3.5,3.9 & 4.2 heads

[90200tdi]

Hi,

A question on V8s and heads,

Whats the difference between 3.5 and 3.9 heads, will 3.9 head fit a 3.5 block is there any advantage ?

and whats the difference between 3.9 and 4.2 heads, and is there any advantage fitting 4.2 heads to a 3.9 block ?

If i am looking at a V8 engine, how do identify if it is a 4.2L block/heads ?

Thanks.

[p76rangie]

Hi,

A question on V8s and heads,

Whats the difference between 3.5 and 3.9 heads, will 3.9 head fit a 3.5 block is there any advantage ?

and whats the difference between 3.9 and 4.2 heads, and is there any advantage fitting 4.2 heads to a 3.9 block ?

If i am looking at a V8 engine, how do identify if it is a 4.2L block/heads ?

Thanks.

There is a difference between pre 1976 heads and later ones. After 76 they basically stayed the same until they went to 10 bolt heads (earlier ones had 14 bolts) around 1994. There are no difference between 3.5, 3.9 and 4.2 heads only 10 bolt compared to 14 bolt. The advantage of 10 bolt is they are designed to use composite head gaskets where 14 bolt heads require a metal head gasket. If you use a composite gasket on a 14 bolt head you drop the compression ratio.

[zim]

Buy a copy of this and all you need to know will be answered Very recommended. It goes into detail about the differences throughout the years.

They're on ebay.

G

[90200tdi]

Hi,

Thanks for the replies, off to buy a book and do some reading.

[g&t]

If you use a composite gasket on a 14 bolt head you drop the compression ratio.

Used comp. gaskets on a 3.5, without having the heads skimmed as engineers confirmed faces were true. No noticeable effect on performance (such as there is!) but it does mean you can happily run on 95 unleaded as opposed to the 97 the engine, assuming 9.35 comp, should use.

[FridgeFreezer]

I'm running 3.9 heads on a 4.6 with comp gaskets, works fine and performance doesn't seem to have suffered.

[Quagmire]

My 3.5 has 10 bolt heads from a 4.0 Disco engine. They were cheap at the time (£23 ) and have allowed me to keep my 9.35:1 compression whilst being able to use composite gaskets.

All tickety-boo so far

[p76rangie]

I'm running 3.9 heads on a 4.6 with comp gaskets, works fine and performance doesn't seem to have suffered.

3.9's came with both 10 and 14 bolt heads, which ones do you have fitted?

[FridgeFreezer]

3.9's came with both 10 and 14 bolt heads, which ones do you have fitted?

14-bolt, with no bolts in the outer holes what with there being no holes in the block for them.

As per the original question though - I haven't seen anything that suggests there's any great gains to be had from any of the various heads out there, certainly nothing worth worrying about. Remember - it's a tractor, not an F1 car.

[Bull Bar Cowboy]

Whats the difference between 3.5 and 3.9 heads, will 3.9 head fit a 3.5 block is there any advantage ?

and whats the difference between 3.9 and 4.2 heads, and is there any advantage fitting 4.2 heads to a 3.9 block ?

If i am looking at a V8 engine, how do identify if it is a 4.2L block/heads ?

Thanks.

There is no difference between the heads of a 3.5, 3.9, & 4.2 ……. Engine capacity is governed by the bore size and stroke (only the 3.5 uses a smaller bore) and the OEM CR is governed by the piston dish. However, you can increase the CR dramatically by machining the heads by up to 40 thou, decking the block by a predetermined amount and using aftermarket cast pistons.

Visually it would be very hard to distinguish a 3.9 from a 4.2 ……. Apart from the engine number as all RR 4.2 engines started with the prefix 40D ……… the only other 4.2 engine (often called a 4.3) was in the TVR Griffith and that had a 47A prefix.

You will loose about 0.6:1 CR by using 14 bolt heads with composite gaskets. The exact CR difference will depend upon the gasket manufacturer and the compressed thickness, but 0.6:1 is most you will lose. Also with the 10 bolt heads you get the benefit of slightly shorter guides and hooded valve seals on both inlet & exhaust.

However, any good machine shop can modify the tops of the earlier guides (14 bolt heads) to accommodate the newer type seals.

As has already been said, all the heads since 1976 with the introduction of the SD1 have the larger inlet valves ……… there are other very minor differences but these are mainly cosmetic. Be aware that there are still many earlier heads appearing for sale on ebay etc………

Owing to a design defect it is best to leave out the outer row of bolts on the 14 bolt heads, or just nip them tight if fitted……….. better still, use ARP studs.

