P6B Cylinder heads

#1
It seems that the Duchess has complete compression loss in one cylinder which the mechanic considers that is most likely due to an ineffective valve. We will know more when the heads are off.
If this is the case it would seem appropriate to change from P6 heads to SD1 heads if I have to have replacement or overhauled heads. My understanding is that SD1 heads have larger inlet valves with improved gas flow and therefore are more efficient.
This was a change I was considering, but had rejected on the basis of "if it ain't broke, don't fix it" . However it would seem may well be broke and so the SD1 fix would seem the best way forward.
Does anyone have any comments on my thinking? Thank you in advance.
 
#3
It's what I've heard too....are SD1 and P38 heads the same? The book I was reading recommended the P38 as the best 'out-of-the-box'. Might be cheaper, too. :)
 

SydneyRoverP6B

Well-Known Member
Staff member
#4
The P6B heads; inlet valves 38.0mm, exhaust valves 33.0mm. Dual valve springs were used with high tensile spring retainers. Spark plugs are short reach with thread length of 12.5mm.

SD1 heads; inlet valves 40.0mm, exhaust valves 34.3mm. Single valve springs were used with manufactured from sintered material that is not as strong as the high tensile retainers as used in the P6B heads. Spark plugs are long reach with 19mm thread length. Flat rubber washer type oil seals were fitted in the factory to the inlet valve guides, whereas the P6B heads were never fitted with oil seals.

Valve springs between the two heads are not interchangeable.

It's what I've heard too....are SD1 and P38 heads the same? The book I was reading recommended the P38 as the best 'out-of-the-box'. Might be cheaper, too. :)
Hi Rob,
SD1 and P38 heads are not the same. They have different combustion chamber sizes, 36cc vs 28cc, different head retention mechanisms, 14 high tensile bolts for the SD1, and 10 torque-to-yield for the P38. The P38 heads also feature Rover Vitesse waisted stem inlet valves and an improved casting that enhances the breathing over all previous Rover cylinder heads. The valves are also recessed into the combustion chambers whereas with all previous Rover cylinder heads, this was not a feature. SD1 heads were designed to be used with tin gaskets, whereas the P38 heads are designed to be used with composite gaskets. The spark plug depth of 19mm is the same for both head types.

Ron.
 

roverp480

Active Member
#6
I seem to remember from when I was at Solihull ,that it was found that the 4 deleted bolts didn't actually do a lot and the 10 gave a more even clamping load
 

SydneyRoverP6B

Well-Known Member
Staff member
#9
Interested to know difference between HC engine and later engine regarding heads / gaskets etc
Hi Pat,

Up until December of 1993, all cylinder heads were manufactured with 14 bolt holes and 36cc combustion chambers, for use with tin gaskets. From January 1994 till 2004, all cylinder heads were manufactured with 10 bolt holes and 28cc combustion chambers, for use with composite gaskets. The compression ratios for the P38 Range Rover and Discovery, in either 4.0 or 4.6 litre capacity varied from 8.13:1 through to 9.35:1.

Ron.
 
#10
So quick question - would putting pre 94 heads on a post 94 engine be possible? Would the larger chambers lower compression to 'boost friendly' levels? Although I see there's a 8.13:1 - that should be low enough right?
 

cobraboy

Well-Known Member
#11
What ever engine you are building, boosted or not it pays to do the math and work out the CR going on what bits you have, or need to get.

With published info out there on combustion chamber cc, piston bowl cc, compressed gasket cc, bore volume etc etc you can work out what parts you need to collect, and what needs modifying to get exactly the CR you want.
I built up my 4.6 to 10.0:1 this way. There is also more than one way to skin the cat.
 

PeterZRH

Well-Known Member
#13
Basically 3 types

1. P5/P6
2. SD1/Discovery - same 14 bolts, same combustion chamber size, same tin gasket but bigger valves different valvegear, different spark plugs
3. Late 10 bolt head - small combustion chamber needs composite gasket to keep same compression ratio. This is a DIFFERENT casting and is cleaner inside and flows better even with the same valve size.

A word on compression ratio. The early engine will drop 0.6 if you use the composite gasket with types 1 or 2. You can machine 0.020" off to keep the same.

There are differences in valve design over the years with the vitesse and later having fully "waisted" types. Use may as well use this type when rebuilding.

Sealing. Most type 1/2 have no valve seals. Discovery engines from about 1992 have crappy rubber seals which are useless and end up in the sump. Later types have "cup" seals on both inlet and exhaust which require the end of the valve stems with a step. These work well.
 

unstable load

Well-Known Member
#14
So quick question - would putting pre 94 heads on a post 94 engine be possible? Would the larger chambers lower compression to 'boost friendly' levels? Although I see there's a 8.13:1 - that should be low enough right?
Short answer, yes. You can boost a standard engine to around 6psi with no adverse effects.
Like cobraboy says, though, do the sums. This is essential to get it right and prevent a very expensive bang from happening.
 
#15
Having now had the heads removed it would seem that a valve seat is no longer seated and that work would be needed to renovate the head. So SD1 heads with normal sized valves will be used as a replacement with tin gaskets.
 

PeterZRH

Well-Known Member
#16
I would do that. Unless you are a concourse restoration then I see no point in investing in the original type of head. When set up properly, should give you 10bhp or so for nothing, pretty much all efficiency gains.

My car has the Montego air filter setup, 10 bolt heads (coming) and a slightly more aggressive cam. This'll go on a rolling road in the coming months and I'm hoping for +30bhp overall.
 

PeterZRH

Well-Known Member
#18
A-ha.... See how ridiculous those air filters are? Those from the 2L Montego (and many others) fit with only a longer bolt to hold them in the standard housing. The effect with a KP or BAK needle is shall we say "transformative" further up the rev range. 59376238_10158499948258009_1715425082077085696_o.jpg 59419222_10158499948243009_1050656894475042816_o.jpg
 
#20
After some setbacks related to carburettor and core plug issues, the Duchess is back on the road and driving well. In a couple of weeks she will have to go the body shop for bodywork treatment - corrosion again! - after the drive from Southampton to Northwich and back this week. Then back to the engineer for "torqueing" the cylinder head bolts.
 
Top