Hi there,
If you can achieve the same levels of acceleration and make the same amount of progress on the new engine with less throttle input then it must be more efficient than the previous engine.
If you've got 4.6 heads sat there on the shelf with nothing to do they need putting on the car ASAP in my opinion!
All 4.6 heads will have a relief machined in to the castings for the injectors as before these were introduced Rover had already completely switched over to EFi anyway. The only exception might be if you've started with a brand new casting? However that's a very very slim chance and its 99% certain they will have the reliefs milled in anyway. Even if you are unlucky and they don't then it's no trouble to "have at it" with a dremmel or a die grinder for a few minutes.
The fact that they are 10 bolt heads as opposed to the 14 bolt heads you have on now doesnt matter - they will still bolt on just fine and the 14 bolt heads clamp the gasket in an odd way in any case. It's a poor design feature really.
You *can* play around with rags and a vacuum on the old setup if you like. However, if you're anything like me suddenly the workshop environment becomes an operating theatre environment when an engine gets built up so the idea of risking ally swarf entering the heads doesn't really appeal to me. Personally I'd take the heads off, strip off the valves, blast the casting with compressed air, bath the casting in petrol and break out the compressed air again to dry.
4.6 heads have a smaller 28cc combustion chamber to allow for the difference in thickness of composite head gaskets.
Making the assumption that the block has not been decked and you are using standard pistons then you will be able to fit 10 bolt heads just fine without any fear of valve contact. If you have been using the SD1 heads on comp gaskets then you will have dropped the compression ratio a bit so there may well be further power improvements to be made.
The only downside is of course you will need to re-set the rocker clearances which can be a faff. I've never liked the half-arsed system Rover used of "a few tight, a few slack and a few just right". It's obvious why they used it - to appease the bean counters in the office! I think it's worth investing in a set of chrome moly adjustable pushrods and get the whole lot set up perfectly.
The fact that there is a known problem re: the distributor is a clear elephant in the room and wants attending to really. I would suggest you either source a new Mallory unit or source a refurbished standard unit from somewhere like the distributor doctor. Of course there's always the stand alone wasted spark systems if you want to upgrade and are happy to play with laptops, but then we're talking about a considerably bigger investment!
*edit* need to type quicker... a whole page of posts between starting and clicking "submit". Perhaps this man flu that I'm suffering from is worse than I thought!