Christ …. I need to get out more ……………

[g&t]

Christ …. I need to get out more ……………

I often think the same!

In your case just make sure you stay in enough to post your words of wisdom, very informative

[p76rangie]

You will loose about 0.6:1 CR by using 14 bolt heads with composite gaskets. The exact CR difference will depend upon the gasket manufacturer and the compressed thickness, but 0.6:1 is most you will lose.

I think you may find that it is actually a bit higher than 0.6:1. The compression ratio is calculated by dividing the swept volume and the static volume into the static volume. Therefore if you change the static volume (the bowl in the head and the piston) by 10%, the compression ratio will change by 10%. Rover reduced the capacity of the head from 36CC to 28CC to compensate for the composite gasket. Therefore a rough calculation would suggest that the composite gasket has a volume of around 8CC. The piston bowl on a 4.6 is around 22CC in a 9.35:1 motor. 4.6 bowls are some of the largest you will find in a rover V8. So with the old heads you would have a total static volume of around 58CC (36 ford head and 22 for piston). So if you increase the volume by 8CC by using a composite gasket you will increase the static volume by 8/58 or 14%. 14% of 9.35 is 1.3 change in compression ratio.

For a 4.0 motor the piston bowl is only 13CC, which would make a total volume of 49CC and the change would be 16% or a change in compression ratio of 1.5

I am not attempting to state that the above figures are 100% accurate in their calculation, but I think you will find that the change in ratio is above 1 or the equivalent of going from a High Compression Rover V8 to the standard Low Compression version.

[Quagmire]

Read this peeps:

http://www.v8forum.co.uk/forum/viewtopic.php?t=133

There are also endless threads on the site for what comp ratio will be derived from which head/gasket combo.

[Bull Bar Cowboy]

I think you may find that it is actually a bit higher than 0.6:1. The compression ratio is calculated by dividing the swept volume and the static volume into the static volume. Therefore if you change the static volume (the bowl in the head and the piston) by 10%, the compression ratio will change by 10%. Rover reduced the capacity of the head from 36CC to 28CC to compensate for the composite gasket. Therefore a rough calculation would suggest that the composite gasket has a volume of around 8CC. The piston bowl on a 4.6 is around 22CC in a 9.35:1 motor. 4.6 bowls are some of the largest you will find in a rover V8. So with the old heads you would have a total static volume of around 58CC (36 ford head and 22 for piston). So if you increase the volume by 8CC by using a composite gasket you will increase the static volume by 8/58 or 14%. 14% of 9.35 is 1.3 change in compression ratio.

For a 4.0 motor the piston bowl is only 13CC, which would make a total volume of 49CC and the change would be 16% or a change in compression ratio of 1.5

I am not attempting to state that the above figures are 100% accurate in their calculation, but I think you will find that the change in ratio is above 1 or the equivalent of going from a High Compression Rover V8 to the standard Low Compression version.

Just a quick mental calc of increased cylinder volume with the gasket compressed thickness would indicate that your calculations are fundamentally flawed ..........

V= 3.142 x 47sq x 0.508 (the gasket thickness is on average 20 thou above that of the tin gasket)

V = 3525 cubic mm

V = 3.5 cc

This equates to a 6% change ………..a little under 0.6:1 with a 9.35:1 CR

[missingsid]

Just a quick mental calc of increased cylinder volume with the gasket compressed thickness would indicate that your calculations are fundamentally flawed ..........

V= 3.142 x 47sq x 0.508 (the gasket thickness is on average 20 thou above that of the tin gasket)

V = 3525 cubic mm

V = 3.5 cc

This equates to a 6% change ………..a little under 0.6:1 with a 9.35:1 CR

That is what I came up with in reverse.

I have 3.9 heads on a 3.5 and I considered using a tin gasket as I could not get a comp gasket for a couple of weeks. I figured this would end up with a 9.95:1 comp ratio.

In the end I waited for the comp gaskets.

I have heard differing quotes about SD1 valve sizes where only the EFI had the bigger valves. the std carb SD1 had better ports only??

Marc.

[p76rangie]

Just a quick mental calc of increased cylinder volume with the gasket compressed thickness would indicate that your calculations are fundamentally flawed ..........

V= 3.142 x 47sq x 0.508 (the gasket thickness is on average 20 thou above that of the tin gasket)

V = 3525 cubic mm

V = 3.5 cc

This equates to a 6% change ………..a little under 0.6:1 with a 9.35:1 CR

Why does everyone on this site like attacking others. The gasket is not 20 thou thicker. Rover machined down the heads 40 thou to cater for the gasket and changed the head capacity by what I have stated. They kept the same compression ratio. So the change in head specifications must have been extremely close to the impact of the composite gasket. That is, 8CC.

