3.5 vs 4.6 Pros and cons....

Oldskoolrob

Oldskoolrob

Active Member
Hey Gurus!
So my project is nearing completion and will hopefully be at the engineers getting certification soon. Geez I hope it passes....
Anyway, so naturally instead of focusing on all the small things that need doing I'm contemplating the next power upgrade. So I currently have a 8.13:1 that appears in good nick (good bearing tolerances, wear seems good (can still see crosshatching in bores, cam looks good, runs ok haven't compression tested it). I did have a 4.6 to rebuild but on my assessment I think it needed re-sleeving, which is cost prohibitive for me. I DO have a 9.71:1 3.5L I'm currently getting ready for rebuild with 10 bolt heads, digital sparkies, Edelbrock performer and Holley carb and a new upgraded camshaft. On market place is a stock 4.6 that is reportedly a 'good-runner' but at a high price as they're rare around here not blown up. I tell myself the 3.5 will be better because there's less chance of liner-slip (and it's only a cruiser) but the rev-head in me lusts for the bigger capacity. What are your thoughts?
Cheers!
P.S. the 4.6 is technically kinda illegal in my custom where the 3.5 IS legal...so there's that too.
 
If it was me.....I would cleanup the latest heads you have (bigger vales, better ports) and fit them. Apart from looking up the engine number there is nothing external that identifies a 4.6 (or 3.9 or 4.2) from a 3.5, so how would any ordinary (ie not Rover specialist) person recognize a 4.6 ? The 3.7" bore blocks all have the bosses for cross bolting - casting HRC2411, but they wont be drilled on a "3.9" version.
 
Hi Rob,

From my experience of having driven a 3.5 from 1985 till 2007 and a 4.6 from 2007 to date, what really stands out is the torque difference. Torque gives your acceleration and pulling capacity which the big engine has in droves. By the time the 4.6 reaches just 1000 rpm it is already delivering more torque than the 3.5 at its maximum of circa 2700 rpm. The 4.6 is a very different engine in its original guise compared to the 3.5, but if you replace the inlet manifold, timing cover, oil pump, run a different camshaft with a nose, and sump, you'll have an engine that looks pretty much like a 3.5, save for the cross bolts, the additional webs on the block, and a blanking plate in the bell housing. The average mechanic not familiar with Rover engines won't have a clue, except for when he drives it, then it will be obvious.

Ron.
 
I doubt that a registration person checking the engine no will notice the cross bolts....and they are not likely to drive it.
 
SydneyRoverP6B said:
Hi Rob,

From my experience of having driven a 3.5 from 1985 till 2007 and a 4.6 from 2007 to date, what really stands out is the torque difference. Torque gives your acceleration and pulling capacity which the big engine has in droves. By the time the 4.6 reaches just 1000 rpm it is already delivering more torque than the 3.5 at its maximum of circa 2700 rpm. The 4.6 is a very different engine in its original guise compared to the 3.5, but if you replace the inlet manifold, timing cover, oil pump, run a different camshaft with a nose, and sump, you'll have an engine that looks pretty much like a 3.5, save for the cross bolts, the additional webs on the block, and a blanking plate in the bell housing. The average mechanic not familiar with Rover engines won't have a clue, except for when he drives it, then it will be obvious.

Ron.
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Hi!
Excuse my for kidnapping the thread but i wonder if you can give me your opinion what a swap from 3.5 to 3.9 in my P6 with ZF 4 speed auto would be like. I understand that the torque improvement will not be as massive as with the 4.6 but do you think this is an upgrade wort doing or should i go for a 4.6 at once?
Peter
 
jp928 said:
I doubt that a registration person checking the engine no will notice the cross bolts....and they are not likely to drive it.
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John, isn't the car taken for a drive at registration time in Victoria? They always do that here in NSW for the purpose of a brake test.

Ron
 
vincenzo said:
Hi!
Excuse my for kidnapping the thread but i wonder if you can give me your opinion what a swap from 3.5 to 3.9 in my P6 with ZF 4 speed auto would be like. I understand that the torque improvement will not be as massive as with the 4.6 but do you think this is an upgrade wort doing or should i go for a 4.6 at once?
Peter
Click to expand...
Hi Peter,

The 3.9 provides more power and torque compared to the 3.5, so you will notice an improvement. It was said to me by a retired service manager having worked with Rovers his entire working life that the 3.9 is "a sweeter engine" than the 4.6.

