polish and ported heads

garethp6

garethp6

Active Member
Hey all does polish and ported gain and more performance as my old engine had post to have them done but not 100% sure on that. I am woundering to have them done for this engine as my old engine was the 9.5 block and it did seem quiker then the 10.5 block i got now. How can i check if the old head had it done cause if that the case i will end up putting those head in this engine but b4 i can do that i will get all the valves ready for the unleaded petrol save my self some work any ideas any 1
 
Hello Gareth,

Polish and porting falls into different categories depending on the level of work carried out. RPI Engineering provide an example...http://www.v8engines.com/engine-7.htm

If you fit such heads to your engine, provided the inlet manifold, carburettors, camshaft, ignition and exhaust systems are all upgraded and tailored to suit, then the outcome will be very noticable. If on the other hand all items bar the heads remain as standard, then very little if any noticable difference will be achieved.

To tell if the heads you own have been port and polished, you will need to remove them from the engine and compare and contrast to a standard pair of heads,...measuring inlet port dimensions etc.

The pistons determine the compression ratio, not the actual block. The reason the 10.5 engine seems slower is probably down to the fact that the compression ratio is too high for todays fuels, so running the ignition retarded is the only option, and of course this will noticably impact on performance.

Ron.
 
Most effective upgrade is to use SD1 heads, as these have bigger valves than the P6 spec engines. Much cheaper than polishing and porting as well!

Chris
 
When I first looked into this back in the 80's, I am sure that the stage one P6 heads were 9% more efficient than standard ones, but standard SD1 heads were 12% more effiecient than the standard P6 ones. So as Chris says, the SD1 heads are better than working on the P6 ones.

Richard
 
hi guys thanks for all that well i got anouther engine and i think is the sd1 but how do i find out if the heads are off a sd1
 
Take a spark plug out and measure it.

The SD1 plugs are 3/4" long whereas the P6 ones are only 1/2".

I am sure someone will put me right on those measurements as I am stating this from memory :?

Richard
 
right well i will do that i know the heads are off the engine does any 1 know wot size the valves would be looking at for the p6 and the sd1. with the sd1 heads i will gain much power from those heads
 
Hello Gareth,

If the heads are already off the engine, then in order to acertain which heads you have you can either measure the depth of the spark plug threads as Richard advised, or measure the valves. The sizes for each type of head are as follows......P6 Inlet 1.5" exhaust 1.312" SD1 inlet 1.575" exhaust 1.350"

You would be hard pressed to really notice any difference I would feel except for possibly when the engine is spinning beyond 4000 or more rpm.

Ron.
 
Not sure I agree Ron. The P6 10.5:1 compression engine and the SD1 9.25:1 (is that right or is it 9.35:1?) engine have near as damn it the same output. That's a considerable achievement for the SD1 as lowering the compression would be expected to knock a fair chunk off the power output. It makes the difference between 150hp and 135hp in an early Range Rover or MGB V8 engine.

Chris
 
Old Thunderdog had a standard (late) P6 block with 10.5 CR and a rubber crank seal.

I fitted a set of 4:2:1 headers, a new (Stock) cam and lifters, high flow/pressure oil pump kit, and some cleaned up SD1 heads. SD1 contactless dizzy and a 30% lightened flywheel. Apart from a header tank and some AAA needles, that was about it.

It ran 197bhp at the flywheel when on a rolling road in Blandford.

It made a significant difference on the road.

Richard
 
You could well be right Chris. The P6B sold in Australia with the 9.25 : 1 CR was rated at 143 net horsepower (flywheel) while the UK spec SD1 running twin HIF6 carbs and a 9.35 : 1 CR was rated at 155 net horsepower (flywheel). I don't know how easily a 12 horsepower variation could be felt, but it may well be noticable given it represents an 8.5% increase.

The P38 HSE Range Rovers sold in Australia running the 4.6 litre engine came in three CR variations...8.13 : 1, 8.37 : 1 and 9.35 : 1. The difference in maximum power between the models was in the order of 5%. However that is not the end of the story. Dyno testing of the engines at Sydney's leading Land Rover specialist revealed that the average power delivery for the low compression engines was better, thus making their use in automatic vehicles superior to their high compression cousins. For manual vehicles though, the high compression engines were favoured.

Dyno results showed that the power delivered and the revs at which this occured was greater for the low compression engines up to approx 3000 rpm after which the high compression engines would catch up and ultimately climb higher as the revs reached 4750. Once that rpm was exceeded power fell away in all cases.

Ron.
 
so if they are sd1 is it worth changing them over or not cause i will get all new valves and stuff and change them over if not i might as well keep with the pair are on there
 
Hello Gareth,

Are you happy with the way your engine pulls at the moment? If so, leave the heads that are currently fitted in place. If not and you know for certain that the existing heads are the problem then refurbish and fit the SD1 heads. Do note though that as a result of these heads breathing more efficiently compared to the P6 heads, that your existing carburettor needles are likely to prove to be too lean. So you may well need to fit the next richer needle in the chain. You will need to experiment and see.

Personally, if your engine is running well, I would be inclinded to leave well alone.

Ron.
 
hi yea she is running great just thought i would try and get some more power out of it but if it involves all that work as you said best to keep it the way she is thanks for that
 
You must log in or register to reply here.
Back
Top