View Full Version : Leaded or unleaded

Phil McCann
14-03-04, 08:07 AM
I have just got hold of an RV8. From the casting no. it looks to be from a P6 3500S.The engine is in extremely good nicjk and has had quite a bit of refurbishment done already.

Question is, how can I determine if the heads are suitable for leaded or unleaded use?



14-03-04, 12:20 PM

there are a couple of problems with P6 engines.

It is almost certainly going to require the valve seats replacing. Most P6's were unleaded only, only post '70-'71 engines have unleaded compatable seats (when Land Rover started using the engine in Range Rover).

The compression ratio of P6 is 10.5:1 - 5 star leaded compatable only. You could run super unleaded (if you can find it these days) but you will still have to do the valve seats. You can back the ignition right off to stop it pinking but that won't do your performance much good. Best bet, apart from fitting some later 9.35 ratio SD1 piston, would be to use composite head gasket which will drop the CR by about 0.75 ratio and allow you to get some of the ignition advance back.

The heads themselves are not great for performance, even fitting standard SD1 heads (with much improved ports and un-leaded compatability) will give a significant performance improvement and even more so if you can find some post '95 heads from a x-bolted engine. You won't get the composite gasket CR reduction with a post '95 head.

Other (and not so important?) issues are the bottom end strength of P6 motors is not great if you want to start doing performance mods. They also have 2 piece 'rope' rear crank seals which tend to leak.

I'm no trying to put you off, if the engine was cheap and you can live with some of it's shortcomings compared to later engines, then it'll be perfectly adequate.


Phil McCann
14-03-04, 01:29 PM
Hi Russ

I'm a bit confused as to the origins of the engine. I found a site on the internet whicj gives casting nos etc. From that it says it is a Rover P6 3500S. However, it is fitted with CD175 carbs and the crank seal is of the "orange rubber" type.
The dizzy is points and condenser. There was no spigot bearing in the crank, so it has been fitted to an auto.
So, all in all it's a stranger.
The reason I asked the original question is that there has been so much work done on the engine, and recently, if the condition is anything to go by, that I wondered if the heads had been done already, and if so, how would I know?
Is it an obvious mod. I will be stripping the heads off and removing the valves as a matter of course anyway. So, will I be able to see if the seats have been done?



14-03-04, 10:06 PM

I don't think there's any easy way of telling if the seats are un-leaded compatable. However, a good guide might be the valve diameters.

If they are 38mm inlet/33mm exhaust then they are P6 sized and not likely to be un-leaded compatable.

If they are 40mm in/34 exhaust, then these are SD1 size and are un-leaded compatable.

It seems unlikely to me that anyone would replace std P6 seats and not take advantage of the larger SD1 valves at the same time.

So I would say if the valves are 40/34 then you are probably OK, if they're 38/33 then probably not. If they're any bigger then you have some performance head mods (lucky!) and almost certainly will be OK for un-leaded.


15-03-04, 04:48 PM
Phil, as Russell will tell you another way is: P6 heads had short reach plugs I think, if the heads have been changed to SD1 with bigger valves then they will be the normal long reach type Eric.