4.0 to 4.6 worth it? Results here.

**FisherX** · 14th October 2016, 12:19 PM

G'day Guys,

Last year I did a rebuild on my old Disco 2 where I fitted a 4.6 crank, rods and pistons after it devolped a tick at 220,000km. Just in time too as you can see by the flexiplate photo.

Anyway here are the performance result of before and after. All that was done was a full rebuild with the 4.6 components. Nothing else was changed. Still the old 4.0 MAF and standard ECU.

Before. Standard 4.0 Low comp

And after 4.6 high comp

The 30KW difference is very noticable and makes the driving experience of a Disco much much better (not that it wasn't great before) but it is a bit hard not to keep using the right boot

.

So I'd say if it is time for a rebuild think of the upgrade from 4 to 4.6 Litres. I got my bits second hand from ebay in the US for under a grand and the rest of the gaskets, rings, seals and other bits from the UK quite cheaply.

The rebuilt engine

And the Flexiplate. The only symptom was a little rattle at idle. Be fore warned!!!

**ozscott** · 14th October 2016, 06:51 PM

Nice one mate. Yep the 4.6 high comp is a thing of beauty. Cheers

**Pedro_The_Swift** · 9th November 2016, 09:04 AM

no torque figures huh,,

that with an auto?
where did it change up?

**FisherX** · 10th November 2016, 12:28 PM

G'day Pedro,

Yep it was an auto. The Dynolicious app uses the "G" meter and GPS in your phone to work out the speed and "G" forces. It then works out the KW on the details of the vehical you program into it. So sorry no torque figures

You can see the gear changes as a drop off in the KW readings as the mighty V8 stuggles to push the brick through the air. Also the ZF 4 speeder is very smoth on the gear changes so a bit hard to pick the shift points. So looking at the graph gears changed into 2nd a 60km, 3rd near 90km and forth about 110km.

It's much easier to see the gear changes and turbo lag in a manual as my sigma below. You can even see the jump in power as the cam comes in at 5500rpm. The power in the Siggy keeps climbing as the speed increases cause its got tourqe out the razoo and the lower gears don't load the engine up enough.

Also the when the VVT of the Jag comes in at 3500rpm. And you can see the wheel spin in the first second. Again the Jag is an auto.

**Pedro_The_Swift** · 10th November 2016, 01:34 PM

I hope you didnt use the 4.6 pistons --

the dish is twice as big---

**101RRS** · 10th November 2016, 09:47 PM

Originally Posted by Pedro_The_Swift

I hope you didnt use the 4.6 pistons --

the dish is twice as big---

If he used 4.6 High Comp pistons there is not an issue but if he had 4.6 low comp pistons you are right - if you use the 4.0 low comp pistons in the 4.6 you end up with about 9.2:1 nearly as good as the 4.6 High Comps.

The other thing to be aware of is the gudgeons on the 4.6 pistons have thicker walls (stronger) so if you use the 4.0 pistons you need to change out the gudgeons for the 4.6 version.

**Pedro_The_Swift** · 11th November 2016, 11:09 AM

is there such a thing as a thor 4.6 low comp?
I thought there was only one piston for both Thor engines, only change is dish to allow same compression for each ??
gudgeons are common to both Thors,,

**Pedro_The_Swift** · 11th November 2016, 11:36 AM

Turns out they made a 33cc dish for a LC Thor 4.6,,( standard was 22cc and the 4L had 13,,)
what a rocket that LC must have been---

Those wonderous 4.6 HC's you talk about Garry must be a helicopter landing pad

**PLR** · 11th November 2016, 03:30 PM

Originally Posted by Pedro_The_Swift

is there such a thing as a thor 4.6 low comp?
I thought there was only one piston for both Thor engines, only change is dish to allow same compression for each ??
gudgeons are common to both Thors,,

G`day .

The last two years of the p38a , 2001 and 2002 or 1 and 2 in the Vin had the higher ratio .

The first two years 1999 and 2000 had the lower ratio , X and Y in the vin i think it is .

The vin is more accurate than the year .

RR had Thor for four years .

Pretty sure the same applies to the RR 4.0 in its last four years .

**101RRS** · 11th November 2016, 05:46 PM

Yes there are 4.6 LC - I have the remains of one here - piston has a deep bowl. The 4.0 LC pistons are different and have a shallower bowl hence if they go into a 4.6 they are sort of mid comp (my guess is about 9.1 or 9.2:1. I did the calc based on the volume of the LC 4.6 pistons (8.3:1) and doing a calc on based on the smaller volume of the 4.0 pistons - all rough.

I am not sure what would happen if you put a 4.0 high comp into a 4.6 but I doubt the engine would last long.

As said, the gudgeons are different between 4.0s and 4.6s - same length, same diameter, but the walls of the gudgeon are thicker on the 4.6s obviously to take the larger forces of the 4.6 over the 4.0.

Garry

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