[p76rangie]

That is what I came up with in reverse.

I have 3.9 heads on a 3.5 and I considered using a tin gasket as I could not get a comp gasket for a couple of weeks. I figured this would end up with a 9.95:1 comp ratio.

In the end I waited for the comp gaskets.

I have heard differing quotes about SD1 valve sizes where only the EFI had the bigger valves. the std carb SD1 had better ports only??

Marc.

A 3.9 head can still be the old style heads and you should use a tin gasket to maintain 9.35:1. The heads changed when the went to 10 bolt and not when they introduced the 3.9.

[Bull Bar Cowboy]

Why does everyone on this site like attacking others. The gasket is not 20 thou thicker. Rover machined down the heads 40 thou to cater for the gasket and changed the head capacity by what I have stated. They kept the same compression ratio. So the change in head specifications must have been extremely close to the impact of the composite gasket. That is, 8CC.

no attack ............... if you use a 14 bolt head with a composite gasket then you lose appox 0.6:1CR .......... that was my original statement that you refuted.

[p76rangie]

no attack ............... if you use a 14 bolt head with a composite gasket then you lose appox 0.6:1CR .......... that was my original statement that you refuted.

It is just that you do not supply any source or method of how you came up with 20 thou. You appear to have posted just something to have a go at my figures no matter wether it was correct or not. Obviously Land Rover got it all wrong. The facts are that Land rover made the bowl on the head 8CC smaller to cater for the change in the head gasket and they maintained exactly the same compression ratio. That is, the 3.9 with 10 bolt heads and composite head gaskets has exactly the same compression ratio as the 3.9 with 14 bolt heads and a tin gasket. If what you say is true that the head gasket only makes a difference of 3.5CC and this equals 0.6 compression ratio change, then changing the heads by 8CC should have up the compression ratio to around 10:1. Can you explain why the compression ratio did not change?

[Bull Bar Cowboy]

It is just that you do not supply any source or method of how you came up with 20 thou. You appear to have posted just something to have a go at my figures no matter wether it was correct or not. Obviously Land Rover got it all wrong. The facts are that Land rover made the bowl on the head 8CC smaller to cater for the change in the head gasket and they maintained exactly the same compression ratio. That is, the 3.9 with 10 bolt heads and composite head gaskets has exactly the same compression ratio as the 3.9 with 14 bolt heads and a tin gasket. If what you say is true that the head gasket only makes a difference of 3.5CC and this equals 0.6 compression ratio change, then changing the heads by 8CC should have up the compression ratio to around 10:1. Can you explain why the compression ratio did not change?

Wrong on many counts .......... you need to understand the changes between pre 94 & post 94 engines. 3.9 ceased manufacture in the 94MY and became known as the 4.0L engine for very good reason. The 4.6L was never manufacturered as a Suffix A engine................

Judging by your posting times and the fact that you have a reference to the P76 4.4 engine in your signature, I must assume that you are a colonial……..

You are assuming that the change from the 36cc to the 28cc chamber was just to allow for the use of a composite gasket ?

It must be understood that the 4.0L / 4.6L engines introduced with this head as part of a package of changes, never used the tin gaskets. The most common noted changes are, crossbolted block, larger crankshaft journals, offset gudgeon pin, larger gudgeon pin, longer conrods, longer crank, hooded valve seals, captive camshaft, and improved oil pump.

It should also be understood that the pistons were a completely new design, being very much shorter, lighter, and stronger due to having a thicker crown (although they were still made by Hepolite and cast as opposed to quality forged ! ). A thicker crown also means that you can get a better bowl shape (see below)

The changes were mainly aimed at reducing vibration by altering the angle of the conrod.

The head chamber is only one small part of the total chamber volume, which comprises of the, head chamber volume, gasket volume, piston deck volume, piston chamber volume, and piston cylinder volume (the void to the compression ring).

The newer 4.0L pistons had changed the piston volume (IIRC the older pistons were 11.19cc the newer pistons were 13.23cc) , the piston cylinder volume and finally the gasket volume. All the changes resulted in the 8cc difference in head chamber volume.

The Rover V8 permutations are huge, but the most common question we get is,

“What do I lose by fitting a composite gasket on a 3.9” (by 3.9 we assume that it is pre 94MY …… post that date would be 4.0L) . the answer is about 0.6:1 (based on a 9.35:1 engine)

Most folk will use after market composite gaskets because they are manufactured for 14 bolt heads whereas LR only produce an OEM 10 bolt gasket pattern.

