956 vs 1108 cylinder heads

[angel]

Last week I took apart a 956cc engine (C1C), in order to enlarge it to 1108cc by fitting 70 mm pistons/liners.
Even by looking at the cylinder head, the combustion chambers seemed to be very deep. I measured their volume to be 36cc, and after calculation, the compression ratio that this engine will have was found to be around 7,35:1 (with flat top pistons, as it is the only type fitted to 1108cc engines). Even the low compression F6 had 8,5:1 ratio, always with the same pistons.
So, here go my questions: Did the 956cc, 1108cc-F6 and 1108cc-GTL engines all have different heads? Has anyone done a direct comparison between these?

 

[Andy McGhee]

Hi Angel
I don't have a 956 head to hand, but here are a couple of pictures of 1108 heads. In the first picture, the lower head is from an F6 - compression ratio 8.3:1; the other is from a GTL (9.5:1). In the second, the heads are reversed; you can clearly see the difference in height.
I know that the earliest R5 956s had a lower compression ratio (8.3:1), and 9.25:1 after 1973. The earliest 1100 R6s were 7.25:1 or 8.0:1; the latest were 9.5:1.
The Revue Technique Automobile gives the compression ratio for R4 956s as 8.3:1.
Can you post a picture of the 956 head?

 

[angel]

Mille merci Andy McGhee! It seems that Renault changed compression ratios by simply skimming the heads...
I'll post a photo of the 956 head in the evening. I guess it is the same as the F6 head, and to compensate for the lost compression ratio,because of the smaller bore, Renault fitted pistons with domes.

7,25 is exactly what I calculate when 1108cc pistons are fitted to the "tall" head. That must had been the way Renault gone with the early 6s.

 

[angel]

Here are two photos from the head. On the bottom face, manifold side there is a step between the cast part and the machined face. Just as in your F6 head. On the GTL head, this does not show, as the head gasket face seems to have been machined that far down.
The cylinder head height is 72,7 mm. Haynes gives a specification (apparently for the GTL head) of 70,6 mm!

So it seems to me that the heads were the same and skimmed more or less at the factory, to obtain the required compression ratio.

 

[David M]

skimmed head

If that is what Renault did, then that's a great way to seek extra performance.

But;

If a significant amount of metal is taken off the head would that mean that the pushrods would have to be shorter or the cam followers shallower?

Maybe I'm missing something?



David

 

[angel]

You are right on both...
There "may" be enough thread left on the adjusters to compensate for the 2 mm skimming though, but I must check it first.
Some years ago, I did exactly the same on a Volvo B18 engine (1,9mm head skimming), to bring it to P1800 specs, and nothing else was needed, all fitted OK.
In the search for extra power from 956 or F6 engines, one could simply fit the GTL head.

 

[Andy McGhee]

A few more pictures:
The first and second are F6 then GTL combustion chambers. The third shows some more GTL heads; two of them have the 'step'. The last one is a low (8.2:1) compression head from a Renault 5GTL (810-19 engine).

 

[angel]

Thank you once again! The first two photos show exactly what I wanted to see. (the spark plug-to-gasket face height is a lot less on the GTL like they skimmed the same head casting).
The "step" that is present on the GTL heads puzzles me. Are you sure they are original GTL ones, or may they had been replaced some time? Or maybe Renault changed the casting to incorporate shallower combustion chambers?

 

[Andy McGhee]

I can't guarantee that they are the original heads, but they all came from GTLs. All of them have the shallower combustion chambers.

 

[John Doe]

This is a 1984 GTL head. This car was made in Slovenia.