Fiat 2 liter TC exhaust manifold setup

[Piero]

Hi Guy,
As I am in the process of having a 4-2-1 manifold made, one thing that would be good to know, is how close I have got to getting the exhaust system right.
Based on a Fiat twin cam, 2 litre, with big valve head, ported and flowed head and valves, CR as high as I can go, on pump station gas, twin 45 webers on a short inlet manifold, rally gravel cams.
I have gone for 18in primarys and 18in secondarys.
Primarys ex dia 1 5/8
Secondary ex dia 1 7/8
Tailpipe/exhaust 2 1/8.
Piero

[Guy Croft]

I think it would be better to play safe and build a 3ft 4-1, with 1.5" bore primary pipes, I don't have nearly enough info to work out a good approximation for a 4-2-1.

GC

[Piero]

Hi,
I am limited to space were the chassis meets the engine, so four pipes at this point could not fit.
I have an Arbath 4-2-1 manifold and have pretty-much copied this, but used your (as per your book) 33mm throat valve size pluss 30%, then done the same extra 30% for the secondry's.
what other engine info would be needed to assised me further.

[Guy Croft]

What I mean is that I cannot actually work out a 4-2-1 with any useful degree of accuracy - beyone copying what I have done in the past - at the moment - as I am waiting for some test/analytical info from someone else, sorry.

GC

[Piero]

Hi Guy,
Would you be kind enough to let me know when you know.
Piero

[SirYun]

You could give the Pipemax win software a try. Never used it myself but the posted results are mostly spot on ( by various users)
It seems very cheap for what it does. $40 US

the guy (Mr Meaux) is very helpfull in answering questions as well.


http://www.maxracesoftware.com/pipemax_win.htm

[Piero]

Thank you Siryun.
Piero

[ICI]

Hi to everyone :)
My first post :)
I found this calculator recently. I don't know is it good, here it is :
http://www.btinternet.com/~mezporting/e ... ength.html

[Guy Croft]

Thanks for trying to assist here.

FWIW I checked out the link and compared with my own known data from advanced engine analysis.
Unfortunately, I suppose to be expected, the primary and secondary lengths offered by that link were far enough out to give a completely different torque response to my optimum, but that said - it's probably better than nothing. It doesn't allow the entering of inlet cam data, so not sure how much reliance can be placed on it. The assertion that a 4-1 will always have 15" primary pipes is definitely wrong, a 4-1 will usually be the same length as the combined length of the primary and secondary of an equivalent 4-2-1.
The fact that the system considers the 'tuned length' as being from the valve head is good, and that it correctly suggests longer primary than secondary pipes, so I imagine for someone who is a bit 'stuck', this will get them going; but it would need a lot more input data to give anything close to 'optimisation'.

Eg: I put in a GC3A ex cam, with 72/48 timing (102 deg FL) and it could not compute some of the dimensions, which suggests to me, along with its push-button repsonse time, that it is simply working off an archive of known data (maybe from dyno) and common cam profiles held by the author - rather than computing it. When I talk about advanced analysis, software of that type takes two days or more to preform all the operations to analyse intake/ex flows and pressure waves.

GC

[simon-spanner]

looks to me like that link is the equations from the A G Bell four stroke tuning book made to look pretty- Ive used those equations to protype exhaust manifolds and then chopped or lengthened them based on dyno/RR readings to obtain the best results
As with anything- youve got to start somewhere!

But be prepared to change things if it doesnt work

[Piero]

Thats just it, you do have to start some where,
but, I have very little experiance with the Fiat TC, and am learning all the time.
All I do have is an Arbarth 4-2-1 manifold that came from a Fiat 130 strada.
I used the lenghts from this (18 inch both primary and secondary) and just upped the bore dia, (sizes at start of topic).
I was hoping to produce more power than the 130 bhp that the starda made.
Also, onced the manifold is made, I dont think it will get changed,
I dont have accses to a dyno to futher develop the engine.
Really could do with some case history from any one plz.

[1969race125]

I can't add anything on the theory, but I can speak from exhaustive dyno-tested experience on my own race engine which has specs very similar to yours: 2L, 41.8/36 valves, high-compression (approx. 12:1), circuit race cams (49-75 / 83-41, 103/111), 149bhp at rear wheels (at 5700rpm).It runs on AvGas, through twin 45 Delortos. A picture and dyno graph are on this thread: (when I get the car back from being repainted this week I will photograph the headers for you)

viewtopic.php?t=246&start=0

We ended up with 1 3/4" primaries @ 19", 2" secondaries @ 36", 2 1/2" main @ 37", high-flow muffler.

I doubt you'll find those measurements specifically recommended by any formulae, but believe me the testing was "exhaustive" (if you'll excuse the pun) and this set-up works (for the characteristics of my motor).