More engine info needed, heads this time, thanks

[Hensen1954]

While rebuilding 3 engines at the same time, and finding a few things that need sorting, I have a few questions.

1. Why do the early engines have 2 springs per valve, but the later ones 1.

2. What makes the compression ratios different, they all see to have the same pistons, and the 10.5 engines actually seem to have inlet valves so dished that it would reduce compression more than the later engines.

3. I have an extra engine - bought 4 , should I rebuild the extra one and put it on ebay, whats a rebuilt V8 worth ?

 

[SydneyRoverP6B]

Hi Hensen,

Double valve springs were used on all Rover V8 engines including those in Range Rovers up until circa 1976 when the SD1 came on the scene. The P5B and P6B engines and early Range Rovers all had small valve heads. From the SD1 onwards including all Land and Range Rovers up until 2004 all cylinder heads had what was referred to as large valves fitted with single valve springs. The pressure exerted by both the double valve springs of the small valve heads and the single valve springs of the large valve heads are essentially the same.

With all else being equal, the pistons set the compression ratio. The 10.5 : 1 pistons are flat top pistons, whilst pistons designed for less compression have dishing within the crown of one form or another. Fitting composite head gaskets in place of tin gaskets in an engine otherwise designed for the latter will lower the compression ratio by 0.6 or so.

I don't think that a rebuilt 3.5 will go for all that much money, especially one originally from either a P5B or P6B. An engine built around the later 3.5 "stiff" block is for many more desirable, although in reality there is absolutely nothing wrong with the original factory spec 3.5 from either P5B or P6B, indeed one properly cared for and used in a manner in which it was designed will run for the best part of 200,000 Miles or more like mine did.

Ron.

 

[Hensen1954]

Many thanks. I am learning Rover V8's very quickly, especially how to get broken bolt studs out !! lol.

 

[LeeEFi]

From the SD1 onwards including all Land and Range Rovers up until 2004 all cylinder heads had what was referred to as large valves fitted with single valve springs.

From what I've read, the larger valves that started on the SD1 3.5 heads were to extract a few more rpm's. Unfortunately the larger valves when fitted to the 3.5 do cause some valve shrouding that was not evident on the earlier P5 and P6 V8's with smaller valve heads.

With the 94mm bore of later(3.9-litre onwards)motors the valve shrouding problem was no more, due to the cylinder wall being further away from the valve. I would imagine the valve shrouding problem caused a slight imbalance of combustion pressure?