Race Engine Forum UK

Dear Guy.

Among other issues I face is lack of power.

The car was not dyno-tested as not much racing was done yet and I wanted to deal with suspension first ( done !) Now my plans are to get its power and torque up :as well as get better response in low to mid RPM range. To do it I plan the following :

1/ install two twin barrel Weber 40 IDF carburettors ( current two twin solesx 34¢ž¢s are just not up to the job).
2/ put in adjustable camshaft drive wheels and adjust it (following instructions contained in your GREAT book !)
3/ install 4-2-1 exhaust modified from original Abarth for Fiat Ritmo abarth 125 TC
4/ rebuild the rest of the exhaust to original Corsa specs


My questions are as follows:

A/ Will IDF 40 be enough for my engine ? Will they require electric fuel pump?
B/ Any specific suggestions re adjustment of the camshaft drive wheels?
C/ I have an original 4-2-1 from Fiat Ritmo abarth stashed under a lot of other stuff somewhere and since I got it I did not think about verifying if this is for 8v or 16v head. If for 16 v - will the diameters and lengths be OK.
D/ The exhaust: What would you suggest to go Corsa or Stradale? Any suggestions on mid silencer or just decompression chamber. Twin flexible pipe before the final ? What type of final. I have an original Abarth but is teems to be from a much smaller Fiat (could it be from a 124 spider ?) (See picture attached )

Miroslaw

Miroslaw, hi

sorry but I strictly enforce copyright rules here (see Protocols in GC v/W) and thus have deleted OE parts manual pictures from your post.

It doesn't matter materially because the Ritmo ex is not what you want. over-baffled and way to complicated for a rally car. You need a 4-2-1 header linked 1&4, 3&2, with say, 25" primary pipes 1.5" bore, secondary pipes 7" 1.75" bore, tailpipe straight to the back end 2" bore and a George Polley AX891 straight thru big bore oval silencer. See

http://www.polleysport.com/polley/index.asp

That's the ONLY silencer I'd use on a TC engine. I know it's good because I have used and recommended it for years. It is not expensive, don't try and copy it! Ring George!

GC

Dear Guy,

Sorry about posting OE parts manual pictures. (Man, you are really strict.) But no discussion - rules are rules, and I will not make the mistake twice. Sorry again!

Thanks for the tip on the final silencer. I have written George Polley and asked for one.

Also thanks for the tips on the measurements of the 4-2-1- and the rest of exhaust. I will have to make one (or are they available somewhere you could recommend).

The Ritmo abarth 4-2-1- wil go on sale !

One clarification. In your post you suggest 1.5" primaries with lenght of 25" and secondaries 1.75" with length of only 7". Studying your book "Modyfying and tuning ..." (on page 190) would suggest that the secondaries should be at leats 25% to 35% larger in diameter than the primaries and have same length. Any reason for suggesting secondaries to be only 7" long ?

Miroslaw

Yes, the view on 4-2-1 primary/secondary length configuration has been overturned in recent years. Certainly from where I sit.

How? Software analysis and dyno test suggests again and again that long primary, short secondary gives more area under the torque/speed curve and thus better driveability (spread of torque) than long secondary/short primary or equal length. And 4-2-1 seems to work better than 4-1 in most cases too even on very highly tuned units.

I cannot say it is a certain rule in every case but it now seems to hold true and it certainly is true in this case; only software could have brought about this rather surprising breakthrough.

GC

Dear Guy,

This looks like science with a touch of art. This is what Einstein always said - "good theory has to be beaufitul".

Thank you very much for clarification. More mid range power and better torqe is what I am after.

Miroslaw