Fiat TC Modification & Tuning Guide

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vcg
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Fiat TC Modification & Tuning Guide

Post by vcg »

Recently I had a wonderful experience with an NGK air/fuel ratio sensor fitted in my exhaust. I am as happy as I ever been with my 124 BC1. Below I describe what I learned from it, and in addition I write modification data collected from 70's catalogs of Abarth, Hoermann, and Alquati Cremona. Please comment and add on it, as I think it's good base to build on it with collected tuners experience.

This guide will help you modify and tune your Fiat TC engine for performance and efficiency. Head, Carbs, Exhaust, and Tuning issues will be discussed. This guide applies to Fiat 1600, 1800, and 2000 TC engines and for 3 types of modification profiles: Low, Medium, High.

With up to 9.0:1 CR go for Low modification profiles, with 9.0-10.0:1 CR go for Medium, and with higher that 10.0:1 CR go for High.

The expected horsepower at the crank in BHP +- 5 is as follows:

Low Medium High
1600 TC 120 135 150
1800 TC 135 150 170
2000 TC 150 170 190


The Head

It is the single most important element of an efficient high performing engine. Take your head of the car and take it to a good machine shop and ask them to do the following:

1. Fit new guides, seats and valves of your choice (see below), and shave the head.
2. Fully port the head shaping venturis related in diameter to the valve throat sizes.
3. Deshroud (smooth all edges between and around the valves) the combustion chamber, using dummy valves.
4. Port the inlet manifold so to match the head inlet ports

The valve sizes should be chosen as follows:

Low Medium High
in/ex in/ex in/ex
1600 TC 42/36 42/38 43.5/36
1800 TC 42/38 43.5/36 43.5/38
2000 TC 43.5/36 43.5/38 45/40

The 38mm ex valves may also be 37.5mm depending on what is available.

The camshaft selection is an important process particularly because of the variations in power/torque curves they have. In absolute power and torque figures the choices are pretty straightforward and that is as far as I will go into that subject. Make sure you chose a cam profile from an experienced cam manufacturer. Personaly I like Abarth (only copies of the famous 068 profile now available), Alquati (now from Pitatore), Piper, Kent, and Bariani & Colombo.

Chose the cam according to overlap (in degrees below) only, and forget max lift or tdc lift, unless you know what specific power/torque graph characteristics you require. Cam Overlap in degrees according to modification profile:

Low Medium High
1600 TC 70-80 80-90 90-100
1800 TC 70-80 80-90 90-100
2000 TC 70-80 80-90 90-up


The Carbs

Fiat TC engines work nicely with 40 IDF/DCOE/DCOM/DHLA, 44IDF, 45 DCOE/DHLA, and 48 IDF/DHLA carbs from Weber and Dellorto. Dellorto DRLA carbs can also be used, but some modifications are needed to fit them both on the manifold.

The single most important element of the carb is the venturi (or choke), and it is should be chosen taken into account all of the above discussed so far. Venturis can be bought, or they can be machined out from a smaller size up.

Choose venturi and carb size according to the modification profile chosen and shaped so far. Remember the carb is only a feeding device, and it¢ž¢s mission in life is to feed the head. It¢ž¢s the head type that determines how much it needs to be fed.

Chose venturi and carb size according to the chart bellow (venturi size in mm / carb size in mm):

Low Medium High
1600 TC 32/40 34/40 36/40,44,45
1800 TC 34/40 36/40,44,45 38/44,45
2000 TC 36/40,44,45 38/44,45 40/45,48

In cases where for a particular venturi size more carb sizes in the above chart appear available, larger carbs are preferred for quicker rpm raise and better overall throttle response. Everything else being equal a car with 45DCOE and 36mm venturis is a faster car than one with 40DCOE and 36mm venturis.


The Exhaust

Exhausts are very important, but not as important as most people think. Fiat TC engines work well with custom 4-1 (equal primaries length) header systems and see-through single silencer only endpipes (absolutely no resonator or second silencer). Countless setups have been tried by tuners worldwide, but unless you look for particular power/torque curve characteristics, stick with the following three for your modification profile:

Setup 1: 1600 Low & Medium, 1800 Low: 38mm primaries + 55mm endpipe
Setup 2: 1600 High, 1800 Medium, 2000 Low: 40mm primaries + 58mm endpipe
Setup 3: 1800 High, 2000 Medium & High: 42mm primaries + 60mm endpipe


Tuning the Fiat TC Engine

First decide what kind of fuel you are going to use: medium octane or high octane. High octane is mandatory for the High modification profile. Low octane is out of the question for an efficient high performing engine. The following is for a Fiat stock points or stock electronic ignition systems:

Get an adjustable timing light and adjust your advance timing at 34-36 degrees at 5500 rpm if you run on medium octane fuel, and at 37-39 degrees at 5500 rpm if you run on high octane. This is a painful experience as it¢ž¢s really hot in there‚¦ Now check what your static timing is at 1000 rpm and right it down for future reference so you don¢ž¢t have to go through the max advance timing experience again next time. Static timing may be 5, 7, 11 or whatever, don¢ž¢t trouble yourself with it anymore.

Next we balance the carbs. First turn in (closed position) all four air bleed screws. Have a synch meter (STE type preferably) handy, warm up the engine first and then turn the idle screw in until the engine reaches 3000 rpm. Now measure the flow in each barrel, and using the balance screw (not the air bleeds) balance the left and right carbs. Now get at idle and measure the flow in all four barrels. Write down the figure for the highest flowing barrel, don¢ž¢t touch it, and then adjust the air bleed screws on the other three barrels so they flow the exact same figure as the highest flowing one that we didn¢ž¢t touch. Then adjust the mixture screws ‚¼ of a turn out from the point where the engine¢ž¢s rpm starts to drop when turning the screw in from wide out.

Now that you have nice set of balanced carbs it¢ž¢s time to jet them properly. Take you car to a rolling road where they have an Air/Fuel Gas Analyzer with a wide band sensor, or buy your own pocket type (LT-1, NGK, etc.). Invest in and take with you (in case of Webers) a set of F11 and a set of F2 emulsion tubes, sets of 160, 170, 180, 190 and 200 air correctors, sets of 55, 60, and 65 idle jets, and sets of main jets from 130 up‚¦

This is a two person procedure and it takes time. With the car fully warm get Air/Fuel Ratio readings from the sensor at 500 rpm increaments from 1000 to 7000 rpm or higher (much) for High modification profiles and write them down. First emphasize on the 3500 - 7000 rpm area and try various combinations of main jets and air correctors with the same emulsions to try and get Ratios ranging between 12.5 and 13.0 at every reading. Remember below 12.5 is rich and above 13.0 is lean. A Ratio of 12.0 is safe but uneconomical and underperforming, and a Ratio of 13.5 is safe and economical but underperforming. After you have worked that out, work the idle area and up playing with some idle jets, that should be easy by now. The right set of emulsion tube, air corrector, main and idle jet will give you Air/Fuel Ratio readings between 12.5 and 13.0 which means optimally tuned engine.

Drive home and enjoy,

(rolling road added by GC for clarity)
Vassilis
124 BS1, 124 BC1, 131 Racing, E Type 4.2 SII, XJ-S 3.6
Guy Croft
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Post by Guy Croft »

MODEL POST

Well done indeed Vassilis, well-written, well-researched, and laid out in a nice, readable sequence. All in all an excellent aide-memoire that has taken time to produce - and just what I like to see here.

GC
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