Fiat 8v Valve Size Vs Seat Size

[mack124]

Hello Guy,

I¢ž¢ve just got some Neway Cutters and I'm about to cut the valve seats on a Fiat 1995 8v head. The valves originally are 35.9 Ex .and 41.9 In. Some 36.5 Ex. & 43.5 In. valves have come my way and I thought I¢ž¢d use them instead. The question is how close to the OUTSIDE diameter of the valve seat can you go? For example if the outside of the valve seat measures 44mm does a 43.5mm valve come to close? Will it crack or cause me any problems?

The engine in question is:

1980 Fiat 132 2.0ltr
Standard rebuilt bottom end
9.25:1 CR
2 x 40IDF on a Waffle Top 34 mm Chokes (jetting to be revised)
4-2-1 124 Exhaust Manifold
Std 132 Cams
The head and manifolds have been flowed. The Ports are around 34mm avg. Short Side Radii is beautifully flat.

If I use the bigger valves will I have to take the ports out anymore? (I know I¢ž¢ll have to redo the SSR) or will 34 average be fine?

What sort of gains will I get out of using the big valves? i.e. Low down or up top power?

Will there be any down side?

Thanks Guy

Kindest Regards

Gill

[Guy Croft]

Gill, hi

you need to leave about 0.5mm seat edge when opening out to stop cracking.

Bigger inlet valves - within reason (there are limits) give - for the otherwise same engine spec) better torque and more power too. So - in brief -all in all a good thing. Bigger exhaust valves, well that's a flowbench analysis really but 36mm will not hold you back with 43.5mm inlet - and maybe bigger.

At 34mm your port size is OK. You won't need to port out more with the big valves. Depending on the effectiveness of your porting and the shape of the SSR your head will fall between 07.132 (final, pale blue) and the yellow line. For my part I like the 07.132 and its high mid range lift and no, I won't be chasing more because (I have dyno tests) that head is good for way over 178bhp/148lbf ft with GC comp cams and high CR. It will give a fabulous torque spread. The 130TC high lift flow and weaker lower-mid is a result of having a little by way of short side radius. The way those ports are cast the port floor runs more or less straight into the seat.
My figures are with race guides fitted, I would not recommend shaving them but if you do there will be an added peak flow gain of about 4cfm. If you have flow figures I'd be interested to see them.

GC