Diff upgrade for 300hp?

Oldskoolrob

Active Member
Hi guru's,
Research continues (blown engine), and as I can get 300hp from a chev for the same dollars I'd spend getting 200 from my blown 3.5 I'm considering what to do. My BW35 needs a rebuild and again a chev box is about the same cost. Lets say I go chev, are there any simple diff upgrades that can be done?
 
Well, you're right - that wasn't simple.....:oops:
What hp would the standard diff cope with, without abuse?
 
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Depends on how mechanically sympathetic you are.
I use all of my 270hp, but don’t apply it like turning on a light switch.
 
Specialist I was talking to said the Jag Unit would take 300bhp, but not much more?

I have seen claims they will take 600+ bhp, so really not sure, except they will take a lot more than the standard Rover unit.

The standard XJS was under 300bhp, with around 400N/m of torque.

I was think the torque figure was more of a problem though, especially with a heavy car (The XJS was around 500Kg heavier than a P6), so 400Nm in one of those would put a lot more pressure on the diff, than a P6.

What is the torque of the Chevy?

Richard
 
Torque is about 330 lbft. Just looking at a 290hp GM crate motor, turbo 300 trans. Which looking at @cobraboy 's post makes me think I might need to do still more research - what have you done to your engine to get 270?

If I stayed "Rover" would there be much benefit in the 3.9 over my 1983(?) 3.5 block? Cheers, Rob.
 
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Bigger and badder = 270.

My other Rover engine is 3.9 and makes 236 bhp, but that is a screamer, not really suited to the refined nature of a P6.

It is possible with 300 lb ft and skinny tyres that the power would bleed away through wheel spin and thus protect the diff.
Just fit the motor you want, then beef up what ever it breaks.
 
Yup, I have heard that before.
I have never had the inclination to light up the rear end. I am looking after the drivetrain but am not sparing any horses when I need to go fast. Sure something may break, but at the moment it is everything bar the diff !
 
I once managed to remove eleven twelfths of the teeth from the offside of the diff (I dunno what the names of all the gubbins inside a diff are!) by injudicious application of full pedal peeling out of a side turning. I made it as far as the middle of the road I was entering, the OS rear wheel smoking, until it caught traction and everything ground to a halt. I think I've boasted about that here before. :rolleyes: Feckless youth, now somewhat wiser.
Ever since that episode I've learnt to feed in the power, a bit less like throwing a switch.
 
So there's a 3.9 for sale locally (I have a 'stiff' 3.5 that needs rebuilding). Is it worth it to spend the $600 he's asking to start with a 3.9? Does 400cc make that much difference in the end? I'm guessing every cc counts - I'm just trying to justify it to myself.

Does the 4.6 have the same mounts/bellhousing and can it accept the P6 sump/timing cover etc?
 
You don't have to buy the 3.9. Assuming it will need a rebuild anyway. If you didn't then you would have a 600 buck head start on rebuilding the 3.5, which with a bit of work can be made to out perform a stock 3.9 anyway.
I only ended up with a 3.9 as it had an incurable overheating problem. Because it was free it was viable to top hat liner it, which made it a fantastic engine to put money into.
You have to be wary of ploughing big bucks into the larger capacity engines without having them top hat linered.
The 4.6 accepts all the P6 stuff, and the block is externally the same dimensions.
 
I'd agree with everything cobraboy said. However, I would posit that if the 3.9 is a cross bolt block, it might be worth the asking price, as that is the best basis for building a really stout Rover V8 motor. Talk him down to five hundred smackers and you'd have made a good purchase.
 
3.9 isn't cross bolted. 4.0L (same capacity as the 3.9) and 4.6L were cross bolted.

There are two different length noses on the crankshafts, so if you're using the P6 front timing cover, make sure you have the correct one.

Richard
 
I really appreciate the advice here. I'd like to build a nice lively motor - but it will rarely see 5K rpm so I don't think I'll have to worry about cross bolting. So I'll stick with the 3.5 I think as budget is an issue.... :)
 
The first couple of 3.5's I built I was wary of the main bolts backing out. As I wanted to rev them I drilled the main bolt heads and wire locked them, I never had a problem and they saw 6K on a regular basis.
 
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