The V8 Owners Forum -

Hi

I have a P6 with a couple of blowing valves in my Spartan. I have been reading up on the posts about different heads and just want to confirm that I can put 4.0lt heads on a P6 block. I understand that I should not use all the cylinder head bolts but don't understand which ones I shouldn't use. Or why. Also which head gasket should I use? Composite or tin? I know that using a composite head gasket will lower the compression but what effect will this have on the engine or performance.

If you treat me as an idiot who knows nothing when you give a reply not only will you be not far short of the truth I might be able to understand it!

Thanks

Fil

Hi Fil, if you use 4.0/4.6 heads with composite gaskets the compression ratio will remain the same. If you use tin gaskets with those later heads you will raise it, which might be beneficial if you want more power, if not you could use the intermediate heads which have the same size valves in as the later ones but you should find them for less money, and use them with tin gaskets or have them skimmed for composites or to raise the comp ratio.

With regards to the head bolts/studs, it is the outer row of 4 that do not need to be used, and if you buy some later heads you wont have a choice as the heads are not drilled to take them! The reason for not using them is that the main 10 bolts are spaced nicely around the cylinders providing good enough and even clamping, when you put an extra row in on 1 side (whoever's idea it was) it pulls down further on that side, which causes a bit of a see-saw action, effectively lowering the clamping force on the inside of the heads, which is why the head gaskets can blow on the inside. Hope that's understandable!

TC

regarding the head gaskets, i have high comp pistons in my engine but late type heads, would i be better using tin gaskets or would that bring the cr up too high?

you can fit the later heads but the capacity of the combustion chambers is different. Yours are 36cc whereas the later chambers are 28cc. This will obviously affect the compression ratio.

If you stick with tin head gaskets then the compression ratio will go up. I'm no expert here but that usually means needing a higher octane rated fuel or using an additive. I believe the P6 engine was a 10 : 1 CR to start with which may result in too high a CR with later heads and tin gaskets. A composite gasket will lower the CR slightly. Alternatively you could change the pistons and/or rods

With the later 10 bolt heads it is the outside 4 holes that are no longer present (the ones nearest the exhaust side of the head). As I understand it, they were no longer used because they had a tendancy to "tilt" the heads slightly and so cause leaks.

I also think your valves are smaller than those in the the later (post 1976)heads. Also the spark plugs were changed to "long reach" items post-1976 and the valve stem oil seals were also changed on the 10 bolt heads.

Hopefully an expert will correct my errors

Pretty much as above, if you use later heads use comp gaskets tin will put your CR up far too high unless you have a big cam in there. Also if your engine is standard and your not planning head work then any heads will do from SD1 on. They all flow pretty much the same. I guess the ultimate using stock parts would be late 10 bolt heads using 3.9 EFI wasted stem valves. They made the ports slightly larger but use straight stem valves so they flow the same as old heads with the wasted valves.

Make sense?

Can't you just replace the blown valves, grind them in along with all the others?

I have some old P6 heads, I can give you some of valves out of one of them if you need them. (Although my mate as my spring compessor at the mo)

Who's to say that any heads that you buy aren't in a right old state anyway?

Pete

Thanks for the replies. I should have said that the only reason I am changing the heads rather than putting new valves in is so that I can use unleaded petrol. Also the later heads have bigger valves so I thought I might get a bit extra power as well. The engine is a standard P6, no fancy bits and to be quite honest I don't need any extra power. The Spartan is very light but with the aerodynamics of a flying brick and probably similar road holding characteristics if you were brave enough to push it!!

So SD1 heads will do what I need then? But not if the have been skimmed?

The only reason I asked about 4.0lt heads was there are some on Fleabay at the moment.

Thanks
Fil

Depends how much they have been skimmed but if its just a clean up they will be ok, certainly no different from skimming your old heads. Only reason I mentioned the older heads are because they will be cheaper these days as a lot of people think a 4.6 is a bigger engine so they must be better heads and their 3.5 will go loads faster. If only it was that easy lol . For you though yes you will get a bit of benefit from the newer heads.

where are you in lincolnshire, as i have 2 spare sets of unleaded heads. i am in sutton bridge.

Hi

I'm in Billinhay so not far away.
Fil

the heads i got are off a 3.9, i htink they have the same size combustion chamber on as your old heads, but are unleaded, they have bigger valves than the early heads.

Hi Poo Slinger

Have sent you a pm to see if we can get something sorted out.

Fil

spartanv8 wrote:I should have said that the only reason I am changing the heads rather than putting new valves in is so that I can use unleaded petrol.

Aha...that's a very good reason.

Well it seems to make sense, save carrying lead addative around and trying to work out how much to put in!
Fil

Fil,

Just to clarify if you use SD1 3.5 or R/R 3.9 heads with 36cc chamber and use comp gasket this will reduce compression by approx .5/1 which is helpful if you currently have 10.5/1

Kevin.