Abarth 131 style 16 valve head question

[tmvolumex]

Guy,
I have an Abarth 16 valve head question for you.
I have heard that the Abarth 16 valve heads (as first used on the Fiat 131 Stradle) and also came on various Fiats and Lancias, were basically the same. Because this head was used on normally aspirated cars, IE the Fiat 131 Stradle (10 to 1 compression), supercharged cars IE the Lancia 037 (7 to 1 compression) and turbo cars IE the group 5 Montecarlo, rumor has it that Abarth shaved the heads a different amount in order to arrive at the desired combustion chamber volume. Rumor has it that this milling was done in addition to domed pistons on the Fiat 131. Do you know if there is any truth to this? I have an old 131 works rally car head and it has been milled more than another heads that I have, but I do not know if it was intentional. Have you seen a variation in head thickness and combustion chamber volume on the heads that you have worked with? I also have 2 different sets of pistons for the Fiat Abarth 16 valve head and they are both flat top pistons. One set is for a Lancia 037 and the other a Fiat Abarth 131 Stradle. Next week I will take some photos and measurements but I thought I would first ask you.
Thanks in advance.

[Guy Croft]

Tom, hi

I shall tell you what I know. Abarth would not have condoned the practice of machining the head down for CR any more than I would, for one simple reason. When an engine is designed the valve to piston criteria are laid down on the eng dwgs and machining the had would have a measurable effect on vertical and of course radial clearances. The latter of course because the radial valve-piston clearance on all inclined valve heads reduces the closer the crown is the head, by an amount according to the valve angle. So a very indeterminate way of raising CR even on a short prod run like the 131 or 037. No, Abarth had access to Mondial where the comp ht of the piston and dome config could be altered far more predictably and simply.

As far as piston shape is concerned the 037s I have seen all ran the pistons low in the block to achieve their lower CR, 9/1 or whatever. The same flat top piston might fit the 131 but the 037 only had VRs for the inlet valves, so far away from the head were the pistons (and the ex valve lift at tdc was modest), whereas the 131 16v piston - if flat top and if run flush would need all 4 VRs and the CR at 85 or 86mm bore would be a bit low for a full spec stage rally unit running normally aspirated. No, again, although I have never seen a set for a 131 16v, I am sure they would have been domed, pretty big too.

GC

[RedLexus]

No, again, although I have never seen a set for a 131 16v, I am sure they would have been domed, pretty big too.

GC

Here's a pic of a 131 Abarth Stradale piston GC if its any use? One I overhauled earlier...

[RedLexus]

re a post elsewhere Guy , that piston is at TDC , I'd call it a flat top , with valve cut outs , as opposed to a domed piston .

[volumetrico]

I am not suprised to see the flat top piston with cut-outs in the 131 Abarth. The 037 (stradale) also uses flat top pistons (no cut outs from memory). At TDC the 037 pistons are no way near the top of the cylinder.

Melo

[tmvolumex]

Guy,
Thanks for your info on the Abarth 16 valve head and pistons. I have a set of Fiat 131 Stradle pistons and a set of Lancia 037 Stradle pistons. I check the wrist pin height and determine the difference in the height of the piston at TDC. Running a piston low in the bore does not seem like a smart thing to do, you would get no quench at all. Cetainly not advisable in a supercharged engine. Thanks redlexus and melo for your comments and photos also. The Lancia 037 Stradle pistons I have only have 2 valve cutouts not four, must be because it runs low in the bore? We will see. I have a friend that has a Abarth 16 valve works rally engine, Ill see if I can get a picture of his pistons also. I post all three photos then.
tmvolumex