**Advice Needed**

need4speed · Post by **need4speed** » Mon Aug 20, 2007 10:30 am

Im planning to build a 4.6 to go into a landrover. Ive already purchased a 4.6 crank which i'll have ground down to fit 3.9 block. My question though is this. Given that i have a pair of stage-3 early heads to fit on top, what rod/piston combo can i use? John Eales has offered me a set of pistons for £240. That then leaves me looking for a set of rods. Whats the cheapest way of making this work? I was just gonna buy standard 4.6 rods/pistons from on here but ive been told that i cant if using my early heads...

ian.stewart · Post by **ian.stewart** » Mon Aug 20, 2007 10:48 am

Cheapest way to do this I would think is to use the 4.6 pistons and machine the tops back to lose compression, how much, I dont know, but I am sure somebody on here will know, I suppose you can measure the piston to deck height on the original engine, then get somebody to measure the same on their 4.6, this would give you a rough idea on how much you need to remove BUT, I think there is a dish in the piston so it may not be straight forward.
I think you have to gain about 8-10 cc in the combustion area, dosent sound much bit it will make a difference, Personally I like a lot of compression, and run over 11.1 on the street without too much problem, I do run it on Shell V and have mapped ignition as well

chodjinn · Post by **chodjinn** » Mon Aug 20, 2007 10:57 am

I keep finding out more and more about this type of conversion that really puts me off considering it! I have early heads as well.

To be honest, its looking like more and more work (and expense!) to get it to work. By the time all the machine work is done plus full ARP kit you may as well have bought a 4.6 to start with lol!

need4speed · Post by **need4speed** » Mon Aug 20, 2007 11:01 am

John said that the early type heads have 36cc chambers, and that as im building a 4.6 i'll need 28cc chambers??

need4speed · Post by **need4speed** » Mon Aug 20, 2007 11:07 am

chodjinn wrote:I keep finding out more and more about this type of conversion that really puts me off considering it! I have early heads as well.

To be honest, its looking like more and more work (and expense!) to get it to work. By the time all the machine work is done plus full ARP kit you may as well have bought a 4.6 to start with lol!

I know what you mean mate. Thing is so far it hasnt worked out too bad. i bought a good 4.6 crank from ebay for £60. 3.9 block wont be expensive to buy either. £200 to grind crank and ive already got stage 3 heads. If i can get away with using stock 4.6 rods/pistons i'll be laughing...we'll see

chodjinn · Post by **chodjinn** » Mon Aug 20, 2007 11:18 am

Gav has his block on fleabay at the moment for a measily 150 sheets.

The way I priced it:

Block: upto £200
Cranks/rods/pistons: upto £300
Machine work: £200
ARP bolts: £300
And that's not including anything else like bearings, machining pistons, crank regrind etc. or if anything goes wrong . . .

I have seen 4.6 short motors on fleabay for less than 800 quid.

need4speed · Post by **need4speed** » Mon Aug 20, 2007 11:36 am

good price but im not in a big rush for block at the mo. 3.9's can be had all the time. its what to do regarding pistons/rods thats concerning me at the mo. i wont need ARP kit i dont think as the engine im building will not be a high revver - its going to be built for big torque between tickover and 5k...

chodjinn · Post by **chodjinn** » Mon Aug 20, 2007 12:04 pm

True, but you do know it is only a certain type of 3.9 block that you can use, and they are quite rare!?

And to be honest ARP bolts are pretty much a must for any Rover that will be abused, high revver or not. They're such fragile little things unfortunately . . .

need4speed · Post by **need4speed** » Mon Aug 20, 2007 12:31 pm

No i didnt know that. when i mentioned to JE about machining my crank to fit a 3.9 block, he did not ask what kind of 3.9 block. He just said that the 3.9 block had 2.3" main bearing housings so i would have to have the crank journals machined to match...
So why is it only a certain type of 3.9 block that can accept the machined 4.6 crank??

Post by **katanaman** » Mon Aug 20, 2007 12:54 pm

Why would you have to do anything spacial to use 4.6 pistons and rods on a 4.6 crank? The bore and deck hight are the same whatever block it is (3.9,4.2,4.0,4.6), so long as you keep the original size 4.6 big end journals it should all just bolt together. CR wise for the heads all you have to do is use the correct gasket and you will keep the correct compression. You dont have to use small volume heads just because your using 4.6 pistons.

You will need ARP studs whether your revving higher or not as its a stroker engine which will put a lot more stress on the mains.

need4speed · Post by **need4speed** » Mon Aug 20, 2007 1:16 pm

katanaman wrote:Why would you have to do anything spacial to use 4.6 pistons and rods on a 4.6 crank? The bore and deck hight are the same whatever block it is (3.9,4.2,4.0,4.6), so long as you keep the original size 4.6 big end journals it should all just bolt together. CR wise for the heads all you have to do is use the correct gasket and you will keep the correct compression. You dont have to use small volume heads just because your using 4.6 pistons.

You will need ARP studs whether your revving higher or not as its a stroker engine which will put a lot more stress on the mains.

I dont have to do anything special to make 4.6 pistons/rods fit 4.6 crank. But what ive been told by John Eales is that i cannot use my early type heads if i want to use the standard type 4.6 pistons. He told me i would need to use different type pistons. Do you feel he is spinning me a yarn?

Post by **katanaman** » Mon Aug 20, 2007 4:38 pm

I would never say John Eales was wrong or having you on but I cant for the life of me think why. There must be loads of 4.6 engines with early heads on them. He must have his reasons for saying this and if he can explain to you why I would like to hear, I am sure others would like the knowledge as well. Maybe something peculiar to this conversion or something?

stevieturbo · Post by **stevieturbo** » Mon Aug 20, 2007 8:08 pm

Dont forget....

4.6 used 28cc heads, and composite gaskets.

older ones used shim gasket, and 36cc heads....

So if you stick with a steel shim gasket, you will regain some compression

need4speed · Post by **need4speed** » Mon Aug 20, 2007 8:42 pm

stevieturbo wrote:Dont forget....

4.6 used 28cc heads, and composite gaskets.

older ones used shim gasket, and 36cc heads....

So if you stick with a steel shim gasket, you will regain some compression

Ahh i see. so thats the reason the 4.6 had smaller combustion chambers - because it used thick gaskets! Thanks for that mate. This sounds like JE would have me buy a set of there pistons, when all i REALLY needed to regain my compression is the thinner tin gaskets.........
So i CAN use my nice stage 3 early heads after all - as long as i use thin gaskets...WOOHOO!!!!

stevieturbo · Post by **stevieturbo** » Mon Aug 20, 2007 9:02 pm

Im not saying that, Im saying calculate and see.

But it could go a long way towards helping you.

I know that when I used a 4.6 short motor, with low compression pistons circa 8.13:1. I did so with thick head gaskets, AND stg 3 36cc chamber heads, to give me around 7.5:1 CR

Calculate how much the 2 gasket thickness's will give. I cant rem exactly how thick they are, but id say 1.2-1.5mm would be a good guess

http://www.csgnetwork.com/compcalc.html