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Competition inlet valve seat prep: Fiat 8v TC head

Posted: November 2nd, 2006, 11:04 am
by Guy Croft
At last a few minute spare to post something interesting. Some development tests here for interest, done 01 Nov 06 on a type 4371507 Fiat TC 8v head, 42.4mm inlet valves, the head is from a 2 liter Lancia Montecarlo head for a GC client.

First I did routine porting as promised, (more or less keeping to standard port dimensions) - smoothing the port at 80 grit throughout and around the fairly generous short side radius.

You can see how much a standard head flows, (black dotted line) and how much more flow you can get just by porting and not even modifying the standard seat (green), and how much more careful seat work adds (red).

In my experience these types of ports do not like a very narrow seat with a very deep throat grind all the way down the insert, and they seem to like a lower parallel section in the seat where it runs into the curved alloy port. The upper section needs to be 70 deg, I have not found anything better, 65 or 75 deg. Half and half 70 deg and parallel is good if you can do it, though I settle for 85 deg lower section which is as near to parallel as the Peg grinder will do for me without spending hours on it. The gain vs cost to bore the last bit out parallel is not huge and is something I'd save for StIII and full race units.

The valve/seat flow gains ultimately shown on the graph below really have come from improving the discharge through the valve and seat, not from just making the diameter of the throat bigger, it has only been enlarged about 0.3mm in fact. If the shape and thus efficiency of the valve/seat region is poor (I'm talking shape here) you cannot get gains just by enlarging the throat diameter. Not on a sidedraft port like this anyhow.

As far as inlet seat width goes I work on minimum 40 thou‚ per inch of valve minimum width, 1.7mm on a 42,4mm valve, and you can go as big as 60 thou‚ without much penalty, although, that said, wider seats seem better at lower lifts and narrower at high lifts, but I cannot be absolutely specific, everything is interlinked. I recommend that the contact face on the valve and seat match for width, anything else seems to generate a loss. I use the back grind as a tool to match the valve contact face when the seat has been finished, the lapped region on the valve shows me where to grind to.

It's interesting how much better the mid lift flow is with the seat top grind of 30 deg. I have done some where more is better but too much top grind can set the valve contact face very deep in the head and make it hard to shim up. I think what might be happening is that the air separates from the back of the valve, skating off at the edge of the 30 deg back grind (rather than flowing round the valve's 45 deg contact face as you might imagine it would) and it responds better to the 30 deg exit angle than it would a flat exit region as it enters the chamber.

Insofar as far as I am able to be specific (not far..!) radiusing the sharp edges of the throat or top grind can lead to a significant flow LOSS. Why? Air doesn't always follow a curvature, sometimes better to let it break away at the sharp edge. My goodness, there are so many unknowns; you really need CFX or some other computational software to figure out what's really best.

We all know nothing is certain in life, and what suits this head will not necessarily suit another with a different port layout. So the beauty of having a flowbench is that (if you don't cheat) it really does take the guesswork out of head prep, so here you can see how it's done. Anyone with a set of Neway cutters (20 x 45 x 70 deg angles) and a porting tool (Metabo, Dremel, Bosch) could get close to these results at home. I would always advise new guides first, esp with Neway pilots, otherwise you could have real problems with concentricity.

The flowbench says the head - even with modest port sizes and standard sized valves - has vastly more potential. There are gains beyond mere flow too; the throttle response (pickup) is vastly better because the air responds far quicker on the new seat when the valve opens, and the crossflow (scavenge) is improved too. From my dyno experience the bhp gain is around/over 18bhp alone just from this 'modest' head work, but then again, so much depends on the carburettor and exhaust layout as to whether it will 'breathe' in situ.

I am sure there are countless combinations that could be tested but I have enough dyno experience of GC units to know that this head will achieve its bhp target comfortably. You can spend weeks doing heads and making dazzlingly clever rules about ‹Å“doing this¢ž¢ and ‹Å“doing that¢ž¢, multi-angle seats etc etc the internet is full of them. But I think the truth is that you are working with an imperfect thing to begin with ¢‚¬Å“ air doesn¢ž¢t like curves ¢‚¬Å“ and however hard you try you will never know for sure if what you have is perfect.