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Range Rover Classic 3.9litre Low Comp


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Hi guys, I'm a newbie to your site.

I own a Rover P6B(manual) that has been fitted with a Range Rover Classic 3.9litre engine. The engine runs fine, but being a Low Comp of 8.13 it is a bit short on power. If I fit a pair of 10 bolt heads, with tin head gaskets what would this do to the compression ratio? Have been told elsewhere that it would increase the C/R to about 9:1 but I don't know if that is true. Can anybody on here verify this for me please?

Many Thanks.

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Indeed, thought all the 3.9s from RRCs were high compression (9.35:1).

Confusingly though, the 4.0 from the P38 came in low compression mode, and could be that the much later Disco 3.9 came in low comp also?

You can work out the compression ratio of you know the cylinder volume, head gasket thickness (and therefore volume increase in the cylinder), piston dish volume and the chamber volume... but you need to have the engine apart to confirm all these.

Having said all this, Rover V8 compression is normally set by the piston, and not the head... by varying the dish size. To raise it, if the engine is at all worn, I would be looking to swap out the pistons and do a hone while in there.

 

 

 

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Fitting P38 heads with a tin gasket would raise compression a bit, although not much.

If it's suffering noticeably poor performance I'd suspect other things first - RV8's will run like a sewing machine even when totally shagged out.

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Engines Nos commencing 37D00001 and 38D00001 are low compression @ 8.13:1. 10 bolt heads have smaller combustion chambers due to fact that they skimmed at the factory to accommodate the increased thickness  of the composite gasket. My thinking was to fit a pair of the 10 bolt heads rather than having the existing ones skimmed. I am just trying to find out how much this would increase the compression ratio.

Adrian.

 

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Yes, I think I am familiar with the thread you are referring to !!  The main contributor said at the end of his last major posting on that thread, that retro fitting post 94 heads onto a pre 94  9.35:1 c/r engine, would result in the c/r being increased by 1.6:1 .  My question is, what would be the effect of doing the same to a pre 94 8.15:1 c/r engine ?

In case you were wondering, I came upon thread you are referring to, as a result of a Google search into the topic. I had hoped by joining this website I might be able to either contact him directly or re-awaken his expertise in the matter !

Am a somewhat surprised that this issue has not come up before because the cheapest source of supply for RV8 engines is Range Rover Classic!

Regards, Adrian.   

Edited by Hernerover
grammatical correction
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IIRC the highest comression V8 other than tuned was the P5 B which was 10:1 cr.

I changed my SD1 V8 in the S1 to 3.9 heads and chose to go with the correct composite gasket rather than try for a tiny cr increase by tin when comp gaskets seal better?

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I think the short version is that of course you can gain a bit of compression by fitting later heads and tin gaskets.

The caveats are that tin gaskets are less good (although they worked OK for decades, so they're by no means rubbish) and that the increase is small.

Depending where you are in the world, the price & availability of parts / donor vehicles skews the economics of the whole thing - for example, in the UK it seems hardly worth the effort of rebuilding a low-comp engine when you can pick up a running 3.9, 4.0 or 4.6 vehicle with either rust or electrical faults for peanuts, swap engines and scrap/sell the remains and have spent very little to gain somewhere between plenty and double the power you started with.

Likewise, some of us might make the suggestion that carbs & points are something of a limiting factor, especially if your carbs where whipped off a 3.5 and haven't been set up for the 3.9.

Much of this depends on your ultimate goals, budget, technical ability, and willingness to carry out work.

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It is important to remember that post 94 engines were heavily revised in all areas, block, crank, rods, pistons and heads mainly !!! The main difference between a 10 bolt head, HRC2479 and a 14 bolt head, ERC0216 (SD1) or HRC2210 (Land Rover) is that the cubic capacity of the heads was reduced from 36cc to 28cc. It is this fact that is key to my original posting on this website.

In the Land Rover Technical archive there is a thread titled: "3.5, 3.9 & 4.2 heads" which is quite exspansive . From that thread, the explanation is quite clear, that fitting post 94 heads to a pre94 engine results in a c/r increase from 9.35 to 10.55 on a 9.35 c/r engine. My question has been, what is the resulting c/r increase to a 8.13 c/r engine? I don't have the relevant technical data to do the calculation myself.

As far as obtaining 9.35:1 c/r engine goes, it is just too expensive and doing an engine change on a P6B is not a job for the faint hearted! The era of good condition RV8s available for peanuts is long gone and such engines when they do become available sell for in excess of £750 - £1,000. I am quite capable and confident of undertaking a cylinder head change though!

 

 

Edited by Hernerover
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On ‎23‎/‎12‎/‎2016 at 10:06 PM, Hernerover said:

Yes there is a reason why they switched to composite gaskets !!

It would appear that the person that I wanted to contact, B*** B** C***** is no longer active here. If he was, he would know exactly what I was talking about.

 

Ian does appear very occasionally - he's retired with grandkids now

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Hernerover............ I have a 3.5 EFI engine which I have had for years, kept for a rainy day. Its only done 22336 miles from new. Also a 9.35 comp 3.9 on 10 bolt heads which has been rebuilt and never run. Might be persuaded to part with either.........or both.

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