Ron
 
The main reason for the better torque in the 4.6 is the much reduced overlap of the 4.6 camshaft. Its significantly less, moving the torque band down much lower.
 
SydneyRoverP6B said:
Hi Peter,

The 3.9 provides more power and torque compared to the 3.5, so you will notice an improvement. It was said to me by a retired service manager having worked with Rovers his entire working life that the 3.9 is "a sweeter engine" than the 4.6.

Ron
Click to expand...
I've also heard the 3.9 is a better over-all unit than the 4.6.
 
Oldskoolrob said:
I've also heard the 3.9 is a better over-all unit than the 4.6.
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I have never driven a 3.9 so I can't comment from actual experience. What I can say is that the 4.6 in my Rover is a much nicer engine than the 3.5 that was there originally. Aside from the massive increase in driveability, the standout feature is just how much smoother the engine is in terms of power delivery.

The later 3.9 engines were better in terms of provding a stronger bottom end compared to the early engines so I dare say they would rev far more smoothly as a result.

Ron
 
vincenzo said:
Hi!
Excuse my for kidnapping the thread but i wonder if you can give me your opinion what a swap from 3.5 to 3.9 in my P6 with ZF 4 speed auto would be like. I understand that the torque improvement will not be as massive as with the 4.6 but do you think this is an upgrade wort doing or should i go for a 4.6 at once?
Peter
Click to expand...
I believe the 4.6 will if you use it in anger will eventually break things like half shafts and I'm assuming it will considerably reduce the life expectancy of a differential and universal joints. The change in torque isn't "just a bit" it's 30-35% properly set up over the 3.5. It's a driveline design of a 100BHP car with a bit of overhead for later engine options remember, so you're not far off triple the original torque, so the fact the thing doesn't self-destruct is actually quite remarkable. I'm guessing that's a slightly moot point after you've fed it through an automatic transmission and it's probably more an issue of clumsy driving with a manual. The ZF is a good start though. I can't imagine the poor old BW35 or 65 is going to have a good time of it.

I've been playing with the idea of a custom build and I'm thinking more of the 4.0 at this time, mostly so I can run slightly lower compression (I have a 10.5:1) and use 95 unleaded more easily without losing anything over the 3.5.
 
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PeterZRH said:
I believe the 4.6 will if you use it in anger will eventually break things like half shafts and I'm assuming it will considerably reduce the life expectancy of a differential and universal joints. The change in torque isn't "just a bit" it's 30-35% properly set up over the 3.5. It's a driveline design of a 100BHP car with a bit of overhead for later engine options remember, so you're not far off triple the original torque, so the fact the thing doesn't self-destruct is actually quite remarkable. I'm guessing that's a slightly moot point after you've fed it through an automatic transmission and it's probably more an issue of clumsy driving with a manual. The ZF is a good start though. I can't imagine the poor old BW35 or 65 is going to have a good time of it.

I've been playing with the idea of a custom build and I'm thinking more of the 4.0 at this time, mostly so I can run slightly lower compression (I have a 10.5:1) and use 95 unleaded more easily without losing anything over the 3.5.
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Hi Peter,

The torque increase of the 4.6 over the 3.5 is actually in the order of at least 50%. It is a massive increase. You won't brake the half shafts as their capacity exceeds 400 HP, but breaking any of the three differential drive shafts is entirely possible if driven with too much vigor. My original BW35 was rebuilt as an M51 in 2009 (according to the transmission rebuilder, a significant strength improvement over the BW35 even with a partial upgrade, which he said it had been given previously). He obviously knew what he was talking about as my current transmission incarnation has out distanced every previous rebuild by a huge margin (more than 3 times), and there had been five. It currently has 219,494 miles (353,385 km) since that rebuild.

You're absolutely spot on, Peter, the fact that components don't fail handing the torque of the 4.6 is a real credit to the manufacturers of all the various parts showing in reality, just how good they were.

Ron.
 
Ron,
Usual practice when doing an engine change here is present to rego office with some paperwork (RWC), they ask to see engine no to verify. In fact when I had a short engine replaced in my Subaru they asked to see the number, I said dont know where it is, might take some dismantling to find it, they didnt press any further. Getting a RWC (pink slip?) the inspector will verify the engine no and drive it along with all the other tests. Some changes require an engineering inspection as well on safety items, which oddly includes a gearbox change.
 
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