Will 4.0/4.6 heads fit on a 3.5?

#1
Old-mate locally selling a reconditioned set. Cheaper than getting my heads reco'd, if they will fit ok to the 9.75:1 engine I've been talking about? 241729935_4132919890168483_5199294866113641469_n.jpg
 

mrtask

Well-Known Member
#2
They'll fit. Good upgrade. Bigger valves. Only ten head bolts, no more outer row of four bolts to increase the chance of head gasket failure.
 

PeterZRH

Well-Known Member
#3
All heads fit any engine (you can leave the extra bolts out if you were mad enough to fit an early head to a late engine)

10 bolt heads must use the composite gasket otherwise you increase the CR by 0.6. Which is doable but not necessarily desirable for your engine but not really for people at 10.5:1 already.

these are the very best heads to use. The large valve heads in the SD1 and 3.5/3.9 Discovery flow 16% more than the P5/P5 head from what I read and the new casting for the 4.0/4.6 is marginally better still. and this is why you never, ever start with the early head for performance tuning. You just won't get that improvement realistically using any other methods.
 
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jp928

Well-Known Member
#6
Those castings - HRC2479 - also came on 3.9L engines, but they are the smaller combustion chamber types, so need composite gaskets. I have a apir on a 8.13:1 3.9L engine.
 

Oldskoolrob

Active Member
#7
Those castings - HRC2479 - also came on 3.9L engines, but they are the smaller combustion chamber types, so need composite gaskets. I have a apir on a 8.13:1 3.9L engine.
I'm getting a bit confused with the advice here and other forums (my fault, no one elses). If they have the composite gaskets they remain the same compression on a 3.9/4.0/4.6 but if I use them on a 3.5 (1985) with a tin gasket there'll be a compression bump of about .6? That's really the only reason I'd get them, to bump up the compression on the 1985 8.13:1....Not much point otherwise from what I've read.....?
 

jp928

Well-Known Member
#8
Yes, if you put them on an 8.13:1 engine it will usefully lift the CR, but will also breathe a lot better. power will improve also, as long as you also ensure the cam, lifters and rockers are in good order - checking the rocker shafts is easy, they are the weakest link in the pre-SD1 engines. Using them on a larger engine with tin gaskets would lift the CR as well, and on a 10.5:1 3.5L might make it even more peaky, and demand 98Octane to prevent pinging.
 

Oldskoolrob

Active Member
#9
Cheers. I'm going to replace the cam and lifters as it looks worn and is easier to do while the engine is out.
So is there much difference in these cams, they're the two I've been recommended by my local LandRover specialist:

PARts B2v4

PARts B2v4

In summary the engine will be an '85 3.5L 8.13:1 bumped up to 8.73 (or so):1 with the 4.6 heads and tin gasket, Edelbrock Performer, Holley 390 (or 450), electronic ign and extractors, with a BW66 or 65 in a sub 1000kg car, probably a standard torque converter, diff gears 4.125:1 (YIKES!) mostly for cruising with the occasional 'spirited' run through the twisites......thoughts?
 

quattro

Administrator
Staff member
#13
With your 8.31 (or 9.75) engine which may or may not had its heads skimmed, and may or may not have its block skimmed, there really isn't a way of telling until you delve into a bit more. Does it have rocker shims now, or more importantly does it need them?

I have used adjustable pushrods on mine, and still have had to use a 15 thou shim.
 

jp928

Well-Known Member
#14
Head skimming can be detected by measuring a boss on the head - documented in the relevant manuals.
Re change in CR - for a 3.5L and 8.13CR, the unswept volume (head and piston crown) should be 61.85cc; if you swap heads to later 28cc from early 36cc, this should reduce to 53.85cc; using CR = (Swept + unswept)/unswept, this volume should give a CR of 9.19 , assuming same gasket as used before - should be very nice! If using a composite gasket , IIRC its volume is ~6cc, would nearly negate the smaller chambers, bringing CR down to around 8.4:1.
 
#15
Thanks guys, I appreciate the technical info.

At the moment I guess I have two options - rebuild the 8.13:1 with tin gaskets or rebuild the 9.75:1 with composite gaskets. The 8.13:1 is in much better condition internally (some black flaking but no sludge - can still see hone marks in the bores - water passages don't appear badly corroded) and the 9.75:1 I'm still pulling down, but there's a lot of Vegemite inside. I haven't pulled the bearings of either, yet. Both were supposed to be good running engines prior to storage.

What indicators would you use to choose which engine to rebuild? The other will stay in storage for the future.
 
#17
Can't you rebuild the better engine with HC pistons? 9.75:1 and rings are less than £250 new.
That is an option - but with the 4.6 heads and stock pistons it will be a touch over 9:1 which is just fine, and the more I think about it the more likely this will be the plan.
 

jp928

Well-Known Member
#18
Note that this ^ plan assumes tin head gaskets to give a bit over 9:1 CR. Good plan - wont need high octane fuel, will rev well, feel good...in my experience.
 
#20
Well I'm pulling down the 9.75:1 engine, and apart from the vegimite it actually looks quite good. There's a bunch of carbon in the cylinders but not nearly as bad as I thought there'd be and it's all dry, not oily. There's no bore lip and I can still see hone marks in the walls. Will journey further into the engine when I get a chance to see what the bottom end is like. I suspect the [previous owner used oil that was WAY to thick -it was like treacle when I emptied the sump. Maybe this caused the vegimite on an otherwise ok engine. Oh, and water jackets look nice and clean too! I'm now thinking I'll rebuild this engine with the same pistons but new rings/bearings and the 4.6 heads with a composite gasket. New cam ordered and on it's way too.
 
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