The V8 Owners Forum -

Hi all,

I understand 4.0 pistons can be used with a 4.6 crank and rods, is this true? The benefit being an approx 10:1 comp ratio i believe.

If so what's required to use them, that being do i need to do any special machining etc or will they "go straight in" with the 4.6 bits?

'I understand 4.0 pistons can be used with a 4.6 crank and rods, is this true? The benefit being an approx 10:1 comp ratio i believe. '

Absolutely correct

'If so what's required to use them, that being do i need to do any special machining etc or will they "go straight in" with the 4.6 bits'

Nothing, straight swap - free performance upgrade that anyone building a 4.6 should consider. Crown height, ring pack, pin diameter, etc all identical - dish volume is the only difference. They come up on Fleebay alot, as bizarely no-one seems to want them.

My spread sheet calcs 10.6:1 with comp gasket and 28cc heads or 9.5:1 with 36cc heads.

Obviouosly check valve clearance - but you'd need to do that with any performance cam even with the 4.6 pistons.

Russ

i will need to get my block machined to take the 4.6 crank wont i (early 3.9)

Does your block have the cross bolt bosses tony?

If not then your block won't clear the 4.6 cranks couterweights i believe.

Thats why i got hold of the interim block. You'd also need to get the cranks mains turned down.

Thanks for the info russel, thats the sort of c/r i had in mind and seeing as i have the tvr heads i can also have the reliability of comp gaskets as they have 28cc chambers so its all good!

I'll be needing to get some cutouts put in the pistons as i want to use one of Rob at v8 dev's stealth cams for this engine no 4.

So what c/r are the 4.0's then? 9.35:1 aren't they? Do the 4.6 rods and so on put the piston further up the bore to get the higher c/r?

no the 4.0 have a smaller dish in them pistons are the same otherwise. Bore is the same but stroke isnt so 4.6 compresses more volume so to keep compression lower has larger dish. Put 4.0 pistons (smaller dish) in the longer stroke gives higher compression.

ihatesissycars wrote:Does your block have the cross bolt bosses tony?

If not then your block won't clear the 4.6 cranks couterweights i believe.

Thats why i got hold of the interim block. You'd also need to get the cranks mains turned down.

Thanks for the info russel, thats the sort of c/r i had in mind and seeing as i have the tvr heads i can also have the reliability of comp gaskets as they have 28cc chambers so its all good!

I'll be needing to get some cutouts put in the pistons as i want to use one of Rob at v8 dev's stealth cams for this engine no 4.

nope mines an early 3.9 block,
can't the block be machines to take the crank surely it can be done?

I looked ino doing tha wih my present engine which is the same block as yours ony, you'd be beter off machining down the counter weights but then you run into balancing issues. It's be alot easier to just source a later 4.0/4.6 block or use an interim block.

SO, what cc is it? still a 4.6??? but higher comp yeah???


GAV you still got that spare intrim block

It stays a 4.6 just higher comp as you say.

And, no i sold it to some guy in malta, sorry!

While we're on the subject of 4.6 bits and so on rumor has it each component (cranks, rods, pistons etc) are made in such a way as to ensure that they all within a certain weight meaning they require no additional balancing meaning i could chuck together any old 4.6 bits and they be ok to use like that with no additional balancing? If you get what i mean?

Yes within the factory balance tolerance but that's nothing new or special to the 4.6. Trouble is your going to kill the balance by notching the pistons for the valves. Course it will still run fine but depending what you want the engine to be you might be better still getting it all balanced. Its all comes down to how much you want to spend at the end of the day but if your revving it I would say get it balanced.

russell_ram wrote:... Absolutely correct

Nothing, straight swap - free performance upgrade that anyone building a 4.6 should consider. Crown height, ring pack, pin diameter, etc all identical - dish volume is the only difference. They come up on Fleebay alot, as bizarely no-one seems to want them.

My spread sheet calcs 10.6:1 with comp gasket and 28cc heads or 9.5:1 with 36cc heads.

Obviouosly check valve clearance - but you'd need to do that with any performance cam even with the 4.6 pistons.

Russ

Russ,

It would be great if you could post stats on the following if you have them to assist others calculating alternatives in engine builds for 30A and 38A RV8 blocks:

RODS
Part No
Rod len mm
Mass gm

PISTONS
Part No
Pin diam mm
Comp height mm
Dish cc
Dish mm
Mass gm

I am particularly interested in specs on the 4.0 and 4.6 pistons (38A), as being in Australia, I am seeking locally produced alternatives.

Owen

Gav, the crank will need rebalancing as I suspect the pistons will have different weights because a dished crown has a different mass to a shallow crown. No doubt you will fit arps, and 4.0/4.6 rods are completely different in length/mass. So, whatever combo you put together it will need a balance job.

I'll post some stuff to you nearer the end of the week:)

Boosted.

Cheers Mike!

I suspected a rebalance would be in order but if the original balance was ok to go then saving a few quid is always welcome

QUESTION!

I have a number of options available to me ref building this engine now.

I can use tin gaskets with these RS heads and the chabers are 27cc.

Can anyone work out what my c/r would be with the 27cc chambers, tin gaskets and 4.6 9.35:1 pistons and then again with the 4.0 pistons?

Gav