V8 rebuild - Keep HC pistons or lower compression ratio?

[ricky8664]

Whilst awaiting the outcome of oiling the bores to loosen up the engine after 10+ years of lying idle, I'm considering what CR I should run with if I have to do a re-bore and new pistons.

You can still get the 10.5 :1 pistons and rings but at £850 plus vat (Rimmers) they are more than double the lower CR pistons, which are under £400.

However, if I go with the much cheaper lower CR pistons, what is the effect on performance if I also use composite head gaskets, which I'm informed will lower the CR further?

I would prefer to keep the engine standard apart from upgrading to SD1 heads, a pair of which I acquired about 10 years ago, so advice on performance from anyone who has fitted lower CR pistons would be appreciated.

Also, if anyone knows where I can get a set of +20 thou oversize (new) 10.5:1 pistons at a more reasonable price than Rimmers, please let me know as that would probably tip the balance.

[Jens Munk]

That's what I did when I overhauled the engine. I.e. 9:75:1 pistons, SD1 heads and modern head gaskets. Can't tell the difference. I replaced the distributor too and with the 98 octane petrol reasonably available here (Denmark), there are no pings.

[enigmas]

The choice of a compression ratio for the engine depends a lot on your intended use of the car and the range of fuel octane available. If you plan on driving it to regions or across other countries where petrol octane varies then build the engine to be more tolerant of fuel quality. Consider using a high torque camshaft and appropriate ignition timing.

As an aside, how much wear is on the bores and pistons? Are you going to check the current piston to bore clearances and the top ring land...really the only one that will show significant wear. The only tool you require is a set of feeler gauges. Invert the pistons and check them on the thrust faces within the cylinders.

[lagain]

I would go for the lower compression pistons, apart from anything else, less chance of blowing a head gasket.
I think that mine are 9.25 and she goes very well.
When the heads are skimmed that will slightly raise the compression.
We like to think that our cars are fast and by 60s standards they were, but now even vans are quicker, so might as well go along in a sedate manner !

[Phil Nottingham]

Apr 5, 2023 19:34:16 GMT lagain said:

I would go for the lower compression pistons, apart from anything else, less chance of blowing a head gasket.
I think that mine are 9.25 and she goes very well.
When the heads are skimmed that will slightly raise the compression.
We like to think that our cars are fast and by 60s standards they were, but now even vans are quicker, so might as well go along in a sedate manner !

I agree but we are becoming "old"

[ricky8664]

I've had the car since 1982 when I was in my early 20s. Coming back from a glider flying holiday down the A1, on a straight stretch I got her up to 105 on the speedo, though probably only about 95 ish. With crossplies and a worn steering box that was hairy and not something I ever repeated. I used to see the P5s in the 1970s when I walked past a BL dealer on my way to the bus stop for the homeward journey from school, and fell in love with the look of them. So I bought one as soon as I could get the money together. Wish I'd bought the coupe that was next to it on the forecourt though, given today's price differentials!

Only had it on the road for 10 years before rust relegated it to the garage and I had to wait 10 years to have the time to learn the requisite skills at college to tackle the bodywork, with my college tutor's massive help. We made the panels at college and he came round for a week in the summer hols for two years running to fit them and a number of JRW's panels, with me mostly doing the grease monkey work of drilling out the spot welds.

I get a shiver down my spine when I think of that ton on the A1 and compare it with the safe unhurried and surefooted manner in which my 16 year old 4wd X type estate gets down the road - though I've never even done a ton in that. I suspect as we get older we get more cautious and sensible.... mostly.

No, when it's back on the road it will be used for car shows and the occasional trip along Southend sea front in a manner more befitting a 50 plus year old car.

Interesting to ponder that there were only 5 years between the end of production on the P5 in 1973 and the start of production of the BMW 635 in about 1978 (I think). That's another classic I want. 

[tonys]

I'd opt for the lower compression pistons: no doubt Rover would have done the same in late '73, as they did with P6, had they kept the P5B production running.

In normal day-to-day driving I doubt that many people could easily tell the difference between driving a 10.5 P6B and 9.25 P6B back in 1975. 50-years later, with 50-year old cars, even less chance as there would be so many other variables over that period of time.

[ricky8664]

It's interesting that the general consensus both on this forum and elsewhere that I've read in online articles, seems to be that for normal road use, the 10.5 compression pistons aren't really required. Just as well given they are nearly 3 times the price of lower compression variants. And, of course, they were originally designed to run on 5 star fuel.

Anyway, this friday is the day for seeing if the oiling of the bores with diesel has freed the pistons up. If they still won't turn by the crankshaft pulley then the heads will come off to see if we can oil the bores up more directly to free them and if they do, fit the replacement SD1 heads with new tappets, push rods, rocker arms and shafts and try to start again with new fuel. 

[ricky8664]

Having got the engine started for the first time in 10+ years, looks like I won't need to get the bottom end rebuilt or at least not yet. I'll know more once it's been run up to temp but there was too much smoke from the transmission oil burning off the exhaust to see if the engine was smoking on the first few runs!

Getting my SD1 heads checked over in the next couple of weeks and will  put them on, which will also allow me to have a look at the bores for wear at the same time.

But thanks for the help/suggestions about piston recommendations which will help if I do need a rebore.