head gaskets and cr, help!!

nosman · Post by **nosman** » Tue Jul 02, 2013 9:27 pm

Ok, split my rv8 3.5 with a nitrous burp, and have bought a lovely 3.9 bottom end to replace it!

However, my motor ran 9.75:1 cr with sd1 heads, i had the heads skimmed 20thou to use composite gaskets and all was well

So seeing as the 3.9 is 9.35:1 cr, could i return to tin gaskets to return my cr to somewhere close? Would the valves and pistons get too familiar?

Help!

Cheers

kiwicar · Post by **kiwicar** » Wed Jul 03, 2013 7:48 am

Hi
The change to tins from composite gaskets generally gives an increase of about .7 CR so the change would b to about 10:1 ish quite safe on a rover. if it is a standard cam and lifters you will be safe for clearance provided you haven't advanced or retarded the cam by more than a couple of degrees, personally I would check, but I would still expect to be safe. You will need to check the lifter pre-load, and shim the rocker pedistals. I am not sure it is worth the effort though, I would do it if you were stripping the top end to fit a new cam, tappets and pocket port the heads (and probably skim a little more off the heads, but by itself it seems a lot of effort for only a gain of 5 to 7 hp at best.
Best regards
Mike

mgbv8 · Post by **mgbv8** » Wed Jul 03, 2013 12:50 pm

What split on the engine?
How much nitrous were you using?
When you say nitrous burp you mean a back fire through the carb yes?
When did the burp happen?

If you want best cylinder sealing then tin gaskets would be best assuming that the faces and decks are flat. But comp gaskets on a 9.35:1 cr engine will take 250 of nitrous all day long if you use it properly.

Why do you want to increase CR for nitrous use?