The manufacturer I use to supply my gaskets gives the following information,

Rover 3.5 tin gaskets .. Bore 88.9mm ….. compressed thickness 0.5mm (20 thou)

Rover 3.9 tin gaskets .. Bore 65.5mm……compressed thickness 0.45mm (18 thou)

Rover 3.5 composite ....Bore 92.96mm … Compressed thickness 1.0mm (40 thou)

Rover 3.9 composite …Bore 95.0mm……Compressed thickness 1.2mm (46 thou)

IIRC the Rover OEM 10 bolt gaskets are also 1.2mm compressed.

I suppose the other question would be, what do I gain buy putting a post 94 head on a pre 94 block …… the answer is an increase in compression of about 1.2:1 (based on a 9.35:1 engine).

Nobody in their right mind would even think about fitting pre 94 heads on a post 94 engine !

I trust that this satisfies your challenge ……………. What you see above is the abridged version , you can have the full half hour discussion if you want………

[FridgeFreezer]

With reference to compression ratios, I would like to point out that...

And that perhaps it doesn't actually matter in a 2-ton tractor with the aerodynamics of a branch of Ikea.

[p76rangie]

Wrong on many counts .......... you need to understand the changes between pre 94 & post 94 engines. 3.9 ceased manufacture in the 94MY and became known as the 4.0L engine for very good reason. The 4.6L was never manufacturered as a Suffix A engine................

Interesting information. I should know that Land Rover are hopeless by now. They put a 3.9 Plenum, 3.9 engine number, and 3.9 timing cover on my motor in my 1998 Disco. They even printed the owners manual wrong which keeps on going on about it being a 3.9. And NO, it does not have cross bolted mains. They should have realised that they did not make any after 1994 and certainly none with 10 bolt heads. I suppose that I should break the bad news to Des Hammill so he can change all his books on Rover V8s. Thanks for the help.

[FridgeFreezer]

Would that Des Hammil book be the one that, in older (&twice as thick) revisions, describes EFI as an emerging technology?

I'd refer you to my previous post, unless you're building a lightweight kit car.

[Bull Bar Cowboy]

Interesting information. I should know that Land Rover are hopeless by now. They put a 3.9 Plenum, 3.9 engine number, and 3.9 timing cover on my motor in my 1998 Disco. They even printed the owners manual wrong which keeps on going on about it being a 3.9. And NO, it does not have cross bolted mains. They should have realised that they did not make any after 1994 and certainly none with 10 bolt heads. I suppose that I should break the bad news to Des Hammill so he can change all his books on Rover V8s. Thanks for the help.

Again you have failed miserably to find my angry side ……………. The post you have just made is the equivalent of a poor politician

The 3.9 ceased production in 94. It was replace by the revised suffix B engine that is known as the 4.0L ……… same displacement, stroke & bore as the older 3.9 but mechanically quite different.

Where did I say 10 bolt engines were not made after 94 ……… FFS the 4.0L/4.6L were 10 bolt engines and introduced in the 94MY ………. 14 BOLT ceased production as of early 94…..old stock engines were used in discos up to the introduction of the series II 4.0L which used the later engine. How many 98 RR's have you found with 3.9 engines ?

There is a big difference between ceased manufacture and the use of old stock in a low volume model, which the V8 disco was.

Also there are a number of strange build engines from around that era which used parts common to both suffix A & Suffix B……………

[p76rangie]

Again you have failed miserably to find my angry side ……………. The post you have just made is the equivalent of a poor politician

The 3.9 ceased production in 94. It was replace by the revised suffix B engine that is known as the 4.0L ……… same displacement, stroke & bore as the older 3.9 but mechanically quite different.

Where did I say 10 bolt engines were not made after 94 ……… FFS the 4.0L/4.6L were 10 bolt engines and introduced in the 94MY ………. 14 BOLT ceased production as of early 94…..old stock engines were used in discos up to the introduction of the series II 4.0L which used the later engine. How many 98 RR's have you found with 3.9 engines ?

There is a big difference between ceased manufacture and the use of old stock in a low volume model, which the V8 disco was.

Also there are a number of strange build engines from around that era which used parts common to both suffix A & Suffix B……………

You sound sooo convincing too. My Disco had a 10 bolt block. There were no extra holes. 10 bolt heads. Small main bearing block with exactly the same crank as an early 3.9. Block was not drilled for cross bolts but was drilled for a cam retaining plate. But I don't know anything, must have had my eyes closed while I rebuilt it. Maybe you should do a google search on the 3.9 "interim" motor.