Tuning Mod

[Deanxm]

The trick to port 'polishing' is blending, ie, get rid of any obstructions in the inlet or exhaust so the flow is unimpeded on its way to and from the combustion chamber.

The ideal is smooth, with the surface down to a good 'flat' finish as left by a 200G flapwheel. It has been shown that a mirror polished surface can cause the atomised fuel to form droplets on the surface of the inlet port, which is wasted effort.

Increasing the size of the port, without understanding the conditions the engine will be working under can have a deterimental effect. Better effort is spent making sure all the components on the inlet or exhaust match, with no obstructions. Mismatched manifolds, gaskets that do not fit, bad castings are all worth checking and easy to rectify and give an easy power gain.

H

I agree totaly, on an Astra i had (my first 4stroke porting project) everything was polished to within an inch of its life, the piston crowns, inlet and exhaust ports, after running the engine and taking the head back off you could plainly see where the fuel had formed droplets on the intake and on the now polished piston crowns, this polishing made a slight difference for the worse, i quickly found i lost power! the gain came from enlarging the throats of the inlet/exhaust ports to match the manifolds and gaskets, also cutting back the casting around the valve guides and blending in the seats made a noticable difference from above 2.5k rpm with wide open throttle, mind you just replacing the valve guides/piston rings will give a power increase in an old engine and i think this is where my efforts will go with the tct engine, that and Xantiamans patented tomcat turbo theft

Steve

Strange how Citroen put so much effort into the 2.1 intakes yet none at all on the tct, both being turbo's i guess that blows my theory out of he water, or maybe they had something to prove with the 2.1 and wanted it to be as good as they could practicaly make it?.
What book was this in, and does it cover all XM models? sounds interesting.

D

[steelcityuk]

Hi Dean,

I don't think it blows your theory out of the water at all.

Rough but not restrictive inlets do work better on carb and monopoint fuel injected engines. Where as multipoint injection tends to be injected as near the throat as possible so nice smooth fast airflow through the manifold is an asset. But with modern/lean burn engines turbulance (swirl) is often generated deliberately by the valve throats to encourage as much mixing as possible to keep emissions down rather than get good power higher up the rev range. Port mismatching can be an advantage if it is done the right way round, say for example the inlet manifold is smaller than the inlet port, this step can help reduce flow reversal at low RPM/flow rates provided the step is on the outside of the bend where gas flow is greatest. On the exhaust side it would need to be the other way around, larger manifold size than port size . David Vizard proved alot of this in the 80s on his flow bench.

Diesels with comet pre chambers depend soley on chamber induced turbulance to provide a good burn so match and polish to your hearts content, air flow rates will be quite high because there's no throttling and because of the relatively low maximum RPM of a diesel there's likely to be a far more mild grind on the cam leading to less flow reversal and pollution of the fresh charge.

Polishing the pistons and chambers can be good in increasing the thermal efficiency of the engine by reducing the amount of heat absorbed. All these things are tiny on their own but they all add up.

The book that mentions casting quality is the Citroen XM book by Alberto Martinez and Maurice Sauzay. It says

"The head is made of aluminium and is in two parts, one above the other. The lower part houses the valves and all the parts associated with the inlet and exhaust. Above it sits the other section which houses the camshaft and stiffens the whole assembly. The lower part was the subject of intensive development to improve gas flow. It's inlet ports are formed during casting by fusible metal cores rather than by sand, for greater accuracy of shape."

Steve.