Wow I didnt know any 211 Libs made it to the UK. They are a real popular boat around here since they were made in Michigan.

If your Liberator has an Alpha drive you have to be very very careful about adding any power whatsoever. A stock 454 maxes an Alpha right out. If you have an OMC King Cobra drive you are a lot better off.

A cam for you engine is a couple of hundred bucks, you will need a new intake for the heads anyways, and the heads will have to be checked for straightness and good valves installed before you use them anyways. If I were you I would pull the motor and do everything now, instead of band aiding the engine together.

A little time and money now could save you a ton down the road

Im in the middle of a build now with same motor its been 3 weeks and ill be lucky if back by next week. Changing the cam isnt as simple as ppl post it to be, you will need all new valve train parts, adjustable rockers, longer lifters, stiffer springs, im also doing it with 781's ill let you know what happens. But my idea is sell it and buy a bigger motor or blower run that ***** till it blows your transom offff.
or be like me blow your bank account out :lolhit:

Heads alone will help a stock 330, even with a stock exhaust. The heads are the weak point of the motor and choke it at 4500 rpm, followed by the cam, followed by the exhaust. In addition, the 781s generally have a 113 cc combustion chamber. The majority of the peanut port heads are the later vintage and generally have 118cc combustion chambers. Lowering the head to 113 helps the compression, and helps to make more power. You may want to run 89 or even 91 octane through the motor with the head change. Run some numbers and calculate the difference in compression ratio. I believe the stock compression ratio of a 330Hp Merc is 8.75 to 1.

With 781 heads the motor will make more power throughout the lower end and add a little upstairs with the larger intake ports. It will also help to match your large oval intake with the large oval heads. I wouldn't expect much (maybe 20hp or so), but they will absolutely help.

In Re hardened valve seats:
You can check around, but the conventional wisdom is that 1971 was the year that Chevy started using hardened seats. Allegedly it was only the Vette and Chevelle heads that got hardened seats in '71, but by 1973 they should all have hardened seats. You can do your own research if you'd like, but I have looked into the subject in depth and that's what I have discovered.

If you don't want to get into swapping your cam for whatever reason, you could do a "poor mans" cam change and add a set of 1.7 roller rockers and likely pick up a little bit. To do anything else you're going to have to upgrade your exh. and cam, and that's a couple thousand dollar investment to boot.

Without looking up I am pretty sure 781's are 118cc. I think the comp on a stock 330 is 8 to 1. The heads alone will get him nothing without the cam and exhaust. But if the old heads need replaced anyway then go for it
I agree with most of what you are saying. I used to have the Gen V 330 engine so that is what I am going by. I also did a ton of research on the old GM heads. Just going by memory.

Pretty certain that stock BBC rockers are 1.7. You will need a set of 1.8's.

Thank you all so much for your help, I would be lost with out you.
I will strip the engine down and fit these heads after a little freshen up as it will make it easier in the future to upgrade. Trying to find decent exhaust manifolds in the UK is a joke but I will keep looking. Once I have those then it's time for a new cam, oh.....and a bravo one!!!!!!

I also say the heads will help...the ports are much better than the oem 330 heads...with a little work you can make some great power with 781's...used them several times...

I can't get a straight answer on compression ratio of the 330. I hear everything from 8.1 - 1, to 8.8 - 1. That said, they generally use a 118 cc combustion chamber in peanut port heads on the motor. The 781s generally have 113 cc combustion chambers:

http://outintheshop.com/faq/casting/castings2.html


That list was taken from Mortech, who for some reason took down the casting number search from their website. They were an online "go-to" authority on GM casting # research.

Great info there, thanks.

The 781's are 118cc

http://web.archive.org/web/200805130...ec.com/bbc.htm

The older 330's were around 8 to 1, they used -3cc flat tops, very low comp. They were made for reliability not HP.