Race Engine Forum UK

I've been trying to enlarge the 1216cc engine of my Fulvia HF by enlarging the bore from 76 to 80mm. (stroke is 67) In that order I had a set of forged pistons made by venolia. The pistons I recieved were in 2 ways not what I expected and one of them is the weight. Compaired to an original 76mm cast piston (with pin and clips) it weights 45gr more. It sounds like a lot but is that so? I wish to get a rev happy short stroke 1348cc Fulvia.
The other thing is the CR. They claim on the box a CR of 11:1 (stroke used for this calculation is 69.7mm (1300 fulvia engine). With my shorter stroke it will drop and when using long overlap 302 deg cams it will drop (effective CR) even more I guess. Is there a Minimum CR relation to a given engine capacity when power and revs are an issue?
In other words, will these pistons do the job intended?
Kind regards,
William

If you don't specify underhead milling to a certain thickness (if they can even do it ) and if the forging is not a 'hollow dome' type, yes forged ones can end up as heavy as the OE ones - or heavier.

But, 45 gms isn't vastly overweight, I guess the Lancia ones were pretty light, like their old engines.

To set your mind at rest - if you can get over 9.5/1 CR never mind near 11/1, on a small motor like that she'll run very well with most cams, the more the better, I would not worry about the CR.

Piston weight is 45gr over 450gr of the stock piston (incl pin and clips). A 'millable' dome was specified but disappeared somehow during the process...
I've allways understood that small engines (like this one) need a high CR to be able to perform. And due to the small capacity and hence small combustion chamber can get away with it without detonation.
I can strike a deal with these pistons and (probably) get my money back. I can have JE pistons made with a large dome and probably a tad lighter as well. Waiting time over 2 months. Price is the same. Worth the wait or just cary on with these ones?
regrds,
W

PS:The ASSO pistons I have now give 15.5 bar on the compression test and hint at pincking even with 98 octane fuel. But with standard 274 deg cams. Peakpower sits at 6800Rpm and it won't rev much further. With new 'big bore' pistons I wanted to get the best of my waiting to be installed Colombo&Bariani 302deg cams.
8000rpm must be obtainable I think.

Sure, William, I just said higher CR is better, but if you think you have the answer why ask me?

Readers - I'm not getting cranky but I don't get time to discuss much in GC Q&A. I mean 'chat about this and that'. That's for other forums. If I say you can do it, be sure - you can and I know because I've done it. I present fact as I know it to be, no more, no less. You will have noticed I rarely post on the other forums at gcre.com. If - from time to time I give you the 'brush strokes' that is the reason. You want reassurance and an answer quickly so there it is in a nutshell. GC brush strokes you can trust.

I repeat - without going into detailed appraisal of the performance of a long list of StII - IV cams for that little motor - that on the right side of 9.5/1 she's going to be fine, and you have much more than that. There is categorically no fixed rule with CR even according to engine cc - except that more is usually better.

Now, regarding weight. Stop worrying about the 45 gms. Look at it this way. Unless you laid down stringent protocols at point of purchase whose fault is it? As for me, the days when I would let any piston firm loose on a model for me are long gone. I design them CP read the cad, check the dwg. If you buy off the shelf or to a loose spec or sketch, this is exactly what happens, over-weight being one minor thing that can go wrong.

Check out the pictures, they will tell you plenty more!

GC