html
looks like i got the 4-1 and 4-2-1 statement above the worng way round. 4-2-1 better for all round and 4-1 better for race and peak power at high revs.

 

it depends of valve timing, port size, you fucking name it

eg a 24v cossie has a little overlap, which is fine with a cat installed, but once you remove it you loose power at the bottom end

 

I dont care about manifolds, primary lengths, i know all the 4-1, 4-2-1 stuff, im purely talking about the main exhaust pipe only, ie after it goes to a single pipe, and a general rule of thumb.

I didnt think my question was that hard to understand

 

sorry.. thought you were on about manifolds, not the system.

 

My mate has the 944 s2 with virtually the same 3 ltr 16v engine in it as the 968. (968 got vvt).

I built him lovely 2 box (straight through) all stainless system. I used 2.5"

It sounds spot on and i tracked down the correct oval tail pipe so it looks like a stock porsche bit of kit. Materials for whole job including two stainless silencers was under £150.

 

I don't think it depends massively on horsepower as you are going to get to a point where for example 2.5" would be good enough to flow what you need for a good amount of bhp than say 4". However, I thought it was dependant on things like length and resonance chambers as well as I know from two stroke engines, fine tuning things like the length, width, chamber shape is all very important unless this also doesn't apply to 4 stroke?

There was an application I saw on the internet a while back that was a Java app and basically you could design your own system and could vary a lot of parameters and it would show you how it would effect bhp and torque. I'll try find it.

 

it will add a little more back pressure, which is not a problem on N/A as they like a little bit of back pressure

 

Wrong!

 

You are right about tuned lengths but with out hours on a dyno and a big budget for labour/materials these things are very hard to get right.

 

garage, if the engine has a little overlap then yes

 

and they are desgned for a CAT

 

No, thats just a work around and not optimum tuning.